Continuing previous aging measurements with detectors based on the Gas Electron Multiplier (GEM), a $31\times 31$cm$^2$ triple-GEM detector, as used in the small area tracking of the COMPASS experiment at CERN, was investigated. With a detector identical to those installed in the experiment, long-term, high-rate exposures to $8.9$keV X-ray radiation were performed to study its aging properties. In standard operation conditions, with Ar:CO$_2$ (70:30) filling and operated at an effective gain of $8.5\cdot 10^3$, no change in gain and energy resolution is observed after collecting a total charge of 7mC/mm$^2$, corresponding to seven years of normal operation. This observation confirms previous results demonstrating the relative insensitivity of GEM detectors to aging, even when manufactured with common materials

Altunbas, M C

Dehmelt, K

Kappler, S

Ketzer, B

Ropelewski, Leszek

Sauli, Fabio

Simon, F

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN{EP/2001-09111 December 2001Aging Measurements with the Gas Electron Multiplier (GEM)M.C. Altunbasc, K. Dehmeltd, S. Kapplerab,B. Ketzera, L. Ropelewskia, F. Saulia, F. SimoneaCERN, CH-1211 Geneva 23, SwitzerlandbInstitut fur Experimentelle Kernphysik, Karlsruhe University, GermanycState University of New York, Bualo, U.S.A.dInstitut fur Kernphysik, Mainz University, GermanyeDepartment E18, Munich Technical University, GermanyABSTRACTContinuing previous aging measurements with detectors based on the Gas Electron Mul-tiplier (GEM), a 31  31cm2triple-GEM detector, as used in the small area tracking ofthe COMPASS experiment at CERN, was investigated. With a detector identical to thoseinstalled in the experiment, long-term, high-rate exposures to 8:9keV X-ray radiation wereperformed to study its aging properties. In standard operation conditions, with Ar:CO2(70:30) lling and operated at an eective gain of 8:5  103, no change in gain and energyresolution is observed after collecting a total charge of 7mC/mm2, corresponding to sevenyears of normal operation.This observation conrms previous results demonstrating the relative insensitivity ofGEM detectors to aging, even when manufactured with common materials.Keywords: Gas Electron Multiplier; GEM; high rate radiation detector; aging measure-mentsPaper presented at:International Workshop on Aging Phenomena in Gaseous DetectorsDESY/Hamburg, October 2001submitted to ELSEVIER PREPRINT1 IntroductionThe Gas Electron Multiplier (GEM) [1] is a two-side copper-clad Kapton foil, perforated with a highdensity of holes. Etched in a photolithographic pro-cess, these holes have diameters of about 70m andare placed with a pitch of usually 140m (standardgeometry), as shown in gure 1.Application of a potential dierence between up-per and lower copper layer creates a high electriceld inside the holes. Figure 2 illustrates, that in-serted in the drift gap of a gas detector, nearlyall drifting electrons are guided into these holes,where they undergo proportional gas amplication.A large fraction of the electrons is released into thevolume below the GEM, resulting in eective gainswell above 103.Stacking several GEMs in a cascade leads to themulti-GEM detector. The total eective gain ob-tained with such a detector can be adapted to theneeds of the application choosing the number ofGEMs and the voltage across each one. The majoradvantage of the GEM detector is the separationof gas amplication and readout stage. This sep-aration results in a fast readout signal (only elec-trons are collected by the readout structure), andprovides a margin of safety in case of discharges oc-curring in the GEM foils. Furthermore, it also al-lows high exibility in the geometry of the readoutstructure so that a two-dimensional detector read-out becomes easily realizable.2 Previous aging measurementsAs already demonstrated in previous measure-ments [2], GEM detectors seem a priori less sen-sitive to aging than other micropattern devices.Figure 3, for instance, shows the result of mea-surements with a small double-GEM detector inAr-CO2(70:30). There, the two standard geome-try GEMs were glued to frames (G10) and stackedbetween a drift cathode of aluminized Mylar anda one-dimensional strip readout inside a breglass(Stesalit) box. A rubber O-ring guaranteed gas-tightness between box and top frame that has athin window. With GEM voltages of 410V each,electric elds of 2.0kV/cm in the drift gap, and4.0kV/cm in the other gaps, an eective gain of2:2  103was achieved. On a spot of 2cm2, the de-tector was irradiated with 6keV X-rays at a rateof 4:75  104Hz/mm2, resulting in a current densityof 3.7nA/mm2. A small single wire proportionalcounter (SWPC) was installed in the same gas line,behind the GEM detector. After a total collectedcharge of 12mC/mm2, the GEM detector did notshow any loss in gain, whereas the SWPC did. Thismight be explained by the presence of outgassingor polymerization products, which triggered agingin the SWPC, but did not harm the GEM detec-tor, demonstrating its relative insensitivity to aging.(We remind the reader, that if operated in clean Ar-Figure 1: Electron microscope photograph of aGEM foil with standard geometry (p = 140mpitchand D = 70m outer hole diameter).Figure 2: Electric eld lines in a GEM foil. Thedotted ring indicates the region of highest chargedensity.CO2, a SWPC would not show any decrease of gainafter this amount of accumulated charge.)As a necessary complement to these measure-ments performed with small detectors and undersomewhat ideal conditions, a triple-GEM detectorfrom the production line of the COMPASS experi-ment was investigated as described in the followingsections.3 The COMPASS triple-GEM detectorsThe COMPASS experiment (COmmon Muonand Proton Apparatus for Structure and Spec-troscopy) is a xed target experiment at CERN'sSuper Proton Synchrotron (SPS), built for the in-vestigation of hadron structure and hadron spec-troscopy using high intensity muon and hadronbeams [3]. In its nal setup, this experiment will2Figure 3: Previous aging measurement of a double-GEM detector with Ar-CO2(70:30): eective gainversus accumulated charge dQ=dA.Figure 4: Microscope photograph of the strip read-out. (Pitch 400m, strip width 70m in the upperand 350m in the lower layer.)use 20 triple-GEM detector modules with an activearea of 31 31cm2for small area tracking.The design of the COMPASS GEM detectors wasoptimized to satisfy the low material budget re-quirement of the experiment while achieving goodspace and time resolution properties.In order to minimize probability and energyof spark discharges, the gas amplication is dis-tributed to three GEM foils, which are on one sidesegmented into 12 sectors plus a central disk, andpowered by a resistive voltage divider. At nominaloperation, the three GEMs have a potential dier-ence of 425V on the rst, 380V on the middle, and340V on the third GEM. The electric elds are cho-sen to 2.5kV/cm in the drift gap and 3.7kV/cm inall other gaps. By lowering the voltage on the cen-tral disk sector of the lowest GEM, this region canbe deactivated to allow operation with the high in-tensity beam traversing the detector volume.The detectors are equipped with a two-dimensional projective strip readout, consisting oftwo layers of 768 strips at 400m pitch (70mwidthin the upper, 350m in the lower layer), engraved onthe two sides of a 50m Kapton foil. With the Kap-ton removed in the interstecies of the upper strips,Figure 5: Exploded view of the COMPASS triple-GEM detector design.both layers are opened for charge collection (see g-ure 4).As shown in the exploded view in gure 5, thestrip readout is glued to a 3mm thick carrier struc-ture made of honeycomb, which is covered by125m Stesalit skins on each side. The GEMs areglued to three breglass (Vetronite) frames of 2mmthickness, which also contain spacing grids to main-tain the gap width constant over the whole area. Toavoid possible charging up eects or discharges closeto these grids, their surface is smoothed with a thinlayer of Polyurethane. The last frame is made of3mm thick Stesalit, and carries the drift cathode,consisting of a 50m thick Kapton foil with 5mcopper. For stability reasons, the whole structureis closed by another 3mm carrier honeycomb, result-ing in a total module thickness of 15mm at a radi-ation length of 7:19o=ooonly [4]. The counting gasis supplied through 0.5m long Polypropylene tubesand distributed via grooves machined into the up-permost frame. The component assembly was donewith the non-outgassing glue Araldit AY103 plushardener HD991 [6]. For completion, all compo-nents and materials used for detector productionare listed in table 1; a more detailed description ofthe detector construction can be found in [5].Operated in Ar-CO2(70:30) at an eective gainof 8:5103, this detector achieves a spatial resolutionof 45m for minimal ionizing particles (MIPs) anda time resolution of 15ns [5]. Thus, with a totaleÆciency of more than 98% and stable operationeven at highest beam intensities, all requirementsof the experiment are met.3Figure 6: Photograph of the aging measurementsetup: X-ray tube and triple-GEM detector withshielding.4 Aging study setupTo investigate aging properties, the setup shownin gure 6 was used to irradiate the GEM detectorwith a high-rate X-ray beam. A small SWPC forreference measurements was installed in the samegas line in front of the GEM detector. Charge sen-sitive ampliers were used to regularly record lowrate pulse height spectra of both coordinates of theGEM detector and the SWPC. Furthermore, thecurrents in the GEM detector and the rate of theX-ray tube were monitored, and sensors for ambi-ent parameters like temperature, relative humidity,and atmospheric pressure were read out.As in the COMPASS experiment, the gas wassupplied with an open ow system. A gas bottlewith Ar-CO2premixed in a 70:30 ratio was con-nected via a pressure reducer and a 10m long stain-less steel gas line to a mass ow control unit. Theconstant ow of 80cc/min was fed to the SWPC andthe GEM detector with short Polypropylene tubes.The GEM detector was directly connected to a hy-grometer and a 10m plastic exhaust gas line, avoid-ing the use of any bubbler. The detector volume of0.85 liters was exchanged 5.6 times per hour, some-what more frequent than in the experiment, wherewe have 3.5 volume exchanges per hour and twodetectors in series per gas line.One quarter of the detector (15  15cm2) wasexposed to the 8.9keV X-ray beam with an approx-imately Gaussian prole of  = 20mm, thus with asurface of 12.6cm2inside the one--line. The de-Figure 7: Ambient parameters versus time. (Theaccuracy is 1oC for temperature, 3% for relativehumidity, and 0:5hPa for atmospheric pressure.)tector was irradiated in two phases: 10 days at6:3 103Hz/mm2in the beam center, and 10 days at2:5 104Hz/mm2. Operated at the nominal eectivegain of 8:5  103, this resulted in a current densityof 2:5nA/mm2and 10nA/mm2, respectively. Dur-ing the whole irradiation, the following measure-ment cycle was performed: after each ten minutesof irradiation, a shutter in front of the X-ray tubewas closed, the low rate pulse height spectra as wellas all other parameters recorded, and after reopen-ing the shutter the irradiation continued. In orderto measure eective gain G and energy resolution,a Gaussian function was tted to the pulse heightspectra, and position and full-width-half-maximumof the photopeak determined.5 ResultsThe variations of the ambient parameters dur-ing the aging measurement are plotted in gure 7.Whereas the relative humidity of the atmospherevaried between 37:9% and 50:2%, the H2O concen-tration in the counting gas measured with the hy-grometer at the output of the detector was constantwithin 2ppm at 60ppm. The crucial importance ofthe H2O concentration for the discharge probabilityin GEM detectors has been demonstrated recently[5]: with a double-GEM detector, a change from81ppm down to 35ppm lowered the discharge prob-ability by almost three orders of magnitude (at again of 8:5  103). During the aging measurementthe currents on the high voltage power supply weremonitored and not a single discharge has been ob-served.The inuence of the variations of tempera-ture (21:4oC to 25:5oC, thus 1:4%) and pressure(1008:4hPa to 1025:7hPa, thus 1:7%) on the gasgain are not negligible and had to be considered.Therefore, a corrected and normalized gain g wascomputed from the eective gain G according togi:=GiAi eBiT=p; (1)4Material Details Producer / SupplierAssembly glue ARALDIT AY103 + HD991 (ratio 10:4) CERN storeGrid conditioning NUVOVERN LW Walter Mader AG,(2 component Polyurethane) CH-8956 KillwangenCarrier structure 2 125m STESALIT Stesalit AG, CH-4234 Zullwilon 3mm NOMEX honeycomb Socol, CH-1020 RenensShielding Aluminium (10m) CERN storeGEM foils 2 5m Cu on 50m KAPTON CERN/ESTDrift 5m Cu on 50m KAPTON CERN/ESTFrame 3mm STESALIT CERN/EP/TA1Spacers 2mm VETRONITE grids CERN/ESTGas pipes PP tube (3mm diam.) Angst-Pster, CH-8052 ZurichGas outlet F-glass + tting CERN/EP/TA1Microstrip readout Cu microstrips on 50m KAPTON CERN/ESTHV boards Custom made CERN/ESTTable 1: List of components and materials used for the production of the COMPASS triple-GEM detec-tors.Figure 8: Correlation of detector gain and temper-ature & pressure: pulse height versus T=p and theexponential ts.where i is an index for the two coordinates of theGEM detector and the SWPC and T=p the ratioof temperature and pressure. Aiand Biare tparameters, determined by tting the exponentialfunctionGi(T=p) = Ai eBiT=p(2)to the correlation plots of gure 8 (in order not tobias the aging study, only points from the beginningof the measurement were used). The exponentialdependence in equation (2) is deduced by assuminginverse proportionality of the Townsend coeÆcient to the mass density , and thus  / 1= / T=p.The results of the aging measurement are givenin gure 9. The accumulated charge dQ=dA isgiven for the one--line of the beam prole, thus thecharge collected in the center was higher by a factor1.65. One clearly recognizes smaller uctuations of7% in the normalized corrected gain of the GEMdetector, completely in phase with the SWPC, andwithout any reduction of the gain. Apart from smallFigure 9: Normalized corrected gain and energyresolution versus accumulated charge dQ=dA. Thespikes marked with a star () indicate an exchangeof the gas bottle.uctuations, also the energy resolution was constantin each of the two irradiation phases, the improvedenergy resolution at the beginning of the secondirradiation phase is only due to a higher signal tonoise ratio. Indeed, in order to compensate the rate-induced voltage drop in the GEM foils, the volt-age on the resistive voltage divider was slightly in-creased, resulting in higher GEM voltages and thuslarger signals during the low rate recording phaseof the spectra.After accumulating 7:1mC/mm2on the one--line (11:7mC/mm2in the center), the measurementwas stopped and a nal scan carried out: pulseheight spectra were recorded in nine dierent po-sitions, one in the center of the beam prole, fourat 19.2mm, and four at 27.2mm distance from thecenter. All spectra were unchanged, without anyloss of gain and energy resolution.56 ConclusionsThe aging properties of the triple-GEM detec-tors built for the COMPASS experiment at CERNwere investigated. These low-mass detectors withan active area of 31 31cm2are built with a choiceof materials imposed by practical constraints, butmaking use of non-outgassing epoxies [6]. Oper-ated with Ar-CO2(70:30) at an eective gain of8:5  103, one quarter of the detector area was ex-posed to an 8.9keV X-ray beam. After accumu-lating more than 7mC/mm2, which corresponds to1:7  1011MIPs/mm2or seven years of normal op-eration in COMPASS, no loss of gain and energyresolution was observed.This observation conrms the results of previousmeasurements demonstrating the relative insensi-tivity of GEM detectors to aging when operatedwith clean Ar-CO2. Two reasons may explain thisresult: the fact, that the gas amplication is local-ized inside the GEM holes (far from any electrodesand walls), and the small eect of possible polymer-ization deposits on the electric eld in this region.References[1] F. Sauli, Nucl. Instr. and Meth. A386 (1997)531.[2] S. Bachmann et al., Development and test oflarge size GEM detectors, IEEE Trans. Nucl.Sci. NS-47 (2000) 1412.[3] The COMPASS Collaboration, COMPASS: Aproposal for a COmmon Muon and ProtonApparatus for Structure and Spectroscopy,CERN/SPSLC 96-14, SPSC/P 297 (1996).[4] B. Ketzer et al., GEM detectors for COM-PASS, Paper presented at the IEEE NuclearScience Symposium and Medical Imaging Con-ference, Lyon, September 2000.[5] M.C. Altunbas et al., Construction, test andcomissioning of the triple-GEM tracking de-tectors for COMPASS, paper in preparation,to be submitted to Nucl. Instr. and Meth. A.[6] R. Bouclier et al., Eects of outgassing fromsome materials on gas chamber aging, Nucl.Instr. and Meth. A350 (1994) 464.6

Aging measurements with the gas electron multiplier (GEM)

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