Gas based detector systems: MWPC, MSGC,GEM and others R. Bellazzini INFN – Pisa

International Advanced School Leonardo da Vinci - 2002 Summer Course R.Bellazzini INFN-Pisa

Gas based detector systems: MWPC, MSGC,GEM and others

R. Bellazzini INFN – Pisa

International Advanced School Leonardo da Vinci


Proportional counters

First came the Cylinder Proportional Counter: developed over a century ago it consists of a thin wire anode coaxially positioned in a gas-filled cylindrical cathode tube.

Depending on the voltage applied on the wire different mode of operation are possible:• ionization• proportional• limited proportionality• Geiger-Müller

entrance window


The MultiWire Proportional Chamber (MWPC)

Disadvantages of counter tubes:• need of an entrance window for low energy particles• positional accuracy, obtained with an array of CT, of the order of centimetres.

In 1968 G.Charpak invented the MultiWire Proportional Chamber:

an array of many anode wires, 12 mm apart, in a single gas volume enclosed between two metalised cathode planes.Each wire acts as an independent proportional counter with a position resolution of 0.2÷1.0 mm.

2D information on the interaction point is derived from the barycentres of the induced charge distribution on the cathode planes.Position information can be further improved by taking into account the

drift time of the electrons towards anode wires (based on this principle are the Drift Chamber and the Time Projection Chamber)


The MWPC

Electrons released in the gas volume by a ionizing particle or a X-ray drift towards the anode wires where the avalanche multiplication starts.

Electric field lines in a MWPC. The effect of the slight shifting of a wire is clearly visible. It has no effect on the field around the wire.

Avalanche electrons are collected very fast on the wire. The development of the anode signal is due essentially to the slow motion of ions towards cathodes.

The avalanche is localized in a restricted volume close to the wire. At high fluxes this can result in space charge effect: the built-up of slow positive ions modify the Electric field by making the effective wire diameter thicker gain decreases

electrons

Increasing electric field

ions

E


From MWPC to MicroStrip Gas Chamber (MSGC)

With the introduction of the MSGC several drawbacks of the MWPC are overcome.

• in MWPC the position of the electrodes can be distorted by electrostatic and gravitational forces. • in MSGC the electrodes position is well defined due to fixation on the substrate.

• an entire sector of an MWPC is disabled when a wire breaks and short-cuts other electrodes. • an interrupted strip in a MSGC has, as worst consequence, a locally reduced position resolution.

• a strong frame is needed in MWPC to maintain the tension force of the wires.

• the possible distortion of the wire position limits the wire spacing to a minimum distance of 1 mm,this limits position resolution and time shaping. Moreover, as the ions are not removed quickly enough, the occurrence of space charge hampers full avalanche development at count rates above 102÷103 counts/sec•mm2.• in a MSGC, the combination of very small inter-electrode distances and high electric field, allows to attain fast charge collection ( 50 ns) and signal shaping (~ 20 ns). The fine detector segmentation allows position resolution of 40 m.


The photo-lithographic process

The lift-off technique


The MSGC

Cross-section of a MSGC

Electric field lines strongly concentrates on anode strips

Simulated current signals

Signals 10 times faster with respect to MWPC signals


Signal development on a multi-electrode system

As for MWPC, the main signal contribution stems from the current induced by the ions motion.

Green’s theorem states for a multi-electrode system the relation between potentials and charges for two state, before and after one or more potentials and charges are changed, in the form:

where are the initial values and the values after the change .

ii

iii

i VQVQ

ii VQ ,

ii VQ ,

due to the motion of along a line with velocity

mq dldt

dlv

1V 1Q mV

Assume is a charge on an infinitesimal electrode and the sensing electrode is 1, then where is caused by and by . The induced current:

mq11VQVq mm

mq

dt

dQi 11

vEq

dt

dl

dl

VVdqdt

V

Vqdi Wm

mm

mm

1

11

Where is the weighting field (determined by applying a unity potential to electrode 1 and zero to all the others). The total induced charge is:

dl

VVdE mW

1

2111

2

1

mVmVqdlEqdtiQ WWm

m

m Wm


MSGC performances:spatial resolution

Ne(25)-DME(75)

Vcath= -530 V

Vdrift= -3000 V

Spatial resolution = 30.5 ± 0.4 m

Residuals distribution


Rate capability

1.1

1.0

0.9

0.8

0.7

0.6

Re

lati

ve

ga

in

6 7 8 90.1

2 3 4 5 6 7 8 91

2 3 4 5

Rate (MHz/mm2)

Vd= -3000V, V

c= -460V

Pestov glass coating

bulk=1010cm

bulk=1011cm

MSGC

INFN - Pisa

Source: 5.4 KeV Cr X-raysNe-DME (50/50)

Vd= -1000V, V

c= -564V

D263 uncoated

surface=510

17/square

Rate capability > 1 MHz/mm2 R. Bellazzini et al., Nucl. Instr. Meth.

From MWPC To MSGC

Rate capability increases 3 orders of

magnitude


Energy resolution

55Fe spectrum

0

1000

2000

3000

4000

5000

1 2 3 4 5 6 7 8

KeV

cou

nts data

fit

Ar escape peak

K5.90KeV

K6.29KeV

2=0.94E/E(FWHM)=10.7%@5.9KeV

Ar-Ethane(50/50)

pitch 125m

anode width 3m


The discharge problem

Characteristics of a MSGC:

• very good spatial resolution• high rate capability• very good energy resolution• very good two-track separation

but, sometimes:

possible discharges at high gain on exposure to heavily ionizing particles.

When the total charge in the avalanche exeeds a value between 107÷108 electron-ions pairs (Raether's limit), an enhancement of the electric field in front and behind the primary avalanche induces the fast growth of a long, filament-like streamer. In the high fields and narrow gaps typical of micro-pattern devices, this leads to discharge, with damaging effects on the strips.

Solution! Cathode edge passivation

Advanced passivation

Standard passivation


High intensity beam test for spark rate studies

Telescope of 32 MSGCs tested at PSI in Nov99

16384 electronic channels


Spark rate measurement

350-400 MeV/c +/p beam.

MIP spectrum with non-negligible HIP rate.32 days exposure @LHC rate! (Max rate ~6 KHz/mm2 )

Spark rate 1/day/detectorin the whole telescope

4 channels (out of 16000!) lost in ~400 hrs run

Measured spark probability 4 10-13


2D reconstruction capability

With the PSI MSGC telescopeexploited

2D-Reconstruction capabilitywith Pion beam

R. Bellazzini et al., Nucl. Instr. Meth. A457 (2001) 22


The MicroGap Chamber (MGC)

The space charge effect, in the MSGC, is strictly connected to the presence of an insulating substrate. When a significant fraction of ions of the avalanche hits this surface, it causes a charge-up which reduces locally the electric field.In the MGC the insulating surface exposed to the gas is strongly reduced.

Scanning electron microscope picture

Electric field lines close to the anodes are even more intense than in a MSGC resulting in a very fast signal development.



Signal evolution from MWPC to MGC

Electric field SignalsMWPC

MSGC

MGC


MGC performances

No evidence of charging-up effect with time is observed

short term measurement of gain stability

The MGC can withstand high rate of radiation (up to few MHz/mm2 ) without visible change in gain.


The Gas Electron Multiplier (GEM)

A new class of position sensitive MicroPattern Gas Detectors, robust and cheap, has been developed using the advanced printed circuit (PCB ) technology: GEM, Micro Groove and Well

Detectors

The GEM is a thin polymer foil (Kapton), metal coated on both sides, chemically pierced by a high density of holes. On application of a voltage gradient, electrons released on the top side drift into the hole, multiply in avalanche and transfer to the other side. Proportional gains above 103 are obtained in most common gases.

F. Sauli, Nucl. Instr. Meth. A386 (1997) 531


GEM manufacturing

Typical geometry:5 m Cu on 50 m Kapton70 m hole at 140 mm pitch

Cu-plated Kapton

Copper etching

Kapton etching

Edge finish


The GEM

R.Bouclier et al., Nucl. Instr. Meth. A396 (1997) 50

Charge amplification and read-out take place on separate electrodes.

The read-out PC board can be structered in a multi-pixel pattern to get full 2D imaging capability.


The GEM

Full width 20 ns(2 mm gap)

Induced charge profile on strips600 m FWHM

The duration of the signal correspond to the drift time of the electrons in the transfer gap.


Multiple GEM structures

Exposed to heavily ionizing particles all the MicroPattern Detectors can discharge at low gains.

Cascades of GEM can provide equal gain at lower voltage so the discharge probability is reduced.

Triple GEM

A. Bressan, Nucl. Instr. Meth. A424 (1999) 361

S. Bachmann, CERN-EP/2001-151


The MicroGroove Detector (MGD)

A new type of 2D position sensitive gas proportional counter fabricated using the advanced PCB technology

Advantages:2D, robustness, big size, low costDisadvantages:Lower field gradient, reduced speed



MGD electric field and signals

Equipotential and drift lines

cathodes

anodes

4 anodes OR signal (triggering)

4 cathodes OR signal (reversed polarity)


Ballistic deficit

If the charge collection time is longer than the shaping time only a fraction (ballistic deficit) of the full avalanche charge is observed in the signal.

MGD has the lowest ballistic deficit if compared to MSGC and MGC:

MGC – b.d.= 90%MSGC – b.d. = 67%MGD – b.d. = 50%

Lower b.c. implies higher gain when a fast electronic is used


MGD performances: gain and energy resolution

Full energy peak (5.4 keV) and Argon escape peak are clearly resolved . Energy resolution ~ 20% .

Gain up to 104 and more can be easily reached with a good uniformity over the whole detector area .


Charging and rate capability

Recovery after Kapton charging is very fast.

Rate capability: no drop in gain observed up to 1.2 kHz/mm2.

Short-term gain stability for high intensity X-ray source. Only a small charging effects is observed at very high gain > 104.


The Well Detector

Based on PCB technology, it consists of a thin Kapton foil, copper-clad on both sides. Charge amplifying micro-wells are etched into the first metal and Kapton layers. These end on a micro-strip pattern which is defined onto the second metal plane and is used for read-out. The pre-pregging technique is used to bond the Kapton foil to a 300 m thick vetronite support.


extremely resistant to mechanical shocks


The Well detector

Microscope photographs of top and bottom of the wells.

The focusing effect of the drift field reaches the maximum at ~ 4 kV/cm where full collection efficiency is observed.

Electric field map of one cell.


Jem-X: a MSGC astrophysical application

The JEM-X Project on INTEGRAL ( a -ray observatory satellite)Danish Space Research Institute

To be launched in October 2002 by ESA

The INTEGRAL spacecraft


The Joint European X-ray Monitor (JEM-X)

JEM-X will make observations simultaneously with the main gamma-ray instruments and provide images with arcminute angular resolution in the 3÷35 keV energy band.

The baseline photon detection system consists of two identical high pressure imaging MSGCs. Each detector unit views the sky through its coded aperture mask located at a distance of 3.2 m above the detection plane. Due to the broad spectral coverage and the ability to detect and resolve cyclotron lines JEM-X will study sources categories such as:

• Active Galactic Nuclei • Accreting X-Ray Pulsars • X-Ray Trasients • Black Hole Candidates

The X-Ray Monitor JEM-X supplements the main Integral instruments (Spectrometer and Imager) and plays a crucial role in the detection and identification of the gamma-ray sources and in the analysis and scientific interpretation of Integral gamma-ray data.


The JEM-X Detector consists of 2 identical, high pressure, 2D-MSGCs.The gas filling is a mixture Xenon-Methane (Gain ~ 1500).

The diameter is 250 mm, corresponding to a collecting area of 500cm2.

The Jem-X Detector

Qualification model2D-MSGC plate


Spectral sensitivity of the JEM-X qualification model

Courtesy C. Budtz-Jorgensen (DSRI)

Jem-X : an X-ray monitor

Response of JEM-X to a full area illumination


GEM + Pixel Read-Out: recent applications

The complete separation between the amplification structure and the pick-up electrodes allows full flexibility on the choice of read-out pattern GEM+Micro Pixel electrode

Recent applications: X-ray PolarimeterTime resolved plasma diagnostic


X-ray Polarimetry with a MicroPattern Gas Detector

ro2

5Z4137mc2

h

72

4 22sin 2cos

1 cos 4

Heitler W.,The Quantum Theory of Radiation

Polarimetry can provide a general tool to explore the structure of compact sources and derive information on mass and angular momentum of supermassive objects.

A new polarimeter, based on the photoelectric effect, using a MicroPattern Gas Detector coupled to a GEM has been developed. The device is highly efficient in the energy range 210 keV, particularly interesting for X-ray Astronomy.

The photoelectric effect is a process very sensitive to photon polarization and with a large cross section at low energy. In the case of linearly polarized photons, the differential photoelectron cross section has a maximum in the plane orthogonal to the direction of the incoming photon. The photoelectron is ejected with maximum probability in the direction of the photon electric field with a cos2 modulation.


GEM+MicroPixel Read-out

8-layer read-out boardPixel size 0.2 mm

Area 2.4 x 2.4 mm2 (128 pixels)

Angle and amount of polarization is computed from the angular distribution of the photoelectron tracks, reconstructed by a finely segmented gas detector. Being fully 2D there no need to rotate the detector as in traditional polarimeters.

The GEM and the drift plane are glued with two fiberglass spacers, respectively of 1.5 mm (transfer gap) and 6 mm (absorption gap) over the read-out plane


Photoelectron track reconstruction

(larger boxes == larger energy losses)

Photoelectron tracks reconstruction(two-step algorithm)

First step - the direction of emission of the photoelectron is reconstructed by finding the major amd minor principal axes (M2

max, M2min) of the charge

distribution on the pixels. The major principal axis is identified as the photoemission direction.

Second step - the third momentum (M3) of the asymmetric charge distribution is computed. It lies along the major axis on the side, with respect to the barycentre, where the charge release is smaller (i.e. at the beginning of the track) The absorption point is obtained going back from the barycentre, along the major axis on the direction of M3, of a distance L M2

max


Reconstructed emission angles of the photoelectron

E. Costa, R.Bellazzini et al., Nature vol. 411 (2001) 662

Unpolarized photons

~100 % Polarized Photons

The modulation factor for 100% polarized radiation is b/(2a+b).

Distribution of the track barycentres with respect to the absorption point

Angular distribution of p.e. tracks


Position resolution

Distribution of barycentres and absorption points of a collimated unpolarized radiation (5.4 keV)

The collimator diameter is 50 m (smaller than the pixel size) and the absorption points are concentrated in a very small spot of 70 m (rms). Barycentres are instead distributed at some distance from the absorption points because of the large energy release at the end of the track.


Observing time to measure at 99% confidence level the degree of polarization of galactic and extra-galactic sources with traditional and MP polarimeters

Degree of polarizationExtra-galactic sources

Comparison with traditional polarimeters


XEUS-1 : a possible application

The tested prototype at the focus of XEUS-1 (the X-ray Evolving Universe Spectroscopy mission) could perform polarimetry at % level on many bright AGN in about 1 day observation, in the energy range 2÷10 keV.

XEUS consist of a Detector spacecraft with the focal plane instrumentation that receives cosmic X-rays focused by a Mirror spacecraft flying at 50 m in front of it.


Plasma imaging with Micro Pattern Gas Detectors

Ultra-fast system for X ray imaging based on GEMNew diagnostic device in soft X range (315 KeV) for magnetic fusion plasmas

Printed circuit board128 pixels (2.5 x 2.5 cm2)

Parallel read-out

Successfully tested on the Frascati Tokamak Upgrade (FTU, Italy) and on the National Spherical Tokamak eXperiment (NSTX) at Princetown (US).


The inner toroid

The Frascati TOKAMAK Upgrade (FTU)

Bt = 8 T, Ip = 1.6 MA R = 0.93 m a = 0.3 m

The experimental set-up at the Enea Laboratories in Frascati


High rate performaces

Linearity of GEM current at very high counting rates.

Counting rate linear up to 2 MHz/pixel (limited by electronics

dead-time)

Imaging at high rates (2MHz/pixel)

Image of a wrench placed close to the detector

exposure time = 50 ms


Setup at the National Spherical Tokamak eXperiment(Princetown USA)


The plasma center is affected by strong oscillations in soft X-ray emission. This effect disappears at r ~ 20 cm

USXR_V TOP USXR_H UP

GEM

High rate imaging of the plasma center


# 107316

Plasma center activity

10 khz


An All-sky X-ray Monitor (AXM) Mission

31 GEM proportional counters provide a continuous monitoring of the entire X-ray sky, except for earth occultation and a small region around the sun. This coverage allow the study of a wide variety of short duration outbursts.

The AXM spacecraft


The AXM camera assembly

One of the 31 GEM cameras


Conclusions I : Gas Detectors

• The field of Micro Pattern Gas Detector is STILL showing great vitality and propulsion.• Competition of solid state devices is becoming HARD• Great potential as X-ray detectors.• Coupling of distributed amplification structures (GEM,…) to separate, pixel read-out structures seems the most exciting perspective.


Conclusions II : X-ray Astronomy

Detectors with two-dimensional imaging capability are the workhorse for X-ray astronomy. In the enrgy range of 1÷10 keV there are two primary types of position-sensitive detectors flown on space-born missions: gas detectors and solid state charge coupled devices (CCDs).

There are tradeoffs between these two types of detectors in the current state of the art of X-ray imaging in the main X-ray band. Where the detector area, time resolution and optimal sampling of the track lenght are a high priority, then the choice of a gas detector would have strong advantages.

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Gas based detector systems: MWPC, MSGC,GEM and others R. Bellazzini INFN – Pisa

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