Nuclear Instruments and Methods in Physics Research A 494 (2002) 128–141

Micro-pattern gaseous detectors

L. Shekhtman*

Budker Institute of Nuclear Physics, Acad. Lavrentiev prospect 11, 630090 Novosibirsk, Russia

Abstract

Introduced at the end of 1980s micro-pattern gas detectors perform much better than classic wire chambers. They

allow to achieve both excellent localization accuracy and high rate capability that make this technology attractive for

charged particle tracking at high luminosity colliders. During its evolution micro-pattern gas technology gave raise to

many different types of devices such as micro-strip gas chambers, MicroMEGAS, CAT and gas electron multipliers.

Essential improvements in the performance of the detectors were achieved especially in what concerned long-term

performance: aging and resistance to accidental discharges.

r 2002 Elsevier Science B.V. All rights reserved.

Keywords: Micro-pattern gas detectors; MSGC; MicroMEGAS; GEM; Tracking

1. Introduction

At the end of 1980s before starting of high-luminosity hadron collider projects it was com-monly understood that gas wire technique cannotrealize full potential of gaseous detectors, namelyin simultaneous high spatial resolution and ratecapability. Intrinsic properties of commonly usedgas mixtures allowed spatial resolution of below50 m: However the large size of amplification cellwith sense wire in the center could not provide ratecapability of more than 10 kHz=mm2: The solu-tion was to put a micro-structure produced withmicro-lithography technique into the gas. Smallamplification cell in such a case could provide bothhigh spatial resolution and rate capability at thesame time. The idea in the form of micro-strip gaschambers was first proposed by Oed in 1988 [1] forneutron detection and then was modified for the

needs of charge particle detection by groups fromINFN Pisa [2] and NIKHEF [3].Later the concept in general got the name of

micro-pattern gaseous detector and many differenttypes of those were proposed during the last 14years.Despite the many types of micro-structures

that are proposed for the micro-pattern devices,major properties of the latter are determined bythe gas mixture and the gap width used for thedetection of charged particles (Fig. 1). From Fig. 1we see that both spatial and time resolution ofthese devices depend mostly on the statistics ofprimary charge clusters deposited by a relativisticcharged particle. Micro-pattern structure is usedfor amplification of the primary electronsand readout of the induced amplified signal andcan thus also affect partly space and timeresolution. Small size of amplification cell of amicro-pattern structure provides fast removal ofpositive ions and low space charge effects at highrates.

*Tel.: +7-3832-394992; fax: +7-3832-342163.

E-mail address: l.i.shekhtman@inp.nsk.su (L. Shekhtman).

0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 1 4 5 6 - 0

In this review we consider different types of themicro-pattern gaseous detectors according toproduction technology and layout of the micro-structure. Main performance parameters will beanalyzed and solutions for most common pro-blems will be discussed.

2. Micro-strip gas chambers

The micro-strip gas chambers (MSGC) consistof thin parallel metal strips, deposited on aninsulating support and alternatively connected asanodes and cathodes. Fig. 2 shows a schematicview of this device with electric field lines andequipotentials computed with anodes and back-plane at equal potentials.Accurate but simple photolithography can

achieve a distance between electrodes of 100 mm;i.e. improving granularity by another order ofmagnitude over that of wire chambers. Fig. 2shows that at appropriate choice of potentials allfield lines from the drift region terminate on thinanode strip. However, avalanche is spread broaderthan anode width (usually about 10 mm) and largefraction of positive ions is collected to theneighboring cathode strips. This effect reducesspace charge accumulation and provides muchhigher intrinsic rate capability than classic devicescan.High gains for a wide range of gas mixtures,

electrodes geometry and types of substrate have

been obtained [4–6]. Fig. 3 shows one example ofabsolute gain measured as a function of anodevoltage in several mixtures of noble gases anddimethyl ether [7].Several improvements of the micro-strip tech-

nology have been proposed mainly towards two-dimensional readout and more compact avalancheregion. This can be achieved by putting the anodestrip on top of thin (several microns thick)insulating layer deposited on top of properly

Fig. 1. General layout of a micro-pattern gaseous detector.

Charged particle track leaves primary charge clusters in the

conversion gap. Electrons drift towards amplification micro-

structure that can consist of several stages. Amplified electron

component of the primary ionization induces charge at the

readout micro-structure.

Fig. 2. Schematic view of the MSGC with equipotentials and

field lines computed close to the substrate. The back-plane

potential has been selected to prevent field lines entering the

dielectric.

Fig. 3. Examples of absolute gain measured, as a function of

anode voltage, in several mixtures of noble gases and dimethyl

ether.

L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128–141 129

segmented cathode metal layer. Such a structurewas called microgap chamber (MGC) [8,9]. Largegains, above 104 have been demonstrated with aMGC. Mixtures of neon and dimethyl ether seemto be particularly advantageous.Another approach utilizing more complicated

photolithography process, namely the microdotchamber [10–12], consists of a dense pattern ofindividual proportional counters made up ofanode dots surrounded by annular cathodes. Field

defining rings can be added to improve theoperation, as shown in Fig. 4.

3. Detectors with parallel-plate amplification region

The evolution of wire chambers and, in parti-cular, asymmetric multiwire proportional deviceshas led to the invention of another micro-patternconcept. It has been recently suggested that in sub-millimeter gaps with strong uniform electric fieldexceptionally large gains could be attained. This hasled to the introduction of the micromesh gaseouschamber (microMEGAS) [13], shown in Fig. 5.The detector consists of a thin metal grid

stretched at a very small distance, 50–100 mm;above a readout electrode. With very high fieldapplied across the gap, typically above 30 kV=cm;electrons released in the upper drift region arecollected and multiplied. The microMEGASexploits the saturating characteristics of the Town-send coefficient at very high field to reduce thedependence of gain on the gap variations, thusimproving the uniformity and stability of responseover a large area. Thanks to the small gap andFig. 4. Schematics of the microdot chamber.

Fig. 5. Schematics and electric field in the microMEGAS. A metallic micromesh separates a low-field region from the high-field

multiplication region.

L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128–141130

high field, positive ions move very quickly andmost are collected to the cathode mesh. Thisinduces very fast signals with very small ion tail,50–100 ns wide, and prevents space-charge accu-mulation in the drift region. Very high gains havebeen demonstrated with the microMEGAS(Fig. 6) showing even the possibility of efficientsingle electron detection [14].

4. PCB detectors and gas electron multiplier

The gain of a parallel-plate counter dependsexponentially on the gap thickness, making itdifficult to obtain a uniform response over largeareas. In the compteur a trous (CAT) holes drilledthrough a metal–insulator sandwich concentratethe field lines converging from a drift volume intoa region of high field, where charge multiplicationoccurs (Fig. 7) [15].This idea combines the concept of parallel-plate

chamber with intrinsically uniform spacer allowinguniform gains over large surfaces. Even withrelatively large holes, the collection and focusingproperties of the field result in good energyresolution at proportional gains up to 104:Combining the idea of multistep avalanche

chamber [16] and CAT, Sauli proposed a newmicro-pattern structure called gas electron multi-plier (GEM) [17]. It consists of a thin, metal-cladpolymer foil chemically perforated by a highdensity of holes, typically 100=mm2 (Fig. 8).As shown in Fig. 8, with a suitable choice of

voltages, all electrons released by ionization in the

overlying gas layer are sucked into the holes,where charge multiplication occurs in the highelectric field. the gain is a property of the GEMstructure and is only mildly affected by theexternal fields, considerably relaxing the mechan-ical requirements. Systematic research efforts haveenabled GEM devices to achieve proportionalgains up to 104; suitable for direct detection ofionization on simple charge-collecting printedcircuit board (PCB) electrodes.GEM foil can work as a distributed preamplifier

allowing cascaded devices with several GEMs orwith GEM combined with active read-out struc-ture such as MSGC. The cascaded device permitsmuch higher gains, or, for given required gain,

Fig. 7. Schematics and field map in the CAT cell.

Fig. 8. Schematic structure of the gas electron multiplier with

electric field lines and equipotentials shown.

Fig. 6. Gain as a function of the mesh voltage in the

microMEGAS with 50 mm amplification gap.

L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128–141 131

allows operation of amplifying elements at muchlower voltages. In Fig. 9 the schematics of adouble GEM detector is shown with typicaldimensions indicated in the caption [18]. Examplesof gain dependence as a function of GEM voltageand for several argon-based gas mixtures fortriple-GEM device are shown in Fig. 10 [19]. Inthis case for all three GEMs voltages were kept thesame.CAT and GEM structures started another

family of devices that can be called PCB detectors.

These devices can be manufactured using simpleand cheap lithography process applied for printedcircuit boards. Two examples of such devices aremicro-groove [20,21] and WELL [22] detectorsshown in Figs. 11 and 12.As one can see from the figures both micro-

groove and WELL detectors realize the CATprinciple in micro-scale.

5. Main properties of the micro-pattern

gaseous detectors

Main properties of the micro-pattern gasdetectors are determined by the gas mixture andthickness of the conversion region. Among otherswe will discuss parameters of high importance forthe detection of relativistic charged particles, suchas efficiency, spatial resolution, time resolutionand rate capability.

5.1. Signal and efficiency

High-energy charged particle penetratingthrough a thin gas layer exhibit a limited numberof interactions with the gas molecules formingprimary charge clusters.Fig. 9. Schematic structure of the double-GEM detector.

Fig. 10. Gain as a function of GEM voltage for triple-GEM detector. Voltages across all 3 GEMs have been kept the same.

L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128–141132

If the gas layer is thin, the probability of zerocharge deposition becomes non-negligible. Thus,in order to get efficiency close to 100% thedetector has to provide high enough amplificationto detect as small number of primary clusters aspossible, and the gas layer has to be thick enough.Typical example of the deposited charge distribu-tion together with efficiency dependence on theoperational voltage is shown for microMEGAS inFig. 13 [23].The measurement of efficiency with very thin

gas layers was reported by several groups (see forexample, Ref. [24]). It was found that even for themost dense gases such as dimetyl ether or

isobutane, efficiency cannot be higher than 95%for the gas layer thickness of less than 2 mm: Anexample of such measurements is presented inFig. 14 [24].

5.2. Spatial resolution

Delta-electrons formed along a charged parti-cle’s track in the sensitive volume, are distributedisotropically in solid angle. Also when primarycharge clusters drift towards the readout andamplifying structure they exhibit diffusion. Thus,spatial resolution of an MPGD is determined bythe combination of gas density, its electron

Fig. 11. Micro-groove detector.

Fig. 12. WELL detector.

L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128–141 133

transport properties and sampling structure (i.e.pitch of the readout structure). Complete simula-tion of the spatial resolution for different gas layerthickness and pitch of the readout structure forparticular gas mixture was performed in Ref. [25](Fig. 15).Sampling with a readout pitch podx; where dx

is RMS of electron distribution, yields optimalposition information, as long as the individualsignals, reduced in amplitude due to the division ofthe total charge over dx=p channels, surpass thethreshold. The case when p > dx; results in acluster width covering one strip and thus aposition resolution equals p=

ffiffiffiffiffi12

p(left figure).

For thick gas layer and large amount of electrons

Fig. 13. Signal distribution and efficiency as a function of operational voltage for the MICROMEGAS.

Fig. 14. Efficiency as a function of incident angle for MSGC

with different gas mixtures and thickness of the gas layer.

L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128–141134

in a primary cluster its width due to the diffusion iscompensated by the improvement of electronstatistics, so that position resolution is saturatedat an optimal value. However, for small gapsposition accuracy is getting worse due to largevariations of charge and non-uniform distributionof ionization along the track (right figure). From

this result we can see that for realistic case with thepitch below 400 mm and gas layer thickness below3 mm one can get spatial resolution well below100 mm: Typical example of the experimentalresult obtained with double GEM detector isshown in Fig. 16 [26].Previous discussion relates to the orthogonal

incidence of tracks on the detector plane. How-ever, when a track is inclined with respect to thedetector, the position resolution is stronglyaffected, as the projection of track on the readoutplane is detected. Thus the width of the chargedistribution induced at the readout structure isproportional to tan y: Typical experimental resultshowing the dependence of spatial resolution onthe track incident angle is presented in Fig. 17 forthe microMEGAS [23].

5.3. Time resolution

Time resolution of an MPGD is determined byfluctuations of the induced charge pulse anddepends on the arrival time of the first primarycluster. Thus, the faster and more dense is the gas,the better is time resolution. In order to detect thefirst arrived cluster, gas amplification has to behigh enough. Also, shaping time of the front-endamplifier plays an obvious role, it has to be of theorder of 10 ns: Typical results that can be obtainedfor the fastest gas mixtures and regular gains

Fig. 15. MSGC position resolution versus pitch p (left figure) and the gas gap L (right figure). In the left figure L ¼ 2 mm for the solid

curve and L ¼ 5 mm for the dashed curve; in the right figure p ¼ 150 mm for the solid curve and p ¼ 200 mm for the dashed curve. Gas

mixture Ar–DME–CO2 (40–40–20).

Fig. 16. Distribution of the differences between fitted track

position and charge cluster position (residuals) for double

GEM detector. Readout pitch is 400 mm; thickness of the gaslayer is 3 mm: Gas mixture is Ar–CO2 (70–30). Sigma of the

gaussian fit equals 38 mm:

L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128–141 135

(10,000) are about 10 ns [26,27]. One of those isshown in Fig. 18.The best result reported for MPGD is time

resolution of 3 ns; obtained for triple GEMdetector filled with gas mixture enriched withCF4 [28].

5.4. Gain stability and rate capability

Presence of an insulator close to the amplifica-tion region led to gain instabilities in the MSGC.Polarization processes and surface charge deposi-tion caused significant changes of gain afterapplication of potentials and irradiation withhigh-intensity particle flux. These instabilitiescould be corrected with proper choice of potentialson drift and back electrodes [29,30] or avoidedcompletely with partially conductive substrate[31,32]. This problem was almost completelysolved in microMEGAS where insulator is presentin very limited amount as spacers and does notmake any effect on gain at high rates [13]. In PCBdetectors and GEM the kapton walls are alwayssurround holes or grooves. They, however, are notexactly in the amplification region but ratheraround it. Some slight charging effect is observed

in these devices as shown in Fig. 19 for micro-groove detector [20].Rate capability of the MPGD is determined by

space charge and surface charge effects. However,while for MSGC surface charge effects playedsignificant role (see, for example, Fig. 20, Ref.[31]), for modern types of micro-pattern detectors,such as microMEGAS and GEM, it is not the caseany more. In Fig. 20 we see that gain drop in

Fig. 18. Time resolution of double GEM detector. Gas mixture

is Ar–CO2 (70–30).

Fig. 19. Gain instability of the micro-groove detector with time

after application of potentials at different gains.

Fig. 17. Spatial resolution as a function of track incident angle

for microMEGAS. Thickness of the gas layer is 2 mm:

L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128–141136

MSGC depends on the resistivity of the substratedue to surface charging. If the substrate has lowenough resistivity, the dominating is space-chargeeffect that cause smooth drop of gain when therate is higher than certain limit (105–106 s�1 mm�2

for 8 keV X-rays in Ar at a gain of B1000). Thisbehavior of MSGC with rate looks very much like

that of wire chambers but at 2 orders of magnitudehigher scale.Unlike MSGC, microMEGAS and GEM de-

tectors have very different gain-rate performancethat is associated with uniform field in amplifica-tion region rather than ‘‘cylindrical’’ as in case ofthe MSGC. The gain stays stable up to very highrates, as shown in Fig. 21 [18], and the measure-ments are usually stopped due to a discharge.Systematic measurement of such effect was

performed by the microMEGAS group in Ref.[33]. Fig. 22 from this paper presents the limits ofgain-rate curves where the discharge occurred. Itlooks like when the charge flow (i.e. the product ofparticle flux and gain) exceeds certain threshold,the amplification gap is discharging. However, thelimits found in all the cases are far beyond therequirements of any particle physics experiment.

6. Main problems and solutions

Despite their promising performance, experiencewith MPGD has raised doubts about long-termFig. 20. Rate capability of MSGC with resistive substrates.

Fig. 21. Rate capability of GEM detector at different gains.

L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128–141 137

behavior. Two major problems, their relevancedepending on the application, have arisen: rarebut often damaging discharges, and slow butcontinuous deterioration (aging) during sustainedirradiation.

6.1. Induced discharges

Discharges during operation are permanentproblem with all micro-pattern detectors. When-ever the total charge in the avalanche exceeds avalue between 107 and 108 electron–ion pairs(Raether’s limit), an enhancement of the electricfield in front and behind the primary avalancheinduces the fast growth of a long, filament-likestreamer. At the gains required for the detection ofminimum ionizing particles in thin gaps, typicallyabove 2000, the accidental release of largeramounts of ionization easily brings the totalcharge above the limit. Large and high-densityionization can be released in gas by slow heavyparticles that appear due to hadronic interactionsin the material of the detector exposed to highenergy hadron beam.Various schemes have been proposed to limit

the probability of an induced discharge and thedamage caused by it. We will mention two ofthem: coating the edges of amplifying structurewith polymide insulator (advanced passivation)[26] and the distribution of amplification betweenseveral GEM pre-amplifying stages [18]. Successfuluse of advanced passivation of MSGC wasreported by one group [34,35], and it providedeffective suppression of the induced discharges up

Fig. 22. Discharge limits of gain-rate dependencies for micro-

MEGAS.

Fig. 23. Probability of discharge as a function of absolute gain in single- double- and triple-GEM detectors.

L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128–141138

to the gains of about 2000. Adding GEM beforeother amplifying structures [36,37] or using multi-GEM structures with PCB readout [18] has beenproved to be effective way of induced dischargessuppression. Example of the performance of multi-GEM structures in an environment with heavilyionizing alpha particles injected into the gas

volume of the detector is shown in Fig. 23.The probability of a discharge as a function ofabsolute gain demonstrates clear advantage ofmulti-GEM detectors with respect to the single-GEM one.This example suggests that Raether’s limit is not

a constant, but rather depends on the electric fieldvalue. Each stage of a multi-stage system operatesat lower field providing higher total discharge gainlimit.

6.2. Aging

Aging, the slow degradation of performanceduring sustained irradiation, is a problem encoun-tered with most gaseous counters and has beenextensively studied experimentally [38]. The ob-served permanent damage of the detectors hasbeen imputed to the production of polymericcompounds in the avalanches, which stick to theelectrodes or to the insulator, perturbing the signaldetection and inducing discharges. MSGC areparticularly prone to ageing, possibly because ofthe small effective area used for charge multi-plication. In Fig. 24 an example of aging undersustained irradiation of MSGC with differentmaterial of electrodes is shown.Several experimental observations show that

microMEGAS and GEM detectors are much lessthan MSGC sensitive to particular conditions of

Fig. 24. Gain drop under sustained irradiation of microstrip

plates manufactured on insulating and semiconducting sub-

strates, for different strip metals.

Fig. 25. Gain as a function of accumulated charge in large double-GEM detector prepared for COMPASS experiment at CERN [41].

L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128–141 139

the measurements: cleanliness of the system andmaterial of the electrodes. An example of theperformance of large multi-GEM detector isshown in Fig. 25, where no degradation is detectedup to 7 mC=mm2 [39]. Similar result has beenobtained for microMEGAS [40].Possible explanation of such stable performance

proposed by authors of Ref. [39] suggests that, asavalanche appears far from any electrode orinsulator in the parallel plate gap or in a GEMhole, polymer products are deposited much slowerthan in MSGC.

7. Summary and conclusions

In the 10 years since the introduction of themicrostrip chamber, an amazingly large number ofstudies have aimed to understand the new detectorand to improve their performance. Althoughsuccessful in experimental setups requiring mod-erate proportional gains, MSGC turned out to beprone to irreversible damage under harsherexperimental conditions. Several new micro-pat-tern concepts increase reliability while preservingor even improving performance. These newdetectors include CAT and PCB detectors, micro-MEGAS and the GEM. Manufactured with ratherconventional technologies, the new devices arecheaper than microstrip chambers and free of theirsize limitations. The GEM has the unique featureof preamplification and transfer of charge essen-tially preserving the ionization pattern into thesubsequent step of amplification. Sharing therequired gain between several stages, each oper-ated at a voltage well below the discharge limit,appears to be a reliable solution to the problems ofsingle-stage devices.Micro-pattern gaseous detectors possess unique

combination of features such as: spatial resolutionof less than 100 mm; rate capability of higherthan 105 mm�2 s�1 at a gain of about 10,000,time resolution down to 3 ns and good agingproperties. These set of features together withcheap and reliable manufacturing technologymakes MPGD a good candidate to fill the gapbetween solid state vertex detectors and large wirechambers.

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