Micro-pattern gaseous detectors

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 highluminosity hadron collider projects it was commonly understood that gas wire technique cannot realize full potential of gaseous detectors, namely in simultaneous high spatial resolution and rate capability. Intrinsic properties of commonly used gas mixtures allowed spatial resolution of below 50 m: However the large size of amplification cell with sense wire in the center could not provide rate capability of more than 10 khz=mm 2 : The solution was to put a micro-structure produced with micro-lithography technique into the gas. Small amplification cell in such a case could provide both high spatial resolution and rate capability at the same time. The idea in the form of micro-stripgas chambers was first proposed by Oed in 1988 [1] for neutron detection and then was modified for the *Tel.: +7-3832-394992; fax: +7-3832-342163. E-mail address: l.i.shekhtman@inp.nsk.su (L. Shekhtman). needs of charge particle detection by groups from INFN Pisa [2] and NIKHEF [3]. Later the concept in general got the name of micro-pattern gaseous detector and many different types of those were proposed during the last 14 years. Despite the many types of micro-structures that are proposed for the micro-pattern devices, major properties of the latter are determined by the gas mixture and the gapwidth used for the detection of charged particles (Fig. 1). From Fig. 1 we see that both spatial and time resolution of these devices depend mostly on the statistics of primary charge clusters deposited by a relativistic charged particle. Micro-pattern structure is used for amplification of the primary electrons and readout of the induced amplified signal and can thus also affect partly space and time resolution. Small size of amplification cell of a micro-pattern structure provides fast removal of positive ions and low space charge effects at high rates. 0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0168-9002(02)01456-0

L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128 141 129 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 microstructure that can consist of several stages. Amplified electron component of the primary ionization induces charge at the readout micro-structure. In this review we consider different types of the micro-pattern gaseous detectors according to production technology and layout of the microstructure. Main performance parameters will be analyzed and solutions for most common problems will be discussed. 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. 2. Micro-strip gas chambers The micro-stripgas chambers (MSGC) consist of thin parallel metal strips, deposited on an insulating support and alternatively connected as anodes and cathodes. Fig. 2 shows a schematic view of this device with electric field lines and equipotentials computed with anodes and backplane at equal potentials. Accurate but simple photolithography can achieve a distance between electrodes of 100 mm; i.e. improving granularity by another order of magnitude over that of wire chambers. Fig. 2 shows that at appropriate choice of potentials all field lines from the drift region terminate on thin anode strip. However, avalanche is spread broader than anode width (usually about 10 mm) and large fraction of positive ions is collected to the neighboring cathode strips. This effect reduces space charge accumulation and provides much higher intrinsic rate capability than classic devices can. High gains for a wide range of gas mixtures, electrodes geometry and types of substrate have Fig. 3. Examples of absolute gain measured, as a function of anode voltage, in several mixtures of noble gases and dimethyl ether. been obtained [4 6]. Fig. 3 shows one example of absolute gain measured as a function of anode voltage in several mixtures of noble gases and dimethyl ether [7]. Several improvements of the micro-strip technology have been proposed mainly towards twodimensional readout and more compact avalanche region. This can be achieved by putting the anode stripon topof thin (several microns thick) insulating layer deposited on top of properly

130 L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128 141 segmented cathode metal layer. Such a structure was called microgapchamber (MGC) [8,9]. Large gains, above 10 4 have been demonstrated with a MGC. Mixtures of neon and dimethyl ether seem to be particularly advantageous. Another approach utilizing more complicated photolithography process, namely the microdot chamber [10 12], consists of a dense pattern of individual proportional counters made up of anode dots surrounded by annular cathodes. Field Fig. 4. Schematics of the microdot chamber. defining rings can be added to improve the operation, as shown in Fig. 4. 3. Detectors with parallel-plate amplification region The evolution of wire chambers and, in particular, asymmetric multiwire proportional devices has led to the invention of another micro-pattern concept. It has been recently suggested that in submillimeter gaps with strong uniform electric field exceptionally large gains could be attained. This has led to the introduction of the micromesh gaseous chamber (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 field applied across the gap, typically above 30 kv=cm; electrons released in the upper drift region are collected and multiplied. The micromegas exploits the saturating characteristics of the Townsend coefficient at very high field to reduce the dependence of gain on the gap variations, thus improving the uniformity and stability of response over a large area. Thanks to the small gapand 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) 128 141 131 Fig. 6. Gain as a function of the mesh voltage in the micromegas with 50 mm amplification gap. Fig. 7. Schematics and field mapin the CAT cell. high field, positive ions move very quickly and most are collected to the cathode mesh. This induces very fast signals with very small ion tail, 50 100 ns wide, and prevents space-charge accumulation in the drift region. Very high gains have been demonstrated with the micromegas (Fig. 6) showing even the possibility of efficient single electron detection [14]. 4. PCBdetectors and gas electron multiplier The gain of a parallel-plate counter depends exponentially on the gap thickness, making it difficult to obtain a uniform response over large areas. In the compteur a trous (CAT) holes drilled through a metal insulator sandwich concentrate the field lines converging from a drift volume into a region of high field, where charge multiplication occurs (Fig. 7) [15]. This idea combines the concept of parallel-plate chamber with intrinsically uniform spacer allowing uniform gains over large surfaces. Even with relatively large holes, the collection and focusing properties of the field result in good energy resolution at proportional gains up to 10 4 : Combining the idea of multistepavalanche chamber [16] and CAT, Sauli proposed a new micro-pattern structure called gas electron multiplier (GEM) [17]. It consists of a thin, metal-clad polymer foil chemically perforated by a high density of holes, typically 100=mm 2 (Fig. 8). As shown in Fig. 8, with a suitable choice of voltages, all electrons released by ionization in the Fig. 8. Schematic structure of the gas electron multiplier with electric field lines and equipotentials shown. overlying gas layer are sucked into the holes, where charge multiplication occurs in the high electric field. the gain is a property of the GEM structure and is only mildly affected by the external fields, considerably relaxing the mechanical requirements. Systematic research efforts have enabled GEM devices to achieve proportional gains upto 10 4 ; suitable for direct detection of ionization on simple charge-collecting printed circuit board (PCB) electrodes. GEM foil can work as a distributed preamplifier allowing cascaded devices with several GEMs or with GEM combined with active read-out structure such as MSGC. The cascaded device permits much higher gains, or, for given required gain,

132 L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128 141 allows operation of amplifying elements at much lower voltages. In Fig. 9 the schematics of a double GEM detector is shown with typical dimensions indicated in the caption [18]. Examples of gain dependence as a function of GEM voltage and for several argon-based gas mixtures for triple-gem device are shown in Fig. 10 [19]. In this case for all three GEMs voltages were kept the same. CAT and GEM structures started another family of devices that can be called PCB detectors. These devices can be manufactured using simple and cheap lithography process applied for printed circuit boards. Two examples of such devices are micro-groove [20,21] and WELL [22] detectors shown in Figs. 11 and 12. As one can see from the figures both microgroove and WELL detectors realize the CAT principle in micro-scale. 5. Main properties of the micro-pattern gaseous detectors Main properties of the micro-pattern gas detectors are determined by the gas mixture and thickness of the conversion region. Among others we will discuss parameters of high importance for the detection of relativistic charged particles, such as efficiency, spatial resolution, time resolution and rate capability. 5.1. Signal and efficiency Fig. 9. Schematic structure of the double-gem detector. High-energy charged particle penetrating through a thin gas layer exhibit a limited number of interactions with the gas molecules forming primary charge clusters. 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) 128 141 133 Fig. 11. Micro-groove detector. Fig. 12. WELL detector. If the gas layer is thin, the probability of zero charge deposition becomes non-negligible. Thus, in order to get efficiency close to 100% the detector has to provide high enough amplification to detect as small number of primary clusters as possible, and the gas layer has to be thick enough. Typical example of the deposited charge distribution together with efficiency dependence on the operational voltage is shown for micromegas in Fig. 13 [23]. The measurement of efficiency with very thin gas layers was reported by several groups (see for example, Ref. [24]). It was found that even for the most dense gases such as dimetyl ether or isobutane, efficiency cannot be higher than 95% for the gas layer thickness of less than 2 mm: An example of such measurements is presented in Fig. 14 [24]. 5.2. Spatial resolution Delta-electrons formed along a charged particle s track in the sensitive volume, are distributed isotropically in solid angle. Also when primary charge clusters drift towards the readout and amplifying structure they exhibit diffusion. Thus, spatial resolution of an MPGD is determined by the combination of gas density, its electron

134 L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128 141 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. transport properties and sampling structure (i.e. pitch of the readout structure). Complete simulation of the spatial resolution for different gas layer thickness and pitch of the readout structure for particular gas mixture was performed in Ref. [25] (Fig. 15). Sampling with a readout pitch podx; where dx is RMS of electron distribution, yields optimal position information, as long as the individual signals, reduced in amplitude due to the division of the total charge over dx=p channels, surpass the threshold. The case when p > dx; results in a cluster width covering one stripand p ffiffiffiffiffi thus a position resolution equals p= 12 (left figure). For thick gas layer and large amount of electrons

L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128 141 135 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 CO 2 (40 40 20). this result we can see that for realistic case with the pitch below 400 mm and gas layer thickness below 3 mm one can get spatial resolution well below 100 mm: Typical example of the experimental result obtained with double GEM detector is shown in Fig. 16 [26]. Previous discussion relates to the orthogonal incidence of tracks on the detector plane. However, when a track is inclined with respect to the detector, the position resolution is strongly affected, as the projection of track on the readout plane is detected. Thus the width of the charge distribution induced at the readout structure is proportional to tan y: Typical experimental result showing the dependence of spatial resolution on the track incident angle is presented in Fig. 17 for the micromegas [23]. 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 gas layer is 3 mm: Gas mixture is Ar CO 2 (70 30). Sigma of the gaussian fit equals 38 mm: in a primary cluster its width due to the diffusion is compensated by the improvement of electron statistics, so that position resolution is saturated at an optimal value. However, for small gaps position accuracy is getting worse due to large variations of charge and non-uniform distribution of ionization along the track (right figure). From 5.3. Time resolution Time resolution of an MPGD is determined by fluctuations of the induced charge pulse and depends on the arrival time of the first primary cluster. Thus, the faster and more dense is the gas, the better is time resolution. In order to detect the first arrived cluster, gas amplification has to be high enough. Also, shaping time of the front-end amplifier plays an obvious role, it has to be of the order of 10 ns: Typical results that can be obtained for the fastest gas mixtures and regular gains

136 L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128 141 Fig. 17. Spatial resolution as a function of track incident angle for micromegas. Thickness of the gas layer is 2 mm: Fig. 18. Time resolution of double GEM detector. Gas mixture is Ar CO 2 (70 30). (10,000) are about 10 ns [26,27]. One of those is shown in Fig. 18. The best result reported for MPGD is time resolution of 3 ns; obtained for triple GEM detector filled with gas mixture enriched with CF 4 [28]. 5.4. Gain stability and rate capability Presence of an insulator close to the amplification region led to gain instabilities in the MSGC. Polarization processes and surface charge deposition caused significant changes of gain after application of potentials and irradiation with high-intensity particle flux. These instabilities could be corrected with proper choice of potentials on drift and back electrodes [29,30] or avoided completely with partially conductive substrate [31,32]. This problem was almost completely solved in micromegas where insulator is present in very limited amount as spacers and does not make any effect on gain at high rates [13]. In PCB detectors and GEM the kapton walls are always surround holes or grooves. They, however, are not exactly in the amplification region but rather around it. Some slight charging effect is observed Fig. 19. Gain instability of the micro-groove detector with time after application of potentials at different gains. in these devices as shown in Fig. 19 for microgroove detector [20]. Rate capability of the MPGD is determined by space charge and surface charge effects. However, while for MSGC surface charge effects played significant role (see, for example, Fig. 20, Ref. [31]), for modern types of micro-pattern detectors, such as micromegas and GEM, it is not the case any more. In Fig. 20 we see that gain dropin

L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128 141 137 MSGC depends on the resistivity of the substrate due to surface charging. If the substrate has low enough resistivity, the dominating is space-charge effect that cause smooth dropof gain when the rate is higher than certain limit (10 5 10 6 s 1 mm 2 for 8 kev X-rays in Ar at a gain of B1000). This behavior of MSGC with rate looks very much like that of wire chambers but at 2 orders of magnitude higher scale. Unlike MSGC, micromegas and GEM detectors have very different gain-rate performance that is associated with uniform field in amplification region rather than cylindrical as in case of the MSGC. The gain stays stable upto very high rates, as shown in Fig. 21 [18], and the measurements 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 of gain-rate curves where the discharge occurred. It looks like when the charge flow (i.e. the product of particle flux and gain) exceeds certain threshold, the amplification gap is discharging. However, the limits found in all the cases are far beyond the requirements of any particle physics experiment. 6. Main problems and solutions Fig. 20. Rate capability of MSGC with resistive substrates. Despite their promising performance, experience with MPGD has raised doubts about long-term Fig. 21. Rate capability of GEM detector at different gains.

138 L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128 141 behavior. Two major problems, their relevance depending on the application, have arisen: rare but often damaging discharges, and slow but continuous deterioration (aging) during sustained irradiation. Fig. 22. Discharge limits of gain-rate dependencies for micro- MEGAS. 6.1. Induced discharges Discharges during operation are permanent problem with all micro-pattern detectors. Whenever the total charge in the avalanche exceeds a value between 10 7 and 10 8 electron ion 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. At the gains required for the detection of minimum ionizing particles in thin gaps, typically above 2000, the accidental release of larger amounts of ionization easily brings the total charge above the limit. Large and high-density ionization can be released in gas by slow heavy particles that appear due to hadronic interactions in the material of the detector exposed to high energy hadron beam. Various schemes have been proposed to limit the probability of an induced discharge and the damage caused by it. We will mention two of them: coating the edges of amplifying structure with polymide insulator (advanced passivation) [26] and the distribution of amplification between several GEM pre-amplifying stages [18]. Successful use of advanced passivation of MSGC was reported by one group [34,35], and it provided effective suppression of the induced discharges up 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) 128 141 139 to the gains of about 2000. Adding GEM before other amplifying structures [36,37] or using multi- GEM structures with PCB readout [18] has been proved to be effective way of induced discharges suppression. Example of the performance of multi- GEM structures in an environment with heavily ionizing alpha particles injected into the gas volume of the detector is shown in Fig. 23. The probability of a discharge as a function of absolute gain demonstrates clear advantage of multi-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 field value. Each stage of a multi-stage system operates at lower field providing higher total discharge gain limit. 6.2. Aging Fig. 24. Gain dropunder sustained irradiation of microstrip plates manufactured on insulating and semiconducting substrates, for different stripmetals. Aging, the slow degradation of performance during sustained irradiation, is a problem encountered with most gaseous counters and has been extensively studied experimentally [38]. The observed permanent damage of the detectors has been imputed to the production of polymeric compounds in the avalanches, which stick to the electrodes or to the insulator, perturbing the signal detection and inducing discharges. MSGC are particularly prone to ageing, possibly because of the small effective area used for charge multiplication. In Fig. 24 an example of aging under sustained irradiation of MSGC with different material of electrodes is shown. Several experimental observations show that micromegas and GEM detectors are much less than MSGC sensitive to particular conditions of Fig. 25. Gain as a function of accumulated charge in large double-gem detector prepared for COMPASS experiment at CERN [41].

140 L. Shekhtman / Nuclear Instruments and Methods in Physics Research A 494 (2002) 128 141 the measurements: cleanliness of the system and material of the electrodes. An example of the performance of large multi-gem detector is shown in Fig. 25, where no degradation is detected upto 7 mc=mm 2 [39]. Similar result has been obtained for micromegas [40]. Possible explanation of such stable performance proposed by authors of Ref. [39] suggests that, as avalanche appears far from any electrode or insulator in the parallel plate gap or in a GEM hole, polymer products are deposited much slower than in MSGC. 7. Summary and conclusions In the 10 years since the introduction of the microstripchamber, an amazingly large number of studies have aimed to understand the new detector and to improve their performance. Although successful in experimental setups requiring moderate proportional gains, MSGC turned out to be prone to irreversible damage under harsher experimental conditions. Several new micro-pattern concepts increase reliability while preserving or even improving performance. These new detectors include CAT and PCB detectors, micro- MEGAS and the GEM. Manufactured with rather conventional technologies, the new devices are cheaper than microstrip chambers and free of their size limitations. The GEM has the unique feature of preamplification and transfer of charge essentially preserving the ionization pattern into the subsequent stepof amplification. Sharing the required gain between several stages, each operated at a voltage well below the discharge limit, appears to be a reliable solution to the problems of single-stage devices. Micro-pattern gaseous detectors possess unique combination of features such as: spatial resolution of less than 100 mm; rate capability of higher than 10 5 mm 2 s 1 at a gain of about 10,000, time resolution down to 3 ns and good aging properties. These set of features together with cheapand reliable manufacturing technology makes MPGD a good candidate to fill the gap between solid state vertex detectors and large wire chambers. References [1] A. Oed, Nucl. Instr. and Meth. A 263 (1988) 351. [2] F. Angelini, et al., Nucl. Instr. and Meth. A 283 (1989) 755. [3] F.G. Hartjes, et al., Nucl. Instr. and Meth. A 315 (1992) 529. [4] C. Budtz-Jorgensen, Rev. Sci. Instr. 63 (1992) 648. [5] R. Bouclier, et al., Nucl. Instr. and Meth. A 365 (1995) 65. [6] O. Bouhali, et al., Nucl. Instr. and Meth. A 378 (1996) 438. [7] T. Beckers, et al., Nucl. Instr. and Meth. A 346 (1994) 195. [8] F. Angelini, et al., Nucl. Instr. and Meth. A 335 (1993) 69. [9] F. Angelini, et al., Nucl. Instr. and Meth. A 349 (1995) 273. [10] S.F. Biagi, T.J. Jones, Nucl. Instr. and Meth. A 361 (1995) 72. [11] S.F. Biagi, et al., Nucl. Instr. and Meth. A 392 (1997) 131. [12] S.F. Biagi, et al., Nucl. Instr. and Meth. A 371 (1995) 12. [13] I. Giomataris, et al., Nucl. Instr. and Meth. A 376 (1996) 29. [14] A. Delbart, et al., New development of micromegas detector, Nucl. Instr. and Meth. A 461 (2001) 84. [15] F. Bartol, et al., J. Phys. III France 6 (1996) 337. [16] G. Charpak, F. Sauli, Phys. Lett. B 72 (1978) 523. [17] F. Sauli, Nucl. Instr. and Meth. A 386 (1997) 531. [18] A. Bressan, et al., Nucl. Instr. and Meth. A 424 (1999) 321. [19] S. Bachmann, et al., Discharge studies and prevention in the gas electron multiplier, CERN-EP-2000-151. [20] R. Bellazzini, et al., Nucl. Instr. and Meth. A 424 (1999) 444. [21] S. Keller, et al., Nucl. Instr. and Meth. A 419 (1998) 382. [22] R. Bellazzini, et al., Nucl. Instr. and Meth. A 423 (1999) 125. [23] G. Barouch, et al., Nucl. Instr. and Meth. A 423 (1999) 32. [24] F. Angelini, et al., Nucl. Instr. and Meth. A 360 (1995) 22. [25] J. Schmitz, Nucl. Instr. and Meth. A 323 (1992) 638. [26] A. Bressan, et al., Nucl. Instr. and Meth. A 425 (1999) 262. [27] D. Tres, et al., Nucl. Instr. and Meth. A 461 (2001) 29. [28] H. Poli-Lener, et al., A systematic study of the performance of a Triple GEM detector for high rate charged particle triggering, Nucl. Instr. and Meth. A 494 (2002), these proceedings. [29] R. Bouclier, et al., Nucl. Instr. and Meth. A 367 (1995) 168. [30] J.E. Bateman, J.F. Connoly, RAL-92-085. [31] R. Bouclier, et al., Nucl. Instr. and Meth. A 332 (1993) 100. [32] Y.N. Pestov, L.I. Shekhtman, Nucl. Instr. and Meth. A 338 (1994) 368. [33] I. Giomataris, et al., Nucl. Instr. and Meth. A 419 (1998) 239. [34] R. Bellazzini, et al., Nucl. Instr. and Meth. A 398 (1998) 426. [35] R. Bellazzini, et al., Nucl. Instr. and Meth. A 457 (2001) 22.

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