Technical Proposal for the Hl D~tector

H 1 Collaboration

.-'\achr.n - Davis - DESY - Dortmund - Ecole Polytechnique - Glasgow -Hamburg - Houston - Lancaster - J,,iverpool - Manchester-·

Mo.c.;cow - MP! Miinch('n - Northea..<;tern - Orsay -Paris - Rome - RAJ,, - Sac/ay -Wuppl'rta.J - Zeut.hf'n - Z1irich

Mar<'h 25, 1986

Technical Proposal -for the H 1 DC'tector

H 1 Collaboration

:'\<l.Ch<'n - Davis - /)f~SY - Dortmund - EcolP Polyt.f>rhnique - Glasgow -TlnmbrJrR - llnu.-.f.nn - /.,;wcasf,pr - f,ivt>rpool - J\,fat1chC'slPr­

l\10.<>row - Ml'! Miinch<'11 - ,\ ·ort.hca..c;tern - Or!'ay -/'ari." • U om<' - RA. I, - Sa.clay -\Vuppnt.a/ - h•ut h<'TI - Z1irid1

Contents

1 INTRODUCTION 4 1.1 Detector Scheme and Design Considerations 4 1.2 The Large Coil Solution . 5

1.3 Calorimetric Measurements 6 1.4 Interplay between Calorimeters and Tracking Devices 6 1.5 Charged Lepton Identification and Measurement ,7

1.6 Summary 7

2 Superconducting coil 10 2.1 General JO :?.2 Magnet ic designs 10 2.3 Forces 10 2.4 Conduct.or . 12 2.5 Coil 12

2.6 Cryostat . 13 2.7 Cryogenic syst.ern 14 2.8 Power Supply and Protect.ion System . 15 2.9 Instrumentation and Control 16 2.10 Assembly of Solenoid . 16 2.] 1 Testing the Solenoid at RAL 17 2.12 Transport to DESY and Installation 17 2.1 :~ Operation and maintenance 17 2.14 Compensation magnet .. 17

3 Iron Structure and Magnetic Field 27 3.1 Description of the lron Structure 27 3.2 Magnetic Field 28 :~. :1 Assembly Procedure 29

4 Iron Instrumentation and Muon Detection 36 4.1 Introduction . 36

4.2 The Streamer Tube-Chamber System . 37 4.3 Mechanical and Electrical Installations . 39

4.4 The Gas System for the Streamer Tubes 39 4.5 Front End Electronic!i 39 4.6 Performance . 40 4.7 Forward Muon Spectrometer 41

5 Calorimeters 5.1 Introduction . . . . . . 5.2 LAr Calorjmeters ... 5.3 LAr Cryogenic System 5 .4 LAr Stack System . . 5.5 LAr Electronic System 5.6 Calibration and Monitoring of the LAr System 5.7 LAr Beam Tests at CERN and DESY 5.8 Backward Calorimeter ... . 5.9 Plug Calorimeter ........ . 5.10 Electron and Photon Taggers .. 5.11 Performance of the Calorimeters

6 Tracking 6.1 Introduction .......... . 6.2 Central Drift Chambers ... . 6.3 Forward Track Detector (FTD) 6.4 Resolution and Performance of the Tracking System 6.5 Trigger Proportional Chambers 6.6 Scintillation Counters . . . 6.7 Drift Chamber Electronics .. . fi.8 Gas Systems ......... . 6.9 Jnstallat,ion of the Tracking Detect.ors

7 Background at HERA 7 .1 Synchrotron Radiation ...... . 7 .2 Jnteractions with the Residua) Gas 7.3 7.4

Off Momentum Beam Particles Cosmic Rays

8 Trigger 8.1 Introduction . 8.2 Requirements . . . . . . . . . . . 8.3 Triggers from Spt-cific Detectors . 8.4 Main Trigger System ...... .

9 Data Acquisition and Control of the Experiment 9.1 Introduction . 9.2 Data Rates ... ..... . 9.3 Data Flow . . . . . . . . . . 9.4 Control of the Experiment .. 9.5 Choice of the Bus System . 9.6 Choice of Online Computers .

10 Off-Line Computing l 0.1 Gen era.I Concept . 10.2 Monte Carlo Programs . 10.3 Software Standards . 10.4 Networks ..... 10.5 CPU Requirements .

2

.'>8 58 58 61 76

95 llO 111 112 117 124 124

142 142 146 158 169 175

181 184 185 188

191 191 191 194 194

200 200 200 202 213

220 220 220 221 223 226 226

231 231 232 233 233 234

11 Luminosity Measurements 11.1 Introduction ................ . 11.2 Main Detector Luminosity Measurements 11.3 On-Line Relative Luminosity Monitor 11.4 Electron Tagging 11.5 Conclusions . . . . . . . . . .

12 Performance of the Detector 12.l Calorimetric Measurements ........ . . 12.2 Charged Lepton Identification and Measurement 12.3 Summary of Expected Detector Performance

13 Installation 13.l Detector assembly ......... . 13.2 Installation of auxilary equipment .. 13.3 Shielding ...... . 13.4 Modes of operation.

14 Safety 14.l Flammable Gases . 14 .2 Inert Gases J 4 .3 Solenoid . . . . . .

15 Finan<'e ~ ResponsihilitiE>s, Timeschedule and Manpower

16 lnfragtructurE' and Requirements from DESY 16 .t Infrastruture in the Experimental Hall .......... . ..... . 16.2 Electrical Power and Cooling for Detector Components . ...... . 16.3 Space for Control Rooms and auxiliary Rooms in t.hc Hall Building . 16.4 Space above Ground . . . . . . . . . . . . .... 16.5 Services for Design, Construction and Installation. 16.6 Requiremf'nt.s from Safety .. 1().7 Offices and Laboratory Space .. 16.8 Test, Beams ........... . 16.9 Data Acquisition and Computing 16. l OReq uirements for Operating the Detector

A List of Participants

3

238 238 238 239 240 240

243 243 244 246

251 251 252 253 254

261 261 261 262

264

271 271 272 272 273 273 273 273 274 274 274

275

Chapter 1

INTRODUCTION

The HERA accelerator is being constructed in order to investigate lepton-quark interactions at very high energy. The experimental program at HERA will include searches for new physics, such as massive new bosons, supersymmetric particles, lepton and quark substucture, heavy leptons and quarks associated with right-handed currents. It. also includes extensions oft.he tests of the Standard Model to regions of high Q 2 and to higher families of quarks and leptons.

To accomplish the goals of HERA, the detector has to meet the following general requirements. It must have a high degree of hermeticity in order to investigate phenomena involving energetic neutrinos or other non-interacting secondary particles. It must. permit excellent energy flow measurements for the inclusive measurements of ~eutral Current and Charged Current interactions based on good energy resolution, fine granularity and absolute energy calibration both for the electromagnetic and hadron calorimeters. Muon identification and energy measurement. must be very good for the New Physics searches, for phenomena involving heavy flavors, and to preserve hermeticity when high energy muons are present. Electrons play a key role for HERA physics which demands that one strive for the best electron energy resolution and identification.

1.1 Detector Scheme and Design Considerations

The basic detector concept has remained unchanged with respect t.o the letter of intent. We have however optimized the design in terms of physics performance, technical realisation and financial implications. Here we give a short description of the overall detector.

The detector arrangement is shown in Figures I.I and 1.2. Following the numbering of Fig.1.1 , we have

• Central and forward tracking provided by a central jet- chamber interleaved with two z-chambers and MW PC's {l} and a forward tracker {2} consisting of a series of radial and planar driftcham­bers interleaved with three layers of MW PC's and transition radiators.

• The EM calorimeter utilizing Pb plates and Liquid Argon in the barrel and forward region { 3} and a lead scintillator sandwich in the backward region {5}.

• The hadron calorimeter { 4} using liquid argon with stainless steel absorber plates. A weight­ing procedure is used to "compensate" for different responses to electromagnetic and hadronic components.

• The superwnducting coil and its cryostat {6} outside t.hc hadron calorimeter.

• Surrounding all of the above a set of iron plates {7} to contain return flux. Interleaved with plastic streamer tubes it acts as a "tail-catcher" for the hadron calorimeter and also as a muon filter and tracker.

4

• Muon detection provided by three layers of muon chambers {9} in the barrel and forward region complemented by a forward muon spectrometer consisting of a magnetized iron toroid {8} and four layers of drift chambers {9}.

• A plug calorimeter to detect small angle hadronic energy {10} built as a. copper silicon sandwich which closes the detector down to an angle of 0.7 degrees.

• A compensating magnet { 11} compensating the axial field of the large coU.

The event topology of HERA collisions necessarily lead::i to an a.symmetric detector design. The center of mass of the collision products moves fast along the proton direction. As a consequence, the quark fragments and especially the decay products of heavy particles produced in the collision are ~oosted along the proton direction. As an example about 50 % of all leptons and of the hadron fragments due to the decay of new heavy Rarticles are typically emitted into polar angles Jess than 25°. The Hl detector design follows these requirements from physics and tries to prov ide a smooth and homog~neous detector response from small forward angles up to backward angles.

• We have chosen a solution with one cryostat for the liquid argon calorimeters in order to avoid detector inhomogeneiti~s and dead material in the transition region between forward and barrel calorimeter (15° to 25°) where energy resolution and energy calibration are of prime importance.

• The cont.ainment of the calorimeters changes smoothly as a function of polar angle following the kinematic energy limits for electrons and quark jets as discussed in chapter 5.2.6

• We provide a Jargf> and homogeneous magnetic field in the forward and barrel tracking regions by the choice of a large solenoid. The drift chamber arrangement gives a momentum resolution of ~p/p2 ~ .003 for isolated tracks in the angular range 7° ~ 0 ~ 150° and good pattern recognition and momentum resolution even for tracks inside jets as discussed in section 6.4. This is vital to measure and identify leptons, to recognize jet structures of events and to make exclusive measurements for heavy particle production.

• Lepton identification requirements change drastically as a function of polar angle since track densities and momenta are much larger in the forward than in the barrel region. As a consequence the transverse and longitudinal segmentation of the electromagnetic calorimeter changes with f) going from small 3 x 3 cm2 towers and four longitudinal segments in the forward direction to 8 x 8 cm2 towers and three longitudinal segments in the backward barrel region. Moreover the identification of high momentum leptons at forward angles has been improved by adding transition radiators for electrons and a toroidal spectrometer for muons.

1.2 The Large Coil Solution

Placing the large superconducting coil outside the calorimeter has the following advantages:

• It provides a strong and homogeneous field at forward angles and in the barrel region for tracking.

• It minimizes the amount of dead material in front of the electromagnetic calorimeter. This is essential for keeping the good intrinsic energy resolution and e/7r separation of the calorimeter as discussed in chapters 5.5 and 12.

• It gives a large field volume in the liquid argon calorimeters which allows to improve and check the muon momentum measurement to an accuracy of 173. This is achieved with moderate requirements on muon chamber precision and alignment (several mm).

5

• It provides a natural iron tail catcher since the thickness of the return yoke is well matched to the requirer:nents of calorimetry. The iron is moreover magnetized so that it bends muons in the r-¢ plane, which can be used for an independent measurement of the muon momentum.

• It reduces the overall size and weight of the calorimeter.

1.3 Calorimetric Measurements

The hadronic energy flow measurement is needed to infer and measure neutral penetrating particles like e.g. neutrinos or photinos, to measure the energy of neutral hadrons such as K2 or neutrons , and in general for accurate inclusive measurements of deep inelastic interactions.

Ht-calorimet ry is based on a large liquid argon calorimeter, backed up by an iron tail catcher and complemented by a warm electromagnetic backward calorimeter and a forward plug calorimeter. The choice of liquid argon for the main calorimeter is motivated by requirements of - stability and easy calibration - fine granularity for e/1£-separation and energy flow measurements - homogeneity of response These advantages of the liquid argon technique should not be spoiled by the technical realisation. We have therefore made a strong effort to minimize calorimeter inhomogeneities such as cracks, dead material (like support structnres) and leakage. The absorber materials (lead for the electromagnetic and steel for the hadron stacks) allow good electromagnetic energy resolution of o-( Ee)/ Ee ~ . 10/VEe . Hadron energy measu rements will use a weighting technique based on the longitudinal and transverse energy distribution to obtain an e/n-ratio of 1.0 and an energy resolution of about <J(EH )/EH = i>:'/.Vc./ \/(EH) 8 2%. This is appropriate for inclusive measurements as discussed in chapter 12.

1.4 Interplay between Calorimeters and Tracking Devices

Calorimeter and tracking devices have some fields of application where they are unique such as hadron energy flow measurements for the calorimeter and sign determination and exclusive measurements for tracking. Apart from these obvious tasks, both detector sections are designed to provide a maximum of supporting and redundant information.

• The rneasurement of the jet struct.ure of events is based on the combined information of tracking and calorimeters. Tracking is the easiest and most sensitive means of seeing jet topologies whereas calorimetry permits precise energy flow measurements

• Good electron identification and energy measurement relies on the combined information from tracking and calorimetry. The comparison of calorimetric energy measurement of electrons with the corresponding momentum measurement in the tracking chambers provides the absolute energy calibration of the large electromagnetic calorimeter segments and improves e/tr-scparation by energy-momentum comparison and the matching of t ra.d impact and shower center.

• The tracking trigger system based on MWPC's and fast drift chamber outputs provides a stand alone trigger and can also be used to cross-check and support calorimeter triggers. At the same tirrw bot.h chambers and calorimeters will determine independently t.he bunch crossing for each trigger . For these purposes the trigger segments of the MWPC's are matched to the trigger tower structure of the calorimeter.

• Background rejection in the different trigger levels can use combined information from tracking and calorimetry. The combination of vertex determination by MWPC's and calorimetric PT triggers is very useful to reject beam-gas events and events due to scraping protons a.s explained in chapter 7.

6

This built-in redundancy in the detector design is intended to give a lot of flexibility to operate the detector, to check its performance and to adjust it to as yet unknown running conditions.

1.5 Charged Lepton Identification and Measurement

Charged leptons are expected to provide important signatures for new heavy particle production. The detector offers charge determination for leptons up to momenta of 150 GeV and polar angles down to 9° for electrons and 3° for muons. It covers backward angles up to f) = 172°.

Electron identification is based on fine transverse and longitudinal shower shape measurement in the electromagnetic calorimeter, the comparison of track momentum with the calorimetric energy and of the shower position with the impact point of the track. It is complemented by dE/dx- measure­ments in t he tracking chambers including transition radiators at forward angles. In summary the detector offers all handles which could be useful for identifying elE:ctrons even in the vicinity of other particles and compares in this respect favourably with other existing or planned detectors. Electron identification will be further discussed in chapters 5,6 and 12.

Muons are identified by their penetration through the liquid argon calorimeters (5 to 8 absorbtion lengths) and the instrumented iron yoke ( 4.5 to 9 absorbtion lengths). They are tracked by two layers of chambers inside, outside and in the middle of the iron yoke and in additional layers of chambers between the iron plates. Muon momenta are best measured in the central t rackers up to momenta of abou I. 50 Ge V. ln ad di ti on the large bending field inside the calorimeter and the return yoke allow the improvf:ment. of the momentum resolution a.t high momenta and, more importantly, to check the momentum measurements of t he inner t rackers. T hus the muon ba.c:kground due to K ~ µ decays inside the tracking volume can be suppressed.

The quest,ion of lepton identification is further discussed in chapter 12.

1.6 Summary

In summary the proposed detector is well matched to the known physics requirements of HERA and will be a powerful tool for the exploration of new phenomena. The design of all detector components and its performance will be given in chapters 2 through 9. A summary of performance is given in chapter 12.

7

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4 HADR. LIQU. AR. CALORIM.

5 BACKW. ELECTR. CALORIM.

6 COIL

7 JNSTRUM. IRON

8 IRON TOROID

9 MUON CHAMBERS

10 PLUG CALORIMETER

11 COMPENS. COIL

12 MACHINE QUADRUPOLE

13 CONCRETE

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39613

Chapter 2

Superconducting coil

2.1 General

The design concept of the detector leads to a magnet configuration with the coil situated outside the electromagnetic and hadron calorimeter . The resulting size of the coil ( ....... 6 m diameter) , the magnitude of the ampere turns (7 x 106 ) and the need for tail-ca.tching calorimetry imply a superconducting solution. A central field value of 1.2 T has been chosen, based on a consideration of the tra.cker performance, coil cost and the need to keep the coil R & D to a minimum.

Figure 2.1 shows a section through the propoHed superconducting soh~noid and the main solenoid parameters are listed in Table 2.J. The superconducting coil is fabricated in four sections; two centre sections of single layer winding and two outer sections of dou blc layer winding to improve the uniformity of the field. The coil is wound directly insidE· an aluminium alloy cylinder which also forms the main structural member for reaction of magnetic forces. The conductor iH cooled indirectly by circulation of two phase he]ium in pipes attached to the aluminium alloy cylinder. To satisfy thermal, mechanical and electrical requirements , each coil unit is a fu lly bonded and insulated structure. The coil is mounted in a stainless steel vacuum vessel designed to cover all safety aspects . The cryostat incorporates intermediate helium gas cooled shields and superinsulation shielding to minimise radiation heat losses.

2.2 Magnetic designs

The coil geometry and its position relative to the iron have been optimised using the axisymmetric design code PEZD. Because the magnetic coupling to the yoke structure is relatively weak, a split coil design with strong end compensation is necessary to provide the homogeneity. Figure 2.2 shows the magnetic circuit with the variation in Bz over the region of the central and forward tracking chambers. The homogeneity satisfies the requirement ABz < ±3% over each detector section.

2.3 Forces

Figure 2.3 shows schematically the various forces acting on the coil structure. In this figure , the radial force component is present.ed as an equivalenL internal magnetic pressure. The radial pressure is supported by hoop tension in the conductor and aluminium alloy support cylinder. In order to avoid excessive non-elastic deformation of the conductor during excitation of the coil , the support cylinder thickness is defined to limit conductor tensile strain f < 0.1 %. Large axial magnetic forces are generated because of the relatively poor magnetic coupling between coil and yoke, a situation which arises from the need to provide cryogenic access to the calorimeter. Integrated axial forces are also presented in Figure 2.3 to illustrate the magnitude of the net force generated between coil sections. Axial conductor forces are transmitted to the support cylinder as a shear stress at the resin

10

Geometric Total length cryostat (m) 5.75 Outer radius cryostat (m) 3.04 Inner radius cryostat (m) 2.60

Vacuum vessel material S.S.

Total length coil assembly (m) 5.16 Winding radius (m) 2.75

Weight of solenoid (tonne) 75 Radiation length (Xo) 4

Magnetic Central field (T) 1.2

Peak field (T) 2.2 Ampere turns 7 x 106

Stored energy (MJ) 120 Inductance (H) 7.84

Total number of turns 1256

Table 2.1: Magnet Parameters

bond interface and are typically~ 2 N/mm2. This is approximately a.n order of magnitude below the shear bond strengths measured on DELPHI coil matrix samples.

The use of an aluminium stabilised conductor within an aluminium alloy support cylinder results in minimal circumferential cool-down stresses. In the axial direction, the coil matrix includes glass fibre/epoxy insulation/bonding layers which increase the thermal contraction of the coil matrix relative to the support cylinder. Axial shear stresses will therefore occur at the coil/cylinder interface during cooldown. Finite element analysis shows a maximum thermal axial shear stress ~10 N/mm2, well within the possible shear bond strengths of ~40-50 N /mm2 . The forces described above are contained within the magnet structure and transferred to the support cylinder at 4.5 K. Nonsymmetric coil location will generate additional forces which must be transferred from the coil structure at 4.5 K to the vacuum vessel at room temperature. The presence of such offset forces has a significant effect on the design of the coil support structure within the cryostat and indirectly on the cryogenic heat loads at 4.5 K. Non~syrnmetrical axial location of the coil in the return yoke will generate an out of balance force with a force constant Fz = 20 tonnes/cm offset. These forces will be transferred to the vacuum vessel at the central position by low heat leak bump stops. Axial location of the coil within the yoke to better than l cm is expected. Non-symmetrical radial forces due to the radial offset of the coil in the return yoke have been computed with a force constant FR= 2 tonnes/cm. Forces at this level will be supported by the normal tie rod system supporting the weight of the coil.

In addition to these forces generated by non-symmetrical location of the coil, additional radial offset forces are generated by the non-symmetrical construction of the iron yoke. Jn order to allow separation of the yoke structure and best access for detector installation, the upper and lower sections of the yoke differ at. the junction between the end cap and barrel (see Figure 1.1). Since the iron structure is laminated, there are differing flux patterns in the upper and lower sections and a computed offset force of approx 20 tonnes downwards on the superconducting coil. This force is compa.rable with the weight of the coil and difficult to compute with accuracy, and so its effect on the coil support structure within the cryostat must be carefully considered.

11

Operating current Operating Central Field Operating Temperature

Critical current @ 3 T@ 4.5K Form of Niobium Titanium Composite

Cu:Sc ratio Cu resistivity ratio

Conductor Section overall aluminium Al resistivity ratio

Contact resistance (Composite/ Al) Shear bond strength (Composite/ Al)

Total length Piece length

5500 A 1.2 T 4.5 K

11000 A Rutherford cable

In range 1:1 to 2:1 80

26 mm x 4.5 mm > 500:1

< 10- 10 om >10 MPa 25.2 km 1.5 km

Table 2.2: Conductor Parameters

2.4 Conductor

The conductor is a Rutherford type superconducting cable stabilised a.nd protected by cladding with high purity aluminium in an ~xtrusion process. The conduct.or section is shown in Figure 2.1, with main paramet,ers listed in Tabl~ 2.2. The conductor is designed t.o satisfy two criteria, stable behaviour against the small energy releases likely to occur in a fully bonded winding under thermal and magnetic stresses and safety under quench conditions when the stored energy of the magnet mu5t be discharged without excessive temperature or voltage. The high purity aluminium substrate with high thermal conductivity and low electrical resistivity is essential to both criteria. For this reason, good electrical and mechanical bonding between the superconductor and substrate is specified in Table 2.2.

For stability and to give a safe working margin, the conductor is designed to operate well below the critical superconducting parameters; the design current of 5500 A being approximately 50% of the critical current in the peak field region. Measurements on conductors of this sect.ion have shown that localised energy impulses of > 10 J are required to initiate a quench. Energy rel(~ases in a well bonded and supported winding matrix are expected to be as much as an order of magnitude smaller than this stability level.

The design current is a compromise between a number of requirements, the linear current density required to generate the design field, stability, quench protection and cryogenic power levels. The quench protection criteria which determine the conductor section are described in section 2.8.

2.5 Coil

The coil is fabricated by winding the conductor directly inside the support cylinder. The coil geometry is very sjmilar to the DELPHI solenoid in both coil diameter and module size. Winding the HI coil will require only minor modifications.

The support cylinder performs the dual role of winding former and reaction member for electro­magnetic forces. The support cylinder is made out of aluminium alloy AL5083, a material which combines good welding arid machining properties with high resistivity at 4.2 K. The high resistivity property is required in order to minimise eddy current losses during powering and run-down of the magnetic field.

The cylinder will be fabricated by the forming and welding of plate material and will be finally machined on the inner surface to provide an accurate winding surface. The cylinder has four main sections which are bolted together after winding and impregnation of the coil sections.

12

Cooling of the coil is achieved by circulating two phase helium in pipes attached to the support cylinder. For successful operation, an indirectly cooled coil must be bonded securely to the support cylinder. This ensures efficient heat transfer to the support cylinder and allows the axia} magnetic forces and thermal stresses to be transferred to the support cylinder.

Conductor piece length is limited to approximately 1500 m by the aluminium cladding process. It is t herefore necessary to make joints between lengths within each coil module and between modules . Low resistance joints < 10- 9 - 10- 10 n will be fabricated by lapping the conductor over one complete turn (16 m) and welding the aluminium along the edges. Throughout the winding process, the conductor is monitored for any shorts to earth or turn to turn.

Ground plane insulation will be provided by a 1 mm layer of glass fibre epoxy laminate bonded to t he inner surface of the support cylinder before winding and electrically tested to 3 kV. Turn to turn insulation will be provided by a double butt wrap of 0. 125 mm epoxy impregnated glass tape applied to the conductor at the winding stage. The impregnated tape has a high abrasion resistance and provides sufficient pre-cure adhesion to hold turns in place as they are wound. During the winding process, a small amount of resin is injected between the turns and support cylinder to ensure 100% contact. This approach ensures that resin is distributed uniformly throughout the coil during the winding process and eliminates the risk of large debonded areas. T he coil is cured with axial and radial pressure applied at a temperature of 120° C in a specially built oven. The technique has been fully developed and tested for the DELPHI coil and all the equipment is available and compatible with the HI design .

2.6 Cryostat

The basic layout of the cryostat is shown in Figure 2.1. The coil assembly is support.ed wit.hin the vacuum vessel by long radial tie rods at each end of the coil. Gas cooled radiation shields, together with layers of superinsulation are i11corporated to minimise radiation heat Joss.

The vacuum vessel provides overall containment for the superconducting coil and allows an in­sulating vacuum of less than I x 10-5 Torr to be maintained. The vessel forms the main structural component for transferring the weight of the coil and radiation shields to the surrounding iron yoke.

By locating the vacuum vessel reinforcing ribs on the inside of the vessel it is possible to utilise the space between the ribs to mount the radiation shields and the superinsulation. Using this approach, a maximum radial dimension for the cryostat of 450 mm can be achieved . Services to the cryostat will he provided by two ports as shown in Figure 2.4. Electrical feeds, cryogenic pipework and monitoring are connected through a port in the centre section of the vessel which terminates in the buffer vessel housing transfer line connections and the main current leads. Vacuum pumping is via a single port at the base of the vessel with pumps housed beneath the base of the yoke structure.

Figure 2.5 shows the radiation length through the solenoid a.s a function of angular position. The angular distribution is presented to illustrate localised variations at flange positions.

The vacuum vessel is made of stainless steel in order to follow the state of the art technology established for DELPHI. The vacuum vessel comprises reinforced inner and outer shells connected by substantial end flanges. Each shell is fabricated in three sections as shown. Under normal operation, t he inner shell is subjected to hoop stress and the outer shell to buckling loads by a differential pressure of one atmosphere. Under fau lt conditions, such as a LHe coolant pipe fracture , L~e would be dumped inside the vacuum vessel. To meet this eventuality, the vessel will be equipped with a pressure relief valve and the inner shell designed for a buckling pressure of 0. 1 bar. Discussions will be held with the DESY Safety Group and t.he design principles fabrication and test procedures will meet the DESY and German codes of practice.

Radiation shielding will be provided by a series of cooled radiation panels installed between the stiffening ribs of the vacuum vessel. Two forms of shield are at present under consideration; full flow shield panels and piped flow with conduction panels. The finaJ choice will depend on compatibility

13

with fabrication, instalJation and cryogenic requirements. The cooling circuit supplying the radiation panels will be configured in such a way that they can be cooled either by helium gas when connected to a refrigerator, or by liquid nitrogen when under test at RAL.

The vacuum system will consist of a Roots pump/ rotary pump set for roughing the system down, with diffusion pumps for establishing a vacuum of sufficiently low pressure to enable the solenoid to be cooled down. An auxiliary rotary pump will also be installed to handle the pumping requirements of the double vacuum ring seals. The pumps will be located beneath the iron base as shown in Figure 2.4.

2. 7 Cryogenic system

The HERA cryogenic system consists of three large refrigerators and a cryogenic ring main, serving the collider complex. Each detector is allocated the following refrigeration power:-

• 500 watts supplied at. a temperature of 4.5-4.7K in the form of single phase helium at 3- 4 atmospheres.

• 1500 watts supplied in the form of gas arriving at 40 K and leaving at 80 K.

• 1.1 g sec- 1 of helium at 4.5 K for current lead cooling.

The helium service lines are as follows:--

• 4.5 K helium in

• 4.5 K helium out.

• 40 K helium in

• 80 K helium out

• Refrigerator bypass

• Warm helium from compressors

• Quench protection and presi;ure relief

• Current lead gas return

Solenoid cooling can be carried out in two different ways either by mixing the 4.5 K and warm gas st.reams in a controlled fashion or by programming the liquid nitrogen precooler in one of the refrigerators. The latter can only be used if the experimental programme allows the refrigerator to be dedicated to a single detector.

The proposed cooling system consists of a valve box for routing the helium service Jines, an in­termediate dewar, the internal cooling system of the magnet and the necessary transfer lines (Figure 2.6). Under normal running conditions the radiation shields are gas cooled using high pressure helium from the 40 K inlet circuit. The gas is circulated through the radiation shields and then returned to the 80 K outlet circuit.

The coil is cooled by circulating two phase helium at a temperature of approximately 4.5 K. The liquid is obtained by expanding supercritical helium supplied by the 4.5 K inlet stream into the in­termediate dewar. The two phase helium is pumped from the intermediate dewar through solenoid cooling circuit and back again into the dewar.

The current lead cooling gas is extracted directly above the liquid level in the intermediat.e dewar.

The estimated cryogenic heat load inventory is given in Table 2.3. Jn line with the practice adopted for HERA cryogenic systems, a safety factor of 2 has been applied to all heat loads except the transient

14

Heat loads at 4.5 K

Solenoid Watts Total Watts

Radiation 80 Conduction 80

Eddy current heating transient 50

210 210

Intermediate dewar plus valve box 200 200 Transfer lines 60 60 - 470

Current leads 2 leads @ 5500 A 1.25 g/sec

Heat loads at 60 K Radiation 800

Support conduction plus heat intercepts 800 Intermediate dewar plus tr<:J.nsf er lines 200

1800 1800

Table 2.:{: Cryogenic heat loads

eddy current heating which occurs in the support shell during coil ramping. Since this component is dependent on the field ramp rate, a factor of 2 in safety could easil y be achieve.cl by extending the coil ramp time to 1.5 hours.

Current lead mass flow requirements are based on two leads of 5500 A with a standard design figure of 1.1 watts/lOOOA. Again a safety factor of 2 has been applied to the mass fl.ow.

With the given safety factor applied, the heat loads at 4.5 K fall within the allotted refrigeration but without any allowance for compensating coil refrigeration. The allotted mass flow and cooling available at 60 K is insufficient to allow a safety factor of 2.

The cooling circuit consists of a parallel network of pipes attached to the support cylinder and the necessary pipe manifolds. In order to simplify cooling circuit fabrication and manifolding , a 1 mm layer of high purity aluminium will be bonded between coil and shell. This aluminium layer will greatly enhance the axial thermal conductivity in the coil sec:t.ion and mjnimise temperature rises between cooling pipes.

Heat loads to the solenoid from the bump stops and support rods will be intercepted by cooling channels which form part of the support cylinder cooling system and will be minimised by thermal intercepts connected to suitable points on the radiation shield cooling circuits .

2.8 Power Supply and Protection System

The power supply and quench protection circuit is shown in Figure 2. 7. The 5.5 kA supply wiJl charge the solenoid to full field in one hour. Under normal run down, the coil will be discharged through the diode-resistor chain by opening circuit breaker CB3. To minimise the cost and space required for discharge diodes a coil discharge time of two hours under normal conditions is proposed.

Under emergency or quench conditions, the coil is discharged by opening the circuit breakers (CBI and CB2), dumping most of the stored energy in the external resistor (RI). Quench protection

15

requirements are dominated by t,wo criteria; peak temperature in the conductor and peak voltage developed. While the ultimate constraints on peak temperature are burnout of insulation at 500-600 K, mechanical damage to the coil due to thermal stressing will occur at much lower temperatures. For temperatures below ~100 K, thermal expansion effects are negligible. Designing for a peak temperature T max ~ I 00 K, therefore represents an extremely safe option with regard to therma) stresses. The peak

temperature will occur at the point of quench origin. The peak voltage is limited by the electrical stresses induced in the ground plane insulation layers.

For the protection circuit illustrated in Figure 2.7, the peak voltage will occur at the instant of opening the circuit breakers and is given by Vmax = l 0 p R1. For design purposes, a maxirnurn operating voltage

of Vmax = 750 Vis chosen wit,h I 0 p = 5500 A and R1 = 0.136 0. The discharge of coil current. is characterised by the time constant r = L/(R1 + Re) where L =

coil inductance, R1 =... dump resistor and Re = internal resistance of coil.

Since the internal coil resistance is a complex function, the conductor section required for safe quench protect.ion has been defined using the adiabatic approximation:-

hr,,,,., c(T) _ 2

L G (T maz) (T) dT - J o ZR

Tu P 1

where c(T) and p(T) are the temperat.ure depend ent heat. capacity and resistivity of the a luminium ~ubst. rat.e, lc1 is the initial current density over the aluminium section and 70 is the operating tem­perat.1m!. For high purity alurniniurn with a resistiv it.y ratio RRR = 500, Tm.a ... ::-: 100 K, G(T max)~ f> / 10 1<; rn 4 sec and ./11 =- 4.$ / 107 Arn - 2 . F0r 10 ,, :-:- 5500 A, this determines t.hc conductor section t.o

be A : 120 rnrn2. For a <:onductor width of 4.5 rnrn defined by t.he linear current. density and operating rnrrtnt., the radial thickm:ss must. be 26 mm.

In practice, the internal rcsist,ance of the coil plays an important part in determining the current dc:cay as does the support cylinder which is closdy coupled magnetically and thermally. At the instant of opening the circuit breaker, a large current. is induced in t he support. shell ar1d this produces rapid

~wat.ing . Comput,er modelling suggests that this induced heating will r1ormalise the complete coil in ~2 secs producing a "q1Jench back". In fact, quench back of the TOPAZ coil has been identified during t.m;t.s at. KEK. Preliminary computer modelling of quench including magneti c aud thermal coupling to t.h<: shell suggests a peak temperature T ma-r. ~ 65 K.

2.9 Instrumentation and Control

Operatio n, contrcil and monitoring of the solenoid will be carried out fully automatically. The trans­ducers, cirr.uits, systems and software have already been selected and produced for the DELPHI solenoid.

2.10 Assembly of Solenoid

Adeyuate space, height, and lifting capacity are required for assembly of the solenoid. A special

hL1ilding has been allocated at RAL for this purpose and suitably equipped. As the lifting capacity at, high level is limited to 10 f.onnes the assembly is carried out on carriages with the assembled

cy lin<lcrs being cantilevered from a large end frame {see Figure 2.8). Most of the equipment exists, some a.ddif.ional equipment will be required , particularly to reinforce the inner vacuum shell which is

structurally weaker than the DELPHI vessel becauae the Hl coil cryostat does not have to support any of the detectors contained inside it.

16

2.11 Testing the Solenoid at RAL

Before transport to DESY in July 1989, the solenoid will be cooled down to 4.5 K and tested at RAL. Powering of the coil to 75% of operating current without the iron yoke will provide a sufficient test of solenoid operation before installation.

2.12 Transport to DESY and Installation

A route from RAL to the English coast at Southampton, suitable for the si.ze and weight of the complete Hl solenoid has been surveyed. 1'ransport from Southampton to the docks at Hamburg should pose no serious problems. The route between Hamburg docks and the HERA North HaH site has been surveyed and, subject to approval by the Hamburg State Authority, the fully assembled solenoid can be delivered via existing roads.

Transfer from ground level to the experimental hall will be in two stages as illustrated in Figure 2.9. The magnet system will be mounted on a specially built trolley and lowered to the level of the hall by crane. Transfer from the base of the shaft into the experimental hall will be made using the specially built trolley. The clearances available within the access tunnel are limited to ~200 mm on height. and width.

2.13 Operation and maintenance

The solenoid will incorporate a fully automatic monitoring system which will automat.ically shut down t.he system safely in thE~ case of a serious problem developing.

At first, it is proposed that suitable t.<!chnicians could he included in the RAL team as members of the Collaboration working at DESY. These technicians would be involved in the installation, test and initial operation of the magnet to become familiar with the system.

The technicians would have sufficient experience to handle most minor problems and would be able to get instant advice from RAL or call out specialist RAL personnel wit.hin 24 hours.

Tht! eventual responsibility for operating and maintaining the Hl solenoid will be subject to agree­ment to ensure that the responsibility is taken by personnel with a long term commitment to the H l experiment. At any rate, any procedure which is established to deal with fault conditions on a superconducting system of t.his siz.c must also take care of the safety aspect.s and a close liaison with the DESY infrastructure will be necessary.

2.14 Compensation magnet

A compensation magnet is required to achieve f B dl ~ 0 along the beam axis and its pa.rameters are summarised in Table 2.4. For a bath cooled magnet, the cooling power needed during operation has heen estimated to be 10 watts or 0.5 g/sec He flow input al, 4 .5 K for the heat. shield and 0.03 g/sec for t.hc current. leads.

Maximum field value 6 T Inner diameter of cryostat 0.5 m Outer diameter of the iron 0.9 m

Iron yoke thickness 0.17 m Length of iron yoke 2.3 m

Table 2.4: Compensating magnet parameters

17

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21

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26

Chapter 3

Iron Structure and Magnetic Field

3.1 Description of the Iron Structure

The iron structure around the superconducting solenoid serves as a flux return for the magnetic field and provides the absorber material for the tail catcher calorimetry and for muon identification. It is divided into three parts , each having its own rolling system. These parts are the base plate, which carries the solenoid, the liquid Argon calorimeter and the tracking devices, and the two barrel structures with end plates . A perspective view of the detector and the iron structure is shown in in Fig. 3.1. In this figure the detect.or is closed, while Fig. 3.2 shows th~ barrel sections opened to give access t.o parts oft h!' detector. Fig. ~~.:~shows a cross- sect.ion through th<> detector perpendicular to

the beam axis. The iron st.ructur~ has t.o support the weight of the solenoid and t.h~ liquid Argon calorimeter.

The corresponding forces are transferred through the iron onto the legs and are finally brought ont.o the rails via rollers.

The iron structure has gaps and holes for accommodating cables , cryo lines for the solenoid a.nd the liquid Argon system as, well as vacuum pipes for pumping the insulating vacuum in the two cryostats. Furthermore there are holes for the beam pipe, the interaction quadrupoles and the compensating solenoid.

3.1.1 Iron Lamination

The return yoke is laminated in order to accomodate the streamer tubes for calorimetry and muon chambers in the gaps between plates . The thickness of individual absorber plates is chosen such that the energy resolution for hadronic showers, which ex t.end beyond the main calorimeter into the yoke , is not deteriorated (cf. Chapter 4). A sampling thi ckness of 10 times 7.5cm is adequate since the energy resolution does not depend critically on the segmentation. The gap height for chamber installation is chosen to be 25 mm in t.he barrel part, to house 18 mm thick chambers, and 35 mm in the end caps for 25 mm chambers. At a depth of 30 cm iron a double layer of muon chambers will be inserted into 50 mm wide gaps. Further double layers will be installed behind the last iron slab and in the gap between the barrel iron and the outer concrete shielding.

3.1.2 Shielding

The experiment is designed to be self-shielding. In order to keep the radiation level in the experimental hall at an acceptable level, even in case of a loss of the entire proton beam in the interaction region , t.he material of the hadron calorimeter and the return yoke is not sufficient to stop slow neutrons. Thus an additional layer of 50 cm concrete has to be mounted onto the barrel iron.

27

3.1.3 Rails

As shown in Fig. 3.1 the three units move on two rail systems, one for the base structure, and one for the two barrel sections. Each unit is driven by hydraulic pistons which are attached to the rollers. The rolling elements themselves have a diameter of 120 mm which is big enough to guarantee a smooth movement with small friction.

The total weight of the detector is about 2500 tons so that an effort has to be made to keep the deformation of the rails and the floor in the HERA haJI small. The maximum deformation occurs when the detector units are wheeled over the cable tunnel w}1ich crosses tbe hall underneath the beamline, when a deformation of~ 1.5 mm is expected which has to be absorbed by the supporting elements in the legs.

3.1.4 Electronics Trolley

A trolley with three floors is attached to the iron structure, and extends into the long side of the hall. There the bulk of the electronics for the calorimeter and t.hc tracking system as well as the first level trigger circuits will be installed. The lower floor moves together with the base plate, while the two upper floors are connected to the barrel structure and move over the lower part of the electronics trolley (cf. Fig. 3.2). Additional space for electronics racks and cryogenic installations is provided on the short side of the hall.

3.2 Magnetic Field

3.2.1 Field Strength

The iron thickness is determined by t.he magnitude of the magnetic flux and by t.he saturation proper­ties of the material. As indicated in Fig. 3.4, the maximum field in the iron occurs at the position of the correction coils. The thickness is chosen such that the estimated fringe field at a distance of one meter from the iron surface is less than 0.005 TP-sla. This requirement is met by 75 cm of low carbon iron for a central field of 1.2 Tesla. Under these conditions the innermost plates will be saturated up to ~ 1.4 Tesla.

The average magnetic field in the region of the tracking devices is 1.2 Tesla. It is homogeneous so that. all second order field dependent corrections for the conversion of drift, time into space coordinates are small. The field homogenuity does not depend on details of the iron struct1m· such as position of holes and exact size of gaps. The maximum variation of the magnetic field over t.he sensitive volume is ±3% as may be inferred from Fig. 2.2. The integrated field or1 t.he axis of the magnet is J Bdi = 8.4 Tesla m.

3.2.2 Forces

Assuming a field strength of 1.2 'l'esla in the center of the solenoid at the position of the tracking devices, a force of about 1300 tons acts on the end walls. This force is transferred to the barrel modules. In order to minimize the deformation of the st.eel plates in the end walls, an iron cylinder is screwed ont.o the laminates around the beam hole and vertical plates arc bolted to the iron structure at the surface of the vertical split where the two halves of the barrel structure meet. The right half of Fig. 3.5, which shows a cross section through an end wall also shows the additional iron attatched to the vertical surfaces.

The force acting on the barrel part of the iron structure is largest at the position of the correction coils. It corresponds to a pressure of about 2 Newton/ cm2 in the direction of the magnet axis. The resulting deformation of the iron structure is small, well within the overall mechanical tolerances.

28

Thf' gaps and holes in the iron structure are not symmetric with respect to the coil resulting in a longitudinal force on the solenoid. This force has to be absorbed by the feet of the cryostat as a shear force.

3.3 Assembly Procedure

The assembly of the iron structure in the experimental hall is complicated by the requirement that the weight of individual pieces is limited to 40 tons by the capacity of the cranes in the hall and the access shaft. In order to meet these requirements each octant barrel segment is split longitudinally into two halves and each half segment is further subdivided into three layers with 4, 3, and 3 iron sheets of 7 .5 cm thickness, respectively. For the iron plates of the end walls the weight limit is reached with two plates. These units, the heaviest with a weight of about 35 tons, are welded and machined at the manufacturer.

The assembly of the iron structure in the experimental hall follows a sequence in which first the rollers are put onto the rails before individual modules are attached to the support structure carried by the bogies.

3.3.1 Assembly of the Base Structure

The prefabricated st.eel slabs are screwed and welded t.o the support structure on the rollers. The handling of the individual modules is done with a crane having two hooks of 20 tons each.

3.3.2 Assembly of the Barrel Modules

The assembly of the barrel modules with the end walls has to meet the requirement that the longitu­dinal axis of the iron structure has to coincide with the beam line which has a slope of 6 mrad with respect to the floor of the hall. Therefore a scaffold has to be erected first which serves as a reference for the alignment of the end walls and at the same time keeps the iron slabs in place and prevents them from topling. The individual modules of the end walls are bolted together. The use of the steel scaffold in erecting the iron structure is indicated in Fig. 3.5 and 3.6.

After erecting the end walls the barrel modules are attached. The joint between the barrel section and the end walls is done first by screws to fix the relative position of the two parts and welding afterwards. Consecutive radial layers of the barrel structure arc also first fixed hy screws and then welded together to give the final rigidity to the structure.

In order to avoid major adjustments of the iron parts during assembly in the experimental hall, the complete iron structure will Le preassembled at the manufacturer by using bolts only to keep the individual parts in place.

29

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32

(

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33

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34

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Figure 3.6: Iron structu~e during the assembly phase. (b) C ross section parallel to the beam line with mounting scaffold and service platform, which is adjustable in higlit .

35

(

Chapter 4

Iron Instrumentation and Muon Detection

4.1 Introduction

The iron yoke for the magnetic flux return is fully instrumented in order to measure the hadronic energy leaking out of the main calorimeters ( see chapter 5 ) and to identify and measure muons.

general layout

The iron structure has been discussed in detail in chapter 3. The barrel part of the yoke consists of 16 wedge-shaped modules ( hexadecants ) : 6 with a length of 7.35 rn form the lower base part and 10 with a length of 9.60 m form the upper part. The yoke is longitudinally divided into 10 slabs of iron, each with a thickness of 7.5 cm. In the barrel {forward endcap) we have three gaps of 2.5 cm ( 3.5 cm ) followed by one ga.p of 5 cm and five gaps of 2.5 cm ( 3.5 cm ). The backward endcap has nine gaps of 3.5 cm.

The gaps are equipped with gas counters operating in the limited streamer mode. The counter system contains essentially two parts :

• The muon detector consisting of 3 ( 2 ) double layers of streamer tube chambers in the polar angular range between 25° and 130° (between 5° and 25° ). The system is schematically shown in fig . 4.1. It allows for efficient and precise measurement of track coordinates of penetrating particles in the bending plane of the magnetic. field inside the yoke and along the beam direction.

• The tail catc:her consisting of 11 single layers for the measurement of the hadronic energy by using the pulse height information of threefold longitudinally segmented towers. The polar angular range is between 5o and 175°. The towers ate indicated in fig. 4.2. Except in the backward direction, these layers are also equipped with strips parallel to the wires to track penetrating particles.

This system is very similar to those which are at present under construction for LEP and SLC detectors. We intend to take as much advantage as possible of all experiences obtained in these experiments concerning fabrication and performance of the tubes as well as the developments for the read out electronics.

The iron instrumentation syst.em is supplemented by a forward muon spectrometer in the proton direction covering polar angles between 3° and 17°. The spectrometer consists of a toroid with a field of 1.5 T and 2 double layers of drift chambers in front of the toroid and behind it. The set up is

36

shown schematically in fig. 4.1. Additional multiwire proportional chambers are foreseen for trigger purposes.

4.2 The Strean1er Tube-Chamber System

The basic element of the streamer tubes is a PVC profile consisting of either eight or seven square cells. The wall thickness of the cells is 1 mm and the active area amounts to 0.9 x 0.9 cm2• The wire with a diameter of 100 µm , made of Be-stainless steel, is mounted at the center of each cell ( see fig. 4.3 ). The profiles are open at the top, so that the wires can be easily inserted. The three inner walls are coated with a graphite varnish yielding a surface resistivity of ,.._,500 kOhm per square ( see fig. 4.3 ). The question of closing the profiles at the top with covers and what their surface resistivity should be is being investigated ( see below ). The wire is at positive potential, the coating is at ground potential.

Two profiles: either 8+8 (forming one 16-fold module) or 8+7 (forming one 15-fold module) are enclosed in gas-tight PVC boxes. The end flange on one side contains holes for gas inlet and outlet as well as the connectors for high voltage and ground ( see fig. 4.3 ). An appropriate combination of 16-fold and 15-fold modules is mounted together to form chambers matching well with the lateral width of the slits inside the iron. The boxes are sandwiched between PVC boards with strips and pads ( aluminium, thickness 40 µm ) coated onto the inner surfaces. For shielding the outer surfaces are also coated with 40 µm thick aluminium. Most of the strips are 4 mm wide positioned every 10 mm; they arc oriented either parallel or perpendicular to the wire direction ( see fig. 4.a ). The induced signals are taken frorn the strips ( digital read out ) for t.racking and from the pads ( analog read out, ) for the hadronic energy measurement. Pads from successive layers are grouped electronically together to form projective towers.The size of the pads is such that the towers match those in the hadronic part of the liquid argon calorimeters ( see chapter 5 ) . The subdivision in the polar angle varies with the polar angle as shown in fig. 4.2 , whereas in azimuth it is 72-fold throughout ( 6.ip ~ 5 ° ).The pad sizes vary from .....,5 x 5 cm2 in the extreme forward direction over -4o x 5o cm2 around a polar angle of ,._,,35o to .....,30 x 30 cmZ at an angle of -9o0 .

The streamer tubes in the barrel part are oriented along t.he beam line. Two chambers of different length are inserted from both ends into the iron slits of the hexadecant:.-;. The chambers arc st.aggered in the beam direction in successive layers in order to avoid discontinuities lining up along a ray pointing to the beam axis. The staggering is indicated in fig. 4.2.

There are three double layers with the tubes staggered by half the tube size : in front of the iron, in the wide gap of 5 cm at an iron depth of 30 cm and behind the yoke between the iron and the concrete. These three double layers constitute the essential part of the muon detector. Each double layer provides 4 read out planes:

• 2 strip layers with the strips running along the wire direction. These allow a fully efficient ( · because of the staggering ) measurement of the phi coordinate in the plane where the magnetic field provides a bending for penetrating particles. The expected resolution amounts to ...._, 1.5 -2.5 mm.

• The inner and outer double layers have 2 strip planes with the strips oriented perpendicular to the wire. The strips are 1 cm wide with a pitch of 2 cm. They allow a fully efficient measurement of the z-coordinate along the beam line with a resolution of 1 cm. Corresponding strips of the two layers are ganged electrically together in order to save electronic read out channels.

37

• The double layer inside the yoke has only I strip layer with the strips oriented perpendicular to the wire direction. These strips are also I cm wide with a pitch of 2 cm. The fourth read out plane of this double layer is equipped with pads.

The remaining 8 slits of the yoke are equipped with single tube layers providing 2 read out planes each : 1 strip layer with the strips oriented parallel to the wires and 1 layer with pads. Although the strips do not allow a fully efficient coordinate measurement for each layer ( due to the walls in the t u be profiles and t he dead area between the iron hexadecants t he average efficiency per layer is reduced to - 85% ), the complete system of 14 layers will provide a powerful tool for recognizing and fitting penetrating tracks even within hadronic showers. Two additional tube layers are mounted in front of the yoke and behind it. They are equipped with pads only . Fig. 4.4 shows the whole arrangement.The assembly of the chambers ( consisting of 16-fold and/or 15-fold modules ) in the first four slits of the barrel yoke is presented in fig. 4.5.

In the forward endcap we have the same structure as in the barrel with the tubes oriented horizon­tally. The chamber planes are inserted horizontally into the slits at the position of the vert.ical split between the two ha.Ifs of the endca.p as indicated in fig . 4.6 .. The 3 double layers with the tubes stag­gered by half t he t,ube size provide again 4 strip read out planes with the strips oriented horizontally ( along the wires) and vertically ( perpendicular to the wires). Here the vertical stri ps are arranged like the horizontal ones, i.e. they are 4 mm wide with a pitch of 1 cm. There will be no double layer outside the yoke in that angular region which is covered by the forward muon spect.rometer . In the rcrrniining layers the strips are oriented parallel to the wires. As in the barre) there will be t.wo additional tube layers with pads only, one in front of the iron and one behind it.

In the backward endcap we have 11 single tube layers : one in front of the yoke, nine in the iron sli ts and one behind t he yoke. All layers will be equipped with pads only.

In the region of the base part of the barrel iron the outer chamber planes ( 2 double strip layers plus 1 pad layer ) do not extend into the endcap regions, because the base part and the two halfs of the iron must be movable independently. Therefore, special chamber modules will be needed around the lower part of the outer circumference of each endcap.

The pads from successive layers are grouped electronically in 3 parts ( longitudinal segmentation ) . The pads of the first layer in front of the iron are read out separately. They serve as presamplers. From the rema.ining 10 layers containing pads 2 towers are formed consisting of 5 pads each.

The number of streamer tubes and the number of the digital electronic channels for the strip read out as well as of the analog electronic channels for the tower read out are given in table 4.1.

Alternative solutions

The performance of the proposed ( so called Iarocci-type ) streamer tubes is strongly affected by the surface resistivity and the quality of the graphite coating of the PVC walls. The resistivity has to be large in order to ensure sufficient t ransparency for the signals induced on the strips and pads. On Lhe other hand , there are indications 1 that the large resistivity (possibly because of unavoidable inhomogeneities of the correspondingly t.hin graphite layer) may lead to the destruction of the tubes after some time due to continuous sparking . At present we are investigating other solutions for the tube design, e.g. profiles with low surface resistivity ( ...... 1 kOhm per square, like that of the tubes used by tbe UAI group at CERN ), either coverless or with a high resistive special cover-foil as used

1V.M.Golova.tyuk et.al DUBNA preprint Dl-85-166 and NIM (1986)

38

by the CHARM II group at CERN. For our .system such a scheme would require a read out of the wires, because induced signals could only be picked up on the chamber side with high resistivity ( which would be equipped with the pads or with the strips oriented perpendicular to the wires ) . In order to save the large amount of high voltage coupling capacitors for the wire read out, the coating would have to be on negative potential and the wires on ground potential.

4.3 Mechanical and Electrical Installations

4.3.1 Alignment of Chamber Planes

Each chamber module will be equipped on both sides with aluminium bars ( cross section i * 1 cm2 )

oriented parallel to thf' tube direction; in addition, springs are fixed to the aluminium bars on one side. After being inserted into the iron slits the chambers are pressed by the springs into predetermined positions. This is provided by small spacer profiles or additional ~tainless steel bars, which are fixed to the iron slabs at several locations along the tube direction and which are aligned by a laser. This scheme is foreseen in the barrel part( see fig. 4.5 ) as well as in the endcaps ( see fig. 4.6 ).The alignment with the laser will be done at the time when the iron structure is erected. The expected accuracy of the chamber ( strip ) position amounts to 0.5 - 1 mm.

4.3.2 High Voltage system

The High Voltage system SY127 of the Italian firm C.A.E.N will be used. In the barrel part there will be 4 independent power lines for each side of each hexadecant.: 2 for the muon chambers of the double layers and 2 for the instrumentation chambers. A similar scheme is foreseen in the endcaps, leading in total to an amount of ...... 255 independent channds.

4.4 The Gas System for the Streamer Tubes

The gas mixture foreseen is argon and isobutane in the proportion 1:3. The whole system contains ,..., 40 m 3 gas. Other mixtures with less content of hydrcarbons - e.g. by adding C02 to the above mixture are being investigated. In order to cope with gas mixture fluctuations a continuous monitoring will be performed by using a small additional streamer 1.uhc in series with the system gas flow and with a radioactive source on it. The gas will be distributed in a similar way as the high voltage leading to ,..., 128 independent lines.

4.5 Front End Electronics

The basic information consists of the analog tower signals and the digital strip signals. The pulse heights induced on the strips and the pads are of comparable size and amount to,...., 5 m V on 50 Ohm impedance.Thus, the strip signals do not need amplification. For the measurement of the hadronic energy we expect ,._ 5 streamers for a deposit of 1 Ge V in the tail catcher.

The small time interval of 100 ns between successive beam crossings at the HERA machine makes the buffering of the detector information necessary, with a depth corresponding t.o the time at which a first level trigger occurs ( up to 2 µs after beam crossing ).

Each strip is connected to a comparator; 4 channels are processed by one chip containing 4 digital pipelines, 4 output shift registers and a 4-fold OR for producing a trigger signal. This electronic is mounted on cards which are directly plugged onto the strip boards in the slits of the iron. When a trigger occurs the data are transferred to the output register from which they are read out seriaHy by

39

a processor which does zero suppression and address coding; it outputs the data onto VME bus under control of a micro processor (MC 68020) in the crate . The scheme is shown in fig. 4.7.

The signals from the pads are taken out by cables ( ,...., 5 m long ). The analog sum over one tower is made at the input of an operational amplifier to reduce the effect of capacitive noise. The amplifier is immediately followed by a FADC with a resolution of 8 or 9 bits and by a comparator which produces an output for t he t rigger. The FADC output is pipelined until the first level trigger decision for the corresponding beam cross arrives, at which time it is either t ransferred to the output register or discarded. Groups of 4-5 towers are read into controllers which process ...... 200 towers each. The processing again consists of zero suppression , offset subtraction and address coding. The data are read onto VME bus under control of a microprocessor ( MC 68020) in the crate. The scheme is shown in fig. 4.8.

An alternative solution with an analog pipeline and a read out scheme similar to the ALEPH solution is under investigation.

4.6 Performance

4.6.1 Calorimetric Behaviour

A calorimetric t.est of streamer tubes was performed at CERN in 1985 by putting 4 planes of tubes between 2.5 cm thick iron slabs behind the uranium / scintillator calorimter ( total thickness 6 nuclear int.eraction lengths ) of t he WA 78 experiment . The pad response of the streamer chambers was directly compared with the response of the scintillators positioned in the same gap ( the results were represented at the Vienna wire chamber conference 2 An approximately linear correlation between the streamer tube chamber response and t he scjnillator response was found up to incident pion energies of 210 Ge V. Thus, we do not expect saturation of shower energy measurements in the tail catcher.

The calorimetric behaviour of the instrumented iron for incident hadrons has been investigated with the GEISHA shower Monte Carlo propgram. Taken as a stand alone calorimeter with the 3 fold longitudinal segmentation as mentioned above, the hadronic energy resolution amounts to o'/ E,...., 110%/VE as expected for a sampling thickness of 7 .5 cm of iron. This resolution is adequate for the system to act as a tail catcher calorimeter which measures the hadronic energy leaking out of the main liquid argon calorimeter of the Hl detector. The detailed performance as a tail catcher is described in chapter 5.

A prototype test of the tail catcher both in the stand alone mode as well as combined with the main calorimeter is foreseen to start at CERN in 1986.

4.6.2 Muon Measurement

The system allows an efficient tracking of penetrating particles. Several problems have been investi­gated :

• ln t he barrel part of the detector the coordinates in the bending plane of the magnetic field inside the iron yoke are measured with an accuracy of ......, 1.5 mm.This allows an independent momentum measurement with a. resolution as shown in fig. 4.9. This independent momentum measurement will help to reduce the amount of muons which are falsely reconstructed in the central tracker as high momentum muons ( see chapter 12 ).

2 see a.Iso G.d-Agostini et.al Internal Report Hl-12/85-37

40

• By simulating ....,3900 exotic events of various kinds containing hadronic jets and -2100 muons the problem of linking tracks as measured in the central chambers with the muon tracks as measured in the return yoke was studied 3 The calculations show that for a muon efficiency > 98% the amount of false or ambiguous links can be kept well below 2%. Fig. 4.10 shows a scatter plot of the true muon momentum versus the momentum of the central detector track which also matched with the track penetrating the iron yoke. It did never occur that a high momentum track ( as measured in the central tracker ) matched with a low momentum muon track ( as detected in the streamer chamber system ) . This behaviour is mainly due to t he large magnetic field volume which provides an additional bending for the extrapolated inner tracks. The amount of ambiguous links is further reduced by applying a momentum cut using the above mentioned independent momentum measurement. In fig. 4.10 the lines labelled 10' ( 3a ) correspond to J ( 3 ) standard deviations, demonstrating that most of the high momentum muons can be ret,ained.

• Due to the large amount of material in the main calorimeter punch-through is expected t.o be negligible. This was checked by simulating hadronic jets with an energy of 200 GeV.

4.6.3 Calibration

Cosmic muons can be used ( at least in a part of the detecLor ) to determine and check the calibration constants. In addition , the use of weak radioactive Co-60 sources, fixed permanently at a few positions on each chamber plane, will be investigat.ed. It has been successfully tested by t.he OPAL collaboration at LEP. The streamer pulseheight c:an be extracted in a selftriggering mode, where the strip signal determines the pad at. which the streamer occured.

4. 7 Forward Muon Spectrometer

In the extreme forward direction the central tracker and the chamber system in the flux return iron are not adequate for measuring muon momenta with sufficient accuracy. A separate forward muon spectrometer is required . It covers product.ion angles between 3° and 17° using 4 layers of drift. chambers arranged in pairs spaced 50 cm apart and located upstream and downstream from an iron toroid ( field 1.5 Tesla, length 1.2 m ), as shown in fig. 4.1. The paired drift chambi!r layers help reject background by providing correlated position and angle measurements for tracks at the exit of the solenoid return yoke to reduce the ambiguities in linking with tracks in the central chambers. The momentum resolution varies from 23% at 25 GeV /c to 32% at 150 GeV / c , as shown in fig. 4.11. Multiple Coulomb scattering is t he dominant source of error below 75 GeV / c while chamber resolution becomes increasingly important at higher momenta. Fig. 4.12 shows the detection efficiency for 100 GeV /c muons as a function of production angle and indicates that the spectrometer is 100% efficient over the specified angular range.

Each of the four layers is composed of eight 2.4 m by 2.4 m drift chambers arranged as shown in fig. 4.13 ( 32 chambers in al1 ) . Each chamber has two parallel planes of 32 sense wires offset by half a drift cell to resolve left-right ambiguities and provide a checksum ( the sum of the drift times from the two offset planes ) to facilitate recognition of good tracks and reject large angle punch-through . There are 2048 sense wires in the entire spectrometer. The maximum drift distance is 3.75 cm. The drift field is defined using copper strips printed onto G 10 boards supported by a central core and outer lids of hexcel, which provides a rigid, low density structure. The proposed system is described in more detail in an Internal Hl report.4 Fig. 4.14 shows the structure of one of the muon chambers used in

3 E. Vogel Internal Report Hl-02 /86-43

'Winston Koet.al [nterna.1 Report Hl-5/85-20

41

the PEP-9 experiment.5 These chambers are nearly identical to those described above ; they operate successfully since five years.

The coordinate along the sense wire is measured using cathode pads coupled to delay lines. This eliminates the need for an extra tilted plane to resolve ambiguities introduced by multiple tra~ks as well as yielding space points to facilitate track finding. A resolution a.long the sense wire of better than 350µrn has been obtained in a small t est chamber using this technique6

Drift and delay times will be read out by fastbus multihit TDCs.

t.built by the UC Davis group of HJ

cK.Maeshim::i. et.alReport UC D::i.vi8 UCD-86-6(1986)

42

STREAMER ANALOG DIGITAL

TUBES (TOWER) (STRIP) CHANNELS CHANNELS

IRON INSTRUMENTATION

Barrel 49216 5832 39072 Forw. Endcap 14592 4320 11520 Ba.ckw. Endcap 16032 3456

S l TBTOTAL 79840 13608 50592

-- ---M UON C HAM BERS

Barrel inside 9088 15008 Barrel middle 9216 16286 Barrel outside 11072 18142 Forw. Endcap inside 2688 7840 Forw. Endcap rniddle 2880 9142 Forw. Endcap outside 3456 8936 Forw. Endcap circumf. 1152 1600 Backw. Endcap circumf. 1152 1600

SUBTOTAL 40704 78554

TOTAL 120544 12930 129146

Table 4.1: Number of streamer tubes and electronic channels

43

TOROID ........_ I

......... !'-...\.. ............. ........._

r----=: >--...

r---.... ........._ ~is• ~·9•

11•

~

-~· ---· - .. . ·--- 30

- MUON ORIFT·CHAMBERS

~ MUON STREAMER TUBES

!__. __e

Figure 4.1: General layout of the muon chamber system

44

/

,,../

39628

TOROID I

\

.. ,,.,. .. ---

-,. - _e _ __e ,_--..,. I ,...--

39629

Figure 4.2: General layout of the instrumentation chamber system. The tower structure and the staggering in successive slits are indicated

45

a) bas ic profile

~,-,] : J cover

paint rv 500 kr?/o wire ¢ fV 100 µm

bl 2Kinds of modules(gas tight)

;:.1 6~f o=ld=---- ---___ J 6 6,6 - ----·-------~

8 tubes

gas in

8 tubes 156 15 fold

c} modules sandwiched between PVC- boards to form chambers

8 tubes

ground

7 tubes

shield 40 µm Al strips 4 mm wide, 40µm Al

Al- bar pads 25 x 25c.m2 40µm Al sh ield 40µm Al

gos out

39623

Figure 4.3 : Design of the basic profile (a) , the 8-fold and 7-fold boxes (b) and the chamber modules {c)

46

PAD-W&D z - ~~:;:=:;:!::;:::;:::::::~::::;::=:;:::;:::;;;¢;;~"==! . . . . . . . ' . .

PAD-~M

..:o=~ z - - \ IP _ I . I I . I . I . I . I . I . I . I . I } MIDDLE

~D w~m,~, .. PAD-whfil itt~R

z - :===~=~~=~=~-=

~:~=~J PRE I I . I . I . I . I . I . I . I . I . I I- SAMPLER

z -- - - - - - -r 1 . 1 - 1 . 1 • 1 . 1 . 1 . 1 . 1 . 1 1} INNER IP,- DOUBLE I . I . I . I . t . t . I . I . I . I . I LAYER z - - - -

J_ 25[351

T J_ 25135)

T J_ 2Sl3SI

T J_ 25(35)

T J_

25(351

T J_ SOISOI

T J_ 251351

T J_ 25 (35)

T J_

25135)

T BARREL IENDCAPS)

39627

Figure 4.4: Arrangement of streamer tubes, strips and pads in the barrel and endcap iron yoke

~ (X)

I

L - v.. ~ ''" IE::3

-;-~

' "

~z ,, (.,.__ ....

ll .(_

\ !~ ,___ /

I - ~

I . nlll

- -+-

-

lS Otaa.t• I

15 I

II l

.. I

" I

II

IS I \S I

" I

" I

" I

~_,,W!t

' N• ft1t1 ttwwt f rd!:,·

Jftl..!.11U.

IS

" I IS I

I :s I

II l 15 I

" I " I

" I II

Typ <I> I I

Ap1tu3'!be!fu/

foA"tlutl<'chh.1'10

" I .. I ,, I

" I " I •• I

II I II I 's I

., I II I II

I .. I .. I ..

Typ <2) 1 1 Typ <3> I ;

·-1

Figure 4.5: Insertion, assembly and fixing of the chambers in the barrel yokes

I tjl ~~~--.. I " " I .. rn1;tlj ,' =; ' /

'

" I •I tR l.1/ 7·< '

I " rn".'.'/'> I " t:R ·V . , /

I

l

IW(l'I <If-I £ '"\lt-t!jll9 .~•I . ,!W!J. 1".!V. .,jc!>~:>ltnf'.

C..-•• nOf ~'''' 111~1 U•!:tt-'iU:~.

39630

-,

<t>- -----~

~·ocu...c s, 11 TU8ES UCtl

11111

4>----+ I

I

-+-- ·- -- .. <t>--- ·---.- ·$-----.. I\ .,

1 I

I ~ --<;> ----<+>----

I

4>

39631

Figure 4.6: Insertion, assembly and fixing of the chambers in the endcap yokes

49

CONNECTED TO CHAMBER BOARDS r---

NR. OF BUNCH i I LOAD

CLOCK CLOCK

IN I A

8 COMP. LATCH DI G. t. BIT t. Bl T

TO ONE STACK SHIFT CONTROLLER c SHOT

D REG. REG.

-- --- -- -- -- -- ---- --- --- --- --J

CONTROLLER

ZERO SUPPRESS

ADRESS MICRO

LOADING

39625

Figure 4.7: Block diagram of the digital read out electronics for the streamer tube system

50

1 TOWER

c.n -

FLASH

..... ADC ~

8-9 81 T

~ /

COMP.

ZERO SUPPRESS

OFFSET SUBSTRACT

AD RESS CODE

CONTROLLER

LOAD

,--- K

9BI T

STACK

REG.

TO TRIGGE R LOG IC

MICRO

,---

9 BIT

SHIFT

REG.

NR. OF BUNCH

TO CONTROLLER

39622

Figure 4.8: Block diag~m of the analog rea.d out electronics for the s~.r_eamer tube system

( Sp/P

0,8

0,6

0,4

0,2

P [GeV/c]

s 10 15

Figure 4.9: Momentum resolution of the streamer chamber system in the barrel part versus the momentum

52

Ph [GeV/c]

100

10

' I 'I I

10

16

I I I JI

100

Pµ [GeV/c] 39624

Figure 4.10: Scatter plo.t of the true muon momentum versus the momentum of an additional central detector track matching with it. The lines are explained in the text.

53

30 -..... c: <l) 0 i... <l) c.

20 -c. ...... -c. -b 10

O L-_1_._L__J~_.l.__L--1~..1--'-----L~.L.--L--'-~L--'--'---'

0 50 100 150 p (GeV/c)

Figure 4.11: Forward muon spectrometer momentum resolution as a function of momentum

54

l.00

>-(,)

0.75 c Q)

(,) --w c.

0.50 .2 -(,) Q) -(l) 0

0.25

0. 0 0 L....k:::........L.-...L-.JL_L_L__L_.l--'--1-.1-L-L.-1-.1-L--'-_L..,_.L.--L--L-....l.-.L.;;:i.L__,I

0 5 10 15 20 25 Production Angle ( Degrees )

39626

Figure 4.12: Forward muon spectrometer acceptance as a function of production angle for 100 GeV /c muons

55

01 O>

"'

~

~TOROID

~ ~

~ :=:t= ~ :=E r \... -

' I ,L

(j ~

~ II ~

i--t-->---

--+--

" ' t:::±::: '

~

~ -

In Place Partially Open for Access to End caps

Figure 4.13: Arrangement of chamber layers in the forward muon spectrometer as viewed ·along the beam line

'

~

I'

~

39621

CJ'I ...;i

G) $.-.... )

" n c: ~ .. Cit

Al rhnnnrl G-10 ~Ar Al ~h~('t

lo

11u1 .~._)-:J I

tOOpF capadtor

I" LI

e:iskct ~pacer

Al hllr ~:t!'ket

cathode wire

PC bo<lrd

Al bar

Al sheet

mothc:ir board

discriminator card

a) V<'rtfcnl cro~q r.~ct!on

G-10 b.ir.- f -~ -~ ----~

2 tn

~I Q)I

w -cl J ti\ "O

Q.I w ..... ;;,

bott<'.lm

h} 011t !':J 01' Vf<'V

Figure 4.14: layout of the PEP-9 muon chamber, very similar to the one proposed for the forward system of Hl

survey blocks

-l1V r<"s 1st or box

.... uv & r11lc:t.!r box

gas in

1 1 ;i ,... i\ l b:irs

lids

s11rvcy blocr.c:

Chapter 5

Calorimeters

5.1 Introduction

The calorimetry of this experiment is based on a large liquid argon(LAr) calorimeter, backed up by an iron tail catcher and complemented by a warm electromagnetic backward calorimeter and a forward plug calorimeter.

The overall system is schematically shown in Figure 5.1, the nomenclature used is explained in Figure 5.2. The forward ( 4° < 8 < 20°) and central (20° < B < 152°) parts consist of LAr calorimeters with lead and steel as absorber materials, the backward region ( 152° < 0 < 176°) is covered by a lead/ scintillator sandwich with photodiode readout, while the very forward region (0 < 60 mrad) is closed by a copper calorimeter sandwiched with silicon (Si) detectors. The entire system is completed by the instrumented iron situated outside of the coil. It serves <1.8 a tail catcher in the central and forward part and as a hadronic calorimeter in the back.

While the majority of all events is well contained and measured in the main calorimeter, leakage of hadronic energy occurs at the back of the liquid argon calorimeter for a small fraction of all events, (see 5.10) . This energy is recognized (tagged) and measured by the iron tail catcher. Energy leakage in the forward direction near the beam hole cannot be avoided . It affects the transverse component of the energy. The plug calorimeter in the very forward direction closes the beam hole as much. as possible and limits the transverse energy in the beam hole which can remain unobserved.

The individual components are described separately in the following sections. The sections 5.2 -5.7 deal with the different aspects of the LAr calorimeters . In section 5.3 we describe the cryogenic system, in 5.4 the stack system including the sensing structures, and in 5.5 the electronic system. The important question of calibration is taken up in section 5.6, while beam tests and calibration with beams are s~mmarized in 5. 7. Section 5.8 treats the backward electromagnetic calorimeter, while section 5.9 covers the plug calorimeter. A brief introduction to electron alld photon taggers is given in section 5.10. In section 5.11 we review the overall performance of the calorimetric system including the influence of the tail catcher.

The data acquisition system as well as the trigger system a.re common to the entire calorimeter setup. They are described in chapter 9.

5.2 LAr Calorimeters

LAr calorimeters have int.rinsic advantages for stable longterm operation and easy calibration. The response of the calorimeters is both homogeneous for all detector elements and shows very small variations with time. In the readout structure, fine granularity and detailed longitudinal segementation can be reached, essentially only limited by the number of electronic readout channels available. LAr calorimeters are, therefore, well matched to the requirements of good e/?r-separation and energy flow measurements.

58

Plu

s

....... \

TRANSVERSAL CROSS SECTION

·-. ---

()

..-/~.J!!.,V,!!!!.!t_~

' =-.!~~QOA \IUUl

U1dron [1lor•mittr

atthAtd Calorllltllt

1,,,

.1'1.t.1 ...... 1 ...

~~f¥--~•IY _,,_,.__ _ _,.__ •.. c110<1o .. hr

Figure 5.1: Longitudinal and transversal cut of the calorimeters

59

-" ... ... 0 m ICI ... -c

" u

Olll

OOOl Oll OOOl

== tpJD•)j:)Df 0 IU ID m ID m

~ I 4, ] 0 - '\ I 0 .. CD -- a.•1 IO

'° (,,) Ol£ \ -\~-

N

0 0

~ \ 11•

" 0 N \ • CD \. h II) (.) ... \ oa

- >-- ------- - LO - I N

/ j ..... ji

0 /j

... 0 M /~if-~ co m Cl u // I

/:ii I

I/) - I 0 ., m osz • • 1· ~ co u.

I N

II

I CJ) I e

oaz I I N 0

I/)

m I OL ll I LO II) .... CD u.. I ....

I I 06 1

' o" - w

I

I 0

f"" u.. LL. ...... ... 0 - .... I I

o'~ N ~ I M u.. u.. 0 .. 0 - 1 ·

.... LO

C) ":""

I OI ....__ ,.., 0

0 LI. N -.... 0 u.. I U>

~ -~ I .

I I

096 -OI ::)

lL

-- 00&1

oooz

Figure 5.2: Schematic vit!W of the segmentation of the calorimeters

60

These advantages should be maintained in the technical realisation. We have made an effort to minimize inhomogeneities in the calorimeter due to cracks and dead material, and to minimize leakage of energy.

The LAt system is composed of e.m. and hadronic calorimetry in the forward and central directions while there is only e.m. calorimetry in the backward direction. All calorimeter modules are contained in one common cryostat. We thus avoid large cracks due t.o cryostat walJs. Such technique was first appJied in the LAr calorimeter system of CELLO. 1

An 8-fold ¢>-segmentation has been chosen to keep the number of cracks in the detector small. Moreover these cracks are highly non-projective in the hadronic part such that we keep good contain­ment. and that the energy deposited around the crack region can be corrected for these losses on an event by event basis. A Monte Carlo (M.C.) study has shown that about 60 % of all events (jets) are u11aff cctcd by </>- cracks and can therefore be used as an unbiased sample to intercalibrate the rest.. The construct.ion of readout gaps and stacks avoids large local inhomogeneities due to support structures and achieves a more or less random distribution of dead material which is at the level of a few%.

The design goals for t.he LAr calorimeter system are:

• E.m. energy resolution of o/E < 10%/,/E for 8 < 152°

• fladronic energy resolution of o / E ~ 55%/v'E for () < 120°

• Good angular resolution of jet directions ( <. JO mrad) for() < 20°

• rr / e rejection of S 10- :~ in part.ir.u lar in the proton direction.

5.3 LAr Cryogenic System

The LAr cryogenic system described here consisl,s of:

• a single cryostat with vacuum insulation c:ontaining th<~ calorimeter modules and the LAr

• t,he storage dewars, transfer lines, corit.r<)h:; and safet.y devices (see chapter 14) required for stable operation as well as for cooling down and warming up.

5.3.1 Cryostat

hi order not to spoil the cn<~rgy resolution of the calorimeter the main design requirements for this vessel are:

• The space available for calorimeter modules should be as large as possible to accomodate as many int,craction lengths as possible for the hadron calorimeter,

• The cryost.at walls should be as t.hin as possible so as to minimize the amount of non-instrumen­ted material:

- in front of the electromagnetic calorimeter

- between the hadron calorimeter and the iron with the constraint that it should withstand a maximum pressure of 3 bars and support the total load of the calorimeter modules (about 500 t).

'H.J. Behr~nd et al., Phys. Script•~ 23,610(1981}

61

Description The cryostat is a cylindrical dewar consisting of two shells separated by vacuum insu­lation and superinsulation. Its shape is designed to take up as much as possible of the space available radially between the central tracker and the inner wall of the solenoid cryostat and available longi­tudinally between the forward and backward muon chambers. Figure 5.1 and Figure 5.3 show the overall layout and Table 5.1 lists the main characteristics of this cryostat. All the walls are made of stainless st.eel (S.S.) except for the inner wall of the vacuum vessel, around the beam-pipe and the central tracker, which is ma.de of aluminium alloy (Al). A stress analysis has been done to define the thickness of the walls and flanges of the inner (cold) vessel as well as the necessary reinforcement structures. The overall deformation of the cy lindrical wall does not exceed a few (3-4) mm when the vessel is loaded with all the calorimeter modules. This limit is important if we want to obtain a good accuracy in the positioning of the calorimeter modules and has to be reached with both forward and rear (cold) end plates not mounted. The increased thickness of the wall at the rear part (pt.C Figure 5.4) as well as the step in radius of the outer wall (near pt.D Figure 5.4) (allowing space for the cable feedthroughs) improve substantially the mechanical strength of the LAr vessel and thus allow to use thinner walls for its cylindrical part. Jn addition, a series of longitudinal spars and circular ribs on the inner part of the cold vessel ensure the mechanical stiffness of the vessel and allow the integration of the stack support system (see below) . The curved walls on the front end are designed to

• minimize t he amount of dead matter in the path of the forward going minimum ionizing particles

• allow for space to mount the supporting feet at the bottom

• allow for space for t.he cable feedthroughs .

The rear walls are shaped t.o accomodate the e.m. calorimeter extending in the backward direction and their thickness is optimized in conjunction with the thickness of the front walls to take up the differences in thermal contraction of the inner and outer walls.

a. Vacuum vessel. For the vacuum vessel (Figure 5.4) the end plates will be mounted to the outer and inner wall using cold seals to prevent leakage in case of rupture of the LAr vessel.

b. LAr vessel. For the LAr vessel the end-plates are welded to the flanges on the outer and inner cylinder walls, using a technique similar to that presently under development for the HERA magnets. Additional stainless steel pieces are bolted onto the flanges, all around the welded joint.s, at points A,B,C and D (Figure 5.4) to give the required mechanical rigidity since the weld is only used as a seal.

Cryostat support The forces due to the total weight of the calorimeter (absorber plus readou t material: 420 t, LAr plus vacuum vessels : 32 t. and LAr weight: about 80 t) are transferred directly down through the cryostat walls to the central iron yoke. These forces are taken through the insulating vacuum on four low thermal conductivity feet made of fiber glass and located between the end flange of the magnet cryostat and the muon chambers (or iron) (Figure 5.5).

Cryogenic ports and expansion vessel Two vertical cryogenic ports are necessary and are presently located on top of the vessel, respectively between the rear and front parts of the iron yoke and the magnet cryostat. The one in the rear part (Figure 5.7) is a LAr expansion volume partly buried in t he top of the iron yoke. This volume of about l .5 m3 is directly connected to the cold vessel through a chimney with no valves to avoid accidental shutting off of this relief volume. The purpose of this vessel is to balance the variations in volume of the liquid due to temperature variations coming from the procedure for regulating the vapour pressure. A 2° C increase of the LAr temperature leads to a volume increase of 600 l. The gas volume is also necessary in the regulation circuit. This vessel houses as well the main lines for feeding the argon, gas and liquid, and for supplying the liquid nitrogen

62

to the heat exchanger coils, as the line to empty the cryostat fast. The diameter of these lines allows to fill the cryostat within~ 24hrs a.nd to empty it within 2 hours. The vacuum jacket of this port is independent of the insulation vacuum of the cryostat. Thus any work or modification on the port is possible without having to destroy the vacuum of the cryostat which would otherwise require to warm up the entire calorimeter. The port on the front end is used for other cryogenic services (e.g. sensor rea.dout cables). The required cutouts in the iron for these ports are 0.5 m2 for the forward one and 0.7xl.5 m 2 for the rear one.

Cable feedthroughs

a. Signal cables. In the present design it is foreseen to use 'warm' electronics (see 5.5). The flat cables for up to 65000 channels are brought out of the cryostat through 16 or 24 special tubular ports distributed at both ends around the periphery of the cryostat. Feedthroughs of similar principle are used in S.C. magnets for feeding in the current leads. The number of feedthroughs will depend on the final choices for the number of channels, type of cable and eventually, whether cold or warm electronics will be used. The 8 or 12 tubes in the forward direction come out longitudinally, between the magnet and liquid argon cryostats (Figure 5.5). The 8 or 12 tubes in the backward direction come out vertically between the end of the magnet cryostat and the backward part of the iron yoke. Figure 5.6 shows the principle of such feedthroughs. The cables within these tubes are packed to form a heat-exchanger which is cooled by the flow of cold argon vapour ( 90°K ) from the boiling off of the LAr due to the heat input of the cables. These cables are embedded in epoxy within the stainless steel flange and vacuum tightness is ensured by locally removing the insulating material around the wires. The argon gas pressure in the feedthrough will be regulated to maintain a. stable equilibrium. This regulation will be done individually for each feedthrough since values for the heat input and the hydrostatic pressure in the LAr will vary from one feed through to the other. The argon vapour exiting from the cable heat-exchangers at a temperature close to 300°K is then recooled by external liquid nitrogen (LN2) heat exchangers and then fed back into the main argon supply circuit. Appropriate shaping of the port tubes ( depending on their azimuthal position ) and gas pressure or temperature regulation will prevent the liquid argon from reaching the feedthrough flange, which is at room temperature. An experimenta.l set·up is presently being built to demonstrate the feasibility of such feedthroughs and to determine the optimal regulating parameters. It should be ready by the fall of 1986.

Figure 5.5 shows how the cables are routed within the cryostat. Located on the periphery of the inside surface of the cold vessel wall will be a series of connectors to which these cables will be connected. Each calorimeter module will be installed complete of with its cables which are then plugged into these connectors.

b. H.V. cables. The 1500 high voltage (H.V.) cables will exit from the cryostat through the top of the expansion ve.91lel.The same type of feedthrough as for signal cables cannot be used in this case mainly because of H.V. insulation requirements and because of H.V. cable size.

Support of calorimeter modules The calorimeter modules (stacks) will be supported by the inner frame of the cold vessel which is made of longitudinal spars and of circular ribs which are welded to the cylindrical part of the cold vessel. Rails will be mounted on to the spars and Teflon coated pads will be put on to the back plate of the stack allowing the stack to slide along the rails. The stacks will be fixed longitudinally by bolts into a circular rib (Figure 5.11). To mount the stacks into the cryostat, we will have an external mounting structure with a revolving frame around an axis parallel to the cryostat axis. This frame will have the same rails and bolts as those inside the cryostat. The rotation of the frame will allow the stack to be positioned correctly with respect to its corresponding rails inside the cryostat. Final alignment is done using a laser beam.

63

Heat losses The main heat losses· are produced by the signal cables. Since we do not know at present the type of cables that will be used, we can only roughly estimate the heat losses due to the cables. These should be in the range from 5.2 to 9.0 kW. Table 5.2 gives the various sources of heat-loss. All t hese heat losses will be minimized as optimisation of the overall design proceeds.

Installation Once the solenoid is installed, t he outer cylindrical walls of the cryostat will be installed ___ . using a special axial mounting support. Because of the reinforcements around the rear part of the

vacuum vessel and of the flange to which the liquid expansion vessel will be welded, this installation can only be done through the rear part of the coil cryostat. The four feet are assembled next and fixed to the iron base. The cryogenic ports are welded into place and all tubings and weldings within the cryostat are done and tested for leaks. The feedthrough tubes are then welded onto the cryostat, between the cold and the vacuum vessels, and leak-tested. The cable heat exchangers, which come as complete assemblies already cold-tested, can then be inserted into the feedthroughs at this stage. Once this is done the loading of the stacks can start, through the front end for the forward calorimeter, and through the rear end for the barrel modules using special elevating structures. Once a.II the connections have been done, the inner vessel is welded shut using a special welding ma.chine. Then the vacuum vessel is closed by mounting the front wall and by inserting the aluminium inner wall using a special support frame. It is bolted to the front wall using flanges with a cold seal. The rear wall is then bolted to the flanges of t he inner and outer walls using also cold seals, and welded to the flange on the outer wall (at C Figure 5.4). All welded joints are designed to a.ccomodate several grind/ reweld procedures in case a.ccess to the stacks is required.

Access to the calorimeter modules To access the barrel part of the calorimeter, it is necessary to remove the complete assembly ABC (Figure 5.4). In this case the central detector has to be removed . Such an operation can only be done once the detector has been moved out of the interaction region . To access the forward part of the calorimeter, the end plates AD have to be removed. This can be done in the interaction region but requires removal of the beam-pipe.

5.3.2 Cryogenics

Principle of operation The layout of the cryogenic system is shown in Figure 5.8. Figures 5.9a and 5.9b show details of the LAr and LN2 circuits. Since the total volume of LAr is close to 60 m3 a storage dewar of about this capacity is necessary to store the LAr. Refrigeration for the cool-down, stable operation and argon storage is provided by the vaporisation of liquid nitrogen which is supplied by a nitrogen liquefier or a storage dewar. The cryogenic system is designed with the following goals in mind:

• maximum cool-down time of 30 days

• transfer of LAr into the cold cryostat in less than 24 hrs

• transfer of LAr out of the cryostat in less than 2 hrs

• maximum warm up time of 30 days

The cool-down and warm-up operations will be described separately. Once the cryostat is filled, an­other mode, for stable operation, maintains the calorimeter at LAr temperature without the formation of bubbles. Table 5.3 gives the main parameters of the system.

64

Cool-down The calorimeter is filled with gaseous argon and cooled down from room temperature by circulation of LN2 in heat exchangers within the cold vessel and the liquid expansion volume as well as circulation of the argon gas. Since the absorber plates form an array of densely packed horizontal and vertical plates, natural convection can be quite poor. It is possible to improve it by adding some baffles and tubes around the arrays. Thus, by blowing the argon gas during cool-down, it is possible to force the gas flow between the plates to reduce temperature gradients which could distort the stack assembly in a harmful way. The cooling is done by means of an extended surface exchanger below the LAr level which will be switched off once the cryostat is full. The boil-off of the nitrogen is vented into the external feedthrough heat exchanger and back into the liquefier. The 30-day cool-down necessitates the vaporisation of 290m8 of LN2 at a rate of 400 l/hr. Once LAr temperature is reached over the whole calorimeter volume, the cryostat is filJed with LAr from the storage dewar.

Warm-up For warming-up, the LAr is emptied into the storage tank and warm argon gas is blown into the cryostat. By correctly adjusting the blower and the heat exchanger capacities, room temper­ature should be reached within 30 days.

Stable operation The main constraints during periods of stable operation which may last for many months are:

• to regulate the temperature to 90 ± 0.2 K

• to regulate the absolute pressure in the vessel to 1.35 ± 0.02 bar

• to avoid the formation of bubbles within the active volume of the calorimeter modules.

Wit.ha circulation of LN2 in a set of coils smaller than those med for cool-down, it will be possible to maintain a stable overpressure of 0.35 bar in the cold vessel. By pressurizing the LN2 circuit the LAr is prevented from freezing around the heat exchanger. The heat losses during normal operation of the calorimeter are estimated to be about 15KW of which about 90% are due to the signal cables. To compensate for these losses, vaporisation of 100 - 150 I/hr of LN2 (depending on the type of cable used in the feedthroughs), is required and can be provided by a LN2 liquefier. This cooling power is necessary to maintain the LAr in a supercooled mode thus avoiding the formation of bubbles in areas where the heat losses can be important. Except for t.he feedthrough tubes, there should only be argon vapour on top of the liquid expansion volume where there will be an argon vapour volume of 800 I also cooled by LN2 in the main regulation loop. Furthermore, 300°K argon vapour from the feedthroughs is fed back into an external LN2 cooled heat exchanger before returning into the liquid expansion vessel.

Purification The amount of oxygen in the LAr should be < 1 ppm. Purification of the argon will be done using a Deoxo cartridge device located outside of the experimental hall. The quality of the LAr will be monitored on a regular basis. In the event of contamination, the purification of all the LAr from a level of 100 ppm to < l ppm will take about 5 days.

Controls All controls will be fully automatic using microprocessors for permanent monitoring with a terminal in the main control room of the experiment.

65

Table 5.1: Cryostat dimensions

Vacuum vessel: Outer diameter (max.) 5.16 m

Inner diameter ( a.t central det.) 1.806 m

Inner diameter (a.t beam-pipe) 0.22 m Length 7.07 m Outer wall thickness (S.S.) 24 mm Inner watl thickness (Al) 8 mm Front end plate thickness (S.S.) 12 mm

Rear end plate thickness (S.S.) 12 mm Total weight 20 t Total volume 120 m3 Vacuum insulation volume 20 m3

Liquid argon vessel: Out er d iameter (n1ax .) 5.00 m Inner di a.meter (at central det .) 1.894 m

Inner diamet er (at beam pipe) 0.33 m Length 6.90 m

Outer wall thickness (S .S.) 15 mm Inner wall thickness (S.S.) 8 mm Front end plate thickness (S.S.) 12 mm

Rear end plate thickness (S.S.) 12 mm

Total weight 15 t

Total volume 100 m3

Liquid argon volume 56 ms

Operating pressure (absolute) 1.35 bar Hydrostatic pressure (top to bottom) 0.8 bar

Table 5.2: Heat losses

Cables ( 65 000) 5.2 - 9.0 kW Feed th roughs 0.26 kW Radiation and conductivity 1.65 kW

Total heat loss 7.1 . 10.9 kW

66

Table 5.3: Parameters of cryogenic system

Operating pressure (abs .) 1.35 bar Uniformity of LAr density in detector volume < 1 % Volume of liquid argon 56 ms Cool-down time 30 days

Enthalpy of cold mass 42000 MJ Vaporisation rate of LN2 for cool-down 400 I/ hr Total quantity of LN2 for cool-down 230 m3

Time to fill-up cryostat, 24 h Time to empty the LAr vessel < 3 h Refrigeration requirement (stable operation) 7- 10 kW

Vaporisation rate of LN2 for stable opera.tion 100-150 I/ hr Added heat load from complete failure of vacuum vessel 45 kW

Storage: Volume of LN2 dewar 30 m3

Volume of LAr dewar 70 m3

Max. operating pressure of LN2 dewar (abs.) 4 bar Max. operating pressure of LAr dewar (abs.) 4 bar

Purification: Number of days to purify 60 m3

contaminated at 100 ppm 5 days

67

(

Figure 5.3: LAr Cryostat: Ma.in dimensions

68

© ..-------------""-----------------------------~_....,

e2 (Alumini um alloy a A.A.)

e4

fi>.- e6 A.A. ~-------

Thickness ( mm) Xo ~

• 2 A.A. 6 0,0898 0,0215

• 4 A,A. 6 tt • II

• 6 A,J., 6 II II (

• 6 s.s • .12 0,682 0,070

• to s.s. 24 ,,.,6}6 O,t403

e 12 s.s. 12 0,682 0,070

e9

forward Barrel

81 .

:l.nnro• wull

•5

--·- -· Thiolmesa (111111) JI) )..

• 1 S.S. 6 0,4545 0,0468

e' s.s. 6 It " • 5 s.s. 8 II II

•1 s.s. 12 0,602 0,070

e 9 ~.s. 15 0,8523 0,0877

• 11 s.s. 12 0,682 0,070 - ·-·---

Figure 5.4: Wall thicknesses of the LAr cryostat: a) Vacuum tank, b) LAr vessel.

69

._..._...__ ___ II

.c .. ....

Figure 5.5: Layout of cable feedthroughs on the LAr cryostat : Front end view, side view showing tout-ing of signal cables within cryostat, and relir end view.

70

-8 ""' ....... ~

~ Cl

n 8 \D

f

Figure 5.6: Principle of the fE-edthroughs of thP signal cables

71

~ Cl 01 ., .. ~ .,, .-i

t: -<

C.Ar L.Ar.

Safety ll HT rabies. ,, LN2 it

I

I

. .L.Ar ' .

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L.Ar

1,Sm Cross section

C:ryost1t

LIQUID ARGON EXPANSION VESSEL

vessel

Figure 5.7: Schematic of the LAr expan!lion ver-1Rd: Principlt> of operation and details of chimney.

72

----- , .-- ------ ----f · L.Ar ~--

30~.

L.N2.

r I

- --<1-rl Liquefier (LN71 1 I

I I

------------'~f 300K

t----1 I I I I I I

~--------~ I I I I I I

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I I I I I

1---

r--1 I

Safety

I I I

70 m3

- - -- - -----l.Ar -

Emptying pump

- _J Reg.T ..

Figure 5.8: Layout of the cryogenic system

73

(

( I I

I fr. I I '---+---

Figure 5.9: a. Details oft.he LAr cryogenic system

74

~ro- ---,~ - 115 - - - - - - ---?1"""1 ,r;;- -, "'4<: ~ ""),, .._ .......... ~ .......... --......... --.-..----,i<.... L.__ - -- , : I I I

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11

STORM!t TANK

' ' [.j~l .u _ "l 1.~:~~r l J II 1. 11!: ST01'11G£_ TllNit

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- ---------"-1 I I I .. r--- - -------~fr- - -,--_ _L ____ _____ _ _______ __ _j

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Figure 5.9: h. Details -of the LN7 cryogE'nic systE>m

75

0 .

Pl'l~~l»C

POSITION

l:."'E!'_J

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PA~T

Cl'Nt:M..-.l. t'l l-lc.'UIT

IN 2

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5.4 LAr Stack System

5.4.1 Introduction

After an introductiory review of the general principles this chapter describes our present ideas on the stack construction (5.4.2), the granularity of the ionization sampling and the number of readout channels (5.4.3), and the structure of the readout cells (5.4.4).

Geometry The general layout of the stacks is shown in Figure 5.1. The orientation of absorber plates has to be such that the incidence of particles be as close as possible to the normal and not smaller than 45° (e.m. resolution x 1.2). Hence the plates are oriented vertically in the forward direction, in the forward barrel part and in the backward barrel part, and horizontally in the central barrel part. These zones will correspond to physically separated rings of stacks. The reference system used is shown in Figure 5.2 and Figure 5.16.

Segmentation and cracks In order to minimize the energy loss due to cracks, an 8-fold symmetry has been chosen for the azimuthal division. Cracks in <p will point to the beam line for the e.m.

( part while the hadronic cracks are not pointing. Electrons passing through the e.m. crack are then measured and identified at the beginning of the hadronic part. Hadronic cracks are non-projective and have an offset angle of ~ 20°. This rather large offset angle ha.rdly disturbs the energy measurement of a jet (see section 5.11).

The cp cracks will be small and filled with a minimum of material (LAr and cables only). A cut perpendicular t.o the beam direction is shown in Figure 5.16. In the z direction the physical separation of the hadronic modules will introduce cracks where some matter is unavoidable. This material will be identical with the absorber material wherever possible.

Absorber material For reasons discussed below, S.S. is preferred for the hadronic part of the stacks. For the e.m. part, the absorber material is lead.

Resolution and Precision The required energy resolutions of< 0.10/../E for the e.m. part and 0.55/../E for the hadronic part lead to choose plates not thicker than 2.4 mm for lead and 12 mm for S.S .. The argon gaps will be 3 mm (or 2 x 1.5 mm). In order to keep the systematic errors small, the total argon thickness in one longitudinal segment is allowed to vary by < 1% in the e.m. part and by < 2% in the hadronic part. This requires a precision of 50 µm in the e.m. part and 100 µm in the hadronic part for individual gaps. For absorber plates the variation in thickness should be < 40 µm

for lead and < 400 µm for S.S.

5.4.2 Stacks

In the following we outline first the general design ideas of the stack mechanics, afterwards we discuss in more detail their application in the different sections of the calorimeter.

Overall Design

a. Hadronic stacks. LAr calorimeters have been built so far essentially for e.m. calorimetry with thin lead plates> and it is not possible to simply extrapolate the typical rod construction used for lead to thick iron plates necessary for hadronic showers. In particular, tolerances on thickness and flatness of 10 mm thick plates as long as 2 m do not allow to use these plates for defining the argon gap with the precision necessary to achieve the overall 2% systematics accuracy required in energy measurements. The precise charge collection functions have to be separated from the bulk mechanics of the absorber. Thus the argon gap is defined by the sensing structure independently of the absorber

76

plates. The readout cells will be independent units which will be inserted into the gaps between the absorber plates.

The best way to provide a rigid structure of the modules is to weld absorber plates together with as little additional material as possible. Welding with electron beams will yield a very clean and precise result due to the small heat input. Modules will be stiff enough to resist severe thermal constraints and handling. S.S. is superior to copper alloys from this point of view (Young modulus 1.7 x better); it allows smaller supports. The ip sides will be used to slide in the sensing structures and to guide the cables.

With respect to eddy currents, S.S. offers another advantage: the resistivity of S.S. (e.g. type 316) is an order of magnitude larger than that of copper a.Boys. Therefore forces due to eddy currents are smaller, especially when the solenoid quenches.

A module will consist of S.S. absorber plates welded to S.S. plates or spacers on two sides of the module. The ip cracks are thus minimized (~ 2 cm) and filJed essentially with argon and cables. The welding is done on the other two sides of a stack, i.e. in the central barrel in planes perpendicular to the beam and in the forward part in planes parallel to t.he beam. Each plate contributes to t.he strength of the module. As shown in Figure 5.lla. each module can slide on rails inside the cryostat by means of a guide block placed on its back. A sliding pad fixed on a corner iron placed on the convenient side of the top takes the torque due to gravitation.

Independent sensing structures wilJ be inserted between the absorber plates. The distance between two absorber plates has to take into account a ll tolerances of the materials involved. With 12 mm thick absorber plat.es and a 3 mm argon gap a repetition of the structu re every 18.9 mm is envisaged as shown in F igure 5.IOc.

The proposed solution offers the possibility of having stacks built industrially. The sensing struc­tures can be built independently. The assembly is foreseen in t.he home institutes.

b. E.m. stacks. Due to the softness of lead and to the different plate orientations and sizes in all the parts of the calorimeter a unique principle of construc:tion cannot be adopted as for the hadronic stacks. The pile up of lead and readout. boards needs rods in certain circumstances. With different technical solutions in t.he different regions of the calorimeter rods may be avoided in some places.

Solutions for the different regions

a. Central barrel part. All plates a.re parallel to the beam axis (perpendicular to the r direction) as shown in Figure 5.lla. The 48 S.S. plates of the hadronic part are welded onto spacers at constanL z. Three-fold segmentation was chosen with 3 identical rings of modules to reduce the bending of the plat.es. Each module measures 880 mm along z and weighs 7 t To minimize the effect of energy leakage in the z cracks, an independent sensing structure will be insertt!d vertically between two modules .

The e.m. pa.rt is made of 43 lead plates with a thickness of 2.4 mm. A construction with rods is envisaged for e.m. stacks (Figure 5.12): a pile up of lead plate, GlO board, spacers on the rods, GlO board, lead pla.t.e can be made with rods distributed every 9 cm. The spacers are dimensioned such that, the lead plates are free t.o move to a.How for the difference in thermal contraction between lead and GlO (3.5 mm/m). On the beam side the stack ca.r1 start with a special readout gap, adapted to the material in front. At the back side the stack is bolted against a S.S. plate being the first plate of the hadronic part. The structure is repeated every 7.2 mm, allowing for 0.2 mm total clearance. Alternative solut.ions without rods are also possible and a.re presently under investigation.

b. Backward part. There will be only lead plates of vertical orientation, linked to a special support at the cryostat rails like a.II other modules. 91 plates are foreseen for a total thickness of 660

77

{

(

l

cm and a radial dimension of 21 cm.

c. Forward barrel part. The plate orientation is vertical and perpendicular to the beam (Figure 5.llb), the hadronic plates being welded together in two planes parallel to the beam. Two slightly different modules are foreseen. Each of them is made of 48 layers for a total thickness (z direction) of 885 mm. Each of the e.m. modules has 123 plates with different radial dimensions (25 and 21 cm) which have been optimized to a thickness varying from 20 Xo at 90° to 30 Xo at 20° .

d. Forward part. The forward hadronic calorimeter has an 8-fold geometry similar in shape to the forward barrel pa.rt . The plate orientation is vertical. Its segmentation is schematically shown in Figure 5.13a.

Due to the cryostat geometry the outer maximal diameter is 4000 mm, the front part correspond­ingly 3600 mm. In a plane perpendicular to the beam direction there is an octogonal inner part surrounded by 8 modules which will be referred to as the outer part. In z-direction the forward calorimeter is divided into three modules of 470, 4:rn and 640 mm thickness, respectively. The central module closest to the interaction point is of the e.m. type, all others are hadronic. The shape of the individual modules is indicated in Figure 5. I 3b.

For the 24 outer hadroni c stacks the plates are welded a.t t heir periphery to a common base plate, thus fixing a constant gap widt h . This base plate also carries the suspension structure which transfers the forces of the st.ackH to the railing devices at the inner wall of the cryostat.. The opposite periphery of the plates is kept in position by rods welded to the edges.

The inner hadronic forward stacks are built from S.S. plates made in two parts and are connected together on the outer periphery only by a supporting structurn. The radial separating face is kept free from any struct.ural material to enable easy mounting of the independent readout boards. All six inner stacks are suspended from the top outer stacks by a flap-and-bolt system. The most forward stacks require a separate suspension construction because t he cryostat wall does not extend over t heir centre of gravity. A steel ring reinforcing t he end flange a.t its inner diameter is used to ta.ke t he weight of these stacks and to transfer the forces into the cryost at structure . The inner e.m. module is made of 65 lead pl11.t.es . The octogone is made in two parts of 2340 mm maximal dimension. For thin lead plat.es of that size it is necessary to foresee a sandwich construction where 0.8 mm G 10 readout boards are glued on each side of the 2.4 mm Pb plate forming a readout sandwich. Every second Pb plate is plated with G IO of the same thickness as before or with a t.hinner metal foil of about 0.1 to O.Z mm> which yield the same thermal shrinking for the sandwich as with 0.8 mm GlO. In contrast to the barrel region this will be a compacted version where the plates are bolted together> and the 3 mm argon gap is guaranteed by spacers on the bolts. This compact version is possible because all sandwich plates have the same thermal shrinking.

Summary of the expected physical characteristics The overall stack system consists of 50 individual Pb stacks with a weight of 57.4 t and 68 S.S. stacks with a weight of 327 t. The proposed stack thicknesses, sampling steps and expected resolutions are summarized in Table 5.4 for 0 = 90n

(central barrel) and 0 = 0° (forward) in the medium plane of one module. Between these two extremes Figure 5.14 shows the variation of the interaction length (>.) and of the radiation length (Xo) as a function of 0. Lines of constant thickness in >. and Xo are shown superimposed on the schematic of the calorimeter. The radial stack dimensions of the e.m . part follow as closely as possible an Xo-line varying from 20 to 30 Xo.

5.4.3 Granularity,segmentation and number of Channels

Good energy resolution has to be accompanied by fine granularity especially in the e.m. and forward hadronic calorimeters. Good e/1T separation depends on both granularity and longitudinal segmenta­tion .

78

Tower and Pad geometry - Transverse granularity Projective towers constitute an attractive solution; nevertheless some of their advantages are weakened by the uncertainty of the J.P. position which alters the notion of projectivity in the barrel part. Furthermore, from a. practical point of view, they present several drawbacks: the pad design is different for all the readout boards; along the non-projective cracks (hadronic part in ip, everywhere in z) several towers are split leading to a larger number of channels.

Another solution employs semi-projective towers (projective in ip, non·projective in z) which would lead to families of identical readout boards and reduce the number of the split towers around the z cracks. Projective towers can then be approximated by staircase towers as shown in Figure 5.15.

The basic dimension of the e.m. pad geometry is~ 2 the Moliere radius (4.3 cm). Jt is measured at t.he entrance of the e.m. stacks perpendicular to the incoming particle: 3 cm in the forward part (necessary for a good separa.tion of the particles); 4 cm for 20° < 0 < 80°; 8 cm for 8 > 80° where particles are well separated. For hadronic stacks the basic dimensions are doubled for reasons of angular measurements and separation of jets.

a. r.p - Granularity. Projective towers in ip {constant z plane) are shown in the left part of Figure 5.16 for the barrel part. For all of the 8 modules (360°}160 ip towers are foreseen (20 per module) corresponding to 4.0 cm pads at the entrance of the e.m. stack. Jn the hadronic part 80 towers are foreseen (10 per module).

b. e - Granularity. Figure />.15 shows a solution for the segmentation in the z direction. In the forward parts (barrel and forward) where all plates and boards are vertical, we envisage fo:r

every part families of identical boards. Examples are shown for the forward barrel in Figure 5.16 and f~r the forward part in Figure 5. 17. For the forward barrel, the required granularity of 4 cm perpendicular to the particle direction defines the number of boards ganged together and the maximum size of the pads in the radial direction. This radial dimension can also be linked to the definition of longitudinal tower segments, since for a module the edge of corresponding pads of several boards defines lines of roughly constant X0 's. With the parallelepipedic elements such obtained one can produce a staircase projective tower as shown in Figure 5.15. Several elements wilJ be ganged together to arrive at · longitudinal segments of appropriate depth (see below). For the hadronic part the size of the pads is defined by the longitudinal tower segmentation and by the size of the approximated projective towers.

For the forward part the size of pads (3x3cm2 ) defines the granularity. The number of hoards linked together defines longitudinal tower segments as in the central barrel part. A staircase approach of projective towers can also be used.

For modules 1 and 2 of the central barrel part the pads are 7.3 cm in z (12 per module) for all boards in the e.m. part. Since towers are projective in ip, for one module all the boards are different. Longitudinal segments (for different depths of the shower) are made by ganging together the corresponding pad of several boards. Module 3 has to make the transition between 4 cm and 7.3 cm pads; 16 pads of variable size are foreseen. The pads in the hadronic part are everywhere twice the size of the pads in the e.m. part.

Jn the backward part,the philosophy of the forward parts is reproduced,with towers of 8 cm per· pendicular to the particle direction.

Longitudinal Hegmentation

a. E.m. part. The knowledge of the longitudinal development of showers is very important for e/TT separation. Different studies {both M.C. and experimental) have shown that the minimum number of longitudinal segments required is 3. Jn the barrel part three segments of 3, 6 and 11 Xo's at 90° are foreseen. For 0 > 30° the same ratio in Xo is kept between segments as the number of Xo mcreases. For 0 < 30° a 4th segment will be added for optimizing the measurement of a larger variety

79

(

of electron energies {up to 250 GeV). Four segments of 3, 6, 6 .and 15 Xo depth will provide good e/'lf separation for the range of electron energies in the forward direction. Detailed M.C. work is presently underway to optimize the number and size of longitudinal segments for an optimal e/7r· separation in a jet environment.

b. Hadronic part. For the hadronic part one segment per ~ 0.9 ,\ allows to use efficiently the weighting method. Therefore 4 segments for (J > 45° and 5 segments for() < 45° are foreseen. Around () ~ 20° one segment per additional 0.9 ..\ will be added. This is summarized in Figure 5.15.

Number of channels The proposed overall segmentation is shown in Figure 5.15. The number of electronic channels obtained for the different parts of the calorimeter is summarized in Table 5.5 together with the range of capacitances per channel. The total number of channels is ~ 61 000. lt includes a presampler in the region 20° < 8 < 30°, i.e. an ionisation gap sampling the energy at the entrance to the LAr system. This measurement is used to compensate for the large amount of material in front of the e.m. calorimeter. The number of channels will slightly increase if a presampler will be needed in a larger angular range. We therefore assume an upper limit of 65000 channels for

( . the final design.

5.4.4 Sensing Structure

The ionization charge will be collected in readout cells for which the following requirements hol<l :

• The capacitance has to be kept small ( < 8 nF / channel) to limit the electronic noise.

• The crosstalk between readout channels has to be kept small(« 1%).

• The high voltage should not be applied directly to the pads or to the absorber plates, but to resistive layers placed either against the pads or against the board opposite to the pads. The decoupling capacitance is thus provided by the readout cell itself. Resistive layers have the advantage of protecting the preamplifiers against high voltage breakdowns. Their developrpent is described below. Should this rather novel technique not work, HV can be applied directly to Cu-foils on the board opposite to the pads.

• The absorber plat.es have to be grounded.

• Since the boards define the gap size, their surface has to be very flat.

• The dead space (due to e.g. spacers or signal lines) has to be kept small ( < 1 %).

• Last but not least, these boards have to be as cheap as possible since the total area is of the order of 5000 m2.

The last requirement excludes sophisticated multilayer boards for the bulk of the readout cells.

Readout cells

a. E.m. part. In the e.m. part below 80° the pad sizes are small (3-5 cm). Therefore in view of the small capacitance a structure of the readout is chosen, where the G 10 board with pads is on one side of the gap (Figure 5.lOb). The leads to the pads are on the backside of this board, insulated by a thin capton foil against the absorber plate. In the barrel region this foil is glued against the G 10 board and it has for screening a copper layer on its side towards the absorber plate. ln the forward region the G 10 board and the capton foil are glued altogether to the absorber plate and the absorber plates act as a screening surface. It has been tested that this screening is important to keep the crosstalk

80

from pads to the leads small . On the opposite side of the gap another G 10 board acts to define the gap , where the size of the gap is defined by spacers as already explained above.

ln the region above 80'\ where the pads are larger (~ 8 cm), the structure will be the same except that the leads to the pads are on the same side of the G 10 board as the pads (Figure 5. lOa). The leads are separated by ground lines. The maximum space needed for 10 pads (1/2 '{)sector) should not exceed ....., 8 mm.

b. Hadronic part. In the hadronic part, especially at large radii, the pads are as large as 20 x 20 cm2 , hence the capacitance of the two board cells is rather large. Therefore a solution is foreseen where the readout boards is in the middle of the argon gap gaining a factor,..., 3 in capacitance Independent readout cells will be made of 3 layers (type c, Figure 5.lOc). We test two alternatives: one in which the two external layers (0.8mm GlO) are coated with copper on both sides and the middle one (0.5mm GlO) has pads printed on both sides with the signal lines on the same side as the pads. Alternatively a solution is foreseen in which thin (1 mm) S.S. plates act as external boards. The advantage of the latter solution is a.n increased amount of absorber material in the given cryostat volume. The capacitance reduces approximately to the argon ga.p capa.citance of 20 nF / m2.

In the inner forward hadronic part the signal lines on the boards are very long. Here the same cell ( structure will be used but with a multilayer boa.rd in the middle in which the signal lines are taken out between the pad layers.

A summary of the readout cell organization is shown in Table 5.6.

High voltage layers The high voltage will be brought. to the ionization gaps via layers with high resistivity, which can t.ben be affixt!d to either side of the gap. In both cases these layers protect the electronics against larg<! discharges through the LAr. The resistance has to be ~ 10 MO/ square. The lower hound is given by the crosstalk to neighbouring channels ( < 1 %), if the layer is attached to the readout board. The upper bound is determined by the maximum possible current in the gap and the corresponding voltage drop.

A method has been developed for the production of such layers. A specific epoxy with an admixture of soot was sieveprinted on 50 µm kapton foils. These foils were then glued with another epoxy to readout boards which had before been glued to Pb absorber plates. Several gaps of an e.m. test calorimeter using LAr have been equipped with these layers, and measurements show that they work well. Other methodes of production are also being studied.

81

Table 5.4: Summary of stack physical parameters

( Absorber Absorber Argon # # # Length i·.VE material thickness gap samplings Xo ),

jmm) [mm] [cm]

e. m . Pb 2.4 3 65 30 ........ l.5 47 0.09 oo

hadr . S.S. 12 2 x 1.5 58 43 ....., 4.5 107 0.55

Total 73 ,.._ 6.0

e. m. Pb 2.4 3 43 20 ""1.0 31 0.09 goo

hadr . S.S. 12 2 x 1.5 48 34 3.8 90 0.55

Total 54 4.8

82

oc w

Pads mean sue

lcm2]

Forward 6 < 20° for e.m. 3.0 x 3.0

Forward barrel 20° < 6 < 45°

Module 2 4.5 x 4.7

Module 1 5.0 x 4.7

Central barrel <l5° < 8 < 152°

Module 3 6.0 x 4.7

Module 2 8.0 x 9.4

Module 1 8.0 x 9.4

Backward barrel 143° < 6 < 152° 8.0 x 9.4

Standard Total

Split channels

Overall

Total # of channels: 60 338

Table 5.5

Electromagnet.ic

# Cap/ # #Pads longitud. channel cha.nnela

aegment. I nF J

3765 4 0.3. 1.3 15060

8x 160 4 0.6- 2.0 5120

8x 160 + Pretiampler 1280

12 x 160 3 0.7 - 2.3 5760

16 x 160 3 0.8 - 2.8 7680

12 x 80 3 0.5 - 6.8 2880

12 x 80 3 2.0. 6.8 2880

6x 80 3 2.2 - 8.0 1440

e.m. 42100

60338 e.m. 42100

Hadronic

Pads # Cap/ # mean sin #Pads longitud. channel channels

jem2) segment. I nF]

7.0 x 7.0 844 5 1.9 •022

15.0 x 13.0 4 x 80 6 2.9 - 9.0 1920

. 5 x 80 400

15.0 x 13 4x 80 6 2.9 - 9.0 1920

18.0 x 13 6x 80 5 4.9- 6.3 2<100

11.0 x 13 8 x80 • 2.9 - <l.8 2560

14.6 x 13 6x 80 • 3.8 - 6.3 1920

li.6 x 13 6 x 80 4 3.8 - 6.3 1920

hadr. 17002

1176

ha.dr. 18238

-

Table 5.6: Characteristics of readout boards in the different parts of the calorimeter

( Part Readout Surface Gap Cap/ Collection Direct

type size layer time crosstalk lmZj !mm] lnF/m2

] Ins]

e.m. pads >7x7 a 750 3 50 600 yes

lcm2]

e.m. pads <7x7 b 1130 3 50 600 no

lcm2]

-

Hadronic c 3400 z x 1.5 20 300 yes

84

::.. ~ :; :;q· ::r c:: ~ ., 0.. ~ .... ..... ~ ..... :? . 0 ,.., ..

O'Q ~ I\' '"I

"O :::-<"O

~ 3 ~: g> ::r ;:.

0.8

.... (")

:::r' 0 "!) ....

3 0.. .... -· ::r ... ~ :::i ~ .... ::I ~ 'Jl Cll -· 0.. 0 0 = c::

0.8

.... ""

.... .... ~ :::;; ;» :ii "O

~ .. ~.I» ..... -'"I "" ~ "'> = ;,,. ... = 0...

'::r ..._., "A =r 0 ~ ,_ :::r ~ -~ e ,.... 0.8 '< "O' <'II ,,.,, 0 3 .... .... ::r 'l)

~ 0.8 3 ~ I»

'"O ("') -(J)

:r 0 ~ ~

Absorber

-----.... ~-~-~~~~~~~G10 Cu

Argon Resistive HT ... , ...,.....'painting

G-1-0pads ~-~-==-=-===-=====...,.. ________ ._. __________ ====-____ .-----Cu

~Absorber

a)

~~~-~~rber ~~~~~~~~~~~~,.G10 .. 9u

' _, ~---,.,

Argon Resistive HT ... _ ...,..,., painting ------------------------------------------- pads

~~~gapton Absorber

b)

0.9 -Q.8

1.5

0.5

; Es 0.8 .9

Absorber

~-----,

G10+Cu or liii~~~~~~~~~~~!!i!J)M._: __ ..... Stainless ---- steel Ar g 0 n -----------=-------.. Re:-isfive ~-!:~G10_ya•ntmg

~---H-T7 ______ A;-g~-n ~;;~:-cu or

!.._ ____ .. Stainless __ steel

~Absorber c)

-

(

/ ·'\ ·~ ~.

..... 8 N

..a' H u c: 0 '-'O _,, ::r::

:El

~

..... ~ .~ -QI c. O" ltJ E 0 t... -~

w

u -QI c: O"

u l'O c E 0 0 '- .i::

"'O u IV ~

::r:: w

.. :El

~

F'igurr. 5.1 J: Mechanical design of the two typf'!'l of LAr Rtacks int he harrel calorimetf'r, left.: horizontal

platf' orientat.inn , right. : vertical plate orientation .

8fi

ELECTROMAGNETIC PART

(

L.A. GAP

Figure 5.12: Mf'chanical df'~ign of the LAr ~tacks in the e.m. ~E>ction oft.he ha.rrf'l calorimeter

87

+

(

z z z z z z .lll: .II( .. .. .,11( ,Jll .,

N CIO 0 0 ID 0 OI '° a» 0 ~ N

II " .. • • " .. CD ca - ., N N N ..r;

"' • z z z z z z ~ ,Jll ,Jll .lll: ,Jiii .J( .J(

4P ... .,, 0 '"' )I( N N N .... ..,, I"-0 ... a. " " II " " " a. ~ U..N u..'"' l.LW ~ U..N

009\ 0

0 0 0 - - -

Figure 5.13: a. Longitudinal and transvprRal Rt-gmf'ntation oft.hf' LAr Rtach in the forward calorimet.Pr

88

QtEz

·- 7 'L

OOOc

(

-·~

®

® OLJJ

Figure 5.13: b. Shape and dimrnsions of the. individual LAr At.acks in thE> forward ca.lorimt>tt>r

89

(

x

0 -G> 0

Figure 5.1 :J : c . Support of t.he forward LAr stacks on the cryostat. wall. The support of an inner forward .o;ta<"k on a top out.er one is also indicated.

90

I

l Q. I

- T I

D

x ~..,_,.,.f'+"-t-'"1-'-il"''f'"l~"'t"I'·· 0

(1')

Figure 5.14: Amount of abAorber maf.<>tial: lso-A and J~o-Xo linfl~

91

I

l

Fi gurP 5 .15: Longitudinal and tramwersal t.ower !legmentation. Rectangles represent the ~erie!' of pad~ gangt>d together t.o form pst>udo-projectivf' t.owen16!1 Ahown in 5 rlaceR. Thickf'r )inC'l'i in t hf' f' .m. part

rorrE'Rpond to longitudinal !'lt>gmf'nl.R.

92

Figure 5.16: Azimuthal cut. through the barrel calorimeter. Left.: Projective towers Right: Pad

!ltructure in the forward barrel stacks

93

(

(

Figure 5.l 7: Pad struct.ure in the forward gtacks

94

5.5 LAr Electronic System

5.5.l Introduction

The design of the electronic system has to take into account the particular HERA beam situation of large energies being deposited at short time intervals (·- 100 nsec) into detectors with large capaci­tances and long charge collection times. In addition the· information has to be stored until the arrival of the trigger signal (....., 2 µsec). Finally the LAr calorimeter has to be operated in a large magnetic field.

Particles traversing the LAr gap ionise the LAr along their tracks. The generated free charge carriers are separated and collected by a high voltage field across the gap. The charge signal is built up by the electrons drifting to the anode, whereas the inHuence of the slow positive ions is small and largely suppressed in the differentiated signal. The drift time is about 200 ns/mrn and depends only slightly on the high voltage. For an ionisation deposited at a distance x from the cathode the measured charge is proportional to the drift path: Q(x) = e(d-x) / d, where d is the gap width. Integration yields half of the negative deposited charge.

In case of high counting rates the length of the signal pulses >. has to be kept short and therefore small gap distances are necessary. In the Hl calorimeter the gap width is 1.5 / 3 mm which corresponds ( to drift times of 300/600 nsec. The integration time has to be at )east of this size in order to accumulate all deposited charge.

The smallest signal is typically that of a minimum ionising particle. In the readout configuration as shown e.g. in Figure 5.IOc the charge Q collected from both gaps onto the high voltage p)ate is given by Q = dE/dx · 2 d / 2 E [eo], where Eis the ionisation potential (26.5 eV /eo) and dE/dx the energy loss of minimum ionising particles (2.11 MeV /cm) in LAr. This gives Q = 11940 eo for the charge deposited in a double gap of 2 · 1.5 mm.

Table 5. 7 summarizes for a typical tower of the forward calorimeter an estimate of the noise using the rather pessimistic values of the "cold'' electronics (see below), the expected signal for minimum ionising particles, and a M.C. result of the expected signal for electrons, pions and jets.

As shown in Table 5. 7 the minimum ionising signals vary between 71800 eo and 405 000 eo de· pending on the longitudinal ganging. To detect this charge efficiently, a low noise amplifier is needed which has to have a variance of the noise peak of q < 20 OOOeo for an input capacitance of about 1 nF as in the e.m. section.

From Table 5. 7 we also derive a dynamical range (defined as the ratio of the maximal signal to the noise) of maximally 16000 for the e.m. part because in a shower of 250 GeV electrons maximal pulses corresponding to 2.8 · 108 e0 are expected. The dynamic range requirement ~n the ha.dronic section is rather 4000 for typical jet energies. For ~75% of all channels a dynamic range of 12 bits is sufficient. For the remaining channels, we aim for 15 bits dynamic range.

Due to the short bunch crossing distances at HERA special care has to be taken to shorten the signal processing time. The probability that pile up occurs in a pad close to the proton beam during the processing of the signal is in the order of 10-5. The pile up problem in one channel does not seem too severe even accounting for the fact that the background is of high energy and spreads over several pads. On the other hand the probability is high (- 10-2) that a background hit happens somewhere in the detector in conjunction with a good event.

5.5.2 Analog Signal Processing

The individual readout planes are ganged together on the side of ea.ch calorimeter module. Signal cables lead to the first stage electronics. Since the calorimeter will be operated in high magnetic field, matching transformers cannot be used. The amplifiers have to be as close as possible to the pad structure to reduce additional noise due to cable capacitance and to avoid to slow signals due to the cable inductance. We discuss two alternative schemes: a "warm" solution in which all analog

95

processing is done outside of the cryostat and a "cold'' solution where amplification a.nd multiplexing are located close to the ·readout boards in the LAr. The obvious advantages of such a solution are:

• Reduction of the cable length between detector and amplifier by ,...., 10 m to < l m.

• Reduction of noise due to the large capacitance of the cables which adds to the detector capac­itance.

• Faster signal response which is slowed by the inductance of the cable.

• Less pick-up noise since the amplifying electronics is well shielded inside the cryostat.

• Reduction of the "white" noise due to improved performance at low temperature.

• Simplification of the cryogenic system due to the reduced number of signal lines (130000 --. 8000) which enter the cryostat. The heat load in the cryostat is largely dominated by the heat leakage due to cables.

There is one obvious disadvantage: bad channels are poorly accessible, there is almost no possibility to replace them. Presently there exist very positive experience on the lifetime of monolithic systems a.t temperatures varying from -20° C to ,...., 150° C, but no experience of long time behaviour at LAr temperatures.

We describe the conventional solution first.

"Warm" Solution It is schematically shown in Figure 5.18. The signal lines are fed out of the calorimeter into charge sensitive prcamplifiers (PA} located just outside the feedthroughs on the cryostat. 16 channels are grouped on one boa.rd. For each board a :;;um of the signals of all channels or of each group of 4 channels is formed and provided for the trigger. The amplified signals are sent to shaping amplifiers which will yield the peak signal into a sample and hold circuit. A multiplexing of 16 channels is foreseen. All these elements a.re contained in one unit. The multiplexed signals are sent via differentially driven twisted pair lines of,...., 25 m to a differential line receiver before entering the ADC and readout system.

The basic considerations for the application of low noise charge sensitive PAs in LAr calorimeters have been previously outlined.2 The input noise charge density is"" (Co + Cpar), where CD is the detector capacitance and Cpn.r the parasitic capacitance (e.g. cables). Typical values are Co ~ 5 nF and Cpar ~ 1 nF. Matching the calorimeter cell to the PA via an optimally chosen transformer makes the noise charge proportional to JCo + Cpar· Input noise equivalent charges of~ 15000 e0 's have been obtained for a cell capacitance of 5 nF and long shaping times. 1 Due to the operation in large magnetic fields other means to minimize the noise for large Co have to be applied in the present case:

• Very low noise JFET's have become available.

• Longer integration times can be used.

• One can employ n JFETs in parallel which would red11ce the noise by .Jn.

While the multiplexing system is still under study, preamplifier and shaping amplifier have been designed for application in our beam test at CERN.3 A single JFET (2SK371 Toshiba) is used at its input as the lowest noise device available. A rather large open loop gain of the amplifier of 95 dB is achieved, the feedback capacitor being CF = 20 pF. An input impedance R1 11 == 70 is found. The output voltage of the preamplifier is given by Vout = - Q/CF where Q is the charge collected in the calorimeter cell. Figure 5.19 displays the frequency response of the PA both measured and calculated.

2 W .J. Willis and V. Radekn., Nu cl.Instr. & Meth. 120, 221 (1974)

"H. Brett!, W. Pimp! and P. Weiflba.ch, MPJ Internal Report, Hl-MPl-017(1986)

96

In the shaping amplifier a diff~ren~al line receiver is followed by a twofold double pole integration/ single pole -differentiation including pole-zero compensation. A bipolar shaping results with a. sh a.ping time (zero to peak} of 2 µsec. With this signal processing the noise depends on the detector capacitance as 4.7 e0 /pF. In the present test setup the input stage of the shaping amplifier accounts for most of the constant term in the noise behaviour. This stage will disappear in the final version and a. rather small constant term is expected.

"Cold" Solution Alternatively we study a "cold" solution in which the analog signal processing is located on the back of the ganging boards on the side of the calorimeter modules in the LAr. Figure 5.20 shows the schematic principles of the arrangement of the electronics components. A monolithic chip contains for each of 16 channels; a charge sensitive preamplifier, a. pipeline of 22 samplings, a signal analyzing difference system yielding upon trigger a filtered and baseline subtracted signal proportional to the deposited charge. Jn addition each channel is supplied with a calibration signal and a reset signal. The output signal is sent via a 16-fold multiplexer to a line-driver both integrated on the same chip. The output is connected to the system outside of the cryostat. The trigger sum for either 4 · 4 or 16 channels is formed at this stage and sent out of the cryostat to the trigger system. From here on the two systems are identical.

CMOS technology was chosen to realize these chips because of its low power consumption and flexibility in circuit design. It operates at cryogenic temperatures and offers the possibility of scaling the current in all branches of the circuit by an external reference current. Speed and noise versus power r.onsumption can therefore be optimized according to the specifi c requirements of the experiment.

The need for low noise-low power-high density rea.dout electronics was encountered in the appli­cation of Si-strip detectors. Jt has led to a collaboration of the MPJ Miinchen with the IMS Duisburg to develop readout electronics for the Si-strip vertex detector and the TPC detector of ALEPH. We draw heavily on this expertise.4 The present project requires t.o extend the investigations to the per­formance at LAr temperatures and for large detector capacities. Particular emphasis has to be put on the requirements in the multiplexing due to the short bunch crossing times at HERA.

For the TPC readout a monolithic combination of a charge sensitive amplifier followed by a sym­metric Jine driver has been developed. A CMOS switch in the feedback loop of the charge sensitive amplifier allows periodic resetting. Thereby an external feedback resistor is avoided resuJting in an infinite time constant. This principle wilt be adopted here. A complete chip of this type was fabricated with the power being adjustable up to 150 mW (12 mW in the amplifier). At 75 mW the risetime is 60 ns. The open loop gain of the amplifier is above 20000. A severe problem in MOS electronics is the presence of appreciable l/f noise which has the unpleasant feature that the signal/noise ratio of a charge sensitive amplifier cannot be improved by filtering (reduction of bandwidth), contrary to white noise where the signal/noise increases with the square root of the shaping time constant. Lowering the temperature reduces the white noise, but does not affect the l /f noise.

The chip was successfully operated at LAr tempera.tures resulting in a 20% decrea.se of the noise. In addition t he feedback capacitor of the amplifier was changed to cope with a large detector capacitance Co. A noise performance of IO 000 e0 +9 je0 / pF]· Co was observed resulting in 55 000 e0 for the typical Co = 5 nF at considerably enlarged rise times of the signal.

Figure 5.20 displays in particular the schematic of t he pipeline and multiplexing system. Each amplifier sends its signal synchronized with the beam crossing into a wrap around system of storage capacitors. The number of such storage capacitors is chosen large enough (22) to have all relevant signals available up to the time when the first level trigger gives the information on the bunch crossing time t 0 of the event. At that point the control logic (Figure 5.21b) starts the multiplexing by switching in sequence through the channels. For each channel a difference signal is formed between the signal levels after and before t0 • In order to achieve a sufficient filtering, the signal levels are determined by averaging over typically 3-5 beam crossings. The principle is illustrated in Figure 5.2la. After the

'G. Lutz, Contribution to the ESSCIRC Conference (1985)

97

multiplexing is done, a reset is applied to each channel. Experience on the important question of the noise injection by switching will be gained from a chip with a multiplexing system for 60 channels without pipelining which has been produced for the ALEPH Si-strip detector.

Although CMOS technology gives quite satisfactory results, further improvement in noise perfor­mance is expected from the inclusion of technology compatible JFETs in the CMOS electronics which have been developed at the IMS Duisburg.

Realization of the "cold" chain. An agreement wa.s reached between the MPJ Miinchen and the IMS Duisburg for the "Development of a multichannel monolithic amplifier system for application at low temperature". It calls for a. joint design between the two institutes with the fabrication to be done at IMS. The specifications are given in Table 5.8.

It is foreseen to build the electronics for 3200 channels ( =200 chips) up to the middle of 1987. It would then be tested with the prototype stacks in the CERN beam tests. A decision for production of the final series of~ 5000 chips would be taken early in 1988. With an estimated production time of< 6 months the entire analog system would be ready for installation with the stacks in fall of 1988.

( 5.5.3 ADC and Readout

The ADC a.nd readout system of the calorimeter must fulfi)J the following requirements:

• 65 000 channels

• fast digitization and readout - ~ 1 msec

• dynamic range of 15 bits for 2.1) % of the channels

• analog cable length up to 25 m

• storage time in crate buff er I ms

• data processing in detector crates

• deliver data in VME standard

• low cost

A major technical problem is the transmission of an analog signal with 15 bits dynamic range over a dist.ance of 25 m at low cost. We are investigating two solutions:

Solution A It is being developed for ALEPH at Saclay. A schematic drawing is shown in Figure 5.22 The main features are:

• Transmit each signal twice at gain-1 and gain-8 over twisted pair cables.

• Digitize both signals in parallel with 12 bits dynamic range.

• If the gain-8 ADC overflows, readout the gain-1 ADC, otherwise readout the gain-8 ADC.

• It thus bas an effective dynamic range of 15 bits.

• It does not require the transmission of very low level signals.

• The ADC conversion time is 7 µsec. Eight pairs of analog signals are multiplexed onto one ADC and each of these signals is multiplexed 32-fold in the analog electronics, giving a total readout. time of~ 2 msec (32 · 8 · 7 µsec+ readout).

98

Solution B It is being developed by LAPP Annecy for UAL A schematic drawing is shown in Figure 5.23. The main features are:

• Digitization close to the amplifier.

• Choose the amplification 2- 1-2H by FADC.

• Digitize with 8 bits.

• Transmit the digital value via optical fiber at or coaxial cable at 20 Mbit/sec.

• Effective dynamic range of 15 bits.

• The transmission of analog signals over large distances is avoided.

• The total conversion time including multiplexing is 1 msec.

Comparison The solution A is being implemented in FASTBUS at SACLAY along with the neces­sary support modules (e.g. sequencer to control the internal operation of the ADC boards, FASTBUS-VME interface). An interesting feature is retention of all ADC data in on-boa.rd memory, allowing ( sophisticated zero-suppression schemes at the board level. There are also two MC68020 based crate processors under development for DELPHI at SACLAY and for ALEPH at CERN which would be suit-able for doing crate level processing such as zero-suppression, gain correction, formatting, monitoring, multi-event buff er management and interfacing.

This system is, however, quite complex and somewhat slow (2 msec). It requires ~ 4000 cables of 25 m length which can bring pickup and ground loops. In addition, it requires suppression of all FASTBUS activity during digitization to prevent pickup of digital noise.

The solution B is faster and avoids electrical problems if optical fibers are used for the long distance transmission. Data storage and processing could use some of the same modules as the solution A, but it is probably better to design new modules to improve the a.cquisition time. We are free to build the system in VME.

The major drawback t,o this solution is that it requires space for the ADC's near the analog electronics where space is at a premium and where the electronics is inaccessible when the detector is closed.

The schematic of the entire readout architecture is shown in Figure 5.24. It will be discussed in detail in chapter 9.

5.5.4 Calibration of the electronic Chain

The response of the electronic cha.in will be calibrated by the injection of charges via a capacitor into the input of the preamplifier. The testsignals are distributed such that at each preamplifier card groups of 4 channels out of the 16 channels at one card are pulsed simultaneously. This allows us to study t.l1e crosstalk between different channels introduced by stray capacitances at the readout boards and within the interconnecting cables. The testcapacitors are mounted-on the preamplifier hybrids. To avoid additional crosstalk, the value of the testcapacitors will not he larger than 4.7 pF. However, for these small values it is impossible to get the capacitors with the required tolerances of a fraction of 1%- From our present studies we can get a preselection by the manufacturer such that the values do not deviate from a mean value by more than 2%. This restriction requires that the final calibration of the testcapacitor must be included in the performance test of the hybrids. The individual preamplifiers will be selected into groups by comparing the response via the testcapacitor on the hybrid with the response via a calibrated testcapacitor which is part of the performance test unit. The individual groups of amplifiers will then be coloured differently and distributed over the Hl detector such that the effort for bookkeeping and spare handling gets minimized. In addition to this calibration procedure many other correction factors must be taken into account.

99

• The termination of the testpulse with the cable impedance .. fotroduces large currents on the groundplane with the result of baseline shifts.

• The cables to the preamplifiers with a length of tom for all channels cannot be backterminated at the readout boards. The reflections on the cables will introduce some differences in the shape of the signals.

• The shape of the testpulse will not be perfectly matched to the shape of the detector signals.

• The large detector capacitances affect the risetime of the preamplifiers.

AJI these points are at present under detailed investigation. This calibration procedure will run under the normal trigger conditions. This means that no extra requirements are introduced on the readout system.

5.5.5 Hjgh Voltage System

The high voltage to be applied to the LAr gaps is typically 1 kV / mm. In the present case this amounts ( to up to 3 kV for th 3mm gaps. The HY is supplied via an external distribution system where voltage,

leak current and sparking a.re monitored . Shielded cables carry the HV to an entrance port at the cryostat where the shielding function is taken over by the cryostat wall. Single lead cables enter via the cryo-supply ports at the entrance into the cryostat. The cables are guided on the inner cryostat wall to the location of the individual st.ads. A plug connects the external HV cable to the HV cabling which is fixed within the stack. The distribution to the individual rca<lout planes is done on the support frame of the stack.

The number of HV c:ounections has to be ker-t to a reasonable minimum to keep the cryogenic losses small. On the other hand the volume of tl1e st~ck which is supplied by one cable has to be kept small enough t o minimize the effect of sparking in a single channel. A reasonable optimum seems to be to supply areas comparable to the longitudinal sampling and oriented on the size of the readout boards. Assuming a maximal size of readout boards of 90 x 60 cm we arrive at a total of 1500 HV cables entering into the cryostat.

100

Tower #Gaps Cap/ El.Noise µSignal

Slice Channel t7

[ nF] [lo! t!o] (103 t:o]

e.m. 1 6 0.27 12.4 71.3

e.m. 2 12 0.54 14.9 142.6

e.m. 3 21 0.95 18.6 249.5

e.m. 4 25 1.13 20.2 297.0

ha.d. 1 6 0 .43 13.9 71.3

had. 2 11 1.07 19.6 297.0

had. 3 11 1.07 19.6 297.0

had. 4 15 1.46 23.1 4-05.0

ha.d. 5 15 1.46 23.1 405.0

Table 5.'T

Characteristic signals compared to electronic noise

in the LAr forward calorimeter

µSignal/ A>1.Signal Max.Signal Av.Signa]

El.Noise r: 250 GeV t 250 GeV ,,. 100 GeV

[to! r:oJ [lo! t:o] 11cri eol

5.8 18150 34600 1150

9.6 152830 198430 5950

13.4 13' 770 191400 12570

14.7 4350 13 550 5930

9.4 2120

15.2 4330

15.2 3270

17.5 1620

17.5 702

Max.Signal Av.Signal Max.Signal

,,. 100 GeV Jet 100 GeV Jet 100 GeV

[lo! t!o] [lo! t!o] [103 r:0 J

6600 3130 7360

31430 21620 58490

50300 16090 45280

27670 4320 16040

8800 1760 5470

17600 2770 8770

16340 1750 6600

7550 1430 5850

3590 700 3210

Tab)e 5.8: Specifications for a 16-channel amplifier and multiplexer in CMOS - technology for use in liquid argon.

( a) Risetime :::; 100 nsec .

b) Noise < 70000 e0 with 5000 pF in parallel to the input, measured after a pulse shaper with time constant 2 µsec .

c) Maximum signal input 5 x i08 e0 •

d) Output 2 x 50 0 differentially, up to 5 V, into a twisted pair cable.

e) Power consumption < 320 mV without input signal.

f) Channel to channel crosstalk < 0.5%.

g) Supply voltage 6 V.

h) Summing amplifier for 16 channels. Amplification by 0.5, including driving circuit.

i) Non-linearity < 1 %.

j) High voltage protection 1.5 kV into 5000 pF.

k) Calibration input for each channel.

I) Reset.

102

-... (..)

z: )( ~£

-z 0 cc ..... (..) w ...J w CJ ( 0 ~ ...J < ~

~ z ~

< I E I .... :B (0

~ ~i I I

I .,... :i: .... I 0 (.) I ·-... I cu E GJ I

.&; (.) ' I en ' ' I

I I

' I

' ,1 ~

Figure 5.1 R: Schematic of the 'warm' analog chain

I 03

(

m "'O 0 0 N -

m "'O .

0 <X> 0

I I ?"'?P I I f~

0 0 0 ("')

i ~ I // I/ I/; ~ I I

~ I I o/ I )/

/ I

/ I / I

II

1 // I

I I I

I I I I I I I I

I 0 I g.I I

I I I I

0 0 0 N

0 0 0 -

0

«> 0 -

""" 0 -co

0 - f"""'1

N :J:

..n '-' 0 - ~

(.)

.c .... °' 0 :J - er Q)

M 0 -

N 0 -

..... 0 -

..... u..

Figure 5.19: FrequE>nr.y response of phMe (ip) and amplitude (a) of the 'warm' PA as measured (solid lines) and cakulatf"d (dai:iht>d lines)

c c

( .

Switch cloud

Swlteh OPffl

( . . , •so - •s11 u '------------_I

(JHfltJ tf ~Hf,..11 n -----' .___ __ ~H(IJ to fHthtl n __ ____, ._

Or _ ____,n ___

Figur~ 5.20: Principf P. configuration and Awitching arrangemP.nt of thf' 1cold' analog chain

105

(

A-t A., A-J A .J A -1

'-s ,_, t., '-1 t .,

Cnobl• Read

l<fP)( Clod

( AJ • A, • A5 • As • A1 )

5

( A -5 • A. , • A-1 • A -1 • A -1 I 5

- __... ADC

An • Somp/u of J/gnal omp/ltud•

5

r----

O•coder 0

11

I Base /In• __ __,_f--c?~--_,s~u.btract,

I so=m~. I to a, -011

I ~----o-~----------__J

L- -

Figure 5.21 : Details of tht' 'cold' ele<"tronio1: a . Prinriple of haseline irnbtract.ion and filtering b .

Control logic of the multiplE>xing system

106

"C c N

u Cl 0 -r1' c.:: <t

"'O c

UJ I ...... c: 0 t-

LL

u 'O\ 3

c:: 0

:~ ~ 0

/

"'O c N

Figure 5.22: Schematic of the ADC and readout of solution A

107

( .

Ill ro

+.... fO

0

t... .a ro :J c QJ ......

...... ClO ro N ...-QJ >

,.. ·- -.... V>

Vt QJ 0:

~

~ 0 __, ~ ·- + c.... .0 QJ

00 > 0

u..<ou N

t­=> a.. :z ......

----_.....-

U...<(CJU .-

QJ v. ....._

c:n .0 c: .,, rn

0:

t:n c: c:n c u

"'O u ·-·-o.- 6 0\ \.J 0\ ·- 0 co t- ._J

W.J

Figure 5.23: Schematic of the ADC and readout of solution B

108

0

E <lJ ....._ V)

V> =>

..0 ~ V>

'-~ QJ - I a.w :J .

l: 8.>=

u ~ QI

Cl c > '--:I: ..e

c: 0 l:

>-. LU V) •

:E ClJ

. -+- > ro '--LJ

"' QJ -+-ro '--u

- V')

c I ::::J cu I

E • ..c D'I • +-QI • "' ""

I ro lL

QI _J

.0 ftJ u

L.. (11

u

' c: cu :::> CT Q)

V)

' W IA

q "E <( ~

en

Figure 5 .24: Readout archit.ect.urf' for t.he L.Ar .calorimeter

109

(·

5.6 Calibration and Monitoring of the LAr System

5.6.1 Energy Calibration

The kinematical range for the measurement of structure functions is limited by the energy resolu­tion and the absolute calibration of the calorimeters. We need an absolute accuracy of 1 % for the eletromagnetic part and 2% for the hadronic part. The possibilities to calibrate the calorimeter with ep events during data taking are however very limited (see chapter 11). In the backward part up to about 135° the scattered electrons are strongly peaked around the electron beam energy. This fact can be used for an absolute calibration of a small fraction of the electromagnetic calorimeter, even without using the momentum determination from the tracking system. In addition momentum analysed electrons from the reaction ep -+ ep ee and from NC events can be used within the whole acceptance of the calorimeter. Due to the limited statistics of these two reactions it is clear that the required precision can only be reached by averaging over large areas of the calorimeter. A similar global check on the calibration can be done for hadrons by ha.lancing the Pi of the current jet to the Pt of the scattered electron for NC events. To avoid these difficult calibration procedures we will take full advantage of the excellent long term stability of liquid argon calorimeters. Ea.ch stack geometry

( will be separately calibrated in a test beam a.t CE;RN with electrons and pions. In addition we also will calibrate some typical cracks between the stacks. To guarantee the long term stability we need to monitor the response of the electronics and the fraction of the charge collection. The geometry of similar stacks and the data processing must be weJI defined. These point are discussed in more detail in the following.

5.6.2 Monitoring the electronics

The response of the electronics is continuously monitored by injecting testcharges via a known capac­itor into the input of the amp]ifier. This procedure is descibed in detail in section 5.5.4.

5.6.3 Monitoring the charge collection

Minimum ionizing particles provide a source of signals for monitoring the charge colledion in different channels and thus a way of intercalibration. The stability of the charge collection will be monitored with a few Ct sources, typically 241 Am which will be installed at suitable places in the cryostat. The heat exchangers in the cryostat will be cooled asymmetrically to get a forced circulation of the liquid argon and thereby a homogenious argon quality. In addition the use of /3 sources is investigated. For a /3 source not only the response at a given electric field , but also the shape of the high voltage curve gives information on the charge collection.

5.6.4 Variation of the stack geometry

Liquid argon calorimeters have the great advantage that different stacks with identical geometry have the same response . As we are planing to calibrate only one stack of each type in a test beam, the tolerances on the converter plate thicknesses and the liquid argon gaps must be small. The required tolerances for the electromagnetic and hadronic part are 50 µ and 100 µ for the liquid argon gaps and 40 µ and 400 µ for the converter plates respectively. The required tolerances allow in principle maximal calibration differences of 1.7% in the electromagnetic and of 3.3% in the hadronic section. However, such large differences in the calibration of two different stacks or sta.ck regions can only occur if a.JI the gaps and plates of the two regions conspire to differ by such amounts. These tolerances lead to calibration differences well below 1 % in the electromagnetic and 2% in the hadronic section for more realistic assumptions how the gaps and plates might vary within the above tolerances. With our independent readout boards these tolerances for the liquid argon gap can easily be met by the high spacer accuracies. The converter plates can be obtained from industry with the required tolerances, so

110

that no further machining is needed. After the stack production the gap capacitances will be measured to establish the mean liquid argon thickness. These measurements can be repeated after installation during beam off periods. In this case the preamplifier card will be rep)aced by a multiplexer card which is connected to a capacitance meter. Jn addition the possibility is investigated to monitor each tower capacitance by applying a voltage step pulse to the detector and measure the equivalent charge through the electronic chain.

5.6.S Weighting technique

With M.C. simulations it was shown that the weighting technique which improves the resolution considerably does change slightly the obtained mean energy of pions and of jets (see 5.10). This weighting technique wilJ be studied in detail with the results from the beam tests and will be applied also to the calibration data obtained for each geometry from the beam tests. The same weighting method must then be used within the experiment for experimental as well as for M.C. data.

The determination of the calibration constants for jets will then be done by the composition of the measured deposited energy of individual particles. This procedure will be such that jets will be generated by the LUND generator5 and for each particle in the jet the corresponding deposited energy will be taken from one testbeam particle. The amount of computer time needed for this methode is ( small. Possible effects coming from differences in the jet topology can therefore be studied with good statistics for the different kinematica) regions.

5.7 LAr Beam Tests at CERN and DESY

A long term test program for the calorimeter is necessary, starting with research and development on various components and ending with a determination of energy resolution and calibration in the various parts of the final calorimeter. The main stages of this program are described in the following chapters and the overall time schedule is given in Table 5.9.

5.7.1 Tests with Preprototypes

An electromagnetic stack with a sampJing thickness of 1.8 mm Pb and LAr gaps of 2 x 1.5 mm per sampling has been tested in an electron beam at DESY in the energy range of l to 5 GeV. This calorimeter has a total length of 21 X0 and transverse dimensions of 42 x 42 cm2 . The readout is done via l mm thick boards of FR4 located in the middle of the argon gaps with pads on both sides of the boards giving fine transverse granularity of 3 x 3 cm2 in the center and a coarser one off center. The longitudinal segmentation is 3, 3, 3 and 12 X0 , thus leading to a total number of 192 electronic channels. Studies on crosstalk and noise were done as well as the measurement of the energy resolution with the result of u /E = 0.085/../E. The measurements will be extended up to energies of about 200 Ge V in the test beam at CERN.

Also, a hadronic stack is under construction to be tested at CERN with pions in the energy range of - 5 - 250 GeV. The sampling thickness is 5 mm Cu and 2 x 1.5 mm liquid argon with G 10 readout boards in the middle of the argon gaps. The boards have strip structure on both sides with a 8 cm width, where the strip orientation is alternating from gap to gap. The total depth of this hadronic section is 6 ~' followed by a tail catcher of 3 ~ with coarser sampling. The transverse dimensions of both sections are 80 x 80 cm2 , the total number of electronic channels is 360.

The main purposes of these tests using electromagnetic and hadronic preprototypes separately as well as in combination, which in construction do not correspond to the final solutions, are to measure the e/1T response ratios, to determine the energy resolution and to apply different weighting methods to get the optimum resolution. These results will be compared with the corresponding M.C. results

vG.Ingelmann, LUND Monte Carlo for Deep-Inelastic Lepton-Nucleon Scattering (1983)

111

(

and will be used to understand and eventually correct. the M.C .. The relatively fine sampling of the main hadronic section will serve to understand the intrinsic shower fluctuations better by keeping the sampling fluctuations small.

Amplifiers and shapers of the electronic channels correspond already to the final solution thus they will undergo tests in a larger quantity under realistic conditions with respect to sensitivity, stability, noise and crosstalk.

A gaseous tail catcher of the finally foreseen solution, located outside the liquid argon cryostat, will replace in late 1986 the tail catcher in the liquid argon with simulating the coil by an equivalent amount of material in front of the tail catcher, thus to obtain results for the resolution as close as possible to the final calorimeter.

5.7.2 Tests of Prototypes

The production of the final modules will start by building at least two prototypes, one with plate orientation parallel and another one perpendicular to the beam. They will be of full size along the beam (,...., 88cm) and in </>( 45°). These prototypes will undergo detailed performance tests in the test beam at CERN with electrons and pions and as far as possible with simulated jets. First values for the calibration constants with and without applying weights will be obtained from these measurements and can be transferred to later modules (see chapter calibration). These constants will have to be checked against a possible energy dependence, which can be taken into account by using the seen energy. The two prototypes can also be combined in a test to measure the resolution and the calibration across the crack in z direction having a change of the plate orientation.

5. 7 .3 Performance Tests and Calibration of Final Modnl~s

One </>-sP.ctor of each type of module or, if of the same type, but fabricated at a different laboratory, will be brought to the test beam at CERN and will be subject to a performance test and to the det.ermination of calibration constants using the weighting technique (see section 5.6).

5. 7.4 Detailed Test of Cracks and Study of Special Conditions

To understand the behaviour of cracks, especially those in the polar angle </>, modules of half size in </> will be built. Because of the overall tight time schedule these modules can only be tested after the completion of the main calorimeter, which means after I 989. For this we ask for a test beam of protons (or secondary pions) at DESY of at least 20 GeV. This test setup would also be used to remeasure calibration constants under special conditions, such as for example extreme directions of particles with respect to plate or era.ck orientation or in general those which cannot be foreseen for the tests at CERN or which might arise only from the actual calorimeter measurements. We believe that it is very important to have this facility available at DESY, where spare or special stack modules could be tested at any time.

5.8 Backward Calorimeter

5.8.1 Introduction

The backward electromagnetic calorimeter covers the angular range 152° 5 B $ 176° and is inserted as a plug into the barrel LAr calorimeter as shown in Figure 5.1. In this angular range we need good energy resolution and absolute energy calibration to the 1% level as will be discussed in chapter 12. We have chosen a solution based on a lead/ scintillator sandwich readout by wavelength shifter bars (WLS) and photodiodes. This is a proven technique and expertise is available within the collaboration.6

GG.G. Winter et a.L,Nucl.lnstr. & Meth. 238,307(1985)

112

Table 5.9: Test beam requirements ( .

Stage Beam Year

Component Tests e, DESY 1986 - 87

Preprototypes e, 11", CERN 1986

Prototypes e, n, CERN 1987

Calibration e, 1r, CERN 1988 - 89

Special Condit.ions p (?r), DESY 1990 - 95

113

(

Despite the fact that such a calorimeter is expected to be less stable and homogeneous than a LAr calorimeter as originally proposed in the letter of intent, such a solution is appropriate in this angular range since the calorimeter segments can easily be ca.lib.rated in situ using the scattered electrons from low Q2 neutral current scattering. These have the nice property that they provide a nearly monoenergetic peak around the beam energy and high statistics. The proposed solution is cheap and more compact such that the gap between the barrel calorimeter and the beam pipe can be closed well.

5.8.2 Practical Realisation·

We have not yet worked out a complete technical solution because we have not yet finally allocated the responsibility for this detector component. Since the technique is at hand, the realisation of such a detector is not regarded to be time critical. However, we can describe in some detail the present ideas.

Since energy resolution, homogeneity and calibration are the overriding requirements whereas spatial shower resolution and e/11: separation are of minor importance, we foresee to use large tower cross-sections and a non-projective tower alignment parallel to the beam line. Each tower is built as a lead / scintillator sandwich with 2.5 mm lead plates and 4 mm thick scintillator planes. The total depth will be 24 X 0 ( 40 cm long ) . Each tower will be light tight and self- supporting such that tower segments can be assembled without major external support structures. The readout is done via yellow WLS bars which are coupled and matched in area t.o photodiodes. At present we discuss two alternative tower structures.

Solution A A trapezoidal tower shape as shown in Figure 5.25 where the scintillat.ors are readout on one side only. Such a scheme has been us~d by the CELLO forward tagging calorimeter6 and has good light collection properties and good homogeneity of response. With typical dimensions of 6 x 10 cm2 we arrive at a total of 432 towers readout by 840 photodiodes. This solution would also provide a spatial shower resolution of o ;:::; 3.5 mm.

Solution B A second solution is shown in Figure 5.25. It uses identical square towers of 14 x 14 cm2

cross-section (apart from a few edge towers) and is readout symmetrically at all four sides by WLS­bars of 30 x 3.4 mm2 • Such a readout scheme has recently been tested7 and is expected to give better homogeneity. In this scheme the number of towers is only 112 (most of them identic:al) readout by 448 photodiodes. The readout. is redundant if the signals of the photodiodes am separately recorded. This gives more safety, can be used to make the response over the tower homogeneous and gives a handle to recognize and reject large signals caused by lea.king particles which traverse the active area of a diode (nuclear counter effect).

Both geometries have in common that the area used by cracks and WLS-bars is small (~ 4%) and they are non-projective. Suitable diodes with an active area of 30 x 3.4 rnm2 are available. The analog electronics has very similar requirements as the one for the LAr calorimeter. We therefore foresee to use the same electronics chain with only minor modifications of the preamplifier to match it to the smaller capacitance of the diodes. The preamplifiers, shapers and a 16-fold multiplexer will be housed behind the calorimeter.

The overall weight of the calorimeter is about 4 t. It could be supported from the outer flange of the cryostat.

5.8.3 Expected Performance

We expect a resolution of q(Ee)/ Ee = 13%/vEe + 1% based on test measurements with similar geometry. The noise per tower is expected to be less than 50 Me V which matches well the requirements.

7F. Fischer, E. Lorenz and G. Mageras, MPI Report (1986)

114

Al WLS-bars

E E Pb ~

(

E

I~ I

--+-'

Figure 5.25: Sc-hf>matic of i;io)ution A of thE> backward c:alorimet.E>rs

115

(

E E

0 ...:ta ~

' t 10cm

Pb

beam hole

vWLS­bar

Figur(> 5.26: Schf>ma.t.ic of solution R oft.he hackwarcl calorimet.er!'l

116

5.9 Plug Calorjmeter

5.9.1 Introduction

The aim of the plug calorimeter is to minimize the missing part of the total transverse momentum due to hadrons emitted close to the beam hole. Therefore the angular accuracy is of much more importance than the energy resolution and hence the following requirements should be matched:

• good angular resolution qo < 5 mrad

• full angular coverage between beam pipe and forward part of LAr-calorimeter

• best overall containment within the available space

• moderate energy resolution for hadronic showers q / E = 100%/../Eh

It is evident,that these requirements can be fulfilled within the given geometrical limitations only by the most compact calorimeter design. In this respect a silicon-instrumented sandwich calorimeter is certainly the best solution. Si-detectors guarantee the minimum required space for detector planes and a full instrumentation around the beam pipe wall.

The application of large area Si-detectors for calorimetry has so far only been tested for elctro­magnetic showers,8 - 10 but is a new concept for hadronic showers. Special emphasis has to be put on radiation sensitivity and long term stability. Hence a modular design is chosen, which offers easy replacibility and a flexible sandwich structure.

5.9.2 Design Parameters

As the beam hole left out by the LAr-calorimeter in the forward direction should be closed as much as possible, the angular range of the plug calorimeter is chosen to be

12.5 mrad ::; 8 ::; 60 mrad

where the lower limit is given by the radius of the beam pipe including a necessary safety gap for alignment and the upper limit is taken from the inner radius of the LAr-calorimeter (see Figure 5.27).

Because of cost considerations we plan to replace tungsten, which was proposed in the Lol as absorber material, by copper in the first stage. This has the additional advantage that fewer layers can be used in order to indude some electromagnetic portion of the shower (> .. / Xo = 10.5 for Cu, but 27.4 for W !). However the longitudinal containment will be smaller since the overall length will be only 4.3 ,\ for Cu instead of 6. 7 ,\ for W. Therefore it is planned, that copper can be replaced by some denser material in a later stage (see also 5.9.3).

From these considerations and the geometrical restrictions we derive design parameters11 outlined in Table 5.10.

5.9.3 Technical Lay Out

Cross sectional views are given in Figure 5.27 and Figure 5.28. The calorimeter consists of two separate parts, which are fixed in the two halfs of the return yoke. According to the present. design of the iron structure, the accuracy of its axis ( 5 mm vertical, 2 mm horizontal) is sufficiently high in order to keep the necessary gap between heam pipe and calorimeter at 8 mm in both directions. This wiJI l>e

8 G. Barbiellini et aJ.,Nud.Instr. & Meth. 285,55(1985) 0 A.Nakamoto et al., ICR- Report-] 18·84-7, University of Tokyo (1984)

"'G.Lindstrom et al., Nucl.Instr.& Meth. in Phys.Res.A240(1985),G3 11 E.Fretwurst, G. Lindstrom, Int. Report Hl-5/85-21

117

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.achieved by remotely controlled positioners located at the outer diameter of the plug. Therefore the diameter of the return yoke hole is foreseen to be at least 700 mm.

As was pointed out before a modular design is favored. The mechanical structure consists of an outer support frame, in which the absorbers as we]) as the detector planes can be mounted. The detector modules slide in. between the absorbers and are locked in slots, which will also provide the electrical connections to the amplifiers at the rear end of the calorimeter. Thus in the case of malfunction a detector module may easily be replaced by a spare one. In addition the absorbers may be replaced by some other m~teriaJ if necessary at a later stage.

Encapsulation of the detectors is necessary for safe operation. Therefore a detector module con­sists of two copper plates, which provide a sealed environment for the detectors (see exploded view, Figure 5.29 and Figure 5.30).

The detectors will be fabricated in our own laboratory. Standard 3" dia Si wafers {thickness 300 µm, p ~ 2 kOcm) will be cut to 5 x 5 cm2 square size. A special edge protection oxide passivation method12 will be employed, resulting in an active surface of 96% of the total detector plane. It is emphasized that the detectors will be totally depleted thus ensuring a reliability of sensitive thickness and calibration, which could not otherwise be guaranteed. The necessary bias voltage for total deple­tion is ~ 200 V and present experiences with large area surface barrier detectors show that the leakage current will be $ 1 µA . Therefore no cooling is necessary to keep the noise at a negligible level. The total power dissipation of$ 150 mW is also negligible and will not produce a temperature increase. The long term stability with respect to charge collection efficiency, which could be affected by radiation damage will be controlled by deposition of appropriate a -sources (e.g. 244Cm ) on sample detectors. Since the detectors have a capacity which is close to that of the LAr-calorimeter, the same electronics can be used (warm solution). The thermal noise of silicon detectors at room temperature is very low and does not contribute to the overall sienal-to-noise ratio. Special equipment has to be developed for monitoring the diode characteristics and for supplying the bias volt'ige for the diodes. It is foreseen, that two subsequent detectors can be ganged together thus reducing the number of electronic channels to 336. Furt.her reduction is not advisable as otherwise malfunctioning detectors could not easily be identified. For the first round of experiments we envisage to install only a few detector planes in order to avoid extensive radiation damage due to the unknown background conditions. According to experience the plug can then be completely instrumented within a short period of time.

5.9.4 Beam Tests

Presently, extensive tests with Silicon instrumented electromagnetic shower calorimeters are carried out at DESY and CERN.3 The further test program includes

I. em Si sandwich calorimeters with 35 cm2 circular detectors and different shower materials (1986).

2. perfomance tests with 5 x 5 cm2 quadrangular detectors (1987).

3. application of such detectors in a Si-Cu test module for hadronic calorimetry (1987 - 1988).

4. extensive studies of particle induced radiation damage and possible effects from synchrotron radiation (starting 1986).

This test program has to be t imed with the elaborate R & D program, which 1s under way.The experimental tests are accompanied by MC- simulation studies.

1lJ.Kemmer, Nucl.lnstr.& Meth. in Phys.Res. 266 (1984),89

118

Table 5.10: Design Parameters for the Plug Calorimeter'

radial dimensions (instrumented pa.rt)

60 mm ~ r ~ 250 mm

angular range 12.5 mrad $ 9 $ 58 mrad

total length 65 cm = 4.3 ~ (instrumented part)

number of layers 8

layer structure 7 .5 cm Cu + 300 µm Si~detector,

mounting and read out boards 1. 7 mm

lateral detector cell size 5 x 5 cm2

total number of detectors 672

number of electronic channels 336

angular resolution 4 mrad

energy resolution ~ 110% /VE

119

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Q)

0 ..c. Q)

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LU

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Vl

0 Ol 0 w en "' .

Figurt> 5.27: Longitudinal cross sect.ion oft.he plug calorimeter

120

,.________ I

~So

·-. - I

~ 250 275

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/

0

\

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Figure 5.28: Cross sectional view of a detect.or plane of the plug calorimE'ter

121

.c. u 0

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Q)

a. a. 0 u

tn

0 -Q)

0

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Figure 5.29: Exploded view of a sealed de t.ector modult>

122

_J_ Absorber (Copper)

Signal board : 1.0 mm

- Signal pods Cu : 70 ;Jm

Detector board : 0.6 mm Al 0 2 3

·- Silicon detector: JOOµrn

- Capt on: 100 µm

Figure 5.30: Layer struct.urc of a det.ect.or segment

123

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5.10 Electron and Photon Taggers

Electron tagging is vital for the study of photoproduction processes at small Q2 which are an interesting part of HERA physics. Some of these processes can also be used to monitor the luminosity and to calibrate the detector {see chapter 11). We foresee small angle electron calorimeters in electron direction which cover the angular range from 0 to 0. 75 mra.d and have good energy resolution and fine granularity. There is expertise in this field in the collaboration to design and build these counters. The work has started only recently and technical solutions have not yet been worked out.

5.11 Performance of the Calorimeters

In this chapter we describe the performance of the calorimeter system, in particular the power of the LAr calorimeter supplemented by the tail catcher to measure electrons and hadronic jets and to separate electrons from pions.

5.11.1 Energy Resolution for Electrons

( . The electrons deposit most of their energy in the lead section of the LAr calorimeter where they are detected due to the fine granularity of the sensing structure with excellent space resolution (a few mm in the forward direction).

For the electromagnetic calorimeter with 2.4 mm Pb plates and 3 mm active LAr (see section 5.4) we expect (EGS13 simulation) a resolution of u(E}/ E = 0.095/../E at normal incidence. However, the resolution of the calorimeter is deteriorated to some extent by the material in front of it. This amounts to about 0.9 Xo at 90° and reaches a maximum of about 2.3 X0 at 20°. At angles below 20° it is about 1 X0 (see Fig. 12.4). We show the effects on the energy resolution for electrons of 10 GeV in Figure 5 . .31a of 35 GeV in Figure 5.3lb. Some improvement is possible by a presampler (dashed curve in Figure 5 .31) which allows to correct to some extent for the material in front of the calorimeter. For energies below 10 Ge V the tracker will supply a better energy measurement than the calorimeter.

At impact angles different from 90° (Figure 5.2) the resolution deteriorates by effectively coarser sampling by at most a factor of 1.2 in the forward and barrel calorimeter except at () ~ 145° where this factor might reach 1.3. With a test set up with Pb plates of 1.8 mm thickness, 3 mm LAr gaps and 3x3 cm pad size we so far reached a resolution of 0.085/vE for beam energies of 2 ~ 6 GeV whilst EGS predicts 0.085/../E.

5.11.2 Measurement of Hadronic Jets

The hadronic energy flow is measured by combining the information from the LAr calorimeter and the iron tail catcher.

Energy Resolution of the LAr Calorimeter For all the calorimeter types involved we expect a difference in response for electrons (or gammas) and pions of same energy (e/7r > 1) which limits, at high energies, the resolution for single pions as well as for jets. The energy yield of a jet depends on its a priori unknown composition of charged hadrons and gammas (Figure 5.32a}. Similarly the resolution for a single pion is limited due to n-0 fluctuations especially in the first interactions in the calorimeter.

Nevertheless we aim to reach an effective jet energy resolution close to the value expected due to intrinsic shower and sampling fluctuations only by correcting the 'Ira fluctuations on an event by event basis making use of the large differences in the transverse and longitudinal development of electromagnetic and hadronic showers. The feasibility of such corrections has been demonstrated by

13R.L.Ford and W.R.Nelson, EGS code, SLAC Rep. 210(1978)

124

several-experiments with ha.dronic bea~s14 To convince ourselves that the energy of a jet can indeed be measured quite independent from its 1ro content (Figure 5.32 b) we have performed extensive M.C. studies using the program GHEISHA.16 Most of these calculations have been performed for a Pb/Cu­LAr calorimeter (1.3-" of lead with 1.8 mm sampling followed by 5.7-" of copper with 7 mm sampling). A weighting method was used to correct for ?To fluctuations where the energy in ea.ch tower segment wa.s corrected by a term quadratic in the visible energy. The main results for fully contained showers have been given in the Hl-Lol with details in an internal report. 16 They can be summarized as follows:

Using jets generated by the Lund program as input to GHEISHA the resolution u(E)/ E varied in the energy range 10 to ·350 GeV from about 0.5/VE to 0.9/ VE without the weighting correction whereas it stayed close to 0.5/../E if applied. For the actual structure proposed by HI (see 5.4) with 2.4 mm Pb/3 mm LAr in the electromagnetic section and 12 mm steel/3 mm LAr in the hadronic section the resolution is somewhat different. We obtain (using GHEISHA) for jets from 10 to 350 GeV with showers depositing at least 98% of the energy in the forward LAr calorimeter resolutions as shown in Figure 5.33. In the 'unweighted' case the calorimeters are assumed to be calibrated by single pions and no further correction is applied. The effect of leakage out of the LAr calorimeter is discussed below. The above resolutions are reasonable estimates at present, final numbers can only be obtained on the basis of the ca.)orimeter tests at CERN (see section 5.7). (

Sensitivity of the Mean Energy to Event Topology If an energy weighting correction is used to correct for ?ro fluctuations within the shower the final result for the jet energy depends on the spatial distribution of the energy in the calorimeter. Therefore some sensitivity to the event topology must be expected. However we verified that this sensitivity is rather limited by evaluating with the same algorithm several extreme cases of "jets" at 170 Ge V and obtained the same response within 3%. These cases were single e, single 7r, u-quark jets (g~nerated by the Lund model), the same u quark jets but all angles widened by a factor 4 and top quark jets (Lund model) (Figure 5.34). The weighting algorithms to be ultimately UBed will have to be developed in context with the CERN test beam results.

Containment and Resolution including the Tail Catcher With the calorimeter described in section 5.4 the number of available absorption lengths >. covered by LAr calorimetry varies with the angle 0 to the proton beam a.s shown in Figure 5.14. There are 4.8 >. at 90° in the barrel region which increase to 8 >. at f::j 30° and fall to 6 >. in the forward region. However, there the LAr calorimeter is directly followed by another 4.5 >. of the tail catcher. It is in the region around 30° where the requirements for calorimetry are most severe for structure function measurements (see section 12.1.1). In the backward direction the tail catcher (4.5 >.) is directly behind the electromagnetic calorimeter.

The design is such that the energy How in the barrel and forward region is determined quite well in the LAr calorimeter for most of the events. The energy leaking into the tail catcher will be small in most cases. The number of absorption lengths needed to contain on average 90%, 95% or 97% of the energy of jets is shown in Figure 5.35 for various jet energies. For single pions of same energy 1.5 to 2 >. more would be needed to get the same average containment.

From the kinematics of e p scattering ( e 30 Ge V and p 820 Ge V) and from the M.C. results in Figure 5.35 we expect in the range 25° < 8 < 130° more than 95% average energy containment in the LAr calorimeter. For 8 < 25° more than 90% can be expected for jets up to 1000 GeV.

But for some events a large a.mount of energy leaks out of the LAr hadron calorimeter. We mustrate this in Figure 5.36 by the simulation of the energy measurement of jets of 50 Ge V at 8 ~ 90°. A tail is visible in the simulated energy distribution which can be removed to a large extent by the tail catcher. But the tail catcher sees less than 20% of the energy in more than 95% of the events as demonst.rated 14CDHS colla.bora.tion,Nucl.lnstr. & Meth. 180, 429 (1981)

H•H. Fesefeldt, GHEISHA Version 6, PITHA 85/02 (1985)

IG J. Gayler, Intern<>! report Hl -5/85-19 (1985)

125

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in (Figure 5.37) for jets of 50 GeV a.t fJ = 90" and of 170 GeV at (J = 45". The expected resolution is given in Figure 5 .38 as a function of the fraction of the energy seen by the tail catcher.

5.11.3 Influence of Cracks on the Energy Resolution

We have chosen cracks a.t constant </> in the electromagnetic section which point to the vertex in order to minimize the angular range with poor electron identification. These cracks contain little material and consist mainly of LAr and signal cables (see section 5.4). Assuming that on either side of the cracks {2 cm width) the electron measurement is disturbed up to a distance of I Moliere radius, we obtain a 10% loss of solid angle for the electron measurement due to the rp cracks.

The 4> cracks continue in the ha.dronic section, but there they are non-projective with an offset angle of 20" (see section 5.4). This is necessary as otherwise energy would leak directly through the crack into the inactive cryostat and coil region and would thus fake missing energy. With the hadronic crackS being non-pointing, the energy of an electron or pion entering the electromagnetic </>crack is still measured in the hadronic section although the e/11'-sepa.ration power is poor. For example it is shown by M.C. simulation of 10 GeV i's at 9 = 90" which enter an electromagnetic crack (2 cm width) in the center that in case of a pointing ha.dronic crack (3 cm width) around 60% of the energy leak out of the LAr calorimeter or is absorbed in the crack. This energy not seen by the calorimeter is reduced to 10% if the hadronic crack has an offset angle of 20°.

The vertical cracks at constant z contain more dead material but they are never pointing to the vertex. It is foreseen to insert vertical readout boards inbetween two adjacent calorimeter modules Lo reduce the deterioration due to these cracks (see section 5.4).

Full understanding of the effects of the cracks will be reached by tests of prototype modules in particle beams (see section 5.7).

5.11.4 Effect of Noise

The noise observed in a typical fraction of the calorimeter comparable to the volume occupied by an event is of importance for the measurement of the energy deposited in this event and for the thresholds of the calorimeter triggers . We obtain e.g. for one eighth of the e.m. part of the forward calorimeter an uncorrelated electronics noise of 260 MeV and for one eighth of the hadronic part of 600 MeV.

The effect of electronic noise on the performance of the forward calorimeter was simulated assuming the noise as given in section 5.5 and the pad capacitance as in table 5.6. If we reconstruct jets or single electrons with the simple cut to consider only those channels which are above the noise level by more than 2 u, we reduce on average the observed signal for example by 2.3 % for jets of 170 Ge V. The same happens to be true for electrons of 20 Ge V. The energy resolution was essentially not affected. neither for electrons nor for jets.

5.11.5 Losses due to Leads on the Hadronic Readout Boards

We simulated the performance of readout boards of the type described in Figure 5.lOc with pads of 8 x 8 cm2 from which always 5 have leads of Imm with gaps of Imm to the same side including shielding lines. With these pessimistic assumptions we obtain for jets of 50 and 170 Ge V a reduction of the mean energy by 6% (compared to no )eads)which can be corrected for. No significant effect on the energy resolution is observed. The effect on the spatial resolution due to the direct crosstalk is negligible compared to the intrinsic resolution from the pad-size.

5.11.6 e/'Tf - Separation

The electron-pion separation has been studied using M.C. generated events (single particles and single particles overlapping with jets) using the EGS and GHEISHA codes for the electromagnetic and hadronic interaction respectively. For the e.m. calorimeter we assumed a Pb-plate-thickness of 1.6 mm

I26

up to 20 X 0 and 3.2 mm beyond (up to 30 X 0 ); the LAr gap .has been kept constant at 2 x 1.5 mm. The longitudinal shower sampling has been varied between fourfold and sixfold sampling in depth. For the lateral readout structure we used a tower size of 3 x 3 cm, pointing to the interaction point.

To separate electrons from pions we followed the method of Engelmann et al. 17 For electron energies of 2, 10 and 50 Ge V the covariance matrix M describing energy deposits a.nd lateral shower distributions including the correlations has been obtained from M.C. generated events:

1 N M = N L (xfn)_ «X, >) (xJnl_ < x, >) n=l

xln) is the energy deposit or the sigma of the spatial distribution in the ith element of the longitudinal sampling for the n-th electron event (total numbers of events is N). To calculate the energy deposit and the sigma of the spatial distribution we considered only those towers, which are within a cone of radius R1 around the particle trajectory.

The inverse matrix H = M- 1 is used to define a test variable ~ for a given event k:

~ = .L: (xitkl_ < x, >) Hi, (x?l- <xi>) i,j

In close similarity to a x2 distribution, the value of ~ represents the probability of an event to be an electron.

As a typical example we show in Figure 5.39 the resulting e/7r rejection for various electron effi­ciencies and for three different R1 - values: Rt = 1.5, 2.5 and 3.5 times the padsize of 3 cm, yielding R1 = 4.5, 7.5 and 10.5 cm. URing a sixfold sampling in depth (3Xo, 3Xo, 3Xo, lOXo, 5Xo, 5Xo) we obtain an e/ ?r rejection of better than 1 : 103 at 95% electron efficiency nearly independent of R1 . A reduction of the longitudinal samplings to four (3X0 , 6Xo, IOXo, lOXo) reduces the e/7r-separation at very high electron efficiencies but has only a small effect below efficiencies of 95%. The separations given do not yet include the energy-momentum comparison which tends to make the differences even smaller and gives further improvements mainly at low energies. As can be seen in Figure 5.39 the e/7r rejection for single, isolated particles does not depend significantly on the Ri value chosen.

The e/7r separation is different for events where the single particle is nearby or within a jet. To study this topology, single particles (10 GeV) have been added to LUND5 u-jets of 50 GeV, varying the angle between the particle and the jet-axis randomly from 0° to 25°. The Figure 5.40 shows the dependence of the resulting e/7r rejection on the electron efficiency. In comparison to single , isolated particles, the e/ 7r rejection shows now a strong dependence on the lateral R1 cut:for R1 = 4.5 cm the separation is about the same as for isolated tracks but going to an acceptance cone of R1 = 3.5 x 3 = 10.5 cm reduces thee: 11' rejection by an order of magnitude. This demonstrates the importance of a fine lateral sampling of the shower profile.

5.11. 7 Performance of the Plug Calorimeter

For most e-p events the energy lost in the beamhole of the forward calorimeter by far exceeds the visible energy. Therefore the transverse momentum inside the beamhole can also be much larger than the resolution o-(PT) for the visible energy. The study of e-p LUND events has indeed shown that the transverse energy loss in the beamhole can be many GeV for events with a hard gluon emitted near the proton direction if the beamhole radius exceeds r ~ 5 cm. For technical reasons the main calorimeter cannot get closer to the beam than - 19 cm - equivalent to ,...., 4° . The plug calorimeter extends the acceptance down to ,..... 0. 7° , reducing the uncertainties of the PT determination to a.n acceptable level. The possible benefits of the plug calorimeter are demonstrated in Figure 5.41 and Table 5.11.Here we have studied LUND events for three appropriate sets of x, Q2 belonging to a current jet, angle of

17 R. Engelmann et al.,Nucl.Instr. & Meth. 21G, 45 (1983)

127

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~ 5°, i.e. still within the LAr-calorimeter. Figure 5.41 shows, that both the absolute value of the lost PT and its standard deviation are appreciably reduced by using the plug calorimeter and that the effect is much larger for events with g)uons than for pure current jets. Table 5.11 summarizes the results. In the worst studied case, i.e for x, Q2 = 0.2, 200 and with gluons we get an improvement for the lost PT from 30% (without plug) to 4% (with plug). It should be mentioned, that these results are due to the geometrical restrictions only. An additional effect stems from the radial leakage out of the LAr-calorimeter, as has been described by BRASSE et al. 18 but was not taken into account here. Therefore the overall possible improvement may be even larger than given above. The accuracy by which this detectable part of the missing PT can be measured, was checked by MC­calculations using GEIS HA 7. In a first step we neglected any dead material in front of the plug calorimeter, as this amounts only to"" V. (Figure 5.42). It should be emphasized that the tungsten slit being located at 1.2m from the interaction point does not obstruct our calorimeter.

The simulation for the present design of the plug calorimeter shows good energy proportionality of ~ I Me V /Eh and a reasonable energy resolution of 200 %/./Eh. Though the design goal of ~ l10%/vf.Eh cannot be reached, due to poor radial and longitudinal containment, this result will guarantee a sizeable improvement. E.g. for the events with x, Q2 = 0.6, 3000 all particles detected in the plug calorimeter would have a total energy of~ 65 GeV. This value can be measured with an accuracy of uE/ E = 25%.

18F.Brasse, F.Eisele, V.Korbel, Int. Report Hl-4/85-13 (1985)

128

Table 5.11: Effect of Beamhole without and with Plug Calorimeter

x, q2 PT.tot (PT,hole) (

without plug with plug

no gluons 1.22 (2.3%) 0.55 (1.0%) 0.6, 3000 53.4

with gluons 1.62 (3.0%) 0.52 (1.0%)

no gluons 1.41 {5.8%) 0.59 (2.4%) 0.3 , 600 24.2

with gluons 2.80 (11.6%) 0.65 (2.7%)

no gluons 1.83 (13.0%) 0.60 (4.3%) 0.2, 200 14.1

with gluons 4.27 (30.3%) 0.59 (4.2%)

129

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--("") <{ -0 x

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without a presn.mpler: a. JO GeV electrons, h . 35 GeV elect.rons .

130

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131

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132

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Ge V aft~r weighting

133

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134

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135

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136

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catcher

137

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138

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141

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(

Chapter 6

Tracking

6.1 Introduction

The tracking devices are designed to provide good momentum and multitrack resolution and to deter-/ mine the event topology. We aim at measuring single tracks and tracks in dense jets with a resolution

of <Tp/p2 ~ 0.3%/GeV. The tracking system also improves the e/1r separation of the calorimeter . In addition , parts of the tracking system are needed for triggering. Fig. 6.1 shows the layout of the system and Table 6.1 gives the basic dimensions.

The technical solutions chosen are the result of the following considerations:

l. Due to the particular event kinematics at HERA the tracking system is divided into two main parts, the Central Trac.king Detectors (CTD) and the Forward Track Detector {FTD).

2. The functions of accurate r<f>- and z-measurements in the central region a.re separated (e.g. as in JADE and OPAL). The central drift chamber (Central Jet Chamber, CJC) has two rings of tilted jet chamber drift cells for r</>-measurement and moderate z·measurement by charge division. Two separate sets of z-chambers, inside and outside the CJC, provide precise z-measurement.

3. Two types of forward chamber modules (radial and planar drift chambers) are required to resolve the particularly difficult pattern recognition problems in the forward direction.

4. Electron identification is provided by dE/ d:i:-measurements in the central and forward drift chambers and by the addition of three transition radiators in front of the forward radial chambers.

5. A track trigger which is insensitive to severe background and allows fast vertex determination is provided by a. layered system of trigger multiwire proportional chambers (MWPCs). They have an integration time less than the bunch separation time and provide an independent determina­tion of the bunch crossing time to. They cover almost the full solid angle. The readout structure of the cathodes of the MWPCs is such that they match the structure of the trigger towers of the calorimeter. Hence a combined trigger can be easily achieved. A further track trigger based on the fast drift chamber information can be added later.

6. A special class of low Q2 neutral current events requires a O·measurement or electrons hitting the backward electromagnetic calorimeter to an accuracy of ~ 2 mrad. The angle is measured by the inner z-chamber as well as by a set MWPCs in front of the backward electromagnetic calorimeter (BEMC).

7. As a veto against background three planes of scintillation counters will be installed. Two of them will be in the backward region and one in the forward direction between the FTD and the liquid Argon cryostat.

142

8. The space between the vacuum pipe and the inner central MWPC is left free for the addition of a vertex chamber in a later stage.

9. Particular attention has been paid towards minimising the amount of material in the tracking region.

The following list summarizes the functions of the different detector components as shown m Fig. 6.1 a.nd refers to the sections where a. more detailed description is given.

• The Central Tracking Detectors ( CTD) consist of

- a Central Jet Chamber (CJC) for measurment of the accurate r<f>-coordinate, of the z-coordinate with moderate resolution and of dE/dx (Sec. 6.2.1),

- Central Inner and Outer z-chambers (CIZ and COZ ) for precise z-measurement (Sec. 6.2.2 and 6.2.3) and

- Central Inner and Outer MWPCs (CIP and COP} for triggering and t 0-measurement (Sec . 6.5.l and 6.5 .2).

Mechanically the MWPCs are linked to units with the respective z-chambers.

• The Forward Track Detector (Sec. 6.3) consists of

- Radial Drift Chambers for measurment of the accurate <,b-coordinate, of the 8-coordinate with moderate resolution and of dE / dx (Sec. 6.3.2),

- Planar Drift Chambers for measurement of 8 and for supporting the pattern recognition (Sec. 6.3.2),

- Forward MWPCs for triggering and to-measurement (Sec. 6.5.3) and

- Transition Radiators to improve e/1f discrimination (Sec. 6.3.7).

• A Backward MWPC (BPC) completes the system of trigger chambers (Sec. 6.5.4).

The rea.dout electronics will be highly standardised. For the drift chamber readout we plan to use two basic types of prea.mplifiers and OPAL type FADCs (Sec. 6.7). Table 6.2 gives the number of electronic channels foreseen.

The complete tracking system has to be mounted into the central hole of the liquid argon cryostat as a single unit. This implies that it has to be entirely pre-assembled and tested above ground to minimize the interference with the installation of the other components. Some technical aspects of the installation are discussed in Sec. 6.9.

143

(

occupied dimensions sensitive

incl. frames and pre amps

region

Component r Zmin Zmaz r Zmin Zmo:i:

(cm) (cm) (cm) (cm) (cm) (cm) beam pipe (initially) 10 inner MWPC + z-ch. 15 - 20 -127 127 15.3 - 19.7 -120 120

CENTRAL jet chamber 20 - 79.5 -132 132 21.5 - 76.9 -120 120 outer MWPC + z-ch. 79.5 - 85.5 -129 129 80 - 85 -120 120 cable duct + clearance 85.5 - 90.3 -135 135

inner surface of cryostat 90.3 -240 262 ( complete FTD 135 259 136 258

forward DC 11 - 85 13 - 78 FORWARD forward MWPCs 11 - 85 16 - 78

transition radiator 11 - 85 13 - 78 BACKWARD backward MWPCs 11 - 75 -144 -138 16 - 70

cable duct + clearance 80 - 90.3 -250 -135

Table 6.1: Basic dimensions of the tracking system

component sense wires pads PMs preamps FADCs central jet chamber 2560 5120 5120 forward drift chambers 2448 2448 2448 inner z-chamber 180 360 360 outer z-chamber 144 288 288 inner MWPCs 1944 1944 outer MWPCs 3840 3840 forward MWPCs 2880 2880 backward MWPCs 1600 1024 2624 scintillation counters 64 64

SUM 19568 8216

Table 6.2: Number of electronic channels in the tracking system

144

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7

6.2 Central Drift Chambers

The functions of the measurement of the accurate r<f>-coordinate and of the z-coordinate are separated in the central region. The central jet chamber {CJC) provides for charge tracks an accurate mea­surement of the r<f>-coordinates and a moderate measurement of the z-coordinates by charge division. A precise measurement of the z-coordinate is obtained from z-drift chambers at the inner and outer radius of the CJC.

The CJC fills a cyl_indrical volume around the beam axis. It is a. pictorial jet chamber of the JADE type, however with cells tilted by ~ 30° with respect to the radial direction and operating at atmospheric pressure. The chamber has two superlayers of cells with wires running parallel to the axis. In total there are 2560 sense wires and~ 10000 field wires. The sense wires are read out at both ends for charge division and dE/dx measurement. In general 64 space points are measured on a irack over a length of~ 550 mm. The design goals are:

• a spatial resolution of crr.p = lOOµm and <lz = 24 mm from charge division measurement (=1% of the wire length)

• a resolution in ionisation loss measurement of adE/dz = 6%,

• a double. track resolution of~ 2.5 mm,

• a thin end wall ('.S 0.2Xo including preamplifiers and cables) and a thin inner cylinder ('.S 0.02X0)

in order to minimize photon conversions and multiple scattering.

The z-chambers are drift chambers where the wires are stretched in a polygon around a cylinder. The electrons from ionizing particles drift in the axial direction and allow an accurate determination of the z-coordinate. The design goals are:

• a spatial resolution of l1z < 350µm and "•t = 25mm (lOmm) from charge division for the outer (inner) z-chamber,

• a double track resolution of:::::: 3.5 mm at 90° and

• a total thickness of '.S 0.02Xo in order to minimize photon conversions and multiple scattering.

Two sets of z~chambers are foreseen, one at the inner surface (CZI) of the CJC and one at the outer (CZO). Each chamber is mechanically linked with its corresponding trigger MWPC into one unit with a common gas volume. ·

6.2.1 Central Jet Chamber {CJC)

Mechanical Design

Fig. 6.2 shows the overall structure of the CJC. It occupies a. volume defined by an inner radius of 200 mm> an outer radius of 795 mm and a length of 2640 mm centered at the interaction point. The sensitive volume of the C.JC is given by an inner radius of 210 mm, an outer radius of 773 mm and a length of 2400 mm. The internal wires are strung to a tension of~ 300 g for cathode> field and potential wires and 60 g for sense wires. Thus the end plates> where the wires a.re fixed, have to sustain a force of~ 30 kN (3 tons}.

Allowing for a move of 0.5 mm under wire tension this would require an aluminum end plate of 30 mm thickness. To get a thinner end plate our preferred solution is to support the end plates by a thin cylinder between the two superlayers. Technically this solution is realized by two independent shells one for each superlayer. Each shell is a complete subdetector with inner cylinder, end plates and wires strung between them. The volume of the CJC is closed at its outer radius by a rigid cylinder. During the wiring of the subdetectors the end plates are kept in position by rods fixed near the outer

146

radius. These rods can be removed through holes in the end plates after assembly of the complete CJC.

To assemble the complete detector the outer subdetector is slid over the inner one and fixed together. After removing the rods the inner cylindrical wall of the outer detector takes over the force of the rods. In a similar way the outer cylinder is mounted to the detector. A possible solution for the end plate is shown in Fig. 6.3, details are still under study. The following parameters have been obtained so far:

• Inner cylinder of subdetectors: wall thickness < 2 mm of cci.rbon fibre reinforced plastic (CFRP) ( < O.OlXo).

• Endplates: 6 mm Al, motion under wire tension 0.5 mm.

• Outer Cylinder: 10 mm Al.

• dead space between inner and outer superlayers ~ 33 mm.

Both sense and cathode wires a.re arranged in planes perpendicular to the planar end walls. This allows the construction of compact feedthrough units which will be inserted into slots of 5mm width in the end plate. Each unit houses 66 individual feedthroughs for all wires of a plane. Details are shown ( in Fig. 6.4. The feedthrough units are made of GlO or a plastic material with similar characteristics. The design is such that they are all identical, 320 units in total. For each wire a hole of 2 mm diameter is drilled on a numerically controJled (NC) machine. Each unit will have two position marks at its inner side. Brass tubes are pushed through the holes and glued in position. These brass tubes serve as feedthrough for the wires. The accurate wire position is determined by a groove in a half-cylindrical insert at the end of a brass tube. The accurate position of the grooves is obtained from a high accuracy NC machine to ~ 20 µm and is measured with an optical device to an accuracy of~ 10 µm. On the outside of the end plate the wires are fixed by two independent solderings on a multilayer PC-board. Pulser lines, coupling capacitors, resistors and a connector are mounted on the board . The connector allows to test each wire individually. This is important for control of the wire stringing procedure and in the set-up phase of the CJC. Amplifiers, resistor chains and HY-cables are also plugged via the connectors to the PC-board. The wires for field shaping need not be positioned with high accuracy. For these wires holes are drilled into the end plate for insertion of insulated feedthroughs for single wires.

The chamber construction will proceed as follows. The feedthrough units are glued into the slots. The position of each feedthrough is then measured again to ~ 20 µm by means of the above mentioned position mark on a high precision measuring machine. The end plates of a subdetector are glued to the inner tube. Rods are fixed at the outer radius to take the force of the wire tension during the stringing procedure. After the stringing procedure the subdetectors are assembled as described above and the rods of are removed.

The brass tubes are sealed by an appropriate insert to get the chamber gas tight. A replacement of single wires is possible with moderate effort by removing the gas seal.

Cell Structure

The cell structure is shown in Fig. '6.5 . A cell of a superlayer consists of 32 sense wires alternating with potential wires. The wire distance is ~ 5 mm. The sense wires a.re staggered by ±100 µm for reasons of electrostatic stability. This also allows to resolve the left-right ambiguity. The cells are separated by planes of cathode wires with a wire distance of~ 5 mm. The field shaping at the inner and outer radius of the cell is also done by wires with a similar distance.

The planes of sense and potential wires and of cathode wires are inclined by ~ 30° with respect to the radial direction in the center of the cell, which allows for compensation of the Lorentz angle. As a consequence stiff tracks (P.L > 2 GeV) traverse the sense and cathode planes about 5 times. In

147

(·

the calibration procedure t.his allows an easy determination of to, of the drift velocity and the Lorentz angle from the data. Also tracks from a different bunch crossings (::::.: 100 ns apart) and cosmics can be identified: the two branches of a track on opposite sides of the sense wire plane are recorded with a seperation of~ 400 µ.m, if the track passes 5 ns before or after the bunch crossing time.

The choice of the cell size is dictated primarily by the aim of a point resolution in ref> of~ 100 µm. This resolution permits a momentum determination to fip/p 2 ~ 0.3%Gev- 1 without vertex constraint in the high momentum limit.

The maximum drift distance in a cell varies between 22 mm and 51 mm. This is a compromise between the following constraints:

• Calibration problems may occur due to nonuniformities of the drift field near the sense wires. Up to a distance of~ 5 mm the electric field deviates considerably from the nominal value. This results in a variation of the drift velocity which has to be taken into account in the calibration. Therefore the distance of the sense and cathode plane is required to be considerably larger than 2 x 5 mm.

• Large drift distances are preferred in order to minimize the number of electronic channels but cause a large memory time as to be compared to the bunch separation time. The proposed maximum drift distance of 51 mm results in a memory time of < 1.5 µs, assuming a drift velocity of vd = 35 µm/ns. Longer drift distances would also require too high voltages for the drift field. In our case the maximum voltage will be ~ 7 kV for a drift field of I kV/ cm in the central drift region.

• At the first and last sense wire of a cell the accuracy of the track measurement is reduced because the electric field is distorted. This can cause problems when track segments of different superlayers are Jinked in the pattern recognition program. Ideally a solution with a single superlayer is preferable. However, this would lead to too big drift distances at the outer or to too small drift distances at the inner radius. As a compromise we have two superlayers: 32 cells for the inner and 40 cells for the outer each with 32 sense wires per cell.

The electrostatics of the cell has been studied in detail by computer simulation. Fig. 6.6 shows field lines and isochronous lines for an inclined cell of 15 sense wires. In most of the drift region the electric field is constant within < 1 %. The influence due to the adjacent cells, the field wires of the other superlayer, and the inner and outer cylinder have been calculated. It produces a change of the electric field of< 2%. This corresponds to a negligible variation of the drift velocity.

Choice of Gas

The selection of the gas mixture for the CJC is directed by the following requirements.

• The drift distance shall be measured with a final resolution of~ 100 µm.

• The drift velocity should be ::::.: 40 µm/ns in order to avoid large memory times. With this drift velocity data from up to 15 bunch crossings will be accumulated in a memory time of~ 1.5 µs .

• The gas mixture should work at atmospheric pressure. This allows a design of the vessel and the end plates with minimum thickness.

• The Lorentz angle in a 1.2 Tesla magnetic field should not be larger than 30°. It is difficult to compensate the Lorentz angle by tilting the wire planes by more than 30°. As the Lorentz angle is strongly correlated with the drift velocity, a noncompensated Lorentz angle gives rise to calibration problems.

148

• The CJC should be operated at an electric field where the drift velocity is maximal. In this case the inhomogeneity of the drift field causes a variation of the drift velocity of

An inhomogeneity of S 2% causes a negligible variation of the drift velocity.

• A moderate dE /dz-resolution should improve the particle identification capability of the calori­meter.

A gas which meets approximately our requirements is a mixture of Xe and C2 H6 at a ratio of about 3 : 1 at atmospheric pressure. This gas is presently being tested at DESY. The resolution should be about 100 µ.m. The drift velocity has a maximum of~ 36 µm/ns at an electric field of 1.1 kV /cm. The Lorentz angle is 30° at 1.2 Tesla. A dE/dx measurement with a resolution of~ 6% would allow an electron identification up to momenta of~ 15 GeV (with 2.5<T s~paration).

The figures given above characterising the expected performance of the CJC are based on published measurements. They must be verified in test measurements using our cell type and our electronics. An alternative gas mixture could be e.g. Ar : C2H6 = 50 : 50. This gas mixture has a somewhat worse resolution (ur~ ~ 120 µm) and a larger Lorentz angle of~ 35°. Also the electron identification from dE / dx measurements would be limited to lower momenta.

The details of the gas system are described in Sec. 6.8.1. The gas system will be designed to provide both Xenon mixtures and standard drift chamber gases. The CJC will be operated at a stabilized absolute pressure of~ 1.05 bar slightly above fluctuations of the atmospheric pressure.

Readout and Performance

The sense wires of the CJC are read out independently on either side by a chain of preamplifiers and of FADC units which a.re of the OPAL type. They are described in Sec. 6.7. This allows a simultaneous measurement of the drift time and the pulse shape. The analysis of the pulse shape will reduce the influence of diffusion on the resolution as shown by recent OPAL results. From these measurements one can extrapolate to obtain an ref>-coordina.te with a final accuracy of O'r.p ~ 100 µm, a z-coordinate from charge division of (J z ~ 24 mm, and the energy loss dE / dx. Tracks at a distance of 2'. 2.5 mm will be resolved.

In the angular range of 30° < 8 < 150° a momentum resolution of

op; = 0.003 Gev- 1

p11'

in the high momentum limit and a 8-resolution of

<Te = 17 mrad · sin28

is expected. If the vertex of the event is taken as additional constraint, the momentum resolution can be

improved by a factor of~ 1.4. A further improvement by a factor of~ 1.4 is expected when a vertex chamber will be added at a later stage.

The pattern recognition is similar to that of the JADE jet chamber. Using a modified pattern recognition program from JADE neutral current events (generated with the Lund program) have been analyzed successfully.

149

I '

c

Test Program

At present we are preparing tests with a small drift chamber. It has a square cell of 90 x 90 mm2,

8 sense wires and a maximum drift distance of 45 mm. The sense wires are connected on both sides to preamplifiers and FA.DC electronics. The drift chamber can be operated in the DESY test beam in a solenoid magnet with a field strength of~ 0.8 Tesla. It will be used to measure properties of different gas mixtures: drift velocity, resolution, Lorentz angle and double track resolution. Later we want to study the influence of the field shaping at the end plate of the drift chamber.

This test program is expected to run through 1986. First results on gas mixtures are needed already in summer 1986 in order to fix the inclination angle of the cells for compensation of the Lorentz angle.

Further test chambers will be built in 1986 in order to study different systems for positioning, feedthroughs, fixation of wires and the gas seal at the end plates. Finally a prototype chamber with full size drift cell structure will be built in 1987. It will consist of 3 adjacent cells with 16 sense wires each and with a length of~ 2 m. It will be possible to operate this chamber in a solenoidal magnetic field of 0.8 Tesla.

6.2.2 Inner z-Chamber (CIZ)

The inner z-chamber is built in a package together with the inner MWPCs (see Sec. 6.5.1). The whole assembly is designed to fit into a cylinder with 15 cm inner and 20 cm outer radius of total length 249 cm. Fig. 6.7 shows a. cross-section. Starting from the inside two thin-wall proportional chambers with 0.4 cm gap are followed by the z-drift chamber of 2 cm active thickness and a last proportional chamber. The chamber set is supported by l.Ocm glass fiber epoxy endplates and comprises a common gas volume. A fast chamber gas, such as an argon/ethane mixture, is foreseen for both chamber types. Signal and high-voltage connections are made at the backward end. Here also the preamplifiers are placed. The active length ot the z-chamher is 240 cm covering the angular range 9° < 0 < 171° as seen from the vertex. ·

The drift chamber (see Fig. 6.8) consists mechanica.lly of 16 azimuthal sectors and 60 cells in the z-direction, corresponding to a maximum drift length of of 2 cm. The cell separator walls and the sheets, which carry the wire feedthroughs are manufactured from 0.1 cm PC-board, while the bottom (top) plates, which support the field shaping strips will be made from RohaceH-Kapton sandwiches (Kapton foils only). The total radiation length of the chambers (including the inner proportional chambers) perpendicular to the beam is less then 1%.

One of the radial separator walls of the drift chamber is double layered to pick up the signal connections. Cables are run out to the backward preamplifiers in the gap between the edges of the hexadecouplet of the drift chamber and the MWPC cylinder on the inside. Ea.ch cell contains three sense wires. Both ends of a sense wire are read out independently in order to allow charge division. The total number of electronic channels and associated number of signal cables is given in Table 6.1, furthermore 120 high voltage cables a.re needed.

The double track resoluti?n is deteriorated by the decreasing crossing angle of the track the further away from the center a given cell is located. The sense wire arrangement shown in Fig. 6.9, which tilts the planes of constant drift time, compensates for this to some extent. With an argon/ethane filling and the field strength indicated in Fig. 6.9 the drift velocity is ~ 35 µm/ns. Using the FADC system described in Sec. 6.7 this converts into a resolution of < 350 µm for perpendicular tracks.The double track resolution can be kept below 0.6 cm for the central 20 drift cells (length 1 m),enough to separate more than 85% of the multihits in a given cell as deduced from Monte-Carlo studies. For the very forward cells a scheme for rotating the drift direction by 90° (i. e. perpendicular instead of parallel to the beam) is under investigation. This would improve the double track resolution though also complicate the design. It is conceivable that the rotation by 90° is even not needed, given an accurate z-vertex point determined by other tracks in the central region. In the backward direction a good double track resolution is not required.

150

Prototypes and Tests

A prototype of a complete ring of drift chamber cells (25 cm length) is under construction at Zurich. Test.s are expected to be completed by late summer of 1986. Be.am tests will be carried out at SIN. With the full chamber design completed by middle of 1987 construction can begin immediately afterwards.

6.2.3 Outer z-Chamber (90Z)

The outer z-drift chamber provides three accurate points in z along the track outside the CJC. Since the inner radius of this chamber (815 mm) is dose to that of a chamber presently in use for the same purpose in the JADE - experiment 1, we intend to copy this design with minor modifications. The chamber discussed here follows radially the two multiwire proportional chambers, which a.re mounted directly onto the CJC. Fig. 6.10 shows a cross-section of the chamber and details of the corners, where special provisions are necessary.

The chamber is constructed in two half shells. The length of the sensitive volume is 2400 mm. 70 mm at each end are needed for the support flange, electrical and gas connections. On the circum­ference each half of the chamber consists of 12 drift cells with 20 mm thickness in track direction, 205 mm width and 100 mm length in z-direction. We will use three sense wire planes compared to ( two layers in the present JADE chamber. This will improve the track finding and linking ability with the central drift chamber. The wires from each half chamber are read out at both ends. The cathode planes are made from wires strung parallel to the anode wires, the field shaping strips on top and bottom of the cell consists of copper layers on Kapton foil. The four cells at the meeting points of the two half cylinders are only half size, because the remaining space is needed for attaching the wires, readout connections and bolts for the interconnection of the two halves. A further dead space in the azimuthal coverage for particle detection occurs at 90° and 270°, where a 30 mm cell, which houses the resistor chain for the field shaping strip is located. The anode and cathode wires are supported by 3 mm wide printed boa.rd running along the cylinder at the 15°, 30°, ... corners in </>.

A z-resolution of< 350 µm and a double track resolution of 3.5 mm at 90° is expected. The JADE chamber reacbed an accuracy for the </>-measurement through charge division of 1.5°. Monte Carlo studies using neutral current Lund events (Q 2 > 1000GeV2 ) showed that at all 9 more than 90% of the tracks can be separated in z without tilting the anode wire plane.

The chamber gas wil) be argon/ ethane and will be the same for the proportional chambers and the inner and outer z-drift chambers.

It is planned to construct a prototype quite similar to the JADE z-chamber to further elucidate the construction problems and test the configuration with three sense wires and a variety of gas mixtures, before actual construction will start. For the construction the existing mandrel can be used with minor modification. Based on the experience of the Rutherford group with the JADE z-chamber it is estimated, that the chamber can be constructed in two years.

6.2.4 Vertex Detector

As well as aiding primary vertex reconstruction and the identification of overlapping tracks within jets, an enhanced track and multivertex resolution can be expected to play a crucial role in the isolation of exotic phenomena at HERA. However we do not foresee the presence of a vertex detector in the first phases of the HI experiment. Only when the sources of background are better understood at HERA and the beam pipe is further optimised we anticipate an upgrade of Hl for good event vertex resolution.

One of the various possible technologies considered is a scintillating fibre system. Development work is being carried out on this . within the collaboration.

1G. Dietrich et a.I., Nucl. Instr. a.nd Methods 217 (1983) IGO.

151

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11· --

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e-==:> --·

""' "-.

·---

-

2640

2~00

·-- .""' . --·~ ~ · -:--'.~t~racti~n po~~·- . __ .-·

0

~ ~

I.() en "' II

'-

<:= p

first wire \ \ / l:ililr ___ _, inner cylinder

preamp I if iers

illllliiiiiiiil'"..__--+-I -endplate

~---outer cylinder

Figure 6.2: General view of the central drift chamber.

..... "' "'

last wire II

first wire II

last wire I

RING I

first wire l

outer cylinder {Al I

--·- ~~.L..L

support rod removable

~

~

o : N

~

~~~~~~-+-----+~~~~~

LO

$2 iMer cylinder (CF1Pl

2 .,1200 , , 20 I s I ,

RING II l'Ocells)

R: 195

I RING [ (32 cells)

R:,85

R = 200

Figure 6.3: Possible solution of t11e end ph11.e of 1,he C.IC.

~~/""i

ndplate feedth_r~ugh

sol~eq:~oints-- . ~ i · ; pos1tronmg insert to fix wires A s

1

'*'£/ <

'

PC board

connector

,L_,~/

Figure 6.4: Feedthrough unit for the CJC with details

of wire positioning and fixation . -

100 y[cm]

• sense wires o field wires

-100 ,______..____.___..__....___.___...______.____.___.__.L--___ _

0 50 100 [cm]

Figure 6.5: Cell structure of the CJC.

154

[mm].-,--.--.-,---,-.-r-~-r--r-r-.-,--,--.--,---,.----,--,-,---.-,--.---r--i 400

350

300

250

-50 0 50 100 150

Figure 6.6: Field lines and isochronous lines of a skew cell with 15 sense wires.

Cross section through the inner chamber array

preamplifier

Z drift chamber

preamplifier

21t9cm

0 2 3 4cm

200 [mm]

•I

a:

E u J' r • a:

Figure 6.7: Forward and back end cross section through the inner z-chamber and MWPC array (schematically ). Driftcells and cathode pads are not shown.

155

(

( 22,9mm

beam/

/ 1908mm

Inner Z- chamber

0 100 mm

Figure 6.8: Perspective view of the inner z-chamber .

sense wire field shaping

wire

Figure 6.9: Field lines and isochronous lines of a z-chamber cell with tilted sense wire planes.

156

200

"' ··· ....

channel for electronics

825mm

Figure 6.10: Cross-section of the outer z-drift chamber (CZO) . The inserts show an enlarged view of the corners.

157

(

(

6.3 Forward Track Detector (FTD)

6.3.1 General Layout

The design aims for the FTD are as follows:

1. momentum resolution of tracks CT / p < 0.003p,

2. angular resolution of tracks" < 1 mrad,

3. efficient pattern recognition of tracks,

4. visual recognition of tracks from raw digitisation to facilitate on-line examination of data,

5. electron identification by means of transition radiation (TR) detection and

6. provision of a track trigger.

They can be achieved using a combination of drift chambers with various wire configurations, with transition radiator modules and with MWPCs. Two designs of drift chambers have been found to meet requirements 1, 2, 3 and 4 above, so called planar {P) and radial (R) chambers.

The radial chambers consist of sense wires radiating outwards from the beam pipe (Fig. 6.11) so that maximum drift length increases with increasing distance from the beam. Each sense wire measures <P (azimuthal angle) from the drift cell and r (radial coordinate) from charge division on each hit . Such chambers have the following important features:

• the drift cell is smallest where the background is expected to be largest,

• a track makes hits which are easily recognised visually as a linear dependence of <Pon z and

• t/> and momentum of a track are measured accurately.

The radial wire configuration suffers from the following two disadvantages. () (the polar angle) is measured by charge division and particles in a jet which have similar </> coordinates move apart in <P only slowly under the influence of the axial magnetic field. Thus any particles unresolved by the minimum double hit resolution in one radial module tend to remain so in subsequent radial modules.

The disadvantages of the radial chambers are overcome by means of the arrays of planar chambers. Each plane in z (Fig. 6.11) has an array of parallel sense wires of variable length and with constant separation interspersed with field wires. Pairs of planes with parallel wires have the sense and field wires interchanged producing a half cell stagger to resolve the left-right ambiguity. Successive pairs of planes are rotated with respect to each other by 30° resulting in the following advantages:

• The spatial resolution is homogeneous in x and y hence giving accurate measurements of 8 as well as useful measurements of ti> a.nd momentum.

• Particles which fall within t he double hit resolution of one plane will be resolved by subsequent planes with a rotated orientation; the planar chambers thus help the extrapolation of track segments from radial chambers.

• If the background near the beam pipe is prohibitively large then inner drift cells can be switched off creating, because of the rotated relative orientation of each of the six pairs of planes in a module, a.n overall dead region of suitable radius.

Though not essential, the inclusion of a. charge division measurement of the orthogonal coordinate on each planar chamber sense wire will be considered because it helps greatly in achieving good pattern recognition of the complicated multitrack events expected in the FTD.

The forward tracker will be constructed as a set of three identical supermodules (Fig. 6.12). Each supermodule contains one planar module, followed by a MWPC plane, then transition radiator and finally a radial module. This sequence 3x(P+MWPC+TR+ R) is chosen for the following reasons:

158

• It is important to have good track space point measurements at the entrance to a supermodule for track linking between the CTD and FTD; because TR must immediately precede the radial modules and because it is intended to support the trigger MWPCs on the TR modules a planar

module comes first.

• Planar and radial modules need to be interleaved for optimum pattern recognition and for the best lever arm for momentum measurement.

• MWPC planes should come close to the front of each supermodule for the best geometric trigger efficiency.

• The TR is detected in the radial modules because of the good X ray detection efficiency possible with such a chamber.

The radial modules in each supermodule are staggered with respect to each other by a rotation through half a drift cell. As a result particles lost "behind" another track because of the finite double hit resolution in the radial drift cell will be detected ,, in front" of the other track in the radial module in the next supermodule. Because of this radial module stagger and because also of the half cell lateral stagger in pairs of adjacent planar chambers, particles which would otherwise be Jost within the double ( hit resolution of each type of drift cell are still likely to be detected though without resolution of the left-right ambiguity.

Each supermodule will be assembled and tested as a single unit. A common dowel system will be used to ensure the long term stability of the relative position of each component. Engineers from each of the laboratories responsible for the construction of these components are already working closely together to this end. The common dowels are then fixed to supports on the inside wall of the gas envelope used for the forward tracker.

6.3.2 Forward Drift Chambers

Radial Module

A radial module (Fig 6.13 and Table 6.3) consists of 48 angular segments (7.5° wide) each with twelve sense wires (Fig. 6.14). To resolve the left-right ambiguity, each sense wire is staggered alternately 500 µm either side of the central plane of potential wires. The drift field is maintained by means of cathode strips which define the necessary potential gradient over the full dimensions of each angular segment. A similar chamber has been built and operated successfully in the CDF experiment at Fermilab 2 . For efficient pattern recognition it is necessary also to obtain a measure of the radial coordinate from these chambers by means of charge division. This will be achieved by reading out sense wires at the outer radius of the module only and by connecting each wire through a suitable resistor on the inner support cylinder to the corresponding sense wire in the angular segment 90° away. Such a scheme both reduces the quantity of material and removes the need for electronics on the inner support cylinder. It also minimises the number of read-out channels necessary.

The main difficulties in the design of a radial module are the inner support cylinder, the positioning and support of the cathode planes with adequate spatial precision, and a construction which provides good X ray detection efficiency through. a thin front wall. All problems are now being studied in the design and construction of a full size prototype. In parallel with this work a single segment prototype has been designed and built and is now under test to study the characteristics of a radial drift cell using both cosmic rays and laser excitation.

2M. Ata.c et al., Proc. IEEE Nuclear Scie11ce Symposium (1984) .

159

Radial Planar No. of wires per plane 48 20 No. planes per superrnodule 12 12 Drift length (max.) ±0.75 to ±5 cm ±4cm Total wires per module 576 240 Tota.I no. of signal channels 1728 720 Module thickness 11.8 cm 12.8 cm

Table 6.3: Radial and Planar chamber parameters

Planar Module

A planar module (Fig. 6.15 and Table 6.3) consists of six pairs of planes of parallel staggered wires orientated at 30" relative to each other. In each plane the drift length potential gradient is maintained by means of electrode strips on the walls of the chamber running parallel to the sense and field wires. The concept of such planar chambers is not new. However the severe demands posed by the required

( · spatial resolution on a large number of densely packed planes makes their construction very difficult. The essential specification for a planar module calls for good point resolution from thin drift cells

in a very compact assembly (12 planes in 12.8cm) constructed using as little material as possible. The requirements both of sufficient mechanical rigidity and of good electrostatic isolation between adjacent cells which are orientated differently pose severe design constraints. The electrodes will be formed by evaporating a thin ( 100 nm) met.al lie film onto Mylar skins which will themselves be bonded to a rigid foam wall and so produce an electrode structure of sufficient precision over the large areas of the drift cells. We have not yet decided on the mechanism for wire support on the inner and outer frames of the chambers. Two solutions are under investigation:

• a glass fiber reinforced epoxy frame divided at the wire plane using standard printed circuit technique and soldered wires or

• a single frame of rectangular cross section which consists of an outer skin of glass fibre reinforced epoxy and a rigid foam body; wires are supported using crimped feedthrough connectors.

A prototype is under construction and will be tested soon.

6.3.3 Pattern Recognition

Many of the criteria used to choose the wire configurations of the drift chambers in the FTD have been based on the requirement of efficient pattern recognition of multitrack events in the forward region at HERA. We describe here the investigations made to establish that such track finding is possible with good efficiency in the chosen FTD configuration.

Track segments in a radial module are to a good approximation straight lines in r and ¢>. Provided that a track does not cross a cathode plane the drift distances also lie on a straight line. These characteristics form the basis of a pattern recognition program which starts with data from a radial module. The procedure is :

1. from the stagger of the sense wires identify the sign of drift,

2. using the charge division measurement of r calculate 4> and

3. starting both at the beginning and end of a module, follow the track by using a straight }ine extrapolation in </>wires tdrift and r simultaneously (tdrift is used only if there is no change of wire plane).

160

A simple program based on the above procedure was tested on simulated data in the forward drift chambers in which we assumed that hits which are due to a track < 3 mm behind another track in the drift direction are lost. A pattern recognition efficiency of 95% was achieved for tracks in simulated jets, that is, 95% of tracks which were judged to be recognisable after allowing for double hit resolution were found correctly. The remaining 5% had errors of a nature which could probably be corrected with further work.

Having thus commenced the pattern recognition using the radial modules roads are formed to identify points in the planar chambers which are associated with the tracks found in the radial modules. These points are then used to determine the polar angle ( 0) and to improve ef> measurements for subsequent linking of the track segments which have been found in different radial modules.

Because it is possible to extrapolate from the radial modules into the planar modules to pick up associated points, little detailed work bas been done on pattern recognition in a planar module. However the configuration of six orientations has been chosen to ensure sufficient redundancy to define a line segment for a track. Consequently pattern recognition using the planar modules alone remains an option which will be further studied.

Initial studies of the problems of track finding in the combination of modules showed that the major difficulty lay in the identification of the points in the planar modules belonging to track segments found in the radial modules. We have made a careful study of any misidentification or loss of data due to ( such difficulties and have concluded the most important consequence is to distort some track fits, producing bad track parameters and large x2 • This effect is included in the performance estimates in Sec. 6.4.2.

6.3.4 Background in the FTD

Background close to the beam pipe is likely to be substantial at HERA. Any detect.or system extending down to small angles must therefore be capable of operating in hostile conditions which, moreover, may change drastically during both the short term and long term operation of an experiment. We therefore intend to use all reasonable methods to enable the forward drift chambers to operate in high background conditions.

The most likely sources of large background are:

• synchrotron radiation from the electron beam and

• beam gas interactions from the proton beam.

Both can be expected to decrease away from the region around the beam pipe. So that the drift chambers are not completely disabled in conditions where such backgrounds

are large, the ability to switch off drift cells close to the beam pipe is desirable. As mentioned in Sec. 6.3.l above, this is conveniently done in the planar chambers by disabling inoperable drift cells close to the beam pipe with little additional loss of geometrical acceptance at radii outside the region of prohibitively large background. In case the background is ever sufficient to swamp the small size of radial chamber drift cell dose to the beam pipe, we are investigating the possibility of operating the radial chambers with inner regions of the drift. cell disabled. This may be possible by running the radial chambers in a mode in which the sense and potential wires are held at the same voltage and the cathode strips are used to control the gas gain so that the inner regions of the cell can be rendered inefficient. We shall investigate this possibility in our prototype tests.

6.3.5 Chamber Gas

A single gas envelope is intended for the FTD enclosing a.ll chambers. A common choice of gas for the drift chambers and MWPCs is therefore desirable. We anticipate a dosed recirculating gas supply {Sec. 6.8.J) operating at a. small (~ 10 mbar) overpressure. The gas is supplied individually

161

( .

to each chamber module, each of which is not necessarily gas tight and each of which vents gas to the common envelope from where it is extracted and recycled (Fig. 6.16). The necessity to flush the transition radiators with a low Z gas (Sec. 6.3.7) requires a small differential pressure (~ few µbar) to be maintained between chambers and radiator with adequate isolation of the radiator from the envelope.

The choice of gas is governed by the following requirements:

• good spatial resolution (we aim at 100 µm but have calculated the performance of the system in Sec. 6.4 assuming 150 µ.m),

• sensible drift velocity (3 to 4 cm/ µs ),

• good ionisation yield and

• efficient detection of transition radiation X rays.

Tests are under way to determine the suitability of Ar and of Xe based mixtures, similar to those tested for the central drift chambers.

6.3.6 Electronics, Cables, Monitoring and Control

The sense wires are connected through the gas envelope to prea.mplifiers mounted alongside the high voltage supply bus and associated termination. The return current loop to all cathodes will be kept as small as possible to reduce pick-up. The signals from the preamps will be routed a.long the outside of the forward and central track detector vessels to the front wall (open end) of the cryostat. Signals from both the CTD and FTD are routed to the electronics chariot (Sec. 6.9.4) from a connector system fixed to this wall.

The choice of cable is governed by the requirements of small cross section to comply with space limitations, low mass to minimise material before the calorimeter and good transmission characteris­tics.

Currently we intend to read all signals from the forward drift chambers into an FADC system identical with that proposed for the central drift chambers (Sec.6.7). Small systems of similar FADCs have recently been delivered and are at present being evaluated. We anticipate that on the experiment the FADC system will be independently controlled by MC68020 microprocessors in a VME bus system. The degree to which data from the FADCs is processed before passing on to the data acquisition system of the experiment will therefore be flexible. This allows us to evaluate the most efficient ways

( of collecting data from the drift chambers commensurate with optimum point resolution, analogue measurement and double track resolution.

The control system envisaged for the forward track detector is outlined in Fig. 6.li. As well as providing access to the focal MC68020 microprocessor dedicated to the FADCs, we foresee that the local control computer will also provide continuous checks of the appa.ratus by remote monitoring and control of technical parameters, again by means of VME bus. This control computer will be part of the local area network {LAN) of computers controlling other parts of the HI experiment.

6.3. 7' Transition Radiators

The purpose of the transition radiators is to support thee/tr discrimination in the forward direction at high momenta. Together with the ionisation energy loss measurement of the drift chambers this enhances the energy deposition of electrons.

We intend to use three modules of transition radiators in front of the radial drift chambers. Polyethylene or polypropylene fibres with a mean bulk density of~ 0.06g/cm3 will be used·as radiator material. The fibres have to be compressed in a suitable frame between two layers, each consisting of a thin layer of Rohacell and a Mylar foil, thereby a.voiding outgassing of impurities into the forward

162

tracker vessel. In addition the radiators will be flushed with a low Z gas (e.g. C02) to minimise the diffusion of impurities through the Mylar foils.

The X rays emitted by electrons traversing the radiators will be detected in the following thin walled radial drift chambers. The energy spectrum cf X ray photons peaks at 9 keV.

The expected performance of the TR detector in the FTD with three sampling units can be deduced from recent measurements 3 . In these measurements polypropylene fibres were used for radiators and 2.5cm thick proportional chambers filled with a mixture of Xenon and Krypton were used for detection. The results (Fig. 6.18) show. that for 90% electron acceptance, the probability that a pion will fake an electron is well below 10% for momenta less than :::: 40 GeV. At higher momenta pions (I > 300) start to radiate and no discrimination is possible. Similar results have also been obtained by Bauche et a.I. 4 but with C fibre radiators (mean density 0.06 g / cm3 ) and I cm thick Xenon filled chambers.

The design of the radiators, which are attached to the radial drift chambers and trigger MWPCs, will be made in Aachen in close cooperation with the groups from the UK and Orsay. Manufacture will be in Aachen.

6.3.8 FTD Production Schedule

It is anticipated that production and shipment to DESY should be complete by early in 1989. This will ( permit sufficient time for assembly and final testing of the complete inner tracking detector consisting of CTD and FTD before installation in the experiment.

aH.J. Butt et a.I., contribution to Wire Chamber Conf., Vienna. (Feb. 198<3); A. Bii.ngener et ;i.l., Nucl. Instr. and Meth. 214_ (1983) 2Gl.

4 B. Ba.uche et a.l.,Proc. Int. Conf. ein Instrumentation for Colliding Beam Physics, SLAC (June 1982).

163

(

electronics

I 0

Radial chamber

1m

cryostat wall

Planar chamber MWPC TRD radiator

Figure 6.11: Axial view of one signal wire plane (where rele"ant) of the components in a supermodule.

50 -

' ' (

Radial drift chamber 1 JRO radiator

\ MWPC

Planar drift chamber

' 30°

~ - - .-~ ----- -=-=-=-=--=~.,____

:;; ,...._....__.._ 3x

Super Module

IP

cryos ta

0

t wa ~ 1m 0

Figure 6.12: Arrangement of modules and supermodules in the FTD.

164

Out~r sup po rt structure

Outer field electrodes

. ;_,,- Inner 3Upport structure.

0 ...

. . . . . . .

.

Petal.s ( 1,.1ith field electrode! end cathode readout) \_ End plate!

ttruor Components of a Radial Chamber Module

Figure 6.13: Exploded view of a radial chamber module.

165

(

(

cattiode p\ane suworl ,/"r-"S;::::,...-.....

el\d \ocalion

Catnode pad readout slr\?S

Seose/Potenlial wires

outer su11port Piece (plos\lt moulding)

H'! conneclions lo pelal electrodes

Figure s.14: Radial chamber drift cell.

0 10mm

~mylar+ electrodes

wires

wires

Figure 6. 15: Cross section showing schematically the layout of a planar chamber .

--------- ---11~• Purge return

I I I Gas return .. •--- ---•.--. ... -• .. ,..-- - ~ .--

; ; :

; :

: :~ ;.;

; : : 1:.

.. I I .. .. ;~ .. Oas supply to~ --- ..... -.i~ ...... Chambers

I I I I I I ~ Purge to TRD ....,__ Radiator

Figure 6. 16: General scheme for gas supply to the FTD.

167

(

(

Controls

( Oetector SuDMsystems) Forward

Tracker

LOQ91!d Oele

Prmure

} Temperature flow Posit IOI\

} Pl"Clai3S Controller

( a:introls own wv1ce and lllqS dela In l~I memory)

Control

Gas system

Control Peremeters Computer

Figure 6.17: General scheme for control and monitoring of the FTD.

---! 10 0

pion electron separation for 3 radiators +chambers

• Butt et al. {1986), preliminary

o Bauche et al. (1982}

•

90°/o electron efficiency

0.6 1 4 6 10 40 60 100 particle momentum (GeV)

Figure 6.18: e/1r discrimination of a setup of 3 transition radiators a.nd chambers as extrapolated from recent measurements. Plotted is the probability that a pion will fake an electron.

168

6.4 Resolution and Performance of the Tracking System

In this section the resolution of the complete t.racking system is considered for single tracks, and the effect of the jet environment is considered for the forward track detector. T}:ie jets in the CTD are broader, so the etf P.ct on jets is less serious and has not been included.

For these studies the following parameters were assumed:

1. vertex measurement error of 0.2 mm in x and y and 2 mm in z,

2. drift length measurement error of 0.15 mm (This figure is conservative and improvement in this figure will lead to a linear improvement in the central region and a slower improvement in the forward direction.),

3. double track resolution of 3 mm,

4. charge division measurement error of IO mm (for radial chambers),

5. z-rneasurement error in the CTD of 1 mm (conservative but may be realistic for the first year),

6. hit resolution of backward MWPCs of 2 mm and

7. magnetic field of 1.2 Tesla.

6.4.1 Overa11 Performance on Single Tracks

The resulting resolutions, calculated using the configuration described in Sec. 6.2 and 6.3 are shown in Fig. 6.19. These assume that pattern recognition is perfect and that all chambers are 100% effi­cient. The curves are for high momentum tracks where multiple scatt.ering is negligible. The peak at approximately 30 ° is due to the dead space between central and forward detectors. Some indication of the sensitivity to these assumptions is given in Fig. 6.20 which shows the momentum resolution if there is no information from either the radial or planar modules. It is seen that the momentum resolution is better than 0.0035 · p2 for all 0 greater than ~ 6 °. The resolution is below 0.002 · p2 in most of the forward region (9° < 0 < 28°).

Fig. 6.21 shows the momentum resolution calculated as above, but including multiple scattering within the tracking system and in the beam pipe. For tracks which pass through the end wall of the CTD, this end wall makes the major contribution to the resolution loss due to multiple scattering. For forward tracks at small angles where no measurement in the CTD is possible, the contribution of the beam pipe becomes important. This is because the vertex detel:"mination from other tracks becomes an increasingly strong constraint on track curvature. The end wall of the CTD (including preamps and cables) is taken rather conservatively to be 0.2 Xo thick and the contribution of beampipe, central inner z-chamber and MWPC and the inner wall of the CJC is taken to be 0.03 X0 . In practice, for low momentum tracks ( < 10 GeV) in the FTD, multiple scattering dominates over the measurement errors. For very low momenta, a more accurate momentum can be obtained. by not attempting to fit the vertex and in extreme cases not even attempting to link the forward and central track segments for tracks that traverse both detectors. However, the most critical measurement, the charge for high momentum particles, is hardly affected by multiple scattering.

The angular resolution in 0 and <P was determined including the measurements from the central z-drift chambers (cf. Fig. 6.19). It is below 1.5 mrad in the region 8° < () < 160°. The effects of multiple scattering are small in most of the angular region and have been neglected in t.his figure.

6.4.2 FTD Accuracy within Jets

The resolution for a single particle is never realised in practice for particles in a jet for several reasons:

l. pattern recognition errors which can transfer hits from one track to another,

169

(

(

2. limited double track resolution which reduces the number of hits and

3. incorrect linkage of tracks between detectors or super-modules.

To study ~he resolution of the forward track detector a "jet" plus an "isolated" track have been generated. The jet axis is at a given 8 and nine particles are taken in a 50 mrad cone about that direction (i.e. for a jet at 0.2 rad particles a.re in the angular region 0.2 ± 0.025 rad approximately). Because of the limited double track resolution a track may be seen in l, 2 or 3 radial modules and also in a varying number of planars. The resolution in momentum is closely correlated with the number of radials in which a track is seen, hence the resolution is given below in terms of number of radial modules (the tracks are potentially seen in all forward chambers • only the effects of double track resolution reduce this number). The correlation is only in part due to the contribution of the radial chambers themselves. It is also due to the correlation of misses in radial modules with those in planar modules which contribute mo.st to the momentum measurement, that is those with wires nearly parallel to the radial wires.

To use these results it is necessary to know the angle of the jet and the probability of seeing a track in 1, 2 or 3 radials. For typical jets the latter numbers are 11%, 25% and 61% respectively. Hence the total track reconstruction efficiency is 97%. These numbers take account of two track resolution and assume that pattern recognition is perfect if the digitisings are not lost through limited double track resolution. In practice the hits from both tracks in a closely spaced pair will be pulled so that the momentum measurement may be distorted more than is calculated here.

The results of these studies are shown in Fig 6.22. The resolution figures are given in terms of u1,/p2 derived from the Runge·I<utta fit. It shows the resolution of tracks which are pattern recognised in one, two or three radial modules (curves a to c respectively). As can be derived from curves b and c 86% of tracks in the jets have an average resolution between 0.002 · p2 and 0.004 · p2 in the angular region of 7° < () < 16°. However, the difficulties of the jet environment lead to non-Gaussian tails. The fraction of tracks which have an error that is more than 4 times the average depends strongly on the number of radial modules in which the tracks are detected. For tracks recognised in only one radial module, 25% of tracks are in these tails, for two radial modules 5% and for three radial modules only 1%. However, it should be remembered that only 11% of particles in jets are seen in only one module.

For isolated tracks outside a jet the result depends on the relative orientation of the track and the jet. If the single track is a.t the same radius as the jet but 100 - 200 mrad from the jet in </>, the track is always seen in three radials and has a momentum resolution closely equal to that of a jet track 'vhich is observed in three radials. If, on the other hand, the single track has the same <f> as the jet but different radius, the track pattern recognition and momentum resolution is nearly identical to that of a track in the jet. In this case its resolution depends on the number of radials in which it is observed.

170

LI)

N 0 N

(PDJW) l.{') ~

~·--·-- / ·-- ....... ._._ /

LO d

• -.. • -.a.. ------- ·-. / -·- ...... ~

LI) --..t d d

;, ....... ---~-- _,. \ . .....

--~-..,,. \ , . ti'

I I I

\

I

\ \

/

/ / bCD

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, 1 / .

/ I I

...... . ' I ,.

~ 1 ' . '

~

0

zd/ d.o

'

0

\ \

I

0

a N ~

a 0 ~

a co

0 lO

0 N

0

-ti) CJ CJ '--CJ) CJ

"O -CD

Figure 6.19: Resolution of the track detectors in the high momentum limit (no losses in efficiency and a. position accuracy of 150 µm in drift length are assumed).

171

(

0.4

Radial chambers /only

0.3

- I ...- I I

> I (l) I

<.::> 0.2 I ,, I

~ I I I

f "1

~ Planar chambers

- only

N 0.1 a. ..._

'8

0'--_.._~~~-----~----0 20 40 60

8 (degrees}

Figure 6.20: Momentum resolution in the high momentum limit with detection in the parts of the system shown.

172

0.35

0.30

0.25

0.20

0.15

Q_

---be. 0.10

0.05

10 GeV/c

Q ____________________ __

0 20 40 60 80 100 8 (degrees)

Figure 6.21: Momentum resolution of the track detectors as function of the angle for different track momenta. The effect of multiple scattering in the beam pipe and the end walls of the CTD is shown by the variation in shape of the curves.

173

(

( I

0.6

0.5

0.4 -""' f .:;:::..

0.3 Q.)

\.D

~ - 0.2 N a.. a) -... b) ~ 0.1 c)

0 0 5 10 15 20

8jet {degrees}

Figure 6.22: Effect of pattern recognition on momentum resolution of tracks within jets in the forward direction. The resolution is shown for three cases: (a) where the track is only pattern recognized in one radial module {11% of tracks), (b) the corresponding curve for recognition in 2 radial modules (25% of tracks) and (c) the case of recognition in all modules (61% of tracks).

174

6.5 Trigger Proportional Chambers

The multiwire proportional chambers of the tracking system have a threefold purpose:

1. they allow a fa.st reconstruction of the vertex position a.long the beam axis for discrimination of beam gas and beam collimator events

2. they provide a fast ray trigger for the detection of low multiplicity events and

3. they independently determine the the bunch crossing time to.

In the central region two arrays of cylindrical multiwire proportional chambets a.re added to the CJC, one with three layers inside (CIP) and one with two layers at the outside (COP). They are each mechanica.Hy linked to the appropriate set of z-drift chambers. In the forward and backward region respectively three and two double layers of chamber are foreseen. The segmentation of the proportional chambers (rays) will be in such a way, that it can be combined with the calorimeter tower structure to form topological ray triggers (see Chapter 8).

6.5.1 Central Inner Proportional Chambers (CIP)

Technical Description

The whole assembly is designed to fit into a cylinder with 15 cm inner and 20 cm outer radius of total length 249 cm. Fig. 6.7 shows a cross~section. Starting from the inside two thin-wall proportional chambers with 4 mm gap similar to those used in the SINDRUM - detector 5 are foltowed by the z-drift chamber of 20 mm active thickness (cf. Sec. 6.2.2) and another proportional chamber. The chamber set is supported by 10 mm glassfiber epoxy end plates and comprises a common gas volume. A fast chamber gas, such as an argon/ethane mixture is foreseen for both chamber types. Signal and high-voltage connections are made at the backward end. Here also the preamplifiers are placed. Table 6.4 summarizes the geometrical parameters.

Mechanical Properties

The proportional chamber walls are fabricated from Rohacell sheets laminated on both sides with Kapton foils (25 µm) carrying the cathode strips (t.ypically 15 mm wide, see Table 6.4). The latter are oriented perpendicular to the cylinder axis and couple capacitively through the Kapton foil to the graphite cathode. Each chamber has one wall, where the Rohacell cylinders are split in order to allow insertion of a further Kapton foil, which carries the 50 µm Cu-Be wires running parallel to the axis. The wires are contacted through the Rohacell to the Aluminium cathode pad and transport the signals to the back end. Fig. 6.23 shows a section through the different layers of a small fiat chamber under test at SIN. The anode wires are strung along the axis and are supported by spacers a.t four points. These spacers also ensure constant gap thickness. The anode wires will not be read out. The cathode pads in the forward region are split into 8 azimuthal sectors.

Electrical Properties

A test with a ffat prototype proportional chamber indicate no serious degradation of the induced cathode signals during the transport along the readout strips perpendicular to the pads and small ( < 5%) crosstalk. Thus standard MWPC preamplifiers and discriminators are foreseen with suitable pipelines. The total number of electronic channels and associated number of signal and high voltage cables is given in Table 6.5.

r,W. Berti et al., Nucl. Phys. Bl60 (1985) 1.

175

(

Central active width of mner.

thickness length cathode MWPC radius length

strip {mm) (mm) (mm) (mm) (mm)

inner 1 151 8 2490 2046 14.2 inner 2 159 8 2490 2117 14.7 inner 3 191 8 1 2490 2400 li.7 outer 1 803 10 2580 2400 36.0 outer 2 813 10 2580 2400 36.0

( Table 6.4: Mechanical Properties of the central MWPC arrays

per layer total no

MWPC of anode HV <P z(R) preamps signal

layers wires cables sectors sectors cables

forward 6 400 ~5 16 30 2880 2880 outer 2 2000 32 32 60 3840 384 inner fwd. 3 600 8 8 72 1728 1728 inner bkw. 3 l 72 216 216 backward 4 400 25 1600 1600

4 16 16 1024 1024 SUM 11288 7832

Table 6.5: Electrical properties of the the MWPC arrays

176

6.5.2 Outer Proportional Chambers (COP)

Two planes of multiwire proportional chamber are mounted with separate support flanges on the outside of the jet-chamber. The anode wires run along the cylinder axis, the cathode pads run perpendicular to the beam axis. 32 azimuthal sectors and 80 sectors in z are formed with the cathode pads, which are read out at the back end of the chamber.

The anodes will not be readout. Each pad will be equipped with its own preamplifier, but 2 sectors in azimuth and 5 sectors in z will be combined after the prea.mplifiers for trigger purposes. The readout lines for the pads run parallel to the beam and are separated from t he Kapton foil carrying the cathode pads by a layer of 2 mm Rohacell. The cha{llber length is 2580 mm, the active length is 2400 mm. An anode spacing of 3 mm is foreseen. The layout of the cathode pads is shown in Fig. 6.24. Ea.ch chamber takes its mechanical strength and rigidity from an inner 4 mm thick epoxy (or glas fiber) full cylinder, which is glued to the 20 mm thick Al end flanges. Gas connections are made at each end, a fast chamber gas, e.g. the argon/ethane mixture of the outer z-drift chamber (cf. Sec. 6.2.3) is suitable to provide a fast signal for to measurement linked to the corresponding calorimeter tower.

The previous experience in the construction of large cylindrical chambers of similar dimensions in Orsay allows to estimate the construction time for a set of two chambers to two years.

6.5.3 Forward Proportional Chambers

Three sets of chambers will be inserted in the FTD between the planar drift chambers and the transition radiator (cf. Fig. 6.12). Each chamber consists of 2 wire planes interleaved with 3 cathode planes. The gap between 2 adjacent planes is 3 mm (cf. Fig. 6.25). All the chamber frames are made of fiberglass epoxy. The mechanical tensions of the wires and the foils are supported by 24 and 8 axial rods on the external and internal frames respectively. In order to ensure that the forward MWPCs are precisely planar and have a constant gap, they must be supported on a rigid frame which is itself attached to the external support structure.

The wire layers contain alternately sense wires and field shaping wires with a spacing of 2 mm between successive wires. They are made of gold plated tungsten and are stretched using soldered connections on printed circuit boards. The size of the cell is 4 x 6 mm2. The first wire plane is offset relative to the second by half a cell. This divi9es the effective drift distance by a factor of two and correspondingly improves the time resolution. The good time resolution of 50ns is needed to uniquely determine the bunch crossing.

Each cathode plane is made of a Mylar foil of 80 to 100 µm thickness. The 4 cathode planes adjacent to the wire planes are coated with resistive graphite paint, whose surface resistivity is typically 100k0/ m 2 . This allows to shape the electric field in the chamber and to eliminate ions . The resistance of the graphite film limits the current in case of discharge between aoodes and cathodes. The induced signal passes accross the film because of its resistivity. On the two external surfaces cathode pads consisting of 100 A Aluminium a.re attached . The pads have a ring shape. Each pad covers an azimuthal angle of 1f/8 and is 1.8 to 3.7 cm wide radially (cf. Fig. 6.25). The pads are read via attached wires.

Finally, to provide a shield against electromagnetic background an aluminised Mylar foil connected to the ground is streched behind the cathodes planes. By requesting coincidence between the two planes, one can reduce by a large amount the synchrotron radiation background if necessary.

The total number of electronic channels and associated number of signal and high voltage cables is given in Table 6.5. No anode wires are read. Preferentially we have a common gas volume with the other forward drift chambers or with the transition radiator vessels.

6.5.4 Backward Proportional Chambers (BPC}

The aim of the backward MWPCs is to detect and measure charged particles (mainly single electron) with a space point a.curacy of 2mm, for events in the very low x, )ow Q2 region . The chambers will also

177

(

(

support the ray trigger. The chambers have a similar mechanical structure as the forward MWPCs: two twin chambers with alternating sense and field wires. Sense wires are read logicaly, the first and second set of chambers have their wires orthoganal to allow x- and y- coordinate measurements. The total number of read-out channels is 1600.

As for the forward MWPCs one can reduce the effect of synchrotron radiation background by requesting coincidence between the two planes. Depending upon the backward calorimeter properties it is envisaged to put pads on the two external cathodes planes (in total 1024 pads). Those pads could help to remove possible ambiguities between the two set·s of perpendicular wires and could be used as a part of the backward trigger.

6.5.5 Electronics

All MWPCs will use the same type of preamplifier. Two solutions are possible:

• After proper combining neighbouring pads, the analog signals will be transported to the elec­tronic racks outside the detector where each channel has a discriminator. Each discriminator output will be clocked into a digital one bit pipeline (shift register) and fed into the trigger logic.

• A more compact solution (4 channels per circuit) is under development. Each channel provides both amplification and discrimination. This has the advantage of transporting logic signals in­stead of analog ones . These signals are then fed into the ray trigger MWPC logic. (cf. Chapter 8). There will be the possibility of a remote disable to suppress noisy channels.

6.5.6 Prototypes and Tests

A prototype of the central MWPC is scheduled for end of 1986. The tests will utilize t.he high intensity, but only low momentum electron, pion and muon beams at SIN. With the full chamber design completed by middle 'of 1987 construction in the Ziirich workshop can commence immediately afterwards. A prototype of the forward MWPC with full length is under construction at Orsay.

178

1µA1 ~~-;L--,, .. ~:<4~Ai~iliM~rnrr:gmwm-0~~#t~Nf%Yt~(l7'tff~:QfNl(iliM3:~WIT:@F-r·~·

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0.8 mm Rohacell/~~k:m%~~~~~~%B~!:mJ ·5-ru~"2

50 µCu Be redout wires

0.8mmRohacell

25 µ Kapton graphite surface~~~~~~~~~~~~~

Figure 6.23: Detail of MWPC construction: The readout of cathode pad~s illustrated for the set-up of the planar prototype. The vertical dimensions of the different layers shown are not to scale.

154mm

~ ...1. '1T 16

.. 2300mm

.,. 32mm 914 ... ""'--~ tooo--~ •1 :fl:2

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PAO STRUCTURE OF OUTER MWPC (COPPER ON KAPTON)

----...---=t=60

DDDD Figure 6.24: Cathode pad structure and layout of readout wires of the CPO.

179

rS.'t;& ......... ~

I:

0

.. ' . . l . <1~ I ·• 1

.• ' . • I I . i~ ' ..

Figure 6.25: Cross section and details of the forward MWPCs.

180

6.6 Scintillation Counters

6.6.1 Performance Goals and Technical Description

Three vertical planes of scintillation counters are foreseen at the end faces of the tracking region. The counters measure the time structure and the radial distribution of the background halo accompanying the electron and the proton beam. Experience at UAl has shown that such counters are vital for the reduction of the t rigger rate to an acceptable level.

Two planes of scintillation counters will be mounted between the backward electromagnetic calori­meter and the iron yoke (cf. Fig. 6.1). The veto function of the backward scintillator wall is based on the time of flight difference between the arrival of the background particles (distance to the interaction point~ 2.20 m) and the bunch crossing time, as given by the RF system. The second backward plane has been added in order to enhance the significance of this veto function by suppressing spurious hits.

The time resolution required will be obtained by the use of photomultiplier (PM) readout. The PMs (Hamamatsu R 2021, 1.5" diameter) have been shown to work in strong magnetic fields 6 when positioned parallel to the field. The amplification obtained so far exceeds 104 at B :::: 1.2 Tesla (cf. Fig. 6 .26). There is also progress in t he development of the field resistant 2" diameter tubes R 2063 and R 2490.

The layout of the scint illators has to be ajusted to the expected background rate and distribution. Fig. 6 .4 shows one possible solution where the area of the segments grows proportionally to the radial distance. The adjacent ends of the segments are connected to a PM mounted perpendicular to the scintillator ring. A total of 24 segments with 24 PMs was chosen t.o ensure that the distance of each physical event to the nearest PM would correspond to less than 2.5 ns delay in the scintillator ( V~ff-::= 14cm/ns}. By this geometrical arrangement an overall time resolution of about 3ns (FWHM), suited for the first trigger level, can be obtained . Due to the fact that each event is seen by two P:\1s, a better time resolution can be obt.ained by averaging and off line evaluation - if required. The scintillator planes can either be put on the rails for the tracking detector or can be mounted direr.t.ly onto the backward electromagnetic calorimeter.

The forward scintillator wall is devoted to beam halo diagnostics with good time resolution. The structure of the wall is identical to the three inner rings of the backward walls (cf. Fig. 6.28). It will be mounted on the cyrostat wall or on the end face of the forward tracker (cf. Fig. 6.29).

We anticipate the installation of additional veto scintillation counters upstream and downstream of the detector.

6.6. 2 Prototypes and Test Planning

So far , the PM gain bas been studied in a magnetic field with light pulses of 530 nm generated from a neodyme yag laser. The effective velocity of light (vef! = 14 cm/ ns), the attenuation length (;\ = 120 cm) and the time resolution AT = 1.5 n.s) has been measured in SCSN-38 slabs of 20 x 1800 x 100mm3 size. The attenuation of light due to a 90° bend of the light guide has been determined (attenuation factor 1. 5).

Tests of the time resolution of PMs in magnetic fields, of the performance of ring detectors, and of the performance of prea.mplifiers and electronics are fore·seen in 1986. A preamplifier with a rise time of a few nanoseconds is being developped.

0 M.Rivoal: Test of a photomultiplier tube with very thin dynodes in a. high magnetic fie ld (Intern~! report Ht · 12/85-36)

181

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\

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-Backward scintillator plane

24 PMs {R2L.90)

A-8: 50

1380 ~

C-0:

o PM ~

suppor I

Figure 6.27: Backward scintillator plane

D

-00 Go.:>

Forward scintil lo tor plane

16 PMs { R 2490}

holes for mounting 50 --1 .

outer wall of cryostol

980~

E-F :

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side view of fastening the sc.in ti 11 a tor rings

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910

665

800

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500

281$

Forward scin ti Ila tor plo ne (side view)

FWD

~scintillator heod ~iece

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2590

Figure 6.29: Forward scintillator plane (side view)

-...

6. 7 Drift Chamber Electronics

It is intended to use the same electronics on the central drift chamber and on the radial and planar drift chambers of the forward detector. A total of 8216 drift chamber channels has to be read out as specified in Tab. 6.2 by a chain of preamplifiers and FADCs.

6.1.1 Preamplifiers

A final choice of the preamplifier has not been ma.de but an attractive candidate is based on a CELLO design, with eight preamplifiers grouped together on a PC board. We aim for a solution with a minimum of radiation length without degrading the performance. The main characteristics of the preamplifier are shown in Tab. 6.6. The final choice of the type of cables will optimise performance, diameter (i.e. radiation length), weight and costs.

size ( 8 channels) 70 x 10mm2

rise/fall time IOns

gain 5mV/µA input impedance = 3900 output load 2 x 500 max.output amplitude ±2.4 V (in 50 O} cross talk < 0.2% linearity 0.1% noise 1 mV power j channel = 150mW average thickness < 0.02 Xo

Table 6.6: Characteristics of preamplifier for drift chamber readout

6. '1.2 Flash AD Cs

For digitisation in the electronic chariot it is intended to use a system similar to the one developed by the Phys. lnstit.ute of the University of Heidelberg 7 for the OPAL detector and which is currently used at JADE. The system uses fast analog to digital converters based on a 6 bit lOOMHz Flash ADC followed by a 256 word deep fast memory. To expand the dynamic range, the FADC's are operated in mode with a non linear response function giving an effective range of 8 bits.

The use of FADCs ensures automatically high multihit capability and provides good (2 - 3 mm} double track resolution. The OPAL FADC system has a high modularity. At present one double height Eurocrate module houses four channels (FADCs and memories). One crate contains:

• 24 modules (96 channels),

• one programmable control unit {scanner) for zero suppression and hit detection for all the 24 modules and

• one VME interface module for data and address transfer to a front end processor (MC68000).

The front end processors perform the pulse analysis for the determination of the drifttime and signal charge for each hit 8 . Each front end processor analyses the hits of the corresponding 96 channels of

7 P. v. Walter ;i.nd G. Mildner, IEEE Trn.ns. Nucl. Sci. NS-32 (1985) 626; G Eckerlin et n.l., "VME-Bus in Physics" Conf., CERN Report 86-01.

8 P. Bock et :i.I., Nucl. Instr. and Meth. A242 (1986)237.

184

one Eurocrate. In addition it ca.n help to analyse all hits stored in the other processors of the same VME crate. The readout of the FADC system is described in further detail in Chapter 9.

At present we study the possibilities of having eight channels instead of four on one Eurocrate module. We also study the possibility of using two cascaded 6-bit FADCs in order to replace the nonlinear response of the OPAL type by a linear response with two different slopes.

6.8 Gas Systems ·

6.8.1 CJC and Forward Drift Chambers

Xenon mixtures are foreseen for the CJC and the forward drift chambers. As Xenon is an expensive gas{~ 12 DM/e), the gas volume has to be recirculated and purified. Fig 6.30 shows a flow diagram of a gas system, which could be used for the forward and central drift chambers as well. For each detector two independent gas circuits are foreseen for circulating and purifying the gas. A schematic view of the purification unit is given in Fig. 6.31. The pressure inside the detectors and the gas flow will be measured with pressure transducers (e.g. Baratron, MKS). The complete system will be controlled by a micro-computer, regulating the pressure inside the detectors, monitoring the gas flow and the gas composition and the correct function of a.II system components. The gas connections will be made of copper or stainless steel pipes.

Because the detectors cannot be evacuated , a molecular sieve for Xe recovery is foreseen (e.g. Linde Typ 5A, see Molesieve for Xenon in Fig. 6.30}. The filling procedure of the CJC for example could be done in the following way: first the detector will be flushed with Helium. Then the Xenon/Ethane mixture will be pumped into the detector and the Helium will be vented through the molecular sieve into the atmosphere . Small amounts of Xe and quenching gas in the Helium are absorbed in the sieve and can later be recovered. As the absorption of Xe-gas by molecular sieves is quite large(~ 55cm3/g , NPT) , the complete Xe content of the CJC of~ 3.5 m3 could be stored in a 60 kg molecular sieve. An alternative solution is a recovery by freezing the Xe (freezing point - 112°C).

6.8.2 Proportional Chambers

The central and backward MWPCs and the central z-drift chambers will be operated with a standard proportional chamber gas. The forward MWPCs will operate either with the same gas or with the gas mixture for the forward drift chambers. We plan on using existing gas systems for the MWPCs with appropriate modifications.

The transition radiators have to be flushed with a low Z gas (e.g. C02) in order to wash out heavier gas contaminations, which would absorb transition radiation .

6.8.3 Development

The Xe gas system is being developed and will be tested at Aachen, making use of experience from similar systems built for experiments at the SPS and at LEP.

185

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187

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6.9 Installation of the Tracking Detectors

6.9.1 Installation Procedure

The installation proceeds in three steps:

1. assembly, testing and cabling of alJ parts of the tracking system in a hall at the DESY site,

2. transporting the complete tracking system to the experimental area,

3. rolling the complete system into the cryostat and connecting the cables.

This scenario has been choosen in order to minimize the interference &nd overlap with the installa­tion of other detector components and to use a minimum of space in the hall for the necessary mounting devices. The proposed scenario uses two relatively big installation devices, namely an "installation crate" and an "installation platform" which are described in below.

Installation Crate

The installation of all components of the complete tracking system will be done on a special crate, which is rigid enough to support all items including all cables and service lines. This device has a rail system which fits to that of the cryostat. It can be supported and transported as one unit.

The dimensions of the insta.lla.tion crate are: length ~ 6 m, width ~ 2.4 m, height~ 2.7 m. It has to support ~ 8 tons (including ~ 4 tons of the BEMC) and has to be rigid enpugh to be transport.ed in one piece without overstressing the mechanical tolerances of the mounted items. It has the form of a cage allowing maximum access to the outer surfaces of the detector for cabling etc .. Parts of the top are removable in order t.o allow crane access.

Installation Platform in tbe Hall

In the experimental area - the detector being in .the "garage position'' - an installation platform has to be used at the - z-side. It has following properties:

• dimensions: length ~ 6 m, width ~ 5 m, height ~ 4.5 m above floor level.

• strong enough to support ~ 10 tons (install~tion crate and complete tracking system including the Backward em-Calorimeter)

• a feature to adjust the position of the crate with respect to the rails of the cryostat

• stairs and handrails to allow safe work on the platform.

It is considered to use the same platform which is necessary to install the calorimeter stacks within the cryostat.

6.9.2 Fixation of the Tracking System in the LAr Cryostat

The masses of the tracking detectors a.nd of the cables have to be supported by the walls of the cryostat. We assume that the CTD has a mass of 1000 kg, the FTD a mass of 500 kg and the cables of 1500 kg. Thus a total mass of~ 3 tons has to be supported. The radial space available between the cryostat ( R = 903 mm) and the tracking detectors ( R = 865 mm) seems to be sufficient for this rail system.

We are studying solutions with two rails mounted horizontally (90°-solution) or alternatively ·a single rail at the bottom of the cryostat plus an additional guiding rail at the top (monorail solution). The favoured solution is the. latter, however, from the statics of the cryostat the other solution is preferred. A decision will be made after the complete stress calculations of the cryostat are available.

188

In this chapter the monorail solution for mounting the tracking detectors has been impHctly assumed. We concentrate on this solution in the following.

In the monorail solution the two main part~ of the tracking device> the CTD and the FTD are each running on two special linear ball bearings on a cylindrical rail mounted to the bottom of the cryostat. CTD and FTD are both connected in a way that keeps their distanc·e constant at one point of their circumference but movements of their respective axes are still allowed. This system is such that each part for itself is mechanically completely defined and can follow the movements of the cryostat independently.

The only task of the top rail is to prevent the detector from turning. Thus the forces are relatively small. A guide raB made of an Al-U-profile is sufficient. Each part of the detector has to have a post at the top which glides in this rail. To minimize friction this gliding should be done with a ball bearing.

6.9.3 Position Monitoring within the Cryostat

Since there is no access to the different detectors of the tracking system, a system of devices to controll the absolute position and relative movements during operation has to be installed. The requirement on precision for the absolute position is far less than that on short term (i.e. 1 day) relative movements ( during operation. To a.void sagitta errors when matching tracks between the central and forward tracker, changes in angle between the axes of these two detectors of> 0.1 mrad have to be monitored. It is expected from the statics of the cryostat that th~se movements may occur (e.g. from changes in atmospheric pressure, switching the magnet). However, torsions between the both ends of the cryostat ca.n be excluded on a level which would be harmful for the track reconstruction.

Absolute Position

To guarantee the absolute position to~ 1 mm precision it is sufficient to have position switches giving contact if the detectors are at the nominal position.

Relative Position Monitoring

Since each of the parts of the detector is supported by two feet on the bottom rail and by one post in the top rail, its position is well determined. Movements of the cryostat will cause movements of the detectors. Thus one has to control the three "spatial axes of each detector. This is feasible with two tilt-sensors (for the x- and z-axes) and a linear transducers between the two detectors (CTD and FTD).

The interconnection between CTD and FTD is done by bars near the bottom rail, which keep the distance fixed but allow angular movements (e.g. a ball - cup connection). Thus the linear transducer has to be monted opposite to this bar. This concept assumes that the detectors themselves are absolutely rigid. Since this cannot be guaranteed it may be necessary to install further devices e.g. at both ends of each detectors.

6.9.4 Cableways

The cabling from the preamplifiers to the readout-electronics is done in two cable sections, namely:

1. Inner Cables: these cables are fixed to the tracking sytem and end at the "cryostat wedge" at the backward side (i.e. place above the prolonged em-calorimeter) of the LAr cryostat.

2. Main Cables: these cables start at the cryostat wedge and go on a ring covering the cryostat ftange (distance between cryostat and iron yoke 10 cm) to the bottom of the calorimeter. There a hole of 0.35 m 2 in the iron guides the cable to the outside. The connection to the electronics trolley (lowest floor) is done by conventional cable ducts.

189

(

Both cableways have to be carefully designed due to the rather limited space within the cryostat and the iron. Furthermore the HERA safety regulations have to be respected. Especially for the inner cables the design has to allow fast unca.hling since any access to the preamplifiers of the CTD implies major uncabling work.

190

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Chapter 7

Background at HERA

The high beam currents at HERA , 480 mA for the protons and 60 mA for the electrons produce three kinds of background:

1. low energy photons from synchrotron radiation

2. interactions of primary protons or electrons with the residual gas in the vacuum chamber

3. off momentum beam particles which hit the vacuum chamber or the synchrotron absorber masks .

In addition , cosmic rays during the e-p interaction are non-negligable because of the beam crossing rate of 10 MHz. Each of the four backgrounds will be discussed in the following.

7 .1 Synchrotron Radiation

The background induced by the synchrotron radiation has been discussed by Bartel et al. 1 Head on collisions between the electron and proton beams at HERA are achieved by deflecting the electrons through 10 mrad into the interaction region. In order to keep the critical energy of the synchrotron radiation as well as the radiated power small,the bending power is uniformly distributed over a length of 13.6 m with a bending radius of 1360 m. For these conditions,the critical energy is 70 keV and the radiated power 9 kW for I = 60 mA stored beam in HERA and 35 GeV beam energy. The number of radiated photons for the same conditions with an photon energy in excess of 20 keV is n(I) = 1018s - 1 . With a beam pipe of 10 cm radius and several synchrotron absorber masks, the nearest to the interaction point located at about 2 m, the rate is reduced to 108s- 1 in the central region (±2.5 m of the interaction point, see Figure 7.1 which shows details about the absorber masks). This is equivalent to JO photons per beam crossing. Drift chambers integrate over about 10 beam crossings depending on their maximum drift length. Experience at PETRA has shown that 100 spurious hits per event pose no problem to pattern recognition . The integrated rate per wire is of the order of 1016cm- 1y - 1 assuming a. gas gain of 104 . Experience has shown that chambers are degraded in performance after 1017 electron per cm on a wire.

7 .2 Interactions with the Residual Gas

Interactions of primary protons and electrons with the residual gas in the vacuum pipe produce high multiplicity events which trigger the calo- rimeter and the tracking devices.The rate depend on the vacuum and on the particle type.

1 W.Bartel et al.,DESY HERA 85-15,1985

191

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192

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The cross section of p-N and e-N reactions scales with the elementary p-p and e-p cross section according to A0

, A being the number of nucleons in the gas atom. The power a: is approximately 0.7 2

The kind of final state particles can be divided in two distinct sets according to their origin and kinematics;

I. the shower particles, which are singly charged hadrons with f3 ;:::: 0.7

2. the target fragmeuts such as p,n,d,t ,He ...

The average shower particle multiplicity distribution Yaries with the nucleon number like A0·14 . The multipliciiy distribution for p-p and p-Pb is therefore only different by a factor 2.1. The momentum spectrum of the shower particles has been measured in p-p collisions at the ISR at a C.M. energy of 63 GeV. Its main features are a limited transverse momentum distribution with a mean P.L of p .L ;:::: 400M eV / c and an almost flat rapidity distribution.

The number and momentum distribution of the visible nuclear fragments at high energies (mea­sured between 6-400 GeV incident proton energy) seem to be independent of the incident energy. It is appropriate to divide the charged target fragments further into two groups, knock-on recoils (grey part icles) and evaporation particles (black t rack particles). 8

The grey tracks exhibit a. differential energy spectrum of the form N(E)dE ex E-"tdE where N(E) is the number of protons per event and energy unit (MeV) and / has the value 1.09±0.02. _. The angular distribution is close to the form

l du

<T dcosO

The black tracks show an evaporation spectrum

with kT = m11'c2 and ha.ve an isotropic angular distribution. The average number of grey and bJad tracks are correlated up to a grey track multiplicity of 10. Above IO the black tracks saturate at 13.

The expected p-N interaction rate is 9000 events/sec/mat 480 mA proton current and io-9 Torr. It was assumed that the residual gas is air. Great care has to be taken at welding joints of the vacuum chamber. Outgasing of heavy metals in the soldering material can increase the interaction by an order of magnitude .

The detector will be sensitive to interactions up to 100 m upstream of the interaction point (I.P.). Simulations have been made of the background assuming particle multiplicity, momentum and angular distributions as described above. The particles where tracked th.rough the detetor taking secondary interactions in the detector material , the beampipe and the absorber masks into account.

In Figures 7 .2 and 7 .3 are plotted the hit multiplicities in t he central and forward tracking devices. The average number of hits from beam-gas interactions is well below the number of synchrotron radi­ation induced hits. For events close to the interaction point, z = 0, the number of hits is comparable to the numbers in genuine e-p collisions.

Figure 7.4 shows the energy flow in the calorimeter as a function of the radius from the beam axis. Except at very smaJJ radii, the calorimeter is well shielded by the backward calorimeter. From Figure 7 .4 and an assumed region for proton gas collisions of -50 m to 3 m around the interaction point, one can deduce a probability of 0.2and 20 GeV in the forward calorimeter in a ring from 12 cm to 15 cm (assuming 10- 9 Torr vacuum).

Beam-gas events are balanced in transverse momentum. The differential trigger efficiency is shown in Figure 7 .5 as a function of the energy window in the total transverse energy. For low energy

,Data of Denisov et al.,Nucl.Phya.B61{1973),62 3 1.0tterlund et al.,Nucl.Phys.B142{1978),445

'Tsai-Chu et al.,Nuovo Clmento Lett. 20(1977),257

193

deposition in the barrel calorimeter, the MWPC trigger helps to keep the trigger rate at a tolerable level (see also Table 7.1).

Electron losses occur at a rate of l08s-1m-1. Due to the lower electron energy, the detector sees only about ±5 m of the beamline around the interaction point. The signature of such events is a high momentum electron E(e) ~ 10 GeV near the beam pipe. They are a background to low Q2 neutral current events.

7.3 Off Momentum Beam Particles

Assuming a lifetime of 10 h for the proton beam, a proton loss rate of 3 x 105s - 1m-1 is expected. This is considerably higher than the interaction rate with the residual gas. A large fraction of these off momentum protons can be scraped off a long way upstream· of the detector. But high momentum muons from far away may penetrate all shielding walls. They have to be vetoed by scintillators outside the detector with a time of flight measurement.

Particularly dangerous are the synchrotron absorber masks about 2 m from the l.P. Off momentum particles which hit these masks will produce high multiplicity events in the tracking region. A few percent will have an energy sum E barrel ~ 10 GeV and due to the high hit multiplicity the MWPC trigger is less effective. The interaction rate with the collimator has to be kept below 1 kHz otherwise the trigger rate will be too high. Table 7 .1 summarises the expected trigger rates under the assumptions of 10-1> Torr vacuum.

7~4 Cosmic Rays

The cosmic background at HERA is more severe than at other colliders because of the frequent beam crossings at 10 MHz rate. The rate of cosmic muons passing through the entire volume of the calorimeter is 3 x 103 Hz. However , after allowing for the shielding effect of the calorimeter, the rate of muons passing through the luminous region of the beams ( and so potentially a background for a pointing charge track trigger) is only about 0.1 Hz.

194

O/o 1QQr---r-~-,---,-~--.--~-r---...-~-r--~.-=---.

R10

60 RS

20 RO

0

60

0

R10 R5 RO

20

20

a

40 60 radius [cm]

80

b

40 60 80 radius (cm]

Figure 7 .2: Hit multiplicities as a function of radius in the central tracking chambers. Shown is the percentage of events from beam gas interactions with zero hits (curve RO),with less than 5 hits (R5) and with less than IO hits (RlO). The two plots are from proton-gas interactions at a) -3m S z S 3m and b) -50m S z < -3m.

195

~1

E u.06 ............ l/} ~

.02

0 20 40 60 rad ius [cm]

80

Figure 7.3: Number of hits per beam gas interaction in the forward tracking chamber in a ring of 1 cm width a.t a radius r from the beam axis for interactions occurring a.t a) -- 3m $ z $ 3m and b) - 50m $ z < -3m.

196

Of . .;)

~00 -e- 0 0 q q a

• 60 R20~ . •

~---·--R10-+-

20 R1 +

radius [cm]

9/o

100 R20-e-~

a e• QI • • R10 I •

e• b

R1 ....

60

2(/-1 !

radius [cm]

Figure 7.4: Energy flow into the forward calorimeter as a function of radius. Shown is the percentage of events with less than 1 GeV (curve RI), with less than 10 GeV (RIO) and with less than 20 GeV (R20) for interactions occurring at a) -3m ~ z ~ 3m and b) -50m 5 <~ -3m.

197

1

10

-- - -· t

L.- ---

1 10 100 GeV

Figure 7.5: Differential trigger efficiency for beam-gas events as a function of the total transverse energy. The solid line is for a calorimeter trigger alone and the dashed line is for a combination of calorimeter and MWPC trigger described in chapters 6 and 8.

198

Table 7.1: Background rates assumi~g 10-9 Torr.

Simulations

Source Calculated Rate ~E.i ~ lOGeV EE.i. ~ IGeV + MW PC z - vertex

lost protons 3 x 105s-1 m- 1

lost electrons 1 x 103s- 1 m- 1

p - air (I0- 9Torr) 9 x 103s- 1 m- 1 90s-I 2 X 103s-I

synchrotron radiation 1 X 108s-l

photons ± 2.5m cosmic rays 3 x I03s- 1 o.1s- 1

199

Chapter 8

Trigger

8.1 Introduction

The trigger has to accept all events either charged current, or neutral current which have p .! greater than some low threshold , typically Pl. 2:'.' 10 GeV , as well as any exotic processes and samples of low pl. photoproduction processes for physics , normalisation, and calibration. Specific types of events that must be inr.luded are those with muons, high P.! jets, or isolated electrons, and samples with back to back charged particles. The trigger is envisaged as having inputs and outputs corresponding to various levels of sophistication at various times . The signals from each subdetector are combined centrally and tbe central system controls all gating and readout functions.

For setting-up each subdet.ector will be able to operate in a stand-alone mode, but in this mode will not have access to the final data acquisition system.

In section 8.2 we discuss the requirements, in 8.3 the formation of trigger signals for each subde­tector and in 8.4 the combining of signals to give the main trigger and gating logic.

8.2 Requirements

8.2.1 Rates

We can be sure that background events (discussed in chapter 7), rather than physics events, are the dominant source of triggers. Some physics event rate estimates are shown on Table 8.1 for a luminosity of 2 x I031cm-2s- 1. The dominant physics process is photoproduction at 100 Hz seen rate.

All rates per beam crossing are small , so the probability of random coincidences in one beam crossing should be negligible. The rates for the proton beam losses are so high that they must be eliminated quickly if dead time losses are to be kept small. In this connection it is educational to compare the rates with those in PETRA. Beam currents in HERA are 20 x , cross sections 400x and the length of the luminous region JOx, so for many backgrounds the rates are 105 x relative to PETRA!

We believe that the greatest difficulty in the trigger is finding methods which will reduce the rate below a few hundred Hz. without using lengthy pattern recognition algorithms which take unaccept· able times to compute on-line.

8.2.2 Determination of Beam Crossing

The 100 nsec interval between beam crossings provides a major challenge. The whole system will be locked to the beam crossing times and all outputs of the trigger system will be strobed so as to be at multiples of this time after the event. The unique association of an event with a specific beam crossing is vital. Errors would give completely wrong drift times and would cause errors in the calibration of the calorimeter and other detectors. Although the total collection time of the calorimeter is several beam

200

Table 8.1: Physics event rates. Luminosity = 2 x 1031cm- 2sec-1 . Units are GeV, GeV /c etc.

Process

NC

cc

Photo­production

ep -+ ep + e t e -

Cuts

q2? 32

Q2 ? 50002

Q2All Q 2 2'. 50002

All Visible E> ;et 2 5.7", E'3ct ~ 10 We+ e - 2'. 1

Rate (Hz) 3

Rate per bunch crossing

3 x io-1

10-4 10-11

3 X 10-g 3 x 10- IO

5 x 10-4 5 x 10- 11

~--~-----10 10-4 102 10- 5

1 10- 1

---------e(e+) e-) ? 2.6° 0. 14 1.4 x io-8

e(e+ ,e-)? 30° 8.0 x 10·-3 8.0x10- 10

ep-+ ep + 1 8°~0(e,1) $ 172° 4.3 x 10- 3 4.3 X 10-IO-ep-+ ep + p 5° ~ 0(11" '.J , 11" - ) ~ 175° 56 5.6 x 10- 6

p - 11"+11" - 30° ::; 6(11"+) 11" - ) < 150° 16 1.6 x 10- 6

ep -+ ep + p 5° ~ 0( e ~ , e- ) < 175 ° - 2-.6- x_ J_O __ -3 --2.-6-x-10--~i~ci--

P - e+e- 30° ~ E>(e+, e- ) ~ 150° 0.73 x 10- 3 0.73 x 10- 10

_e_p ___ e_p_+_J /-1/J--5-0-~-e-( e_,__+) e- ) $ 175 ° 1.0 x 10-2 1.0 x 1 o-9--

J j 1/;-+ e+e- 30° $ e(e+,e- ) ~ 150° 0.3 x 10-2 0.3 x 10-0 ------~- ~---

e p-+ ep +cc 45° $ E>jet ~ 135° 2 X 10-2 2 x 10-9

c -+ jet P.l (jet) 2'. 4 c-+ jet

201

crossings it should be possible to derive a trigger which is stable to much better than the 100 nsec inter-beam-crossing time. The beam crossing will also be defined from the MWPC's. and from the jet-chamber trigger. Provided suitable gas is chosen for the MWPC's the variations of drift time will be sufficiently small and simple discrimination on the chamber pulse will define the time sufficiently well. The jet chamber gives t0 from the short drift times close to where the track crosses the sense wire plane.

There is a difficulty in cases where different methods of getting the beam crossing number disagree. This is no problem if the occurance is rare as the ambiguous events could be rejected but if any subdetector is marginal in its ability or has individual bits slightly out of time, then we will have to derive a rescue route. This is a particular problem for the calorimeter and is discussed in section 8.3.l.

8.3 Triggers from Specific Detectors

The most powerful trigger selector is the deposition of substantial energy in the calorimeter. There are, however, two major problems, low energy deposition (in the barrel) for some charged current events, which must trigger on the hadronic jet, and false forward deposition from background events.

lt is essential that the trigger combine signals from different subdetectors in a correlated wa.y. Thus it is necessary for different subdetectors to have correlated "trigger rays". This applies to calorimeters, trackers and muon detectors. By use of these correlations we will be able to tighten, for example, the barrel c:alorimeter with tracking information, or combine different detectors when they conflict as to the beam crossing number.

8.3.1 Calorimetry

Trigger signals from the calorimeter will be provided at several levels of solid angle granularity ranging from a. few msr to 4?r, and at two levels of precision in the energy measurement, single bit and eight bits. For stand-alone triggering, the signals are gated to suppress noise and to associate the signals with a single beam crossing. Equivalent signals are also provided which are gated only for noise suppression. These signals can be used to form combined triggers with other components which determine the beam crossing.

Beam Crossing Determination An essential requirement for a stand-alone trigger is the ability to determine, uniquely, which beam crossing of the machine produced the trigger. Liquid argon calorimeter signals are triangular in form as shown in Figure 8. la, with a total fall time of 300 or 600 ns for the two argon gaps under consideration. We propose to shape the signals for trigger use with a fast bipolar shaper having a. time constant of the order of 200-300 ns.

As indicated in Figure 8.1 b, the signals from a particular beam crossing then pass through zero at a precisely determined time, approximately half the shaping time, after the beam crossing. By requiring that the signal be both above an upper threshold (Tu) and below a. lower threshold (Ti) within a time window produced by a beam crossing clock, we will detect this zero-cross and uniquely determine the beam crossing from which the signal originated. Trigger signals gated by the presence of a zero-cross at. the appropriate time are thus associated to exactly one beam crossing. The thresholds Tu and T1 will be separately settable and measurable by the trigger control computer.

Trigger Tower Signals Trigger towers (TT}, formed by grouping normal towers, are the smallest units of the calorimeter for which t.rigger signals will be produced. In the electromagnetic calorimeter, signals from sixteen laterally adjacent towers will be combined in the second and third depth samples in the forward part and in the second depth sample in the central and backward parts. Four neigh­bouring towers in the each of the first two depth samples will be summed in the hadronic part. The groupings will be chosen such that the same solid angle is covered in the hadronic and electromagnetic

202

parts. Figure 8.2 indicates schematically how the TT signals will be generated. The signals from the electromagnetic and hadronic towers in each TT are first separately added. In the adding process, t he signals are weighted to account for differences in geometry and tower ca.pa.citance so that a.11 signals have the same energy sea.le and dynamic range. This weighting is built into the hardware and is only manually adjustable. The sum signals are then shaped to produce the desired zero-crossing. The electromagnetic and hadronic energy signals are summed and this total energy signal subjected to three separate conditions for each beam crossing.

1. Does the signal cross zero at the time expected for this beam crossing (to .::::: tbc)?

2. Is the energy above the level expected from noise (Etot > Enoi~e) ?

3. Is the energy above a physics minimum (Etot > Emin) ?

The timing of the signals is then adjusted so t.hat the logical signals are coincident with each other and with the peaks of the shaped analog signals. The energy thresholds are separately settable and readable by the trigger control computer for each TT.

From the two analogue and three digital signals discussed above, five output signaJs are produced per TT for each beam crossing. Electromagnetic and total energy analogue signals gated by the noise threshold and zero-cross are provided for constructing stand-alone energy triggers. The digital signal (Etot > Emin /\ to = tbc) is available for constructing stand-alone topological or multiplicity triggers. Finally, the electromagnetic and total energy signals, gated only by the noise t hreshold , are also provided for use in combined triggers with signal from other det.nctors which determine the beam crossing.

Calorimeter Trigger Signals In order to associate geometrical information with energy informa­tion and to facilitate set.ting energy thresholds in solid angle regions smaller than the entire detector, we introduce the concepts of big towers(BT), and super towers(ST).

BT's are groups of TT's - there are typically four BT for each calorimeter stack, two segmentations each in z and phi, for a total of 256 BT. Similar groupings into BT are also formed for the forward plug calorimeter and for the backward electromagnetic calorimeter. The groupings into BT are indicated in Figure 8.3.

The shaped TT analogue electromagnetic and total energy signals, gated by ( Etot > Enoise /\ to = tbc) and by (Etot > Enoi~e) alone are summed to make four signals per BT. The four BT signals are then digitized to 8-bit precision by flash adc's. The digitized values are available in a VME module for readout . All further manipulations of the calorimeter energies are carried out digitally with these values.

The BT are grouped into four overlapping sets of ST, each set consisting of 64 ST. The ST sets differ by the position of the boundaries in theta and phi. A blob of energy placed on a er.a.ck between two ST in one set will be centered in an ST in another set. One combination of BT to ST is indicated in Figure 8.4; the other three sets a.re obtained by shifting the boundaries by one BT in either theta. or phi or both.

One set of ST is used to associate geometrical information with the energy information. This is done by digitally computing a weighted sum of energies over the set of ST, using weights controlled by computer. We will provide circuitry for six weighted sums - each set of weights can be used together with either the electromagnetic or total energy digital signals from the ST. Two sums will have weights equivalent to unity to compute the electromagnetic and total energy in the entire calorimeter. Additional sets could, for example, be used to compute transverse energy by weighting with the average value of sin(8) for ea.ch ST.

Sums for the entire calorimeter are then compared to corresponding thresholds to give first )eve! trigger signals. The single-bit results of these comparisons will be available in a VME module for readout. Table 8.2 gives a summary of the trigger signals provided by the calorimeter. Note that,

203

with the exception of the single TT signals> all signals are provided twice, once gated to provide a beam crossing determination and once without beam crossing gating for use in combined triggers.

8.3.2 MWPC Triggers

A considerable background trigger rate will come from beam-gas interactions which may only be ex­cluded by the calorimeter trigger if unacceptably high thresholds are set. One particular case is events with rather little energy in the barrel calorimeter> where there could even be a major contribution to the trigger rate from noise if the threshold is set so as to accept all the interesting physics. For these reasons we propose a fa.st MWPC trigger which determines

ZV the z-vertex with an accuracy of 6 cm for events with large numbers of tracks>

NT the number of tracks coming from the interaction region and simple geometrical correlations between them for low multiplicity events.

The types of trigger produced are summarised in Table 8.2 and are discussed in more detail below. For ZV only the information of the inner 3 layers of the central MWPC's is used, while for NT

all available MWPC information will be combined. We will use a hardwired logic system for ZV and a fast programable memory system for NT. Both systems will work in a dead-time-less way and the output information will be ready after ......, 200 ns and can therefore easily be used as input for the level I trigger (discussed in 8.4). Intermediate output signals of the NT system can al!'m be used in coincidence with the calorimeter trigger-towers to determine the bunch crossing time to.

Vertex Trigger The met.hod was first developed for the Jade experiment where it was implemented as a software filter. In order to match the high beam crossing frequency at HERA we plan to build a hardwired logic (see Figure 8.5), which histograms the intercept in z with the beam axis deduced from all possible hit combinations of the inner MWPC's. The electronic histogram will consist of 16 bins (width 5 cm each) and will cover the whole interaction region. Events occuring outside the interaction region and the false hit combinations of the tracks will produce a ft.at distribution in the histogram> while all the correct hit combinations will show up in one or two single bins. A significant peak in the histogram will therfore signal a good event occuring in the interaction region. The innermost central MWPC consists of a double layer (see Figure 6.10) and a logical 'AND' of corresponding z-pads will be required in order to be less sensitive to synchrotron radiation.

The effect of the background reduction has been demonstrated in chapter 7. The efficiency for events with low calorimetric energy is around 95% from a full Monte Carlo simulation.

Ray Trigger The first level ray trigger will be built from the fast signals from the cathode pads of the MWPC's. There will be 6 pad planes (in 3 pairs) in the forward direction, 5 in the barrel part (3 inner, 2 outer) and 2 in the backward direction. The associated logic will find rays by making use of fast 10 bit address RAMs. The granularity of the trigger rays for different angular regions is given in Table 8.3. Groups of pads will be associated to form a road by feeding the signals from the pads to the address lines of the memory. Each road is fourfold for four different z-bins of 25 cm width around the interaction point. The memory is anticipated to be lk x 4 bit and a given pad can contribute to 1 or 2 roads depending on the particular geometrical case. The depth of storage depends on the delay till the first level trigger but 21 will certainly suffice. In practice we can use 64 depth and put 3 roads per memory. Using this scheme requires a total of 3776 fast RAM's.

One attractive method of adding this large number of channels is to use an analogue optical technique. An led is pulsed into an optical fibre for each ram and the fibres brought together to a linear optical detector which then gives both the trigger pulse and the trigger multiplicity. A schematic diagram is shown in Fig 8.6.

204

n

signal

n+l

shaped signal

l:B tot

Strobe n Oiscr.

to

st robed delayed signal

I •

n+2 n+3 BC---

BC

n ~ ~

I I ( \

Figure 8.1: Pulse forming for liquid argon calorimeter signals.

205

... "' i

~ A

s :t ... ~ .:;; .;;

...... ¥ !

I -rr, 11~1 I U' ..:-.!..

.. ..

. . .. ~

Figure 8.2: Calorimeter trigger signal generation logic.

206

Figure 8.3: Segmentation of super towers in the liquid argon calorimeter.

207

00

-~ ·'"""""""""""""""""""""""~"""""~~"""""~ '""'-"""""'-"-"-"-"-"-"-"-"-"-"-"-"'-"'-~ "'\.'-..X-, -

~

-co CD -

0

<( ~-·- ·-IO.-

w

-

co c:c u

oo ___ C7'

-

u CD u

01.n J-·-·-

-< in u..

(%) u..

3 u..

9:... 0 0 l

Figure 8.4: Theta/phi segmentation of BT and one set of ST.

208

The segmentation of the rays will be tailored to match the trigger towers of the calorimeter. In particular super-rays will be formed to match very closely the acceptance of the super towers of the calorimeter.

8.3.3 Drift Chamber Triggers

In addition to MWPC triggers, a finer granularity, and consequently a better background rejection, is available from the drift chambers. Such a trigger is not particularly suitable for combining with the calorimeter triggers as it does not !laturally have the pad structure that is needed for this purpose. It is, howeve~, sensitive to different background than is the MWPC system. It does have the ability to give superb angular segmentation and thus give very accurate P.L cuts for low multiplicity events and form stand alone triggers for them. This is a unique trigger for this class of events.

The drift chambers provide very powerful triggering possibilities. The information can be obtained by getting crude drift times by clocking a discriminated chamber pulse at 10 MHz into a shift register. This gives about 3 mm granularity. Hard wired logic based on RAM's will be used to search for these characteristic patterns in times of order of 1 µsec. A more sophisticated pattern recognition using microprocessors on the same data is very simple and powerful but tends to be slow. A hard wired approach is both sufficiently selective and is available in lµsec so that the data could be used in the first level trigger (see section 8.4.1). A detailed scheme for the jet chamber (purely r/</>) has been explored. A schematic diagram is shown in Figure 8.7.

For the forward drift chambers a similar approach is also viable using the forward radial chambers. Again docking into a shift register would be used.

Such approaches give a strong selectivity for a pointing trigger. In addition there is the possibility of using a system from CELLO which has the ability, using

analogue division of the signals on opposite ends of wires and FADC's, to give linked z-coordinates for tracks found in r/phi by the above technique. Because an accurate r/phi is required as a starting point, this method is too slow to be usable on the granularity in r/phi provided by the MWPC system but is fast enough to be availab)e at level Z starting from the drift chamber r / <P trigger.

We consider drift chamber triggers have major long term uses in HI but recognise that some delay in implementation would not be disasterous.

8.3.4 Muon Detect.ors

It is important that all events with muons be retained. An indication of a muon from coincidences between pads in the instrumented iron will be available within 0.5 µsec. This will be a ray trigger matched to the calorimeter trigger towers and inner tracker rays, and can be correlated with these. Thus the muon trigger will be pointing in order to minimise the contribution from backgrounds. In addition strip triggers will be readily available from the wire readout.

The forward muon system has drift chambers with maximum drift times of 0.8 µsec so a trigger signal can be available by 1.0 µsec directly from the wire.

The muon data available to the trigger is summarised in Table 8.2

8.3.5 Scintiliators

It is proposed to install segmented scintillators in order to permit rejection of backgrounds due to interactions of the proton beam up stream of our apparatus. These scintillators, as discussed in chapter 6, will be reasonably close to the axis of the apparatus and give nearly complete coverage for backgrounds coming along the machine tunnel. Coincidences between well shielded counters will be needed to prevent very high rates from soft backgrounds causing false vetoes. It is expected that the accurate timing from such scintillator will give major rejection power by use of time-of-flight.

209

":i:j aq. c .... (0

?' ~

N I

t,:) < (0

~

0 ... c+ (0

>C c+ ... oq.

()q (II ..., 0

OQ

;:;·

Z4 HWPC cathodes

ii HWPC cathodes

~1 HWPC cathodes

~ ·Origin ~onsiru:tion I histogram

digital pipeline

· · · \ ' outside ~I ..,_s_p_e-ct_r_om_e....,ter

• ., 1

O'.i l !§ I 2 1 ~I - 1

I

"tc .. mean··

find significant peak

digital pipeline

A>B

A>B

OR

3

r--------, .-L-------, I

chamber 3

chamber '2 Amp. Discri.

chamber 1

MPU Ad

MPU Data out

MPU Data in

I. Config.1 +Multiplicity

1

Sync.

Adder

OKG

1

Be

3

10

MUX

fan out

-{>--

MUX

Ad

Do Di

MEM 1

10

10

MUX

Ad

4 L---4--- -1 Do Di

R· X3 Contig. ,,Multiplicity to Contig's mem.

Figure 8.6: Optical adder circuit .

211

MEM R

R X10 Bits

to Pads mem.

Table 8.2: Trigger signals

signal solid type number beam cross likely angle time resolution level

~ lOOns > lOOns of use

CALORIMETER

TTmult 411' 1-bit 1 yes no 1

Eem 2:: Eminl ST 1-bit 4 x 64 yes yes l Etot 2 Emin2 ST 1-bit 4 x 64 yes yes 1 Eem? Emin3 471" l·bit l yes yes 1

Etot 2 Emin4 41!' 1-bit 1 yes yes 1

Eem.L 2.: Emin5 411" 1-bit l yes yes 1 Etou 2 Emin6 41f 1-bit 1 yes yes 1 Eem BT 8-bit 256 yes yes 3 Etot BT 8-bit 256 yes yes 3 Etot 2.: Emin7 TT I-bit 1024 yes no 3

MWPC mult?multmin- 411" I-bit few yes no 1 super ray ST ]-bit 64 yes no l ray ray ]-hit 5632 yes no 3

MUON SYSTEM

mult2::multmin 4'1' I-bit few yes no 1 super ray ST I-bit 64 yes no 1 ray ray 1-bit fewx 103 yes no 3

DRIFT C HAMBER mult 471' 1-bit few yes no 1 r/ <f> r/ ¢ 1-bit many yes no 1 r/z 10 (; 1-bit few yes no 2

Table 8.3: Granularity of MWPC trigger rays

region 8 No 4> No r/z No MWPC No bins No No degrees sectors sectors hits at JR rays RAMs

forward 5-30 2x16 30 5-9 4 3840 2880 central 30-150 16 12 3-5 4 768 384 backward 150-165 16 16 3-5 4 1024 5I2

TOTAL 5632 3776

212

8.3.6 Cosmic triggers

Cosmic triggers will be useful for calibration and setting-up. The occurance of a cosmic event will be best signalled by the coincidence of the appropriate signals from the tracking and muon detectors. There is however an additional requirement that these trigger signals cannot sensibly be restandardised in time by the ma.chine bunch crossing signal. The pads on both the muon and MWPC systems are capable of giving fast signals so that a time reference for the drift chambers can be provided. In addition, offiine, the jet cha~ber can provide an accurate to.

8.4 Main Trigger System

The trigger is envisaged as being at 5 levels varying in time from l µsec to 100 msec after the event occurred. This is needed in order to have a data acquisition system that has sufficiently small dead time. The aim is to reject all background events at the earliest possible time. It is important to note that unless a trigger is made at any given level it cannot be made at subsequent ones. Figure 8.8 shows the approximate timing diagram for the t rigger . The signals available are listed in Table 8.2 together with the trigger level at which they will be most useful.

In principle , at a given level, any trigger signal can be combined with any other - some of t he possibilities suggested by physics considerations are shown in Table 8.5. The selection of specific trigger combinations will be possible from a trigger control programme in the main control computer .

The details are d isc:ussed below.

8.4.1 Level I

This is dead-time-less and based on discriminator outputs and hard wired logic only. The rate must. be less than 104 Hz so that the dead time introduced at this level is less than 10% for a level 2 timing of 10 µsec. In practice we hope that the rate will be substantially lower than 10.( as it is possible to do quite sophisticated logic in the time available. The requirement of a dead-time-less operation implies that the data must be pipelined and that the trigger logic itself must be able to process independently data from neighbouring beam crossings. Data used by the first level trigger will come from the calorimeter, the inner charged particle detectors, the muon detector, and the veto counters outside the main experiment.

The triggers available at first level will be:

• stand alone triggers from any detector; these correspond to small numbers of connections to each detector,

• triggers at the granularity of super-towers from each detector; these use many more connections,

• special triggers such as monitors and cosmics.

Jn principle, finer granularity is quite possible on timing considerations for many triggers but numbers of cables from the detectors and the usefulness of the additional information for hard wired logic limits realistic possibilities.

The earliest timing of the first level trigger is constrained by the calorimeter signals and the transit time. The minimum time between a beam crossing and the appearance of a. first level trigger at the trigger box is 1.2 µsec. There are however other considerations . The FADC's on the tracking detectors will have digital pipelines of 2.5 µsec on present plans. Hence for the tracking drift chambers it would be permissible to have the first level trigger at about 2.0 µsec. Such a timing would permit very sophisticated logical operations on calorimeter and other signals involving complex matrix coincidences and OR's. The longer integration time of calorimeter signals, while improving final energy resolution, will inevita.bly increase backgrounds as one is summing over more beam crossings. It is not yet possible

213

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to be sure what is the optimum, but it could well be quite a critical parameter. For this reason we assume for the present that the first level trigger will occur at about 1.2 µsec keeping a delay in reserve.

The trigger signal is used remotely from the trigger box so the appropriate delay must be added. This imposes significant constraints on the location of various parts of the electronics.

It is expected that level 1 will initiate all the read-in and digitisation of the detector information.

8.4.2 Level 2

This is still based on discriminator outputs but can have much more sophisticated logic. The time at which this occurs is less critical than its rate. This must be at most a few hundred Hz (which with a typical time to level 3 trigger of 0.5 ms gives a dead fraction = 200 x 5 x 10-4 = 10%).

The level 2 trigger is hard wired and occurs at about 10 µsec. This order of time is a maximum if level 1 is approximately 104 Hz since the dead fraction would be 10%.

If there is no level 2 trigger it will be necessary to abort digitisation and readout and reset all the electronics. Such a process takes a few µsec.

It is possible that the functions of level 2 triggers can be carried out fast enough to be available at level 1. If this turns out to be the case level 2 will become redundant. We intend however, to forsee it in the hardware and logic so that we retain flexibility and the ability to react to adverse background conditions when HERA comes into operation.

8;4.3 Level 3

This is a microprocessor based trigger using only the information available up to between 10 and 100 µsec. Thus we shall have information from front end discriminators and data specifically designed for trigger purposes such as the FADC's from the calorimeter trigger towers rather than data from full digitisation. However the use of DMA transfers to the micros will permit very substantial amounts of data to be available for computation. Hence quite selective ray triggers will be used for each detector and correlations between rays from different detector will also be done. All trigger tower information, and all pad data from the MW PCs would be used. In addition some fast trigger information obtained from crude (3 mm) drift chamber digitisings could be available.

Level 3 will also check that different parts of the detector are agreed on the beam crossing number from which the event originated. Provided conflict is rare then such events will be rejected. If it is not rare a warning will be given.

Provided the level 2 rate is of order a few hundred Hz an average processing time of several hundred µsec is permissible. Using a micros with a power of a few mips would be realistic but would still lead to significant dead time at this input rate. The maximum rate of the level 3 trigger is approximately 50 Hz, corresponding to an acceptable dead time loss with the full digitisation and read-in time of about 2 msec. In order to minimise dead time losses the level 3 trigger will be produced at variable times. As soon as a specific candidate trigger from level 2 is computed to be uninteresting then a fast reset is sent out to aU the digitisation and read-in processors. It is therefore possible to envisage some difficult cases taking much longer times. The upper limit is really imposed by the full digitisation and read-in time, as there is no point in continuing to use partial data when fuU data is available.

8.4.4 Level 4

Again this is a microprocessor trigger but now working on fully digitised information. This cannot have full data till about 2 msec. At this time the data is still in blocks of memory associated with particular detectors. While data reduction can occur at this point, there seems little advantage in Level 4 working on the data until event building has been completed. This is because buffering within each detector data col1ection line is quite realistic and so no dead time need be introduced by waiting.

216

After event building there is easy access to all data from any detector. Buffering at some point ahead of the input to the Level 4 trigger has t he further advantage of evening the work load on Level 4 trigger microprocessors because events are de-randomised at t heir input. The Level 4 trigger can take of order 30msec to be formed. (Because data is de-randomised a.head of level 4 the requirement is that input rate x time < 1 not «I.) The output rate of the Level 4 trigger must be no more than 5 Hz in the early days of the experiment as it is anticipated that level 4 will initially be the final level of the trigger. It will initiate transfer of the data down the link to the main storage medium.

Once data is fully digitised and transferred to the event building buffer then the gating of levels 1 to 3 trigger can be removed and another event be processed through to this stage.

8.4.5 Level 5

This is a data filtering trigger and although it wilJ probably not be implemented at HERA switch on, yet it must be foreseen. The trigger can be derived from the results from the on-line processor farm. It will consist of selections working on clustered data, eg found tracks, or jet energies computed by the online data reduction of calorimeter data. This trigger is needed to save off-line analysis as well as reducing the final data recording rate when the luminosity of HERA becomes larger. Its rate should be as low as possible consistent with negligible rejection of useful events (i.e. physics events).

8.4.6 Recording of Trigger Data

All the data used specifically for the trigger will be read out with the rest of the data from the accepted events. This is summarised in Table 8.4. The calorimeter, MWPC and muon data has to be recorded for the beam crossing to which the event is a.scribed as well as for neighbouring beam crossings in order to give the possibility of checking the correctness of the assignment . Once we get, full confidence in the correctness of this assignment then the data from neighbouring beam crossings can be dispensed with. The drift chamber trigger data naturally covers many beam crossings because the drift time can greatly exceed the int.er beam crossing time.

8.4. 7 Gating

The basic gating is simple. A gate is closed on all the data systems immediately the first level trigger fires . This is then re-opened if level 2 did not occur at its expected time or is left closed until a third level trigger has sent a clear pulse and the apparatus has had tirne to settle. The gating must also check that there will be spare buffering space available at the event builder stage before opening the gate. (see chapter 9.)

All gating will be controlled by the main trigger system via direct wired connections. We must not wait for signals going over LAN,s or other multiplexed routes.

8.4.8 Implementation of the Trigger

All aspects of the trigger will be capable of ~ing altered under software control by the main monitoring computer (see section 9.4) and the subsystems will, when permitted by this control, be able to operate in stand-alone mode for setting up, calibration and other purposes. To this end, individual detectors have their own local gating and trigger generation. They will operate on their own input to the trigger and on pulse generator and cosmic triggers. The use of a detector in local trigger mode will mean that the data will not get to the event builder and links to the desy computer centre will not be available for recording data.

217

8.4.9 Standards

In order to operate a satisfactory system, certain standards will be followed. These include the form and response to signals associated with the trigger, gating and event building. These standards are necessary to avoid unusable data such as parts of different events being put together as a single event.

All signals to and from the trigger box will be ECL on fl.at twisted pair cables. In order to ensure that subsystems always act as slaves to the main trigger control no hand shaking will be permitted and a.ll data control in subsystems will respond to central instruction from the trigger box within pre­specified times. Each local trigger system will encorporate standard units to keep the time reference locked to the beam crossings and respond appropriately to gate signals.

218

Table 8.4: Summary of data used by the trigger

Detector number of bits number of bits per beam crossing total

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Muon detectors 51< 15k Totals 61k 103k

Table 8.5: A sample of the uses of trigger elements to form triggers

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219

Chapter 9

Data Acquisition and Control of the Experiment

9.1 Introduction

The primary function of the data acquisition system is to readout event data from the various sub­detectors and to store the information for subsequent physics analysis . For the data to be useful, it is necessary to control and monitor the operation of the detector in such a way that malfunctions are detected as early as possible and that a record of operating conditions and detector performance is available during the data analysis. Import.ant aspects are the communication with the operator, the display of histograms and an event display to debug the detector and the trigger online. A facility to "Jogon" to the DAQ system from the DESY site and from outside DESY to check and monitor the performance remotely is highly desirable.

To obtain adequate speed , it is necessary to acquire data in all subdetectors concurrently and to combine the data to logically complete events only afterwards. It is also necessary, in a. detector of this size and complexity, to be able to test, debug and, to some extent, calibrate the various subdetectors and their readout systems independently. These independent systems must also integrate smoothly and efficiently for physics data taking.

It is essential to do as much filtering and data processing as possible before recording the data. It is also advantageous to do some high-priority analysis, at lea.st crudely, online with very short response time. Therefore, a major fraction of the computing power available to the experiment is concentrated in the data acquisition system.

9.2 Data Rates

For practical reasons we aim for a final data recording rate of below 5 Hz . The estimated amounts of data and readout times for the major detector subsystems are summarised in Table 9.1. The largest amount of data comes from the tracking system and we will read out only after zero suppression. Normally, the data in each subdetector will be zero-suppressed and, in some subdetectors, further compressed before recording. However, for monitoring and calibration purposes, it will be possible to record data without zero-suppression or compression at the cost of higher deadtime.

Data. will be transmitted to the DESY computing center via a. link to be provided by DESY and logged there. We require that the link support a. data. rate of at least 0.5 Mbyte/ sec.

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9.3 Data Flow

The data flow is down a tree structure, shown schematically in Fig. 9.1, with the lowest level detector hardware in the leaves, single detector data flow in the branches and event flow down the trunk towards the data logger in the root.

Processing such as data compression, formatting, filtering, monitoring, and physics analysis is distributed at all levels of the tree. It is also possible, without disturbing the data flow, 1o copy events for use by processors not in the main data flow for e.g. event display, hardware debugging, software testing, physics analysis, etc.

Data buffering is provided wherever a processor must work on the data. In particular, buffers are provided before detector branch processors, event builder, trigger levels 4 and 5 and before the data logger. These buffers are simply computer memories on the data bus with software to manage them as first-in, first-out queues. The data flow is controlled locally by the buffer managers in cooperation with the corresponding processors. Except for the data logger, the policy is to pass the data along as rapidly as the processes and buffer sizes allow. It may be more efficient to manage the data logger buffer with a policy which allows only full blocks of a given size to be transmitted over the link to the DESY computing center.

Globally, the data flow is controlled by the trigger box which can halt the input of data at the leaves by inhibiting the trigger. Clearly it is necessary for the managers of the readout buffers in each detector branch to communicate the presence of buffer space to the trigger box. Since the level 1 trigger starts the readout, the trigger box must inhibit the level 1 trigger until all detector buffer managers signal that they have space.

The processor bus system is VME. Other systems are acceptable in the branches provided they deliver the data to the event builder at the top of the trunk in VME.

9.3.1 Calorimeter Branch

Calorimeter data will normally be zero-suppressed during readout. With the threshold set to 1 u of the noise spectrum, we would read about one-third of the channels as indicated in Table 9.1. This represents a conservative strategy. In parts of the calorimeter where signals from minimum ionizing particles are many q above the average noise, as indicated in Table 5. 7, we would raise the threshold and read less data. For test and calibration purposes, we would read all the data. The readout time will be 1-2 msec depending on the final choice of ADC and multiplexing, but independent of the zero-suppression threshold. Very low thresholds would, nevertheless, bring a deadtime penalty at high trigger rates due to saturation of the IBM link.

A more sophisticated form of zero-suppression is under consideration in which not only channels above a high threshold are read, but also their nearest neighbours which exceed a lower cut. Such a scheme could be implemented on intelligent ADC boards as in the calorimeter readout solution A, or by reading all channels above the lower threshold into a buffer and performing the higher threshold suppression with crate processors.

9.3.2 Tracking Branch

In the tracking system, the pulse heights of all drift .chamber signals will be sampled at a rate of 100 MHz with an FADC. At present, the DL300 system originally developed at the Physikalische Institut of the University of Heidelberg and currently in use at JADE is favoured. A version with a 256 sample pipeline is envisaged.

The FADC's are non·linear and can be either 6- or 8·bit. As far as reconstructing drift times is concerned, 6 bits shou)d be adequate. On the other hand, charge division can result in rather large dynamic range requirements, so 8 bits may be needed. This can be decided later when the cost of

221

the 8-bit chip ma.y have come down sufficiently to make the overall cost difference small. Transition radiation and dE / dx measurements would also profit from 8 bits.

The system has a fast scanner to search the memo:ry for valid hits. As soon as the scanner detects a "start of hit", a fast microprocessor reads the data for this particular hit out (about 16 words). Assuming an event produces 50 hits per DL300 crate (=5000 hits per detector) with 16 words per hit, the readout time for a 16·bit MC68000 CPU running at 12.5 MHz would be~ 3 msec. The readout time could be reduced to~ 1 msec by using a 32-bit MC68020 running at 16.7 MHz.

In order to be able to set up the system independently of other parts of the detector , it is proposed to combine all data from each of the forward and barrel drift chambers in separate system crates which will be VME. It has to be connected with the VME crates into which the data is read out by the FADC system. There will be 3 of these crates so three Interface Ports are needed. This crate must also contain a VME branch driver so as to communicate with the ma.in event builder.

In order to convert raw digitisings to pulse heights and times of signals corresponding to deposited ionisation, considerable computing power will be needed. It is proposed that this be within the data acquisition path and could be either in the system crate or associated with the memory of the event builder. It is vita] that there be considerable buffer memory associated with these processors so that one event can be processed in parallel with ma.ny others. We envisage that this function will be carried out by dedicated VME compatible 32.bit micros such as MC68020's. Initially we assume that ,....5 of these will be required. The final requirements depend on the trigger rate at this stage and on the complexity of the code needed for the separation of close tracks and backgrounds. Clearly, in the initial operation of HERA, we shall also record the zero·suppressed FADC digitisings in the final data.

9.3.3 Muon Branch

In the central muon system, the data is zero-suppressed in the front end modules and read onto VME bus under microprocessor control. TotaJ readout time is 1 µsec per non.zero channeJ with, typically, a few hu·ndred non.:zero channels per event.

For the forward system, we plan to use the LRS 1879 96-channel pipeline TDC system in FASTBUS with subsequent readout onto VME. The readout time is 412 µsec plus 50 nsec per hit for a total of ~ 500 µsec for a typicaJ event. At this time the data has been zero-suppressed and stored in local memory so the trigger gate could be re-opened.

9.3.4 Trigger Branch

All trigger information provided by the subdetectors plus all results of level 1 and level 2 trigger processing will be read into the VME system for use by the trigger level 3 processor. The reading will be done in parallel for the different subdetectors to achieve the maximum speed. If the result of level 3 is positive, the data will be zero-suppressed by the processor and transferred to the event builder just like the data from any subdetector. Intermediate results of the level 3 computation will be included to assist in monitoring.

9.3.5 Event Building and Processing

Our current thin)<in g on the event builder is based on the idea of a VME-VMX matrix as in the 0 PAL event builder. The structure is indicated schematically at the right in Fig. 9.2. There is one crate per detector containing memories which each hold the detector data from one event. The memories containing the parts of one event are connected to each other and to an event processor (e.g.MC68020) via its VMX local memory bus. These processors have access to all the data in one event and carry out the trigger level 4 processing and any monitoring tasks which require data from more than one subdetector.

To guarantee that the event builder does not collect parts of events from different beam crossings into a single event, it is necessary to synchronise all the components with a single clock (e.g. the

222

beam crossing number). This clock signal is be broadcast by the trigger box to all components. Each component counts the clock pulses locally and delivers the clock count with the data for each beam crossing. The ''time» of the first level trigger is recorded. The event builder uses this information to ensure that only data from the same beam crossing are combined.

Events passing level 4 are passed on to the buffer of the level 5 filter processors. The buffer consists of many (say 10) interfaces to level 5 processors. In addition to the interfaces of level 5 processors, there may be additional interfaces to other computers which require full events for some special purpose not covered by the standard processing done in the level 4 and level 5 processors.

In level 5, we consider running more sophisticated standard programs such as event filtering and first pass reconstruction of the sort most experiments in the past have run off-line as the first stage analysis package on the raw data tapes. For an accelerator like HERA one should envisage dedicated processors for this activity. It will, of course, be possible to run filter programs at this level in a flagging mode where the decision is recorded with the data and all the data are passed.

Surviving event.s are passed to the data logger buffer to be blocked for efficient transfer to the DESY computer center where the data will be recorded on magnetic tape or optical disk.

9.4 Control of the Experiment

9.4.1 Control System Architecture

Fig. 9.2 shows the architecture of the control system. The tree structure of the main data flow will be built as a di~tributed multiprocessor system in VME. All data flow and computation necessary for actual data acquisition will take place within this system. The VME system will be the only access to the data logging link.

The subdetector branches will also be connected to dedicatc,d, user-friendly subdetector computers capable of stand-alone control and data acquisition in their own branches for setup, calibration and debugging. These machines will be responsible for control and monitoring of subdetector support systems, e.g. high voltage, gas, cryogenics. They should, ideally, be of the same type and run much common software, e.g. histogram package, bus diagnostics.

The main data flow in each branch will go either into the event builder or into the subdetector computer. The choice can be made independently for each subdetector, but only data leaving via the event builder will go to the data logger. However, it will be possible, during data acquisition via the event builder, to spy on the main data flow by copying events out of the VME system into the subdetector computers.

There will also be a central multi-user, multi-tasking computer for program development, archival storage and general experiment related computing. Programs for the processors in the VME system will be deve)oped and stored on this machine using appropriate cross-software. The overall control of the experiment will be via terminals connected to the central computer. It will have spying access to events after the event builder for special monitoring or physics tasks. There could also be substantial disk storage available for detector constants. We will investigate the advantages to be gained by keeping a.H constants there, under control of a data base management system as opposed to keeping the constants on the subdetector computers where they are first determined.

The central computer, the subdetector computers and key microprocessors in the VME system will all be connected to a Local Area Network, e.g. Ethernet, for exchange of programs, data or control information. We prefer this to transmission along the main data paths during acquisition on the grounds of simplicity and security. However, during setup, when many programs and large numbers of constants may have to be loaded into the VME system, we would use the higher transfer rate available on the VME bus.

During normal operation, users would not use the LAN to logon to processors in the VME system, but would communicate with them only via. the central computer or one of the subdetector computers.

223

Cblorimele ADC Data

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This provides the user with a more comfortable system while, simultaneously, protecting the VME processor in the data flow from user mistakes.

ln case of severe hangups of the data acquisition system, the LAN also provides us with an alternative route into the processors during troubleshooting.

9.4.2 Organisation of Monitoring

The primary requireme~t of the monitoring system is to ensure that the detector is functioning properly and to provide clear up-to-date status information to the shift crew. It is essential that all events be validated and monitored, and that warnings of serious errors be transmitted immediately to the main contro) terminals. There will be a set of standard histograms that are always filled.

There are three types of data which will be monitored:

1. subdetector non-event data such as gas pressures, temperatures, H.V. values, magnet currents. There is no need for the speed of VME and these data may be collected by the subdetector computer via a lower specification bus.

2. subdetector event data. This is readily available in the subdetector branches, but, as data is compressed in passing down the tree, not all information will be available lower down. I ·

3. subdetector event data which requires data from other subdetectors for evaluation, such as e.g. efficiency or resolution. Such monitoring will have to be carried out in or after the event builder.

Monitoring of subdetector non-event data will be done in the subdetector computers; it is one of their primary functions.

Online monitoring of event data can be performed at different rates on different parts of the data in three places:

l. on every event in the VME readout system

• on single subdetector data in each subdetector branch

• on entire events in the trunk

2. in spying mode on single branch data in the subdetector computers

3. in spying mode on entire events in the central online computer

Results of monitoring done in the VME system, e.g. histograms, efficiencies, will be sent to the subdetector computers or the central online computer via the LAN for display or archiving.

9.4.3 Graphics

Graphics devices are required for histogram and event displays and status information. Careful thought has to be given to selecting graphics devices. In the past, storage tube devices have been employed but nowadays we expect to use raster-scan displays with built-in processing power. Those to be used on subdetector computers need not have high resolution since the displays will be less complica.ted than for whole events. The graphics devices for the main experimental control room should be more sophisticated. High resolution is required - CELLO and JADE use Tektronix 4014,s giving a reso­lution of around 4000 by 3000 points. There are many attractions in using a sophisticated graphics workstation (Apollo, Sun) for the displays and controlling the experiment via graphical input. At least two such devices are required for the main control room. The event display program on such a device can also be used as part of offiine analysis. The online event display should be able to access events after the final online analysis in order to be able to select specific types of events according to software classification or trigger source. Sophisticated 3-D displays which will certainly be desired offiine may not be necessary online.

225

9.5 Choice of the Bus System

The trunk of the data acquisition is concerned with data flow and processing and it seems most appropriate to use a computer bus at this point. The size of the system and complexity of the data flow requires a multicrate bus system.

We have chosen VME bus, a standardized, modern computer bus supported widely by industry. Although VME is only defined at the single crate level, there are currently several multi-crate systems available:

• UAl system

• OPAL system

• Creative Electronic Systems (VB 8212/3)

• Performance Technologies (VP-VME 901)

A detailed comparison of our requirements with the capabilities and costs of these systems will be done before a choice is made among them.

FASTBUS, the other modern, standard bus system well-known in high energy physics, is much more complicated than VME and does not have the wide industry support for microprocessor boards which are needed for this part of the system. For front end electronics, considerations of board size and crate power, strong points of FASTBUS, become more important. Although we recommend the VME electrical standard with Euromechanics standards for board size, we realize that it may be irresistible to use some FASTBUS in this pa.rt of the system. Therefore, we only require that data be delivered to the event builder in VME standard.

9.6 Choice of Online Computers

The entire data acquisition system is a multiprocessor system capable of performing most of the tasks which were performed by "the" online computer in previous experiments. There is still a need for a standard, multi-user, multi-tasking computer (SMMC) for program development and archival storage. However, there is no need for this computer to be in the main data stream since it will not be doing data logging. It would be possible to do all program development and archiving on the DESY computers, control the experiment with a work station and have no main data acquisition computer at all. However, this would require sophisticated soft.ware on the IBM link in order to avoid strong restrictions on software testing activity during data. taking. It would also make online software activity too dependent on the DESY computing center. Thus we should still have an SMMC at the experiment.

The SMMC should be capable of running physics analysis programs to give us fast feedback on special categories of events. We also envisage a farm of processors, e.g. 370/E, 3081/E or ACP-type, for doing online filtering and physics analysis. The SMMC should be capable of serving as host to these machines. Although a computer of the VAX family is an obvious candidate for the SMMC, it is certainly not the only candidate, and possibly not the best one. The requirements for the SMMC will first be formulated and then a systematic search made for the best machine.

We expect the experiment to have several mini- or micro-computers acting as subdetector com­puters. A high degree of software compatibility between these machines and the SMMC is clearly desirable. Small subdetectors with simple software requirements could weJI use a personal computer.

Level 5 will require considerable computing power, possibly of the order of 100 msec equivalent IBM/370 cpu-time per event at an event rate of order 5 Hz. It is dearly necessary to find a very cost effective way of providing this computing power that is still convenient to use for the physicists who must write the programs.

226

The most promising approach seems to be to utilize the inherent parallelism in the problem and to provide a farm consisting of many cheap processors all running the same code on different events, independently but in parallel. So far, two types of processors have evolved for use in such farms

1. the IBM emulators 168/E, 370/E and 3081/E developed at SLAC and at the Weizmann Institute

2. nodes based on standard microprocessors available from industry such as those in the Fermilab Advanced Computer Project (ACP), the Southampton Transputer Project and the beam orbit computer at DESY

Since the approaches are technically very different, it is necessary to choose one or the other and stick to it. Parameters for comparison are cost effectiveness, ease of maintenance and ease of use.

The choice between emulators and microprocessors is not obvious. The primary advantage of an emulator is the ability to run exactly the same object code that you develop and debug on your standard ofHine computer. The main disadvantage is that you have to build and maintain them yourself, and that the technology is inevitably a step behind. The main argument in favour of microprocessors is that you can buy them from industry a.nd change them to keep up with the latest technological developments. On the other hand, the rapid development often means that the software support is lagging a year or more behind. Let us now examine these points in a bit more detail for two examples, the 3081 /E and the ACP(MC68020).

The 3081/E is completely different from the previous IBM emulators such as the 168/E and 370/E. It uses off-the-shelf, multiple sourced TTL components and modular design, making it cheaper to build and easier to maintain. Speed is obtained through the use of high-speed static memory chips and separate execution units for integer arithmetic, floating point add/subtract, floating point multiply and floating point divide, all of which can operate in parallel. The processor executes its own microcode which is obtained by translating IBM object code. This makes the migration of running programs from an IBM main frame to a 3081/E particularly simple. The translator is capable of reordering the code to take particular advantage of the parallel execution in different execution units and pipelined execution within a single execution unit. One drawback, since the emulator uses expensive fast memory, is that the microcode requires more memory space than the original object code. However the expansion factor is typically of order two and never exceeds three.

True 32-bit microprocessors with IEEE-standard floating point coprocessors are now on the mar­ket (Motorola 68020/68881, AT&T 32100/32106, DEC 78032/78132). They allow the building of extremely cost effective computers from few components with well documented characteristics. They are programmable in high level languages, in particular FORTRAN-77, thus providing source level portability of existing programs. The most information is currently available to us is on the Motorola devices so we now compare these with the 3081/E.

A typical performance measure is the computing power of the VAX 11/780 mini-mainframe com­puter, which corresponds to roughly 1 Mips (million instructions per second); an IBM 3081K has a power of about 20 of these VAX units. Typical performance figures for MC68020/ 68881 cpu/fpu systems are in the range 0.5-1.0 VAX units. Average performance is expected to shift toward the high end of this range as the present day, non-optimized compilers are improved. Increasing the clock rate from 16 to 20 MHz and use of high speed cache memory could also increase performance. A single emulator is much more powerful, with typical performance in the range 3-5 VAX units. Performance improvement is conceivable through refinement of the translator which converts IBM object code into the microcode which is executed by the emulator. Quoted prices for emulator boards (29 kSF + 11 kSF /Mb from CERN) applied to reasonably sized node with 4Mb of memory lead to a price of about 88 kDM. This gives a price/performance ratio of 18-29 kDM/VAX. MC68020 based nodes

, of the Fermilab Advanced Computer Project will cost ~6 kDM when produced in standard series. Prototype nodes have been tested with real HEP programs and gave an effective power of 0.6 VAX units. They are expected to produce at least 0.8 VAX units when the compilers have been optimised

227

for the MC68020. Thus, the price performance ratio for MC68020 systems lies in the range of 6-12 kDM/VAX. Table 9.2 gives a summary of the price performance figures with VAX 11/780 and micro-VAX II included to help set the scale.

We will continue to observe developments in this area and expect to take a decision in mid-1987. Support of processors by DESY is highly desirable and would weigh heavily in ou'r decision, given the similarity in price/performance ratios of the two types of solution.

228

L

A

N

D subdet.eclor computer

' . .............................. ................................................................... ... ............ ..............

D subdetector computer

. ' ... ............ ......... ...................... ................................ ......................... ........ ............. ..

D subdetector computer

•ubdet..:toT •l~tro11k•

. ' ....................... , ....................................... ......... ,_, ,, ,. ,, ..................... .. ........... ....... .

D subdetect.or computer

D central

computer

subd~tector

elec\ronlc•

.. ....... ..... .... !. ............ ........... ............................................. , ....................... ..

Figure 9.2: Control System Architecture

229

event output to level 4

e v e n t

b u I 1 d e r

Table 9.1: Amount of Data

Trigger Tracking DC Calorimeter Muon Number elect. chan. 8000 65 536 166 000 Data before 0.-sup. 12 kbyte 2 Mbyte 256 kbyte IO kbyte Data after 0.-sup. <l kbyte 100 kbyte 88 kbyte 1 kbyte Data after compaction 20 kbyte Readout time 0.5 ms I ms 2 ms 0.5 ms

Table 9.2: Comparison of Emulators and Microprocessors

Processor (including 4Mb memory) VAX 11/780 micro- VAX 11 3081/E ACP(MC68020+68881)

power in VAX 11/780 units

l.O 1.0 3-5

0.5-1.0

230

Price/ power kDM/VAX unit

578 73

18-29 6-12

Chapter 10

Off-Line Computing

10.1 General Concept

The general off·line software concept for the Hl experiment is sketched in Fig. 10.1. It shows the break down of the off-line design into different tasks (represented by circles) and the data. flow between the experiment, the tasks and the common data base. The purpose of the major elements is briefly described in the following: -

• Reconstruction P:rograms. They perform pattern recognition and particle identification in tracking devices and calorimeters from real and Monte Carlo input data. They may include monitor programs for diagnostics of detector components and reconstruction efficiencies.

• Analysis P1·ograms. They depend on the particular ir.terest of the user and the physics to be investigated. General versions could contain well established classification criteria for different physics processes and be used for a fast survey of rates or to set up a.n analysis express line. They also comprise interactive a.na.lysis.

• Display Programs. They are needed for pattern recognition development, detector perfor­mance checks and event scanning.

• Monte Carlo Programs. They are used for event generation and detector simulation to any desired detail. They provide the necessary input to develop and check out the complete r~construction and analysis chain before the start up of the Hl experiment.

• Common Data Base. It contains all geometrical data. of the detector, calibration constants and all run informations (energy, luminosity, polarization, ... ) for real and Monte Carlo data. All tasks should have access and use the same information from the run library.

The tasks themselves will be highly structured into smaller program modules for each detector component (or possibly parts of it). The data flow between the modules has to be flexible and accessible to all modules, such that independent event processing for each detector component is possible.

It is too early to be specific on the algorithms to be used in the individual tasks. Most work h~s been 'done on Monte Carlo programs, which will be described later in more detail.

As a first step towards the coherent realisation of such a complex system a careful design with the definition of the tasks and modules, and the data flow between the experiment and the tasks has to be made. This work has been started. A structured design is provided by consequent application of a modern dynamic memory manager. We also investigate whether commercially available software engineering tools for the design, realisation, coding and testing phases and the overall management could be an effective help for the whole project.

231

It is obvious that there are many common aspects with the on-line system and some of the algo­rithms developed will be used in an eventually simpler version in the on-line data analysis.

10.2 Monte Carlo Programs

10.2.1 Event Generators

Several event generators simulating the physics to be investigated at HERA are in uae wi.thin .the Hl collaboration. They are baaed on the LUND Monte Ca.rlo for standard model physici or the LUND fragmentation scheme for exotic proceHee. The au.ilable event generators deacribe the following reactiona:

- neutral currents e p ... e X

- charged currenta e p - 11 X

- photon-gluon fusion of heavy quarka, e.g. e p -+ et i X

- SUSY particle production ep- iqX and ep...,. .VqX

- heavy lepton heavy quark production e p -+ E Q X

~ excited electrons and quarks e p - e• X and e p-. ew q+ X

·- leptoquark production e p - (Lt) 1

There also exi11t programs to 11imula.te background from electron brem11trahlung, beam ga1 inter­action~, a.nd cosmic rays. Generators to des9ribe reaction• for luminosity measurement and "'t"'t phye1ics are under development.

10.2.2 Detector Simulation

During the construction phase of an experiment detector 11imulation11 are of prime importance in order to check the detector performance a.11 given by the hardware desjgna and to feed back the phy1ics requirements to the hardware 1peciflcations. In later stages all sort1 of analy1i1t rely more or IHI! on Monte Ce.rlo simulations. Different approache1 have been followed: (i) a crude and fast 11imulatlon and (ii) a detailed and slow detector 1imulation.

(i). Fast simulation methods have been developed which deacribe the gro11 fee.tures of the propoHd H 1 detector. They include the smearing of charged track fl ae given by the rHolutione of the tracking devices and the calorimeter response to electromagnetic and ha.dronic interacting partlcleB. Energy deposits in the calorimeter element• are parametrized uaing existing data for longitudinal and lateral shower profiles. The energy reaolution and the t/'lf responH aro free parametcm.

(ii). Detailed detector 1imulation11 are of particular interHL to 1tudy detailed qu11tion1 like energy and spa.ce reeolution of particles and jet11, application of weighting techniqut1 for energy re1olution improvement, e/7r separation, 'Ir h overlap, trigger conditions, on~line filtering, the overall performance, and to develop pattern recognitio11 algorithme. Up to now theto tochnique1 havo been ueed to 1imulate te~t aet-up like or simplified detector configurations of the forwe.rd tracking By1tom including the TRD, the electromagnetic and hadronic calorimetera, the coil and the forward and central muon 1y11tem (tail catcher).

For calorimetry studie1 we u1e the ca1cade type Montt Oarlo1 EGS for electromagnetic partlcl11 and·QHEISHA (includin1 EGS) for hadron1. Theee codes are easy to apply for 1imple 1tructure1. For· the simulation of the complete and very complex Hl detector we intend to UH the GEANT program, in particular it1 geometry package hae very attractive features. GEANT al10 has &n inter­face to G HEISHA. One might also con1id1r ·the incorporation of the GEANT geometry handling ln

282

the GHEISHA code, which has already been realized. A complementary shower program hss been developed which combines the physics processes of EGS and HETC together with the geometry of GEANT.

Since shower type Monte Carlos are extremely computer time consuming ( ....., l sec/GeV at IBM 3081K ), they are inappropriate for large scale production. We have to develop fast and sufficiently precise simulations, e.g. by constructing correlated shower parametrizations or by using a hybrid method replacing within a shower cascade the )ow energy electromagnetic particles by prefabricated showers from a store.

10.3 Software Standards

Many programs or program modules will be developed and used in various institutes with different computers (see Table 10.1). Therefore a minimal set of rules and soft.ware standards are necessary.

Decisions have been taken concerning

• FORTRAN77 as programming language avoiding machine dependent code.

• BOS77 as dynamic memory management system. It is a flexible and powerful system, very easy to use and can readily be installed on computers like IBM and VAX. It provides machine independent tape 1/0 and offers a data base management for our run Hbrary on direct access file. Long and positive experience has been gained with the BOS system by almost all PETRA collaborations.

• GKS standards for graphic display programs, in particular its 3 dimensional extension (now under test at DESY and CERN).

Still open questions are

• Source Code Management. For the time being we envisage transfering program source code via data links or to preparing PATCHY-Ceta files for the exchange of large libraries. There is no available system which seems appropriate to our wishes. We are looking for better solutions.

• Histogram Package. Here each user should be free to choose his favourite program. How~ ever, an interactive analysis and plotting system (like GEP) providing standard output formats transportable to other installations would be very useful.

We encourage the DESY computer centre to provide, support, and maintain universal standard software systems satisfying the requirements of experiments of the new generation. Discussions with the other HERA experiment facing similar problems are taking place and will continue.

10.4 Networks

For successful and efficient data handling and analysis, the interconnection between computers at different locations is indispensable. ln a large collaboration it is necessary to transfer electronic mail, documentation, on-line messages, and source code, to provide access to common data bases and event data files (real and Monte Carlo), and to have remote control for detector component checks. This has to be established through data links. Local connections between the on-line computer, emulators, work stations, and the DESY main computer are provided via DESYNET. Long distance connections to almost all collaborating institutes in Europe and US are available via EARN, BlTNET, JANET, CHUNET, INFNET, and/or X25. These networks are considered sufficient during the set up phase of Hl and its software. Once the experiment is running, high speed links for transfer of large amounts of data wiU be required (e.g. via satellites). The non-resident physicist should be able to perform calibrations, detector checks, and to participate in first round fast physics analyses. The specifications

233

of the required data links need further discussion, in particular with the DESY computer centre and the other HERA experiment.

10.5 CPU Requirements

A reliable estimate of the computer time needed for data processing and analysis during the data taking is extremely difficult. The major uncertainties come from the facts that (i) the rate and nature of the beam induced background processes as well as the achievable trigger purity depend strongly on the HERA machine performance and are essentially unknown today and (ii) up to now pattern recognition algorithms and programs allowing reasonable time estimates for data reduction and processing have not yet been developed. We tried to combine our experience from PETRA experiments and some input from UA1 to estimate the CPU time requirements. All CPU times will be given in units of IBM 3081K, which has two processors delivering 48 hours/day.

Our CPU time estimate is based on the following assumptions:

• A RUN TIME of 200 days/year at a trigger rate of Z-3 Hz gives "' 3 · 107 events in total or "' 1.5 · 105 events/ day (independent of the luminosity). The average event length is "' 100 kbytes.

• All events have to be passed through a FILTER PROGRAM employing partial or crude but efficient event reconstruct.ion algorithms. After this step a high degree of background event recognition with no further data pr()cessing is anticipated. Assuming an average time of -0.1 sec/event this will require a minimum of 4 hours/day. It is envisaged to do this filtering on emulators or microprocessors. However, these facilities may not be fully available at the start of HERA.

• The filter is supposed to reduce the da.ta to be investigated in more detail by a factor of 5. (Typical reduction factors of -10 are achieved by PETRA experiments.) This leaves us with a sample of,....., 3 · 104 events/day for which COMPLETE EVENT RECONSTRUCTION with precise tracking, calorimetry and particle identification will be done. This data corresponds to an amount of .....,20 tapes per day, a quantity which seems still managable. Assuming an average reconstruction time of 5 sec/event requires a minimum computing time of 42 hours/day.

• ANALYSIS TIME for detector calibration, physics, refined reconstruction for specific events, interactive computing, and program development is requested to be a minimum of 12 hours/day (this may be translated to ,....,1Q minutes/ day for ea.ch physicist involved in the data analysis).

• For MONTE CARLO SIMULATION of physics processes, detector response and event reconstruc­tion we assume roughly the same computing time needs as for complete event reconstruction. This time can be spread over the year leading to 25 hours/day.

The total CPU requirements add up to 85 hours/ day. In order to be competitive and produce new and interesting physics results in due time, it is necessary that the raw data reduction and complete event reconstruction will be done in paraJlel to the data taking allowing for no major backlog. This means that a cosiderable amount of work, in particular the evaluation of an express line type analysis, has to be done on-site. A large amount of computing capacity has to be supplied by DESY, and we aim at having a strong centralized off-line computing group to provide the collaboration with reconstructed events on data. summary tapes. In addition CPU time should be allocated to visiting scientists to perform analysis during their stay.

Our requirements from DESY are 60 hours/day of IBM 3081K equivalent CPU time. This is slightly more than half the computing capacity of DESY as of today. It should again be pointed out that our estimates are based on certain assumptions, the reality may require more computing power.

234

Experience a.t PETRA shows that a substantial amount of CPU time will be provided by outside institutes e.g. for Monte Carlo and data reduction, and the computing power available from these sources are listed in Table 10.1 together with the DESY contribution. The available CPU contingencies seem adequate for the immediate future software development. Most time will be used for Monte Carlo studies, test data analysis, and the development of pattern recognition programs.

235

Institution Accessible Computer Installations

FRANCE

Ecole Polytechnique IBM 3090, VAX 750, Cray I Orsay IBM 3090, VAX 785 + 8600 Paris VI + Vil IBM 3090, VAX 750 + 785 Sacla.y IBM 3081K, Cray, VAX 750 + 780 GERMANY

OESY IBM 3081K + 30810 Aachen DESY IBM, 370/E, PDP 10, µ-VAX 11 Dortmund ND 500

1 Hamburg DESY IBM, Siemens 7882

I MPI Miinchen IBM 4381, Siemens 7880 Wuppertal VAX 780, Cyber 175 + 176

-GERMAN DEM. REP. Zeuthen BESM6 ITALY Rome VAX 8600 SWITZERLAND Ziirich IBM 3033 + 3083, VAX 8650 U .K. RAL IBM 30810, lCL Atlas 10 Glasgow IBM 4361 Lancaster VAX 750 Liverpool IBM 4331, VAX 750 Manchester GEC 4090, µ-VAX II U.S . Davis VAX 780 Houston VAX 780 Northeastern IBM 4083, VAX 750, µ.-VAX II

Table 10.1: Accessible computer installations

236

MC Generator Banks Data Base Run library

Rec. Track Bankt

Figure IO.I: Structure of Off-Line Computing

237

Experiment

Chapter 11

Luminosity Measurements

11.1 Introduction

Processes useful for luminosity measurement involve low multiplicities and so are sensitive to the potentially very high background at HERA; thus requiring sophisticated event selections with their resulting uncertainty. Several different processes, using different triggers, detector parts, and analysis methods, will allow multiple checks to reduce systematic errors, and control at the same time the detector performance. In addition, due to their distinct kinematics, many luminosity reactions provide independent calibrations of various detector elements.

Two general classes of luminosity monitors will be used. High rate monitors with restricted absolute accuracy will be used for luminosity optimisation and

relative normalisation of different data sets. As fast monitor for beam steering, rates higher than single beam background are needed ('""' 104 Hz) and a coarse relation with luminosity is acceptable.

Absolute luminosity measurements will be based on reactions with known cross section and a unique signature. A good monitor reaction for HERA should have a cross section larger than neutral current reactions at large Q2, larger than say -100 pb (as for NC at Q2 > lOOO(GeV/c)2). Back­grounds and systematic errors should allow a final accuracy in the absolute measurement better than 10%.

The processes considered by Hl for the luminosity measurement are listed in Table 11.1, which summarizes our considerations on the subject, including the related calibrations.

11.2 Main Detector Luminosity Measurements

Low Q2 (- 50 (GeV /c)2) neutral current reactions can be used as a relative monitor. The scattered electron will be detected in the backward calorimeter with E' > 0.8 * E. The x acceptance is very large (0.001 < x < I) and the counting rate is only weakly dependent on energy and angular cuts on the scattered electron. The observable cross section is of the order of I nb. This reaction also gives an absolute luminosity measurement to an accuracy of about 20% using the structure function measurements of present experiments.

An exclusive process with a very clea.nexperimental signature, providing an useful relative lumi­nosity monitor, is diffractive J /'l/J electroproduction at Q2 ,.._ 0 (GeV /c)2

ep ....... epJfi/J-ept+1-

(l = e, µ) with an observable cross section of -600 pb. Changing the polar angular cut from 5° to 30? reduces the counting rate by a factor of three. The lepton pairs can be detected by a. charged two-track trigger. The sharp effective mass peak provides not only a clean signature but also ihe possibility of a good ( < 10-4 ) absolute calibration of the average magnetic field. Background sources are beam gas collisionswithin the interaction region and the 'elastic' diffractive process

238

e p -+ e p p0 -+ e p 7r + 1T - •

Possible methods for rejection a.t trigger )eve) are a p1 cut of -800 MeV /c for charged tracks and the use of calorimetric and muon triggers to identify electrons and muons.

Uncertainties in the theoretical calculations and the possible onset of large inelastic J /t/J and open charm production limit this process to the role of a precise measurement of relative luminosity in the long term. The calibration factor could be obtained for each run energy performing a cross check (with the same final state, trigger and detector biases) using the calculable (QED) 1/ production of lepton pairs at low Q2

ep-+ept+1-.

The last process is suitable for an absolute measurement. The detection of a 'central' lepton pair (without anything else) would be a relatively good signature, even without the small angle tagging system described in section 11.4. The cross section is large for small masses of the lepton pair, e.g. for Mu > I GeV one gets:

Angular cuts

2.6° < 8u < 177.4° 10° < Ou < 170° 30° < 011 < 150°

Cross section

13.6 nb 3.8 nb

0.78 nh

Backgrounds are similar to the previous case. Applying more restrictive cuts on Pl. II and the acopla­narity angle will compensate the lack of a narrow peak. A low multiplicity charged trigger in the forward and central regions, selecting on 0 and <l>aco. is needed. Antitagging of the initial electron puts a limit on the mass of the quasi real photons.The best resolutio11 for the electrons is achieved by the calorimetric measurement.

The main theoretical uncertainty in the luminosity measurement comes from effects of the inelastic form factor of the proton, so direct experimental measurements of this are important. At the same time this process (jn the long range conrolled with the J / 1/J) allows monitoring of hadronic calorimeter using muon pairs, and self calibration of drift chambers.

Another QED process, wide angle bremsstrahlung ('virtual Compton scattering'), provides events with one electron and one photon scattered in the main detector, with visible energy larger than the electron beam energy. The angular distribution of both the electron and photon peaks in the backward direction. The cross section is ,.....200 pb for 10° < Oe, O..., < 170° and about 1 nb if the backward electromagnetic calorimeter is included.

A purely calorimetric trigger can detect tpis process with a very clean event signature. Relying on it and on electron/photon recognition, this process will be a very useful monitor of the efficiency of the trigger and of the tracking and shower matching (where the complementary 21 process above is predominantly given by a charged particle trigger). It can also be used for calibration of EM calorimeters in the central and backward direction.

The reliability of the computation of virtual Compton scattering (up to small uncertanties in the proton inelastic form factor) allows in principle an absolute luminosity measurement.

11.3 On~Line Relative Luminosity Monitor

The luminosity and eventually the electron po}arisation will have to be measured by the machine g,roup using zero degree bremsstrahlung photons and/or laser-beam Compton back-scattered photons. The detector foreseen is a simple shower counter situated about 90 m downstream the electron beam with ~n expected angular acceptance of -0.25 mrad. As the angular divergence of the electron and proton beams are of the same order, the acceptance of the detector will depend on machine conditions.

239

Such a detector is therefore unsuitable for an absolute measurement, but its high rate will provide a coarse and fast relative monitor. The average photon angle is ....,Q.2 mrad, so a good fraction of the radiated photons will be accepted. The most serious backgrounds are synchrotron radiation photons and beam gas bremsstrahlung. The former can be removed by low Z (e.g. carbon) absorbers in front of the shower counter, the latter will be discussed in the following section.

11.4 Electron Tagging

The scattered electron from beam-beam bremsstrahlung can be detected by using the horizontal bending magnet downstream in the electron beam as a spectrometer 1 (Fig. 11.1). With a 4 cm wide vacuum tank at the exit of Bl the range of scattered electron energies accepted is 21 < E' < 26 GeV. With a special vacuum tank in Bl the acceptance can be extended to 14 < E' < 26 GeV, with an angular coverage up to -1 mrad.

The d~tector will be situated behind a special exit window in the vacuum chamber of the electron ring (Fig.11.1 ). The counting rate is estimated to be of the order of 1 MHz (including off momentum electrons), corresponding to - 0.10/0.15 electrons per bunch crossing (assumptions as in Ta.b.11.l}.

A coincidence with the o0 1 detector will have a rate of-60 KHz due to beam-beam bremsstrahlung, providing another relative monitor. This coincidence wiH be needed to calibrate the small angle I detector as well as the electron tagging detector.

A coincidence with the charged track trigger will provide tagging of quasi-real photon events, allowing,for example, much more precise measurements of the luminosity by the QED process e p --+

e pi+ 1-. The rejection of inelastic contributions (like e p--+ e N · t+ 1-)achieved in this way and the constraint Q2 - 0 (GeV /c) 2 will improve the systematic error of the absolute luminosity measurement, though at the expense of statistics. The precision of the calibrations performed using the lepton pairs in the main detector will also substantially improve.

A major use of a tagging system is the measurement of electroproduction processes at Q2 -

0 (GeV /c)2. Photoproduction is the most frequent process at HERA: as such, it can be used as relative monitor of luminosity in a backup solution when, due to background conditions; the charged two-track trigger could not work.

11.5 Conclusions

The eight monitor reactions envisaged (see Table 11.1) provide considerable redundancy allowing for cross checks and calibrations of most of the Hl apparatus. We expect to achieve an overall precision in the luminosity measurement of better than 10%.

1 D. Kisielewska et a.I., DESY HERA 85-25

240

t~

""'

TABLE 11. 1

Sunnory of reoctlona ond bock9round• conald•r•d fot IU1tlno1ity raeoovrera•nt1 and for colibrotions

PROC£SS USE:F'UL fOR (In () the RELEVANT ACCEPTANCE TRlCGERS RATE W.tN BACKGROUNO CALIBRA-und•t•cted DETECTOR (Ht) P R 0 C E S S E S TION portlcl .. ) Of'

NC Bock. E~ • 001< x <1 Coloriraet . -2 to·t ... Q' .... so (GeV/c) F'orw. dot • ~ 80X b•Clfll on. A I •p->(•p)+J/'r s• < 9,. ,. < 11~ - l

ep->op ~o N 41• .- 8ockw . ChciTged -t 10 Mo9net le F'orw. M t•r> 2 GeV/c, t rig9or 1--+'lt•it-: .... flold

H ~ ... -~ (2 troeka) SBG 1 •• 1· - .,. ..... pJ.>0.6CeV/c U8G +.

Inelost le J/""f ptod . A

ep->[ep]+1• 1· lnolostlc op->0N•1• r p eockw . Previ ous row + Che1r9ed Cho'?. tr I 9. p forw. (p .1.)~.-> 1 GeV t ri99er Holo lkuona Et.C bock.+ A

~'"'~ .... )

·ro < 20 (2 trocka' UBQ forw.)

R }"•r·J 2x 2.6 <~<10) F'orw. + 10} ·t ep->op r• Hod tonic A 2x 10 < 11<30) central 3 10 4 1ttlf.1• f co Io r ita. T 2x JO <9 <150) (l trl9.) .6 (by t" pairs) u Solf col lbr. s of drift ch.

•P->(p)+• 'r Sock. Ewe 10 < 9e< 110 Colori111. 4 101 Color.tri9g.

10 < 13-c < 170 Boc:kw. EWC Bof re I Et.C o- '¥ recogn. Plug colorlro.

• ep->(•p)+'{(o•) o• '( - !' 0.25 ntrod Special 1.4 106 8e<11a goa 8rtnt11stral.

do he tor £ "t. > 3 c.v Colorl111. Synehrotr. rodlotion

£ o• "! - 3 10"' L •p->(p)+e'IW) £ y > 4 GeV Speclol Off ntOll. eleclrona El•c;tr.ta9g.

E dohctor 21<Ee.<26 GeV + el Photoprodvctlon forw. 'r det. c Electron · 0·1 iarod tagger T to99in9 R

1o·t 0 ep-[p)+e1•1- Electron 21<E,<Z6 G•V Chorg•d Off ntort. electrons £1.(:(back+t or) H toggln9 0-t 111rod trigger Elostlc ecotterin9 Hodr. col.

forw. + row J + oloc. Photoprodvctlon (t" pol ro) T Sockw. logger Self collbr. A drl ft c;homb. G G op->[p}+o+X Electron 2t<Ee<26 GoV Chorged JO Off 111011. electrons I (photoprod.) te199ln9 CH 111rod trigger uoo H F'orword + hadrons + eloc. G Plv9 c;ol. logger

• • MACHINE MON!lOR 31 -t. ., Assumed IUl!llnoo1ty: 2·10 e111 aoe uec-. upslteQll boo:m-goa intoroctions ; sec-. apollotion beam-gos in the Interaction region.

PRE Ct-SJON {LUMI MON.)

•20X .

Rt lot.

·10%

:1~

Coor••· re lot.

Re lot.

Absol. (lo• a tot.)

Re lot.

"".! oq c: ... ~ bends --;... .. t""' I» '< ' 0 c: ~

0 ...., t~ I» ~ :s ~

~

lb'" ("> ..... ... 0 0 .... "' oq

()q

5· oq

Q. ~

£" ("> .... 0 ...

2m ..___...

a,

1.25mr 2.5mr 1.56mr

4.69mr

SCHEt1AT1 C LAYOUT FOR A ::s0° ELECTRON TAGGING DETECTOR

ACCEPTANCE: 0.13<Y<0,3 C2l<E'<26 GEV) e

0 , 0<1'<0 , 75 MRAD

AT y=Q,23 E

<v=L.) Ee

TAGGING EFF. 0.2-0.4

Chapter 12

Performance of the Detector

This chapter deals with the overall performance of the detector making use of the combined action of several detector components. It refers to the performance discussions for individual detector compo­nents which have been given before together with their technical description.

12.1 Calorimetric Measurements

The calorimeters are designed for good energy flow measurements which are relevant for missing Er-measurements and for inclusive measurements of charged current and neutral current interactions.

Missing Er-Measurements

Missing transverse energy has to be detected and measured precisely in order to recognize new particle production involving missing neutrals (e.g. neutrinos or photinos) and to be able to interpret and kinematically reconstruct these events. The missing Er distribution for ordinary events contains a gaussian pa.rt determined by the intrinsic energy and angular resolution of the calorimeter arid a tail at high missing Er due to unidentified muons and neutrinos from e.g. charm decay but mainly due to leakage effects and absorption of energy in dead areas of the detector. The tail events are the limiting factor for the separation of new particle production and background due to ordinary events. They will be minimized in our det.ector by it's hermiticity and the small amount of dead material in the calorimeter. Events with large energy leaking out of the liquid argon calorimeter are tagged efficientJy by the iron tail catcher; events with large missing ET inside the beam hole can be tagged by the plug calorimeter. The energy and angular resolution of the calorimeter determines the accuracy of the kinematic reconstruction of exotic events involving missing neutrals. We expect a typical resolution u(Er) ~ 2.5 GeV for our detector in the kinematic range W 2 ~ 1000GeV2 .

Inclusive Neuti·al and Charged Cunent Measurements

An important aim of HERA physics is the accurate measurement of neutral and charged current structure functions in the largest possible (x,Q2)-range. Apart from event statistics and geometrical acceptance, the accessible kinematic range is limited by errors on the structure functions due to a systematic shift in the energy calibration of the calorimeters and by the magnitude of the necessary corrections to account for the experimental resolution in x and Q2 (smearing corrections). Based on the experience of present day deep inelastic scattering experiments we have chosen reasonable bin sizes in x and Q2 and acceptance criteria. Bins are accepted if the ratio r of true event population to the measured population (smearing correction) is in the range 0.8:::; r $ 1.2.

Neutral current inclusive measurements can be based either on the measurement of the scattered electron or on the hadron energy flow (»jet-measurement" ). The accessible (x,Q2)-ra.nge for both measurements is shown in Figure 12.1 . Also shown are lines which indicate a systematic shift of the

243

structure function F2 by 10% by a systematic error fJ E in the absolute calibration of the electron resp. hadronic energy. The kinematic region which is accessible for the electron measurement assuming an average resolution of u(Ee)/Ee = 10%/../Ee in the barrel and 13%/../Ee for the backward calorimeter covers the region of high energy transfer up to the kinematic limit and extends down to an energy transfer 11 ~ 5000 GeV (or equivalently y ~ .1 ). This lower limit is given by large smearing corrections in the energy and requires at the same time a very good absolute enrgy calibration to about 1 %. We have also used an angular resolution for the scattered electron of u(8)=2 mr which is provided by the z-chambers and by the backward MWPC's. The jet-measurement complements and extends the electron measurements. This method gives the most precise measurement at low energy transfers and deteriorates towards the kinematic limit a.thigh 11. For x;:::.03 the jet-measurements extend the kinematic range by nearly a factor 10 down toy~ .01, where this limit is caused by the beam hole and can be avoided by running at lower proton energy. At high y practically the whole kinematic range up to the kinematic limit is accessible though with substantially worse resolution compared to the electron measurement. In addition jet-measurements in the high y region require very good absolute calibration. Nevertheless it is extremely important to have the largest possible overlap of the accessible kinematic ranges for the two methods in order to intercalibrate them since for charged current scattering only the jet-measurement can be used.

At very low values of x (x::; .03) event statistics and experimental resolution of the electron measurement are ather good. This offers the unique possibility for HERA to measure neutral current structure functions down to xi=::: 2 * 10- 4 in a Q 2-range which is relevant and interesting to test perturbative QCD. Backward and barrel electromagnetic calorimeters are designed such that this interesting field of physics is well covered.

Charged current inclusive measurements rely entirely on jet-measurements. The accessible kine­matic range is shown in Figure 12.2 together with lines separating regions where the structure function F2 changes by more resp. less than 10% by a systematic error f>EH in the energy scale. Our conclusion is that based on the expected resolution a(EH) /En::::: 55%/VEH $ 2% the detector is able to make full use of the available kinematic range at HERA a.part from the high x region which contains a very small fraction of all events .

12.2 Charged Lepton Identification and Measurement

Charged leptons are expected to provide important signatures for new heavy particle production.

Leptons from New Particle Production and Background studies

Extensive Monte Carlo studies have been performed to simulate a wide spectrum of exotic processes involving particles such as SUSY scalars, heavy quarks and leptons, exdted electrons and leptoquarks in a mass range between 20 and 150 Ge V. The majority of these events has at least one charged lepton. Multilepton signatures are frequent. These studies have given us insight into the momentum and angular distributions of prompt leptons and on their separation from neighbouring particles ( pions and gammas) as presented already in the letter of intent. In the meantime we have complemented this information by background studies simulating e.g. number and distributions of conversion electrons, the identifications of leptons inside jets , and decays of pion and kaons. The main results of these studies can be summarized as follows: - The forward region (B ~ 30°) plays an essential role and poses by far the most stringen~ requirements on lepton identification due to high track densities and high momenta. A very large fraction of prompt leptons (typically around 50%) is emitted into forward angles and moreover these are the leptons which carry. the best signatures. • conversion electrons are abundant mainly at forward angles since typically more than 2 conversion electrons per event are expected with momenta above 2 GeV /c. Whereas symmetric conversion pairs can be recognized by tracking or by a rough dE/dx-measurement and eliminated, others decay

244

asymmetrically , look like genuine electrons and have to be rejected on a kinematic basis. An effective separation can be based on the following requirement: The direction of total hadronic energy flow excluding the spectator fragments (eliminated by a cut in 0 ?: 5°) is measured. A cut on the minimum transverse momentum PT(e-) of the electron with respect to this direction of about 2 GeV /c is then able to reject conversion electrons rather effectively without a severe loss of prompt leptons of high signature. - muons from pion and kaon decays have similar kinematic properties like asymmetric conversion electrons and can be eliqiinated on the same basis. This requires however that the muon momenta are decently measured.

It should be noted that such a strategy of background separation is possible because the detector has good tracking both for isolated tracks and for tracks within jets from small polar angles up to backward angles (see section 6.4).

Electron Identification

Electron identification is based on the transverse and longitudinal shower shape measurement in the calorimeter, the comparison of track momentum with the calorimetric energy and of the shower position with the impact point of the track. It is complemented by multiple pulsheight measurements in the drift chambers.

The calorimetric energy resolution and identification is degraded in the vicinity of cracks and by dead material in front. At present we prefer projective ~cracks in the electromagnetic calorimeters in order to miniruiz<~ the angular range which is affected by cracks. Since the cracks in the hadronic stacks are highly nonprojective the energy of electrons (and gammas) is always measured and a reasonable electron identification is still possible for isolated tracks as needed e.g. to separate neutral and charged current events. The present estimate of the thickness of dead material between the tracking volume and the electromagnetic calorimeter as a function of (J is shown in Figure 12.3 In the forward and backward barrel region, we may expect to have more than 1.5 Xo in front which would seriously affect the energy resolution as discussed in section 5.5. We forsee to add a presampler gap in these regions in order to improve the energy resolution and e/11'-separation and will make an effort to reduce the thickness.

Simulations of calorimetric e/1f separation have been done for isolated tracks and for electrons superimposed to hadron jets as discussed in chapter 5.11. They result in separations better than 10-3

for energies above 2 Ge V. Electron identification is backed up by multiple pulsheight measurements in the drift chambers.

About 50 pulsheight measurements for each charged track are obtained in the drift chambers from 5° up to 150°. The expected performance depends on the final choice of the gas. We hope to use a gas with ? 50 % Xenon which would provide an additional e/n separation by more than a factor 10 up to momenta of about 15 GeV. At forward angles this separation is improved and extended up to momenta of about 50 GeV by the addition of three transition radiators as described in section 6.3 .In summary the detector provides e/1f separation of the order of io-4 to 10-5 even for electrons in the vicinity of other tracks which changes smoothly with polar angle and matches well the requirements of e-p physics.

Muon Identification and Measurement

The identification of muons is based on their penetration of the liquid argon calorimeters ( 5 to 8 absorbtion lengths) and the instrumented iron yoke (4.5 to 9 absorbtion lengths). As discussed in chapter 4 muons are tracked by three layers of double muon chambers which give muon coordinates in both dimensions to an accuracy of 2 to 3 mm. In addition the muon track is recorded by 8 layers of streamer tubes between the iron slabs which record the muon coordinate in the bending plane to

245

an accuracy of about 5 mm. These additional chambers allow a clear identification of a muon track even within a leaking hadronic shower.

The outer muon track segment has to be linked to the muon track measured in the central chambers. Monte Carlo simulations using jet events have shown that this link is unique for about 97% of all events (with 98% muon efficiency). In about 3% of all cases a pion inside the jet with similar momentum vector also gives an additional false link.

The momentum measurement of muons up to momenta of about 50 Ge Vis best done in the tracking chambers. The large field volume inside the calorimeters improves the momentum measurement at higher momenta and gives an easy and reliable way to check the momentum measurement of the inner trackers. Muons have to pass between 60 and 90 radiation lengths before they reach the first muon chamber. This results in an uncertainty in the predicted muon position in this chamber of 80 mm/pr(GeV) compared to a displacement due to magnetic bending of 600 mm/pT(GeV) inside the calorimeter. In consequence a rough measurement of the muon position to an accuracy of several mm in the muon chambers checks the muon momentum to about 17% for all momenta down to polar angles of about 20°. This is to be compared to the independent momentum measurement which measures the magnetic bending inside the return yoke by the three muon chamber planes. This meaaurement is limited to an momentum accuracy of 25% by multiple scattering and moreover would require a chamber alignment to a fraction of a mm for momenta above 20 GeV /c. The proposed muon chambers give useful independent momentum measurements up to about 10 GeV /c only, which is very useful to reject false track links of low momentum muons as explained above.

The accuracy of both the momentum measurement and the probability of a unique track link deteriorates severely at small forward angles due to small magnetic bending and high track density. In this region the forward muon spectrometer gives an independent muon momentum measurement with an accuracy of ap/p $ 32% up to p=150 GeV at angles 3° ~ e $ 17°.

Ka.on decays within the central tracking chambers occur in about l % of standard events giving a true t ransverse muon momentum PT 2: 3 GeV. These decays can mimic high momentum muons due to undetected decay kinks. This sort of high PT background is eliminated in our detector by the second momentum measurement in the muon chambers.

In conclusion, the proposed muon system will be able to identify and measure muons in an angular range 3° ~ 0 ~ 125° with an efficiency of 95% and a resolution up/P == 0.003 * p. The second momentum measurement provides a. resolution of up/p $32% up to to 150 GeV.

12.3 Summary of Expected Detector Performance

Table 12.l gives a summary of the most important parameters which characterize the performance of the detector. They have been discussed earlier in detail in the sections indicated in the la.st column of table 12.1.

The numbers given in table 12.1 have partially still to be verified in experimental tests. We are confident however, that they are realistic estimated and will be reached at least in the long run. If so, the detector will be well prepared both for the extension of known physics and for the unexpected.

246

I calorimetric measurements para.meter electromagnetic ha.dronic section u(E)/E

barrel +forward $ 10%/J{Ee) 61 1% 55%/ V{EH) $ 2% 5.11 backward region 13%/ V{ Ee) ffi 2% 80%/V(EH) $ 4% 5.8,5.11

energy calibration 1% 2% ·-

5.5 containment 20 to 30Xo 4.8 to 8Aabs(L .A.) 5.11

4.5 to 9Aabs(Yoke) effective stack Xo == 1.5 cm Aatia = 23 cm 5.4

quantities RM= 3.9 cm 3 x 3 cm:.: (0 $ 20°) 2 x

transverse tower 4 x 4 cm2 (0 $ 80°) electromagnetic 5.4 size ~ 8 x 8 cm2 (8 $ 150°) in angle

14 x 14 cm2 (0 ~ 150°} number of 3 (0 ~ 45°) 4

longit udinal segments 4 (8 $ 45°) 5 to 6 5.4 length of 3,6,11 Xo ~ 0.9Aab .. 5.4 segments 3,6,6,15 Xo

missing energy resolution u(Er) ~ 2.5GeV Q2 ~ 1000 GeV 2

jet angular resolution 0"(8H) ~ 40 mr Q2 2 1000 GeV 2

n tracking parameter value reference

magnetic field B=l.2 T 3. field homogenity ±3% in tracking region 3.

measured coordinates / track 64 to 73 (5° $ () $ 150°) 6.6 point accuracy

central jet chamber - lOOµm 6.2 forward drift chambers 100 to 150µm 6.3.5

z-drift chambers < 350µm 6 .2.3,6.2.4 muJtitrack resolution 2 to 3 mm 6.2,6.3

u(p) /p2 (high momentum tracks) isolated t racks < .003 (7° s 8 s 150°) 6.4

tracks inside jets .002 to .004 (9° $ 0 s 150°) 6.4 u{O) $ 1.5 mr (0 ~ 30°) 6.4

S 0.5 mr (0 s 30°)

III charged leptons parameter electrons muons

angular coverage 7° S Oe S 175° 3° < 0 < 125° - µ -sign determination

forward fJ S 28° Pe$ 150 GeV Pµ S 150 GeV 6.4 barrel 0 ~ 28° up to kinematic limit up to kinematic limit 12.1.2

e / 11'-separation (single tracks) identification - io-4 (0 ~ 25° Pµ. ~ 3 GeV/ c 5.11 ,6.2

,..., 10-• to 10- 5 (0 s 25°) up to 9 = 125° 6.3.7

Table 12.1: parameters characterizing the performance of the detector

247

x

104 10~

Figure 12.1: Accessible region in (x,Q 2) for neutral current measurements due to the experimental resolution of electron and jet measurements. Also shown are lines indicating a 10% shift in the structure function measurement of F2 by a systematic error in the energy determination of the electron (8Ee) resp. the hadrons ( 6 EH)

248

CC inclusive measurements to-------------~-------------

x Q9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

cl di ,, ~-

/ . I I

I •

-; l ~·

I I •

I ..... ...._

.§

o~~~~~~===:::.__~~--1-~~~~--'

102 103 104 10!>

Q2 CGeV2/c2J

Figure 12.2: Accessible region in (x,Q2) for charged current measurements due to the experimental resolution and the beamhole. Also shown are two lines indicating a 10% shift of the structure function F2 by. a systematic error {,EH in the hadron energy measurement

249

~~3 pJeM>1:>eq

(..)

~ UJ -G> ~ .... «S .a

N .... c0 x J 1epaiew peap

Figure 12.3: Dead material in front of the calorimeter

250

0 0 Q) .... 0 0 CD ......

0 0 ...,;z ......

0 0 N .... 0 0 0 .... 0 0 co

0 0 <O

0 0 ~

0 0 N

0 0

CD CP -c:n c cu ._ ns -0 a.

Chapter 13

Installation

The installation of the detector in the North hall is constrained by the limited space available and the interference with the service installations for the machine. Therefore the lay out of the experimental area is done in close contact with the DESY groups building HERA. The space requirements of the HERA machine in the North hall are not yet specified in detail so that changes may become necessary at a later stage.

13.1 Detector assembly

The dt>tertor consists of three independent parts moving on rails, namely the base structure, which rarrieH the main detector components, and the two barrel modules with the end plates. An advantage of this division is that in the assembly phase work may proceed simultaneously on all three units, provided the two 40 ton cranes are carefully scheduled. At the time the iron structure is erected, the hall should be free of machine components. In particular no beam pipe should run through the hall. Tbe time schedule is adjusted such that the iron structure is assembled immediately after the completion of the hall before the HERA vacuum is closed.

The installation of the main detector components on the base structure will be done in a position where the base structure occupies the centre of the hall and the barrel modules are moved as fa.r apart as possible to have sufficient space around the central part. The installation of major detector components has to follow a time sequence, which is dictated by their arrangement around the beam line. Most of these components are sensitive to dust, and by the time they are installed the ha.II should be clean and dust producing work should be terminated.

13.1.1 Installation of the Solenoid.

After the assembly of the base plate the super conducting solenoid will be installed and the He cryo lines will be connected so that. the magnet can be tested at full field with the complete iron structure. This test can also be used for a measurement of the magnetic field.

13.1.2 Installation of the Calorimeter.

The component. which is to be installed next, is the LA cryostat vessel. It will be inserted into the coil from the electron side of the haU. The insertion of the calorimeter stacks into the cryostat and the ca.belling is a major job, which has to proceed in a clean environment in order to avoid electrical shorts between detector planes.

It is foreseen to insert calorimeter stacks from both sides into the cryostat. A procedure has been developed, which allows prefabricated stacks to be roHed into the cryostat on a system of rails. For this purpose the stacks will be mounted on a movable platform, which can be adjusted in height a.nd position so that the stacks can be attached to the rails in the cryostat.

251

13.1.3 Installation of the Tracking System.

After the installation of all calorimeter stacks the cryostat will be closed and the cool down will start. During the cool down of the calorimeter system the installation of the tracking system may start . The insertion of the premounted tracking system and the beam pipe into the detector is described in chapter 6.9. The same lifting gear which was used for t.he installation of the calorimeter stacks will be used here.

13.1.4 Installation of the Iron Instrumentation.

The installation of the streamer tube chambers into the barrel sections and the end walls of the iron structure is independent of activities around the central unit. The streamer tubes will be inserted into the slots in the yoke from a scaffold, which is adjustable in height and position. There will be a platform which is large enough to accommodate one chamber module with a rotation device for turning the module into a position so that it can slide into the slots.

More restricted scheduling is needed for the installation of the iron instrumentation of the base section because of a possible interference with other activities in that area.

13.1.5 Installation Schedule.

T he schedule for the assembly of the Hl detector in the experimental hall is summarised in Table 13.1.

13.1.6 Electronics.

The electronics trolley will accommodate in the first floor, which moves with t he base structure, those unit.s belonging to the tracking system so that a fixed cable installation can be made. The calorimeter electronics and the first level trigger will be housed on the second and third level of the trolley which move with the barrel units. In this case cable t rails will be used allowing a displacement of the barrel modules of a.bout 3 m.

Electronic racks for the iron instrumentation will be installed at several places. Some racks will be on the third floor of the electronics trolley, some will be attached to the base structure on the short side of the hall and some racks will be mounted on the barrel module on the short side.

The main control room for the experiment with the central on line computer will be housed in one of the rooms attached to the underground hall.

13.2 Installation of auxHary equipment.

The Hl detector with its superconducting coi l and its liquid argon calorimeter needs substantial service installations in the underground hall.

13.2.1 Geometry of the North Hall.

In Fig. 13.1 a sketch of the North ha.II is shown with the experiment in the beam position. Through the asymmetric arrangement of the beam line the hall is divided into a long section and into a short section, while the offset of the position of the interaction point separates the hall into a wide proton section a.nd narrow electron section perpendicular to t he beam line. By these names we indicate the directions in which the the two particle beams traverse the hall.

13.2.2 Installations for the solenoid.

The liquid Helium supply for the superconducting solenoid and the compensating magnet is derived from one of the HERA cold boxes on the small side of the hall. While the HERA installation occupies

252

the corners of the small section, the valve box for the experimental magnets is situated at a position opposite to the interaction point. This arrangement minimizes the distances over which liquid Helium has to be transported. There are short transfer lines from the HERA cold box to the experimental distributor and a short transfer line to a dewar vessel which is attached to the base structure of the experiment and moves with the experiment. The latter dewar accomodates the current leads and a valve system which allows disconnecting of the incoming transfer line. A service turret between the dewar and the coil cryostat completes the cryogenic circuit.

The electrical services for. the solenoid are situated at the far end of the long section of the hall. The major components which have to be installed are a 5500 Amp power supply, a dump resistor, diodes and a breaker. The connection to the experiment is done with cables laid out on the floor of the hall.

The heights of the dewar vessel housing the cold current leads and the main distribution box for the experiment is limited to a size which allows the barrel section of the iron structure to pass over these cold boxes so that access to the central pa.rt of the experiment is possible without interfering with the cryogenic installation.

13.2.3 Installations of the Liquid Argon Cryo System.

The transfer lines providing cold argon and nitrogen for the main calorimeter enter the experiment through two ports on top of the iron structure at a height of some 10 m above floor level. Therefore it is convenient to install as much of the cryogenic equipment as possible at about the same height, so it is forseen to use a 2 m wide platform which occupies the wall on the eletron side. The height at which the plat.form will be mounted is such that there will he sufficient space for the crane bridge to pass without endangouring personnel working on the liquid argon system. A sketch of the installation ,in a vertical mt through the North Area, is shown in Fig.13.2.

A set of 12 transfer lines will connect the experiment to the installations on the platform. The major components on this platform are three heat exchangers each about 3 m long , some control pannels, a pump for emptying the Hquid argon into a storage vessel above ground , two sets of connecting systems which a.How the operation of the experiment and connection to the liquid a.rgon system in two positions in the hall.

Besides service systems in the experimental hall additional auxiliary equipment for the liquid argon system has to be installed outside the hall. Jn order to empty the calorimeter croystat, a storage dewar for about 60 m3 of liquid argon has to be provided above ground. The primary cooling agent will be liquid nitrogen and a nitrogen dewar of similar size is required above ground in order to be independent of refrigerators.

For normal operation of the LA calorimeter the refrigeration power will be provided by a nitrogen liquifier, which will be housed in one of the underground halls attached to the main hall. Jn the same location further space is required for an argon purification plant so that the tota.} area needed is of the order of 100 m2. The connection between the equipment in the hall and outside the hall will be provided by transfer lines which are fed through a shaft on the far end of the long section (labelled 154 on official HERA drawings).

13.3 Shielding.

The experiment has to be shielded against background penetrating from the machine tunnel into the experimental area. The background associated with the proton beam will be shielded by a wall of 2 m of iron and I m of concrete. The background associated with the electron beam is easier to suppress and concrete shielding, with lead at strategic places, will be sufficient. The detailed design of the shielding is in progress and it will be finalised as soon as the machine configuration in the interaction region is better known.

253

13.4 Modes of operation.

Various modes for the operation of the experiment have to be accomodated by the installation. They will be sketched in the following sections.

13.4.1 In the beam position.

In the normal mode of operation the experiment is closed, centered around the beam and it is connected to the machine vacuum. This situation has been discussed in the preceeding sections. It is shown in figures Fig.13.1 and Fig.13.2.

For repairs in short maintenance periods (weeks) the experiment may be opened in the beam position by moving the barrel sections with the end plates to the side the vacuum connection remaining intact. This situation is shown in Fig.13.3.

13.4.2 Out of the beam position.

Before the experiment is wheeled into the beam it will be tested as a complete assembly outside the beam. The position for these tests has to be chosen such that commissioning of the HERA machine ( is not affected so that the beam line has to be shielded by a 2 m thick concrete wall. This mode of operation is shown in Flg.13.4. Jn order to power the magnet, a transfer line will be used, which is permanentely installed on the floor of the hall and can be connected to the dewar containing the current leads in a position in the center of the hall. Also the liquid argon transfer lines will have two positions to which they can be connected.

ln case the experiment should be rolled out of the beam when it is cold, the helium and ar­gon /nitrogen transfer lines may be disconnected for the time of the movement without affecting the cold parts of the experiment.

Also in the parking position the experiment may be opened by moving the barre) sections apart. An extreme case for this situation is shown in Fig.13.5. Here one of the barrel sections is moved against the wall on the short side of the hall.

254

Component 1987 1988 198.,

. • I t t I I . ' r . Rails '

Crane

re-structure

Solenoid -Test -

L.A. cryostat

L.A. cryogenics

Stacks I

Tests

'Tracking -Tests -~ --

Iron Instrumentation

I

Table 13.1: Time schedule for the installation of the detector in the North Hall.

255

Electrical components for H1 solenoid

H1 cold box

Electronics

Power

Hera cold box

He transfer line I

LA transfer lines

LA cryo installation

Figure 13. l: Hl experiment in beam position. The experiment is closed for normal operation.

256

c .Q ........ "'O g ·.a 0 c U1 Q)

. I; ~

g, 5 L.. -u

Figure 13.2: Hl experiment in beam position closed for normal operation. View rn the electron direction .

257

Electrical components for H1 solenoid

H1 cold box

.. .. , ..

Electronics

Power cables

LA transfer. tines

Hem cold box

Shaft 154

LA cryo installation

Figure 13.3: HI experiment in beam position. The barrel sections with end plates are opened giving access to the central part of the experiment .

258

Electrical for H1

H1 cold box

Electronics--+-

Power cables

u" ,,

Ill] 'II I i11

1,11

' 1' 1 1 I ~ 111

I

LA cryo installation

Hero

LA transfer lines

Figure 13.4: Hl experiment in parking position. The experiment is closed for tests with magnetic field on.

259

Electrical components for H1 solenoid

H1 cold box

·~~ ':."."";.'.:..r· ·-:-:• f:":".- -: ~~!. ' • -·· . • . ··;ff:· • " .... -. ..:: .. i·:~ ..

. ~-·'~!r;,' ..£.

Electronics ---1---

L I

Power cables

LA transfer lines

Hera cold box

Shaft 154

LA cryo installation

Figufe 13.5: Hl experiment in parking position. T he barrel modules are opened giving access to the central part of the detector.

260

Chapter 14

Safety

The construction, installation and operation of a large detector operating deep underground in the confined space of the HERA experimental Hall requires stringent safety surveillance. The mechanical design is done in a collaboration of experienced engineers who have built magnets, pressure vessels and iron structure in the past. Equipment built by laboratories outside DESY will follow DESY safety rules and regulations. The DESY services concerned will be kept informed cont inuously and together with them the best solutions will be found. The most important risks posed by the Hl detector are due to : the presence of flammable gas (risk category 5) , the presence of a large amount of liquid argon (risk category 3) and to the electrical operation of a large superconductor solenoid.

14.1 Flammable Gases

A summary of the proposed gas mixtures for the various detector components of H 1, the volume of gas, the pressure , the content of hydrocarbon and the materials of the detector walls is listed in Tab.14 .1. From this t.able it can be seen that the total amount of hydrocarbon will be 85 kg. This puts the Hl detector below the limit of 100 kg, where serious risks of explosion exist. This aspect of the safety problem has not yet been studied in detail. However it should be noticed that the largest fraction of flammable gas is contained in the many streamer tu bes of the tail catcher, located between thick iron plates in the non confined part of our experiment. The most serious problem comes from the chambers housed in the inner part. of the calorimeter, where special care will be taken for detection of flammable gas and to provide forced ventilation. In the magnet iron yoke holes are provided at the top and bottom and will be large enough for the ingoing and outgoing flow pipes of the high speed ventilation system.

14.2 Inert Gases

The matters of concern for safety in the liquid argon system are accidental interruption of the cooling power, collapse of the insulation vacuum or argon leak from the liquid argon vessel into the vacuum vessel. The latter being the most severe. To avoid this risk, the complete liquid argon vessel is welded. Calculations show that in the event of a break in the insulation vacuum the resulting heat loss could reach 45 kw ,thus vaporising 0.2 lit/ sec of argon. This increa.sed evaporation rate is disposed of to the atmosphere by means of safety valves and rupture disks (see Fig.14.1). A test set-up is in preparation at Sa.clay to confirm the amount of heat loss in case of a collapse of the insulation vacuum. The liquid argon pump does not belong to the safety devices .In case of danger in the hall it is possible to force the liquid out of the detector into the outside storage tank within 3 hours.

261

Detector Gas Volume (m3) Pressure (bar) Hydrocarbon(kg) Streamer tubes 25 % Ar

+ 40 1.02 80 (Ta.ii catcher ) 75 % Isobutane

Forw. DCs 50 % Xe (Ar) and + 2 1.05 1.2

Forw. MWPCs 50 % Ethane 50 % Xe (Ar)

CDC + 5 1.05 3 50 % Ethane

Centr+forw MWPC 80 % Ar and + < 0.1 I.05 < 0.03

z chamber 20 % Ethane

Table 14.1: Gases in the HI Detector

14.3 Solenoid

Two important domains are carefully examined with regard to safety: mechanical design and electrical operation. The coil is mounted in a stainless steel vacuum vessel which represents more radiation lengths than an aluminium vessel but is optimised for all safety aspects. Computed shear stresses due to electrical forces between the coil and &upport cylinder are approximately an order of magnitude lower than in DELPHI coil. As dicussed in section 2.8 electrical safety is mainly concerned with the quench response system. A quench is detected by a bridge network. The quench detection will activate the circuit breakers CBI and CB2 (Fig. 2.7) switching the magnet current through the fast dump resistor Rl, thus causing discharge of the coil stored energy { 120 MJ) during the approximate 60 s of exponential decay. The resistor will not be required to discharge the solenoid more frequently than once in two hours. Cooling shall be by natural air convection. The other important aspect for safety in the event of a. quench is the mechanical protection against forces induced by eddy currents. As discussed in section 5.4.2 this is one of the reasons which have dictated our choice of stainless steel as absorber material in the calorimeter.

262

01

SSS

70 rrr1

E ..,., N

-e SS1 r---J

-----<li..oA.r -!RYOSIAI _

-=-- --

l.Ar Vessel

-Operating absolute pressure:

-P.L 1 :: Abs .pressure

·-SS1 : Abs . pressure

- 01 : Abs. pressure.

L.Ar dewar

-SS2 : Abs. pressure.

-02 : Abs. pressure.

Vacuum tank.

SS3-SS4 .· Abs.

Vacuum

SS3

0 ..,., E -0

dewar·

. Abs.

pressure.

pressure

. - .

Hydroshtic pressure : 910 mb

Figure 14.1: LAr Cryogenic Safety System

263

1,35 b

1,6 b

1,8 b

2,2 b

1,4 b

1,6 b

1,2 b

1,2 b

Chapter 15

Finance, Responsibilities, Timeschedule and Manpower

Costs and Funds ·

In Table 15.1 estimated prices of all Hl detector components are shown. Breakdowns of costs for the major detector components are listed in Table 15.2.

Tracking: Calorimeters : Central jet chamber 4.63 LAr cryogenic system a) 17.60 Forward DCs 3.50 LAr calorimeter 23.60 MW PCs 1.53 Backward calorimeter 0.40 Track triggers 1.43 Plug calorimeter 0.30 z chambers 1.00 Calorimeter trigger+DAQ 1.20 Scintillators 0.22 Gas system 0.60 Cabling 0.50

Sum 13.41 Sum 43.10

Magnet+ Iron structure: Trigger+DAQ: Superconducting coil 10.68 Central system 2.40 Iron structure 10.86 Compensation magnet 1.00 Beam Tests 2.10 Iron instrumentation 6.28 Forward muon DCs ] .51 Toroid 0.80

Sum 31.13 Sum 4.50 TOTAL COSTS 92.14

a} Costs to be negotiated

Table 15.1: Costs of Detector Components (MDM)

264

Forward Drift Chambers Mechanical construction Electronics ..... Monitor, power supplies

Sum Central Jet Chamber Vessel Wire fixation, tools, wires Mounting device Electronics

Sum Trigger and DAQ Event builder Computational farm Monitoring, graphics Computers Trigger boxes

Sum LAr Calorimeter System Stacks Sensing structures Electronic system High Voltage system Monitoring system

Sum

Table 15.2: a) Cost breakdown for Major Components

MDM 1.42 1.75 0.33

3.50

1.37 0.26 0.15 2.85

4.63

0.27 0.36 0.17 0.85 0.75 2.40

8.00 5.50 8.80 1.00 0.30

23.60

I Iron structure Iron Design effort Machining of iron Moving gear Mounting gear Preassembly, transport RaiJs Assembly, scaffolding

Sum Iron Instrumentation &. Muon Chambers Material, tools, wiring Strips, pads Assembly, transport Gas system Cables Digital electronics Analogue electronics Power supplies, computer

Sum Superconducting Coil Design and development Coil Cryostat Refrigeration Electrical System Test at RAL Jigs and fixtures Transport, installation

Sum

Table 15.2: b) Cost breakdown for Major Components

265

MDM 3.20 0.50 4.26 1.23 0.20 0.20 0.82 0.45

10.86

0.82 0.70 0.20 0.36 0.45 1.50 1.59 0.66

6.28

0.61 3.34 2.94 0.78 0.86 0.57 0.80 0.78

10.68

The presently expected funding contributions from the collaborating institutes are shown in Table 15.3. These figures are subject to approval by the various funding agencies. The total investment money available to finance the Hl detector is presently not fully determined because of the following reasons :

1. Not all funding agencies are able to quote figures beyond 1989

2. The contribution from DESY beyond 1988 is not yet established

3. The contributions from the USSR and some institutes in the USA have still to be negotiated.

Our aim is to have a working detector ready when beams become available at HERA at the beginning of 1990. This implies that we are initially restricted to those funds available in the years 1986 • 1990. We assume that funds for 1990 ca.n be used for delayed payment of items (for example electronics) delivered in 1989. The funds for this period amount to,..,,, 70 MDM. In order to match the detector costs to the available funds we are obliged to delay some investments beyond 1990. We have identified the following possibilities for staging:

• 60 % of FADCs for drift chambers (2.52 MDM)

• backward MWPCs (0.33 MDM)

• 50 % of the tracker MWPC electronics (0.2 MDM)

• drift chamber triggers (0.92 MDM)

• part of cryogenic equipment for the LAr system (3.6 MDM)

• 60 % of the LAr calorimeter electronics (5.2 MDM)

• 60 % of the plug calorimeter silicon planes (0.1 MOM)

• part of the calorimeter trigger system (0.2 MDM)

• 70% of analogue and 40 % of digital electronics in the iron instrumentation, one muon chamber, electronics of two muon chambers (2.03 MDM}

• 40% of costs of the forward muon DCs (0.66 MDM)

• forward toroid (0.80 MDM)

• part of the DAQ and monitoring system (0.3 MDM)

Assuming for the sa.ke of argument, we adopt this whole staging scheme, the costs for the first phase of the Hl detector are reduced to "" 75 MOM. The resulting cost profile for this period is shown in Table. 15.4. Assuming that the negotiations with institutes from USA and USSR result in appropriate contributions to the large items, costs and funds can be balanced. We are also investigating the use of existing equipment which is expected to give some reduction of costs in phase one. Taking into account the possibHity of further groups joining the collaboration, we are confident that there is enough margin for covering unexpected expenses.

The staging scheme envisaged reduces mainly the number of electronic channels available in phase one of detector operation. These reductions result in a coarser granularity of the calorimeter system and some limitations in momentum resolution and pattern recognition capabilities of the tracking devices. We will however be able to do good physics a.t the HERA start up. The final implementation of the complete detector is relatively easy and does not require major shut down periods.

Within a reasonable extrapolation of the funding budgets, the detector is expected to be completed at the latest by 1992.

266

1986 1987 1988 1989 1990 1991 1992

Great Britain 0.20 0.88 2.29 1.65 1.65 1.52 Fr a.nee IN2P3 0.90 0.90 0.90 0.90

. Saclay 1.22 1.21 1.22 1.21 Germany DESY 5.25 10.00 7.50 a) a) a)

BMFT 1.40 1.90 3.20 9.00 4.00 MPI 0.62 0.62 0.63 0.63

Italy 0.20 0.20 0.20 0.30 0.20 Switzerland 0.32 0.32 0.82 0.82 0.32 0.32 0.32

USA c) 0.72 0.72 0.72 0.72 0.72 DDR d) 0.10 2.70 0.20 USSR b) b) b) b) b) b)

SUM 10.93 19.45 17.68 15.23 6.89 1.84 0.32

a ) Co n . " . " Funding level f . r th1!< period 11 . t ye1 e. t .. 1 hl1. hed

b) Fund i11g lt'vel I.:. be negot i:tt.r.d

c) Fundin~ )(•vel for D avi$ only . Other inst it11t.e ~ yet t o be negotiat.ed

d) Fundi11g l«vel depend~ on sharing 0:i f re!'-p(•H~i bilit.ies f,JI' iron struct ure

Table 15.3: Estimated Funding Profiles (MDM)

COMPONENT 1986 1987 1988 1989 1990 ? 1991 SUM

Tracking 0.51 2.52 3.37 2.24 0.80 3.97 13.41

Magnet + Iron- 7.59 11 .49 4.85 2.12 1.59 3.49 31.13 Structure

Calorimeters 3.90 7.30 8.60 9.80 4.40 9.10 43.10

Trigger + DAQ 0.10 0.40 0.80 0.70 0.10 0.30 2.40

Beam t ests 0.65 0.95 0.50 2.10

SUM 12.75 22.66 17.62 14.86 6.89 17.36 92.14

Table 15.4: Total Cost profile (MOM)

267

Responsibilities, Time Schedules and Manpower

The distl:'ibution of responsibilities for the different detector components is listed in Table 15.5. The responsibilities for some smaller detector components (outer barrel MW PCs and backward calorimeter) have not yet been assigned . Eventually new collaborators may provide these items. For the iron instrumentation and the LAr calorimeters the sharing of responsibilities has not yet been completely settled.

The manpower necessary for hardware design and construction is listed in Table 15.6. There is available manpower within the collaboration to cover the requirements, exept in the case of the iron instrumentation and the LAr calorimeter where a shortfall exists. We are actively investigating a rearrangement of effort in order to solve this latter problem. In addition we are engaged in bringing new collaborators into our experiment.

Table 15.7 shows the anticipated construction schedules for the major detector components under the assumption that we can solve our manpower problem.

COMPONENT INSTITUTES

Central Jet Chamber DESY, Hamburg II Inner Barrel MWPC Zurich Inner Barrel z Chamber Zurich Outer Barrel z Chamber Zeuthen Outer Barrel MWPC not yet decided Scintilla tors Hamburg I Forward MWPC Orsay, Paris Gas System Aachen III, DESY Ray + z-Trigger Orsay, Paris , Ziirich Forw. DC + TRD All UK Institutes, Aachen III Supercond. Coil a.} RAL Iron Structure a),b) DESY, Moscow, Zeuthen Iron Instrumentation b) Aachen I, Dortmund, Rome, Wupperta) Forw. Muon DC Davis Compensation Magnet Ziirich LAr Cryostat a) Saclay LAr Calorimeter b) DESY, Ecol.Poly, Houston, MPI,

Northeastern, Orsay, Saclay Forw. Plug Calorimeter Hamburg I Backward Calorimeter not yet decided Electron Tagger Moscow Trigger & DAQ All institutes a} Costs to be shared by a.II institutes

b) Shi\Ting of responsibilities not yet completly settled

Table 15.5: Distribution of Responsibilities

268

Required Manpower COMPONENT (Man years)

p E T Central Drift Chamber 24 16 20 Inner Barrel MWPC + z chamber 5 3 9 Outer Barrel z Chamber 6 4 6 Outer Barrel MWPC 6 6 6 Scintillators 3 3 3 Forward MWPC 6 6 10 Gas System 3 3 7 Ray + z-Trigger 3 3 3 Forward DC + TRD -35- 45 Superconducting Coil 40 Iron Structure 2 3 I 2 Iron Instrumentation 20 40 Compensation Magnet - 0.5 -LAr Calorimeter 50 24 120 Forward Plug Calorimeter 5 2 6 Backward Calorimeter 3 2 6 Trigger & DAQ 3 3 5

P= Physicists E= Engineers T = Technicians

Table 15.6: Manpower estimates for hardware design and construction

269

COMPONENT -1986- -1987- -1988- -1989--Central Jet Chamber Detector Ve9sel Wire fixation Mounting device Wiring Endplate infrastructure Testl!I -Inst allation -Forward Tracker Test modules Mechanics Testl!I & assembly Test at DESY --Installation -Iron Structure Design, tenders Fabrication Assembly -Coil instal . + field test -Superc:.onducting Coil Over&ll design

,_

Conductor Coil CryD!ltat Cryogen.+ electr. equipm. As~mbly - i-Test at RAL ·-LAr Cryostat LAr vessel Vacuum tank Test, transport, instaJ. Cryogenica Tests . ......._ LAr Calorimeter Prototypes Stacks Installation -Cool down -Iron instrumentation R&D, Design, Tools Fabrication, aesembly Installation

Table 15.7: Schedule for construction and mechanical asl!lembly or major detector components

270

Chapter 16

Infrastructure and Requirements from DESY

DESY is asked to provide equipment, facilities, floor space and services as listed and described in the following sections.

16.1 Infrastruture in the Experimental Hall

Because of the short timescale it is necessary to assemble two large detector components in parallel. Therefore two cranes of 40 teach are heeded , one having two bogies each of 20 t, and with continuously adjustable smooth drives. For the assembly we ask furthermore for devices ,

• to transport heavy equipment (e.g. stacks) from the shaft to the hall,

• to transprort the coil from the shaft to the hall,

• to transport the calorimeter cryostat from the shaft to the hall.

• A systen of platforms at different heights (scaffold) to mount the iron structure and to insert the instrumentation and

• two platforms ( 4 x 6 m2), adjustable in height with gears to insert calorimeter modules into the cryostat from both sides and to install the tracking device are requested.

The system of rails, on which inner and outer parts of the detector can move, should be supplied and installed.

Besides the special detector requests for electrical power , cooling and water, list€d below, we ask for general supply of water and compressed air.

Cable trays for the electronic connections to the counting room and pipes for chamber gases must be designed and installed. Pipes with cooled air and water (ca.. 22 °) are needed to cool the electronics inside the detector. Cable trays for the electronics connections between the detector and the carriage must be designed and installed.

271

16.2 Electrical Power and Cooling for Detector Components

Electrical power in the counting room, taken away by a.ir conditioning

Electrical power for coil

Cryogenic system of calorimeter (6.Floor)

Electrical power in the experimental hall. Of this about 100 kW are on the detector, to be taken away by cooled water, 250 kW on the trailer to be taken away by cooled water or forced air flow, and the rest is in the hall.

120 kW

120 kW

200 kW

500kW

Cooling power for superconducting coils at 4 .5 K at 60 K main magnet 4 70 W 1800 W comp. magnet 10 W

Water cooling for cryogenic of calorimeter (6. Floor): 100 kw

16.3 Space for Control Rooms and auxiliary Rooms in the Hall Building

Control rooms proper 250 m2

Mechanical workshop 60 m2

Electronic laboratory 40 m 2

Offices for physicists, engineers and for discussions 135 m2

Stores 30 m 2

Racks for control of chamber gases 20 m 2

Compressors, liquefier and heat exchangers for calorimeter (6. Floor) 100 m2

272

16.4 Space above Ground

For construction, assemblys and tests the fol-lowing areas will be needed on or near the DESY site:

Construction of calorimeter stacks (15 t crane)

Warm tests and maintenaqce of stacks (10 t crane)

Room for chamber construction, tests and assembly of the complete tracking system (5 t crane), of which 70 m 2 need to be a cleanroom

Tests and storage of iron instrumentation

Model of detector with cables and electronics

Intermediate storage of detector components

For gases and liquids the following areas will be needed above the experimentel hall:

Liquid argon dewar and liquid nitrogen dewar

Chamber gases {bottles> mixing units, recycling)

100 m 2

600 m2

120 m2

260 m2

300 m2

300 m 2

100 m 2

100 m2

16.s· Services for Design, Construction and Installation

It is expected that the design of the iron structure with all interfaces to the other detector components and to the hall (rails and gears ) will be done by DESY (already started). Also the construction of the iron structure should be supervised by DESY in conjunction with HI. We expect that the responsibility for the assembly of the iron structure as well as of the whole detector with respect to its main components will be taken by a senior engineer from DESY (Hallendienst) and that he will be supported by the necessary staff of people (draftsman> cranedrivers, etc.). Furthermore, manpower for surveying has to be provided for each detector component prior to and during installation.

Manpower for design, construction and installation of the required power systems, cooling systems and their service lines to the detector components has to be provided.

16.6 Requirements from Safety

Special equipment required by safety such as a collection and monitoring system for inflammable gases below and above the detector should be provided by DESY. In the vicinity of the calorimeter, oxygen as well as gaseous argon has to be monitored .

16. 7 Offices and Laboratory Space

For those physicists from outside DESY and Universitaet Hamburg participating in the experiment, 60 desks of office space are required, preferably in one building. The laboratory space presently used by the DESY and the external groups of HI will continue to be needed.

273

16.8 Test Beams

The test programmes 'for different components are given in chapters 4, 5 and 6. The requests for beams are summarized in the following table:

DETECTOR e-BEAM e/p-BEAM p-BEAM DESY CERN DESY(20 GEV)

Calorimeter 1986-92 1986-89 1990-95 Tracking 1986-89 1987/88 1989-92 Iron Instrumentation 1986-89 1990-92

Beam and time at CERN has already been allocated to Hl for 1986 for calorimeter tests. Extending to 1987 already leads to a problem with respect to the available beamtime and for 1988 and 1989 the foreseen beamtime is completely open. Therefore negotiations between the CERN and DESY directors are needed in order to ensure that sufficient beamtime will be allocated. Also as a direct consequence of this situation, a proton (or pion) beam at DESY is needed as early as possible , at least from 1990 on, as requested in chapter 5.7.4.

16.9 Data Acquisition and Computing

The following items are requested from DESY:

• A link to the central computer system with a. capacity of at least 0.5 Mbyte/sec

• Data recording in the computer center including an appropiate medium (e.g. optical disk or magnetic tape} capable of writing at least 0.5 Mbyte/sec

• A repair facility for VME crates and standard VME modules

• Support for a processor farm for the experiment. This should clearly be a system agreed between both the HERA experiments and DESY.

• A remote input/output station (terminals, graphics> printer) of the DESY central computer in the North Hall

• 60 hours/day of IBM 3081K equivalent CPU time at the DESY computer center.

16.10 Requirements for Operating the Detector

• A supply of liquid nitrogen and argon

• Maintenance and control of the calorimeter cryosystem must be ensured

• Supply of chamber gases (argon, isobutane,ethane, Xe, C02, He, N2, N2H2)

• Quick access to the main workshop for small repairs

• Access to the electronic pool for spares and repairs

• Maintenance and regular checks of the chamber gas systems

• A car service to the North Hall

• A cafeteria service at the North Hall

274

Appendix A

List of Participants

Ch. Berger, W. Braunschweig, H.Genzel, F .-J. Kirschfink, H.- U. Martyn, F . Raupach , J . Tutas, E. Vogel RWTH Aachen, I. Physikalisches lnstitul1 Aachen, Germany

P. Bosetti , H. Grassier, W. Schmitz, W. Struczinski RWTH Aachen, 111. Physikalisches lnstitul, Aachen, Germany

Winston. Ko, R.L. Lander, K. Maeshima., D.E. Pellett, J.R. Smith, J .T. Volk , W. Wagner , M .C.S. Williams , P.M. Yager University of California (Da~s), Physics Department, Davis, USA

W. Bartel, L. Becker, H.-J . Behrend, F. Brasse, H. Bruck, J. Burger, L. Criegee, F. Eisele, R. Feist, J. Field, W. Flauger, G. Franke, J. Gayler, D. Haidt, G. Knies, V. Korbel, H. Krehbiel, J . Marks, R. Meinke, J. Meyer, B. Naroska, J .E. Olsson, E . Schenuit, V. Schroder , P.Steffen, P . Wa.loschek , G. Winter , W. Zimmermann DESY, Hamburg, Germany

:R. Mankel, K. Rauschnabel, M. Schmelling, D. Wegener Universtat Dortmund, Jnstitut f4r Physik, Dortmund, Germany

E. Barrelet, V. Brisson, D. Lelouch, M. Urban , C. Valle Ecole Polytechnique, L P N H E, Palaiseau, France

P. Bussey, A. Campbell, I. Skillicorn University of Glasgow, Department of Natural Phi'.losophy, Glasgow, UK

G. Andersson-Lindstrom, H.H. Duhm, E. Fretwurst, R. Langkau, V. Riech, W. Scobel Universitat Hamburg, I. Institut fii,r Experim entalphysik, Hamburg, Germany

F .~W. Busser , V.Blobel, G . Heinzelmann, F. Janata, H. Spitzer, P. Stahelin, G. Weber Universitat Hamburg, II. lnstitut fiir Ezperimentalphysik, Hamburg, Germany

H. Blume, E. Hungerford, K. Lau, J. Pyrlik , H. Wald, R. Weinstein University of Houston1 Physics D epartment, HoU1Jton 1 USA

A.B. Clegg, D .C. Darvill , R.C.W. Henderson , D. Newt on University of Lancaster, Physics Department, Lancaster, UK

275

J.B. Dainton, J. Fty, E. Gab,a.thuler, R. Ga.met, P. Mason, J.Morton, G.D. Patel University of Liverpool, Department of Physics, Liverpool, UK

B. Dickinson, A. Donnachie, R.J. Ellison, M. Ibbotson, D. Mercer, H.E. Mills, R.J. Thompson University of Manchester, Department of Physics, Manchester, UK

..... A.S. Belousov. P.A. Cerenkov, A. I. Lebedev, S.V. Rusakov Lebedev Physical Institute, Moscow, USSR

W. de Boer, G. Buschhorn, H. Greif, G. Grindhammer, B. Gunderson, C . Kiesling, D. Luers, H. Oberlack, P. Schacht Max-Planck-Institut, Werner Heisenberg lnstitut fiir Phy6ik, Munchen, Germany

E. von Goeler, J. Moromisato, D. Sha.mbroom, J. Sleeman Northeastern University, Physics Department and College of Computer Science , Boston , USA

A. Courau, B.Delcourt, M.Jaffre, J. Jeanjea.n, V. Journ~, C. Pascaud LAL, Centre d'Orsay, Orsay, France

J. Duboc, R. George, M. Goldberg, 0. Hamon, H.K. Nguyen, M. Rivoal, T.P. Yiou LPNHE, Universites Paris VI et VJJ, Paris, France

F'. Ferrarotto, M. Iacovacci, B. Stella INFN e Universita di Roma 'La Sapienza' Dipart. di Fisica , Roma, Italia

R. Apsimon, D. Clarke, P. Clee, P.Flower , W. J. Haynes, R. Hedgecock , R. Marsha.JI, J. V. Morris Rutherford Appleton Laboratory, Chilton, Didcot, UK

H. Acounis, P. Bareyre, P Bona.my, G. Cozzika, M. David, J. Ernwein, J. Feltesse, L. Gosset, G. Goujon, A. de Lesquen, J.C. Lottin, A. Patoux, J Tichit, P. Verrecchia, G. Villet GEN Saclay, DP H PE, Gif-sur- Yvette, France

H.J. Daum, H. Meyer, U. Pietrzyk Universitiit Wuppertal, Fachbereich Ph.ysik, Wuppertal, Germany

H. Biirwolff, M. Klein , P. Kostka., Th. Na.uma.nn lnstitut fur Hochenergiephysik Akademie der Wissenschaften der DDR Berlin-Zeuthen, DDR

S. Egli, R. Eichler, U. Straumann, P. Truol Physikinstitut der Universitiit und IMP /ETHZ, Zurich, Schweiz

276

l

l ).