In this study we report the results of a systematic study of the gain, rate
and the position resolution limits of various micropattern gaseous detectors.
It was found that at low rates (<1 Hz/mm^2) each detector has it own gain
limit, which depends on the size and design features, as well as on gas
composition and pressure. However, in all cases the maximum achievable gain is
less than or equal to the classical Raether limit. It also was found that for
all detectors tested the maximum achievable gain drops sharply with the
counting rate. The position resolution of micropattern detectors for detection
of X-rays (6 to 35 kV) was also studied, being demonstrated that with solid
converters one could reach a position resolution better than 30 micrometers at
1 atm in a simple counting mode.Comment: 10 pages, 3 figures, Presented at the PSD99-5th International
  Conference on Position-Sensitive Detectors, 13-17 th September 1999,
  University College, Londo

Fonte, P.

Peskov, V.

English

arXiv

In this study we report the results of a systematic study of the gain, rate and the position resolution limits of various micropattern gaseous detectors. It was found that at low rates (<1 Hz/mm^2) each detector has it own gain limit, which depends on the size and design features, as well as on gas composition and pressure. However, in all cases the maximum achievable gain is less than or equal to the classical Raether limit. It also was found that for all detectors tested the maximum achievable gain drops sharply with the counting rate. The position resolution of micropattern detectors for detection of X-rays (6 to 35 kV) was also studied, being demonstrated that with solid converters one could reach a position resolution better than 30 micrometers at 1 atm in a simple counting mode.In this study we report the results of a systematic study of the gain, rate and the position resolution limits of various micropattern gaseous detectors. It was found that at low rates (<1 Hz/mm^2) each detector has it own gain limit, which depends on the size and design features, as well as on gas composition and pressure. However, in all cases the maximum achievable gain is less than or equal to the classical Raether limit. It also was found that for all detectors tested the maximum achievable gain drops sharply with the counting rate. The position resolution of micropattern detectors for detection of X-rays (6 to 35 kV) was also studied, being demonstrated that with solid converters one could reach a position resolution better than 30 micrometers at 1 atm in a simple counting mode

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LABORATÓRIO DE INSTRUMENTAÇÃO EFÍSICA EXPERIMENTAL DE PARTÍCULASPreprint LIP/01-065 June 2001Gain, Rate and Position Resolution Limits ofMicropattern Gaseous DetectorsV.Peskov1,*, P.Fonte2,31 - Royal Institute of Technology, Stockholm, Sweden2 - CERN, Geneva, Switzerland3 - LIP/ISEC, Coimbra, PortugalAbstractIn this study we report the results of a systematic study of the gain, rate and the positionresolution limits of various micropattern gaseous detectors. It was found that at low rates(<1 Hz/mm2) each detector has it own gain limit, which depends on the size and designfeatures, as well as on gas composition and pressure. However, in all cases the maximumachievable gain is less than or equal to the classical Raether limit. It also was found thatfor all detectors tested the maximum achievable gain drops sharply with the countingrate.The position resolution of micropattern detectors for detection of X-rays (6 to 35 kV) wasalso studied, being demonstrated that with solid converters one could reach a positionresolution better than 30   m at 1 atm in a simple counting mode.Presented at thePSD99-5th International Conference on Position-Sensitive Detectors13-17 th September 1999, University College, London                                                          * Corresponding author: V. Peskov, Physics Department, Royal Institute of Technology, Frescativagen 24,Stockholm 10405, Sweden (telephone: 468161000, e-mail: vladimir.peskov @cern.ch).I. IntroductionIn the last few years there was an “explosion” of inventions of new micropattern gaseousdetectors: Microstrip [1], CAT [2], MICROMEGAS [3], GEM[4], Microdot [5],Microgap [6],Well [7], etc. Although many of these detectors are very differentgeometrically, one can notice, however, that conceptually they are rather similar [8]: inall cases the gas amplification appears in a small region of strong and focused electricalfield. Since this feature is common for all micropattern detectors, it will be interesting toconsider the limits of these new micropattern detectors. Results of earlier studies can befound in refs. [9-14], while in this paper we present our latest results and conclusions.II. Experimental set upA detailed description of the experimental setup can be found elsewhere [9,10]. The mostimportant “basic” micro-pattern gaseous detectors were studied: micro-dot [15], micro-gap (obtained from [16]), “compteur a trous” (CAT) [12,15], GEM, MICROMEGAS andthin-gap (0.1 µm) resistive-plate chamber (RPC). The cathode of the RPC (3 3cm2) wasdone from aluminium and the anode from Pestov glass with resistivity ~1010  /¨. Theinner surface of the anode was covered with chromium strips 10 µm wide, 30 µm pitch.In some measurements the inner surface of the RPC cathode was covered with a CsIsecondary-electron emitter – Fig 1 (see [17] for more details). Two types of emitterstructures were used: uniform (300 nm thick) and porous (~1 µm thick). We also studiedsome combinations of these detectors with preamplification structures: GEM or parallel-plate avalanche chamber (PPAC) [11]. Tests were done in various mixtures of noblegases with quenchers at pressures of 0.05-5 atm. For position resolution studies we used awell-collimated (30 µm) x-ray beam (6-35 keV).Some tests, in particular with thin gap RPC, were also done in T9 and T10 pion-beam testfacilities at CERN [17].III. Results.1. Gain limit at low rate.We found that the maximum achievable gain (A) in micropattern detectors is limited by:A   Qmax/n0 (1)where n0 is the number of primary electrons and Qmax is the maximum achievable chargein the avalanche, depending on detector design and gas composition and pressure.Typically at 1 atm Qmax~107 electrons, but each type of micropattern detector has it ownlimit, which drops rapidly, almost inversely proportional, with pressure.With two or more multiplication steps Qmax increases and may reach ~108 electrons.The value of Qmax~107 electrons is always valid for large values of n0 (alpha particles, forinstance), but for smaller values of n0 (1-100) Qmax could be smaller as well. This isbecause in order to meet the limit (1) at small n0 values one has to operate the detector athigh gains and therefore at high voltages as well. At such voltages breakdown may occurdue to construction defects, which are rather common in micropattern detectors. At smalln0 one can reach Qmax 107 electrons only with two or more of preamplification steps.2. Gain limit at high rates.We found that for all micropattern detectors tested, the maximum achievable gain alwaysdrops with rate, in some cases almost inversely proportional to it (see Fig.1 in [12]). Thesame feature appears in some other gaseous detectors, for example in single wire counters[18] or in PPAC’s [19], so it seems to be a universal feature of fast detectors [13]. Theseobservations suggest that there is a common physical mechanism, limiting the detectorsperformance at high rates.3. Position resolution limitIt is clear that in general the position resolution limit in micropattern detectors isdetermined by the path length of the electrons created by the incident radiation in the gas(primary electrons), by the diffusion in the drift region and by the geometry of theamplification region. Obviously, in order to achieve the best possible position resolutionone has to optimize each of these parameters. The minimum possible electron path lengthcan be achieved by using solid converters of radiation [20, 21]. In this case the primaryelectrons are created mostly inside a thin solid layer and some (sometimes all) primaryelectrons can be extracted (by applying a strong electric field on the cathode surface) intothe gas media and multiplied there. The minimum diffusion spread can be achieved byusing the minimum possible gap between the converter and the amplification region. Theminimum avalanche size can ensured by using the thinnest possible multiplication gap.These considerations lead to a concept of a micropattern detector with a solid converterimmediately followed (without any drift) by an amplification region. One possible designfor such a detector is shown in Fig. 1Fig. 2a shows an image of a 30 µm wide slit placed in front of the detector,perpendicularly to the anode strips and to the electrodes plates. Figures 3b and 3c showthe images of the same slit shifted each time by 15 µm in the direction perpendicular tothe strips. From the image contrast (ratio of counts from neighbouring strips) one canconclude that a position resolution better than 30 µm was achieved. Note that in aparallel-plate amplification structure, whose maximum multiplication is obtained only forelectrons created near the cathode, almost the same position resolution could be achievedwithout a converter, being however the efficiency very low.IV. Discussion1 Gain limitOne can see from Fig. 3 that the gain vs. rate curve has different slopes, which webelieve, reflect different physical mechanisms, responsible for breakdown. In thefollowing paragraphs we will present possible explanations.1.a Very Low rate (<1Hz/mm2)One can safely assume that at a low rates breakdown occurs when the space charge fieldbecomes comparable with the applied field (see [22] for more details).In general, the space charge effect depends on the applied field and on factors that affectthe dimensions of the avalanche: pressure, length of the drift region, gap size and gascomposition. This is probably the reason why each micropatern detector has its own limitand why it drops rapidly with pressure. As it was shown in a previous work withpreamplification structures, Qmax increases due to the diffusion [10].1.b Medium and high rates (10 to 106Hz/mm2)In previous studies, at least two mechanisms were identified that may contribute to thebreakdown of micropattern detectors at medium and high rates: cathode “excitation” [13]and spots with high electric field near the cathodes (“hot” spots [23]). More experimentsare needed to fully clarify the main mechanism.As it was mentioned above, the gain drop with rate is valid for detectors having no “hot”spots, for example single wire counters or PPAC’s. One may therefore speculate thatcathode excitation dominates in these cases. Breakdown then appears due to jets ofelectrons, photon or ion feedback loops or their combination [12,13].1.c Very high rates (>107Hz/mm2)At very high rates (>107Hz/mm2), additionally to the cathode excitation effect,plasma-type effects may appear as well. They include the modification of electrical fieldin the cathode region due to the steady space charge, multistep ionization, gas heatingeffect and accumulation of excited atoms and molecules [24]. As it was shown in otherstudies, these mechanisms may create instability leading to breakdown [18].2 Position resolution limitThe use of solid converters places a real physical limit on the position resolution ofmicropattern detectors. However, despite the already very satisfactory result obtained, thesimple counting method used in this work is far from optimum and the ultimate value ofthe position resolution should be measured by applying a centre of gravity method. Anincrease of the gas pressure would further improve the position resolution.VI. Conclusions.As any other instrument, micropattern detectors have some limitations. We believe thatidentification and understanding of these limits will be useful for those who considertheir various applications. The lack of understanding of such limits has led, in the past, tomicropattern detectors being proposed for applications that demanded sometimesunrealistic performances.VII. AcknowledgementsWe thank M. Lemonnier, F. Sauli, L. Ropelewski and Y. Giomataris for usefuldiscussions.References:[1] A. Oed, Nucl. Instr. and Meth.A263 (1988) 351[2] F. Bartol et al., J. Phys.III France, 6 (1996) 337[3] Y. Giomataris et al., Nucl. Instr. and Meth. A 376 (1996) 29[4] F. Sauli, Nucl. Instr. and Meth. A386 (1997) 531[5] S.Biagi et al., Nucl. Instr. and Meth. A366 (1995) 76[6] F. Angelini et al., Nucl. Instr. and Meth.A335 (1993)69[7] R. Bellazzini et al., Nucl. Instr. and Meth. .A423 (1999)125[8] G. Chaplier et al., Nucl. Instr. and Meth. A426 (1999) 339[9] V. Peskov et al., Nucl. Instr. and Meth.A397 (1997) 243[10] P. Fonte et al., Nucl. Instr. and Meth. A416 (1998) 23[11] P. Fonte et al., Nucl. Instr. and Meth A419 (1999) 405[12] Y. Ivaniouchenkov et al., Nucl. Instr. and Meth A422 (1999) 300[13] P. Fonte, V.Peskov, B.Ramsey, "The fundamental limitations of high-rate gaseousdetectors”, IEEE Nucl. Sci. Symposium, Toronto, Canada 1998, IEEE Trans. Nucl.Science, vol. 46, n.3, (1999) pp. 321.[14] A. Bressan et al., CERN-EP/98-139.[15] V. Peskov et al., IEEE Trans. Nucl. Sci. 45 (1998) 244.[16] INFN, Pisa, R. Bellazzini group, Delf. Inst of Technology, C.Van Eijk group.[17] P. Fonte et al, “Micro-Gap Parallel-Plate Chambers with Porous Secondary ElectronEmitters”, Symposium on Applications of Particle Detectors in Medicine, Biologyand Astrophysics (SAMBA), Siegen, Germany, 1999, Nucl. Instr. and Meth. A 454(2000) 260.[18] V. Peskov, Doct. of Sciences Thesis, Moscow 1981[19] I. Ivaniouchenkov et al., IEEE Trans. Nucl. Science 45 (1998) 244[20] A. Breskin, Nucl. Phys. B 44 (1995) 351[21] A. Ceron Zeballos et al., Nucl. Instr. and Meth.,A392 (1997) 150[22] P. Fonte et al., Preprint SLAC-PUB-7718, 1987[23] T. Beckers et al., Nucl. Instr. and Meth. A346 (1994) 95[24] I. Kaptzov, “Physics of gas discharge devices”, Atomisdat, Moscow 1965(inRussian)Figure captions:Fig 1: Schematic picture of a thin gap RPC with a CsI converterFig. 2: Number of counts measured in a thin-gap RPC with solid converter irradiated byx-rays through a 30 µm slit: a) slit is placed in front of the strip #12; b) slit is placedbetween the strips #12 and #13; c) slit is placed in front of the strip #13.Fig. 3: Typical dependence of the maximum achievable total charge in avalanches vs.rate. 1- micropattern detectors. 2-micropattern detectors with preamplification structure.Fig 1: Schematic picture of a thin gap RPC with a CsI converter02000400060008000100001 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Strip num b e rCountsa )01000200030004000500060001 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Strip num b e rCountsb)02000400060008000100001 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Strip num b e rCountsc)Fig. 2: Number of counts measured in a thin-gap RPC with solid converter irradiated byx-rays through a 30 µm slit: a) slit is placed in front of the strip #12; b) slit is placedbetween the strips #12 and #13; c) slit is placed in front of the strip #13.1.E+001.E+021.E+041.E+061.E+081.E+101.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08Rate (Hz/mm2)Total charge  (electrons)12Fig. 3: Typical dependence of the maximum achievable total charge in avalanches vs.rate. 1- micropattern detectors. 2-micropattern detectors with preamplification structure.

Gain, Rate and Position Resolution Limits of Micropattern Gaseous Detectors

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