The Pad HPD as Photodetector of the LHCb RICH Detectors

Pad HPD Proposal 3 Nov 1999 C. Joram

The Pad HPD

as Photodetectorof the

LHCb RICH Detectors

General Design L0 Electronics R&D: Results + Future Tube Fabrication + costs

C. Joram

for the

Pad HPD team


Granularity ca. 2.5 mm

High filling factor Single photon sensitivity

Robust operationCost effectiveness

LHCb RICH Requirements

Large area High sensitivity for E<5.5 eV

LHC speed readout



The Design of the Pad HPD

Total (active) diameter: 127 (114) mm 69% filling factor when hex. closed pack + 3mm gap space for mu-metal + insulation

Bialkali photocathode + UV extended borosilicate window good compromise between E and

chrom.

Fountain shaped electron optics purely linear adjustable demagnification strong E-field over full tube robustness against magnetic fields

Work at maximum high voltage (25 kV) large signal (ca. 6000-6500 e-) low point spread function (pixel error) robustness against magnetic fields

Round Si sensor with pads of 1 x 1 mm2

no dead space, no unused pads pixel error matches other error sources

E

n


Analog readout electronics full exploitation of information continuous online pedestal determination + subtraction can cope with common mode + other noise sources

Tube design allows to re-use all components

Tube can be manufactured by LHCb RICH group or by industry


Summary of the Pad HPD characteristics

General:

total diameter 127 mm (5")active diameter 114 mmgranularity at entrance window 2.3 x 2.3 mm2

entrance window type UV-extended borosilicate glassentrance window transmission T=50% at ca. 250 nm, T=0 at ca. 200 nmphotocathode bialkali K2CsSb, semitransparentquantum efficiency > 20% at 390 nm

Electrostatics:

electrodes 4 concentric ring electrodesfocusing fountain typedemagnification 2.3, constant over full active diameter

Silicon sensor:

Si sensor diameter 50 mm active diameterSi sensor thickness 300 m (160 m will be tested, which will result in a

faster charge collection)segmentation 2048 pads of 1 x 1 mm2, no dead spaceE-loss in dead layer (n+ + Al) ca. 1 keV

Readout electronics:

Level-0 readout Analog readout scheme. 16 SCTA128 chips, inside tubeSCTA128 features comply with LHCb specifications (25 ns peaking time, 4s L0 latency, 32 fold multiplexing)Operation:

max. cathode voltage -25 kVsignal 6600 e- (expected)pedestal noise 650 e- (expected)S/N ratio 10 (expected)single el. det. efficiency 92.8%, 4 sigma cut (expected)

Mounting:

Tube arrangement Hexagonal close packingTotal number of tubes 216gap between tubes 3 mm (allowing for -metal shielding)active area fraction 69%


L0 Electronics of the Pad HPD

SCTA128 modified according to LHCb specifications. SCTA128_LHCb

Analog readout chip in rad hard DMILL technology. Chip is fully identical with proposed vtx chip.

preamp shaperanalog pipeline

4 x

32 fo

ld a

nal

og

mul

tiple

xin

g

silicon diode

128 channels

derand.buffer

diff

ere

ntia

l an

alog

ou

t

SCTA128 exists and has demonstrated peaking time 25 ns analog multiplexing at 40 MHz expected noise figures: 398 + 55 pF-1

For the Pad HPD (ca. 4 pF) we expect a pedestal noise of 650e- ENC S/N = 10 det. = 92.8% (4 cut)

Necessary LHCb modifications will be discussed under future R&D


R&D programme of the Pad HPD

1996 1997 1998 1999 2000

Concept, design, fabrication of HPD components

Test of electron optics, Si sensor, VA electronics (CsI PC)

Design, fabrication, commissioning of HPD plant

Development of tube processing (PC, sealing, getter)

Further optimization of processing. Final electronics.

Main Results of the R&D

Difficulties and problems

Future R&D


Main Results of the R&D

0

3

6

9

12

15

18

21

24

27

Q.E

. (%

)

Photocathode 59

Center of cathode

Non-uniformity <10%

300 350 400 450 500 550 600 650

wavelength (nm)

4.00

3.69

3.39

3.39

3.09

2.83

2.50

2.24

2.09

1.99

energy (eV)

47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

cathode no.

0

2

4

6

8

10

12

14

16

18

20

22

24

Q.E

. (%

) a

t 4

00

nm

Q.E. of a good HPD

Summary ofall cathodesproduced sinceMarch 1999

Quantum efficiency of bialkali cathodes


Comparison of lab. measured Q.E. and test beam data

PC59 tested with 180 cm long C4F10 radiator

• Expect 48 p.e.• Find 54 p.e. (noise subtracted)

• Agreement withinuncertainties

Online displayof single eventring

-30

-20

-10

-0

10

20

30

x silic

on (

mm

)

m = 2.7

114 mm

0.00 31.75 63.50 95.25 127.00

xcathode (mm)

Electron Optics of the Pad HPD

• 114 mm active

• Linear demagnification,• adjustable

• Fit residuals 200 m


Electron optics with mu-metal shield

-25

-20

-15

-10

-5

0

x silic

on (

mm

)

m = 2.2

57 mm

60.0 77.5 95.0 112.5 130.0

xcathode (mm)

160

mm

63.

5 m

m

110

mm

128 mm, 0.9 mm thick

Linear opticsnot affected by grounded shield.


Point spread function

measured on Si plane

14 15 16 17 18 19 20 21 22 23 24 25Cathode Voltage (kV)

200

250

300

350

400

450

500

Po

int S

pre

ad F

unct

ion

(m

)

Pixel Error


Measurement in B-field (Helmholtz coil)

11 12 13 14 15 16 17 18 19 20 21

UC (kV)

15

20

25

30

/B (

mra

d/G

au

ss)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

R

/R/B

(%

/Ga

uss

)

Axial B-field, no shield

0 5 10 15 20 25

Baxial (Gauss)

0

10

20

30

40

50

60

(m

rad

) a

t U

C =

16

kV

Axial B-field, 160 mm long mu-metal shield

/B = 2.2 mrad / Gauss

R/R negligible

pad granularity: 1 mm = 40 mrad

Axial B-field, no shield

Axial B-field, with shield


-1 1 3 5 7 9 11 13 15 17 19

By (Gauss)

-1

0

1

2

3

4

x-sh

ift (

mm

) a

t U

C =

14

kV

Transverse B-field (in y direction)

with and without 160 mm long mu-metal shield

x = 32 m/Gauss

x = 0.39 mm/Gauss

• A 160 mm long mu-metal shield is very effective forBE : reduction of by factor 12, R/R0BE: reduction of x by factor 12

• For reasonably low fields (B<10 Gauss) the shift and the rotation becomes negligible

• A short shield (110 mm) has been tested but found to be less efficient (only factor 3 reduction)

• If a long shield has to be used, a pointing geometry is possible (12% fewer tubes required)

measured on Si-plane

x

y

B

Transverse B-field,

For reasonably low fields (B<10 Gauss) the shift and the rotation become negligible

pointing geometry

16 6


0 2 4 6 8 10 12 14 16 18 20 22

voltage (kV)

0

10

20

30

40

50

60

70

80

90

puls

e he

ight

of

1st p

hoto

elec

tron

(A

DC

bin

s)

0.53 kV

num

ber

of e

/h p

airs

1000

2000

3000

4000

5000

6000

Pedestal cut (4)

Single pad

S/N = 20

1 p.e.

2 p.e.

3 p.e.

Viking VA3 chip (peak = 1.3 s, 300 e- ENC)UC = -26 kV

ADC counts

Signals on the Si sensor

thanks to a thinned n+ and Al layer


Exposure to direct charged particles

•Data at 0º and 25º incidence angle

•Photons from C-effect in window well localized 20-40 multi photon hits.

•Charged particles in Si sensor easily identifiable

•Results in agreement with Malcolm’s simulations.

•Analog information useful for event cleaning.

C-photonsfrom windowx = x 1.2 mm

Signals fromcharged particlesx 1.3 mmY 2.2 mm


Difficulties and Problems

Phototube fabrication is not extremely difficult, but...

we started essentially from zero many parameters have to be precisely tuned difficult diagnostics of ambiguous symptoms only few trials

Two problems caused significant delays

Some envelopes develop leaks during bakeout

Problem understood and fixedby SVT. A good fraction of our

envelopes are affected.However: All tubes can be repaired!

Some tubes with bialkali cathodedo not stand full cathode voltage

Problem understood but not fully fixed.K and Cs atoms adhere to impurities on glass surface. Surface conductivity Field distortions

atomic excitation under HV light emission

Improved (chemical) cleaning helps !Walls can be protected by mask. To be tested.


Future R&D

Further optimization and stabilization of theof tube fabrication process

Build tubes with final LHCb electronics

Modification of SCTA analog frontend

• FE identical to existing ABCD chip

• expect 650 e- noise in Pad HPD

• submission still in 1999

Modifications of SCTA backend

• 4 s pipeline• 432 multiplexing• reduced set-up time

• submission by 2/2000


ID Task Name

1 Complete tests w ith VA electronic

2 Bakeout test of exist. SCTA128

3 Vacuum test of exist. SCTA128

4 Process SCTA128 w ith mod. FE

5 Process SCTA128 w ith mod. BE

6 Fabricate sealed HPD w ith exist SCTA

7 Fabricate sealed HPD w ith f inal SCTA

28/2

30/6

Oct Nov Dec Jan Feb Mar Apr May Jun Jul AugQtr 4, 1999 Qtr 1, 2000 Qtr 2, 2000 Qtr 3, 2000

In the meantime…

demonstrate vacuum operation of existing SCTA128HC

demonstrate baking of SCTA128HC

Finally…

produce sealed HPD with existing SCTA128HC produce sealed HPD with final SCTA128_LHCb

R&D finished by mid 2000 !


Tube Fabrication

We considered 3 possible fabrication scenarios

A) In-house production at CERN (baseline of proposal)

B) Distributed in-house production (CERN + other institutes inside LHCb)

c) Industrial fabrication: 1 offer available (Thomson, France), a 2nd offer in preparation (Photek, UK)


ID Task Name

1 SCTA128 w ith mod. FE/BE available

2 Reserve for possible 2nd SCT iteration

3 Purchasing of HPD components

4 Design of fabrication plants

5 Fabrication/commissioning of plants

6 Photodetector fabrication/tests

7 All Photodectors available/tested

1/5

1/

H1 H2 H1 H2 H1 H2 H1 H2 H1 H22000 2001 2002 2003 2004

Our baseline scenario

240 Pad HPDs are required to be ready and testedby mid 2004.

2 new optimized fabrication plants have to be built (re-using existing special components)

4 Pad HPDs are produced per week

240 tubes can be produced in 22 months assuming average yield of 80% 40 production weeks/yr

HPD fabrication should start by beginning of 2002

There is sufficient time to

• design, build and commission plants• re-iterate on SCTA128 if required

All technical labour is included in cost estimate !


Cost estimate

(Details will be given by Dave Websdale.)

Total cost of 216 tubes (incl. encapsulated electronics)

A) In-house: 2.0 MCHF (ca. 30% labour)

B) Distr. in-house: unknown, could be less than A)

C) Industry: 3.2 MCHF


Conclusion

The Pad HPD is a photodetector which has been coherently designed to fulfill the LHCb RICH requirements in an optimum way.

The achieved performance and the expected results of the remaining R&D phase make it an excellent candidate.

The (distributed) in-house fabrication represents a cost-effective scenario, which allows to produce all tubes well in time and still provides sufficient reserves.

Many thanks to

all members of the Pad HPD team,

our industrial partners,

and all people who generously supported us.

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The Pad HPD as Photodetector of the LHCb RICH Detectors

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