Amplification process of a gas electron multiplier simulated by PIC-MCC model

Nuclear Science and Techniques 23 (2012) 203- -208Amplification process of a gas electron multiplier simulated byPIC-MCC modelYANG Lanlan TU Yan Harm Tolner ZHONG XuefeiZHANG Panpan QIN NanaMA ShanleSchool of Elecronic Science and Engineering, Southeast University, Nanjing 210096, ChinaAbstract The performance of a single gas electron multiplier (GEM) in pure Xe at an atmospheric pressure isinvestigated by Particle in Cell-Monte Carlo Collision (PIC-MCC) model. The micro development processes withelectrons and ions distributions in space have been revealed. Based on the micro development processes, themacroscopic parameters such as GEM gain and the effective eficiency have also been obtained. The simulationresults indicate that after tens of nanoseconds, electrons are collected by the readout electrode while the ions still existin the gas space for several microseconds. The main signal current is formed by the electrons ariving at the readoutelectrode, but electrons and ions are also cllected by the copper electrodes near the GEM hole and the thin Kaptonfilm boundary. The simulated gain of GEM exponentially increases with the applied GEM voltage. With thePIC-MCC simulations, both the physical amplification and charging mechanisms in the GEM device can be wellunderstood, which is beneficial to the device design.Key words Gas electron multiplier, Particle in Cell-Monte Carlo Collision, Gainmultiplication processes in GEM holes, which an1 Introductionimportant for understanding the GEM amplificationGas electron multiplier (GEM) is a robust device forand charging mechanisms. In this paper, aproportional amplification of electrons released in anParticle-ln-Cell/Monte-Carlo Collision (PIC-MCC)iradiated gas by charged particles or photons. A GEMsimulation method is used to simulate the electronconsists of a thin polymer film, coated on each sideavalanche in GEM, which can distinctly reveal thewith metal, applied with suitable bias voltage, andavalanche process in the GEM apertures. Also, it canperforated by a high density of holes. It is used in ainvestigate the electron and ion motions in the devicewide range of conditions and gas fllings, for detectingseparately, and can analyze the charge distributions inGEM electrodes and GEM foils.charged particles, X-rays and single photoelectrons.The advantages of GEM-based detectors include high2 GEM simulation cell structurecounting rate, excellent spatial resolution， goodimaging capability, operation in magnetic field, largeThe basic component of GEM is a metal-coated thinsensitive area and low costl-51.Kapton film, chemically pierced with a high density ofSimulations based on the GARFIELD code andholes, typically φ50- 100 μum in a 100 -200 μm pitch.MAXWELL/ ANSYS eletric field analyzer 68 areThe polymer film is 50-μm thick, with 5-um copperuseful for optimizing electrical and geometricalcoating on both sides!9!. The holes are made byparameters of a GEM. However, a GARFIELDchemical erosion, and their typical geometry structuresimulation often fails to provide detailed particleis double-conical. The drift electrode is placed severalSupported by the Nationsl Natural Science Foundation of China under Contract No.50907009 and 60871015.中国煤化工cholarsorSoutheast University●Corresponding author, E-mail adres: jujube. yang@seu.edu.cn'fHCNMHG .Received date: 2012-02-23YANG Lanlan er al.1 Nuclear Science and Techniques 23 (2012) 203- 208203.(aBecause of their heavy mass and smaller2.5|velocity, the Xe ion distribution in space can depict the- Electronsavalanche shape. During the avalanche process, theXenon lons曼.。electrons and Xe ions are collected by the Kapton filmg 1.5boundaries and the lower copper electrodes. At 31 ns,1.0 tmost electrons pass through the aperture and move0:toward the readout electrode. And the ions movingupwards are collected by the upper copper electrode.00 200 300 400 500 600Finally, the residual electrons are collected by theTime /nsreadout electrode and generate a signal current, while0012m800F 31ims-(ban ion will either to be collected by GEM upper Cu600600-electrode or move towards the drift electrode. The里400一专400i号400process lasts for several microseconds, depending on00 ttheir drift velocity. In our simulation model, the whole50 10x1umx1 umprocess is simulated by Monte-Carlo method, which isclose to the real physical mechanisms, but moreB00F 75mprecise results can be achieved with the PIC-MCCsimulation in longer computer hours.I a00E..oo200GEM characteristics50 100x/um4.1 GEM gainFig.2 Changes in the number of electrons and xenon ions (a),While knowing the number of electrons and ions inand electron (b) distributions and xenon ion (c) distributions inspace as a function of time, we need to know thethe amplification process in the GEM.macroscopic parameters, such as the gain and theThe amplification process of electrons andeffective efficiency, so as to compare the simulationxenon ions can be observed clearly in Fig.2(b) andresults with experimental data. Here, we define the2(c). Within one nanosecond, the primary electronsvariables as follows: The total GEM gain, Gror=No/Nini,emit from the drift electrode. Then until 12ns, thewhere Not is the total number of electrons afterelectrons are at the drift region and move towards themultiplication and Nini is the number of primaryGEM hole by the drift electric field. The primaryelectrons. The effective GEM gain, Gef =Nt!Nini,electrons may collide with gas atoms, but nowhere Nerr is the number of electrons collected by theionization occurs in the drift region as the elecric fieldreadout electrode to generate a signal current.is not strong enough to cause ionization; hence noappearance of xenon ions in the drift region. In theirmovement to the GEM hole, some electrons may ber Simulated, PIC-MCC合Gep Measured, Ref[13]collected by the upper copper electrodes, while theGot= 0.1274@0196Vothers enter into the GEM hole. However, a 100%transparency of primary electrons can be achieved inGom= 030913some cases (related to the GEM hole geometry, theδ=2910electric field, the gas type, the gas pressure, etc). Inour simulation structure, no electrons are collected bythe upper copper electrodes. That is to say, all theprimary electrons enter into the GEM hole, and are300500multiplied inside the strong electric field, at several中国煤化工tens ofkV.cm' and even higher.Fig.3 The tota!l and.JYHcNMHGionofGEMvoltage (&: relative06YANG Lanlan er al.1 Nuclear Science and Techniques 23 (2012) 203- 208Figure 3 shows the simulated total andan exponential growth with GEM voltage, where theeffective gain Grot and Ger as a function of GEMslope a -0.0197 is similar to that in Fig.3. The numbervoltage applied to the copper electrodes on both sidesof electrons deposited on the upper copper electrode isof the Kapton film. Both Grot and Gef exhibit thezero (not shown in Fig.4), i.e. a 100% primary electroncharacteristic exponential avalanche growth with theentry into the GEM hole in our GEM cell structure.GEM voltage, as measured in Ref.[13]. At about 500 VThe number of electrons lost in the GEM lowerof GEM voltage, the avalanche tends to discharge, socopper electrodes is the largest, and only a smallthis point is omitted when fiting the data points.number of electrons collcted by the Kapton flmDefining a ftting formula of G=Aea", we haveboundaries. Defining the effective efficiency as thethe simulated effective gain of Ger =0.0393e0197", andratio between the effective gain Gef and total gain Giot,the simulated total gain of Gror=0.1274e.0196F Then,despite the statistical error, the effective efficiency ofthe Giot and Gefr increase with the GEM voltage atXe is around 30%，which is consistent with theexperimental data in Ref.[9]. At VGEM=500 V, thealmost the same slope (a).By rewriting the gain results as G=(4-aYVrusimulation and the experimental results are 34.1% and32.8%, respectively, with an error of 3.8%. Most of thewhere Vhr is the effective threshold voltage, and Vrefis the effective voltage, i.e. the voltage needed tosecondary clectrons due to avalanche in the GEMaperture are lost in the GEM lower copper electrodes.ionize xenon, here Vien=1/a. So, the difference incoefficient A can be described in terms of a differencein Var and Vef. Experimentally we have Ver=38 V,o Totalut electrode心GEM foilswhich is a little lower than the simulated value of+ Lower Cu electrodesabout 51 V. According to Roth J Cl4, the effectivevoltage Vieff should be 3- 4 times of 12.1 eV (xenonionization potential). Then, both the simulation andFlting: G=Aexp(aV)experimental results fit the Roth's theory, while the置10measured V-eff for pure xenon is a perfect match.At the GEM voltage of around 500V, the real30355000ions in the space is approaching 10'-108 (RaetherVauiVlimit), the accumulated space charge effect causes the40avalanche discharge. So upon this voltage, th一o- Smulaxionexponential law is broken. The quick gain increase ancExpomentthe discharge will causes damage to GEM equipments.Fig.3 also shows the gain measured in Ref.[14]at the same conditions using a single-GEM flld withXe working gas of 1.01x10' Pa. By shifting theg3oexperiment curve to the left, one can see that Ger andthe experimental gain Gexp fit quite nicely. The mainreason for this difference is due to the simplified cell350structure of our 2D model.Fig.4 The number of electrons ollcted by electrodes and4.2 Charge deposited on the boundariesGEM Kapton flms (a), and effective efficiency (b).The charge deposition on different boundaries, i.e. theFigure 5 shows the distribution of electronselectrons and ions lost at the electrodes and theand ions deposited on the left Kapton film boundary.boundaries of the Kapton flm, is simulated, too. Fig.4As the cell structure is symmetric, the distribution ofshows the total umber of electrons deposited by all electrons and ions中国煤化工Kaptonthe boundaries. It can be seen that the numbers exhibitboundary is similar|YHCNMH GelectronsYANG Lanlan et al.1 Nuclear Science and Techniques 23 (2012) 203- 20820are collected by the Kapton film near the lower copperelectrons are amplified inside this aperture by theelectrode, while most of the ions are collected by theavalanche process. However, the electrons have thKapton film near the upper copper electrode. Thesame uniform distribution at the readout electrode,number of electrons is much smaller near the uppercompared with the uniform distribution of the primarycopper electrode before avalanche, so the electrons areelectrons. It proves that GEM can be used as ancollected by the lower Kapton film boundary.imaging device. More detailed test should be followedXe ions are generated in the space near theusing special primary electron distribution shapes inlower copper electrode due to electron avalanche, butseveral cell structures.the electric field prevents them from depositing on theFig.6(b) shows the signal current on thelower part of the Kapton flm. Under the electric field,readout electrode deduced from the time gradients ofthey move upwards like a fountain, and some of themthe particles charge. The current can be measured.are deposited on the Kapton film near the upper CuTherefore, the macroscopic parameters, such as theelectrode. We note that the number of ions depositedgain and the current can be extracted from the particleson the Kapton film is tens of time less than that of thetracking.electrons. Actually, the number of charged particles100000has little effect on the external electric field.●300V. 350V .400V 450V . 500V (a).6 t10000(a- 300V.2 t1000_ 5000000040 60 80 100 120 140500000.4x1 um).2t"0.0000(b400405410415420425430435440445450y1μms -0.0005b)00008000-0.0010600050040TimeIns00 t440Fig.6 (a) The distribution of electrons collected at the readout40041420 43045electrode and (b) the signal current in the readout electrodewhen VGEM *500 V.Fig.5 The distribution of electrons and xenon ions deposited onthe left Kapton film boundary, (a) the electrons distribution, andConclusions(b) the xenon ions distribution.A 2D PIC-MCC model is presented to simulate theFigure 6(a) shows the electron distributionparticle avalanche process of the GEM. It describes incollected on the readout electrode where they generatedetail the avalanche process in the GEM holes, anda signal current. The primary electrons emit randomlyshows the special spatial dynamics of the electron andin time from the drift electrode, hence the uniformion movement in the device separately. In particular, itdistribution. They pass through the GEM hole, whichacts like an electron lens, with the upper part beinglso analyzes the ah中国煤化工the GEMelectrodes and the, the GEMconvergent and the lower part being divergent. Themacroscopic paralMH。N M H Gsimulation208YANG Lanlan et al. /Nuclear Science and Techniques 23 (2012) 203- 208model can be directly compared with the experimental3 Buzulutskov A. Nucl Instrum Meth A, 2002, 494: 148-results. The simulation results indicate that after tens155.of nanoseconds, electrons are collected by the readoutLiuJ, Lai Y F, Cheng J P, et al. Nucl Sci Tech, 2005, 16:electrode while the ions still exist in the gas space.225- 230.(in Chinese)Besides the effective charge collected by the readoutAnSH, Li C, Zhou Y, et al. Nucl Sci Tech, 2004, 15:electrode, electrons and ions are also collected by the290- 293.in Chinese)lower copper electrodes of GEM hole and the thinSharma A. Nucl Instum Meth A, 2000, 454: 267-271Kapton flm section. This however has only a minorBouianov O, Bouianov M, Orava R, et al. Nucl Instrumeffect on the GEM performance, but it is important toMeth A, 2000, 450: 277- -287.understand high voltage breakdown effects.Tikhonov V, Veenbof R. Nucl Instrum Metb A, 2002, 478:The physical mechanisms in macro and macro452- 459.are well described by the PIC-MCC simulation.Bachmann S, Bressan A, Ropelewski L, et al. NuclHowever, in order to thoroughly and deeply unveil theInstrum Meth A, 1999, 438: 376 408.physical mechanism of GEM, more detailed numerical10 Verboncoeur J P, Langdon A B, Gladd N T. Comp Physstudies are required, that also include the photon- andComm 1995, 87: 199 -211.ion-mediated secondary processes. Finally, moreBruhwiler D L, Giacone R E, CaryJ R, et al. Phys Rev STpractical geometry shape and the influence of the ga:Accel Beams, 2001, 4: 101302,type and gas pressure need to be further investigated in12 YangLL, Tu Y, ZhangPP, et al. Nucl Tech, 2011, 34:the future.543- -548. (in Chinese)References|3 Amaro F D, Conceicao A S, VelosoJ F C A, et al. NuclInstrum Meth A, 2007. 579: 62 -66.1Sauli F. Nucl Instnum Meth A, 1997, 386: 531- -534.14 Roth J C. Industrial plasma engineering, Vol.1, Principles,Bachmann S, Bressan A, Kappler S, et al. Nucl InstrumIOP Publishing, 1995.Meth A, 2001, 471: 15S-119.中国煤化工MHCNMH G