cPc(HEP&NP),2010,34(1):8387Chinese Physics Cvol.34,No.1,Jan.2010Experimental study on the performance of asingle-THGEM gas detectorAN Zheng-Hua(安正華)21) LU Jun- Guang(呂軍光) WANG Zhi-Gang(王志剛)2 CAI Xiao(蔡嘯)DONG Ming-Yi(董明義)2 FANG Jian(方建)} Hu Tao(胡濤)}LUQi-Wen(呂綺雯)23NING Fei-Peng(寧飛鵬}3SUNL-Jun(孫麗君) suN Xi-Lei(孫希磊)2WANG Xiao-Dong(王曉東)3 XUE Zhen(薛鎮(zhèn))14 YU Bo-Xiang(俞伯祥)ZHANG Ai-Wu(章愛武)2 ZHOU L(周莉)} ZHENG Yang-Heng(鄭陽恒)21 Institute of High Energy Physics, CAS, Beijing 100049, China2 Graduate University of Chinese Academy of Sciences, Beijing 100049,China3 Shanxi University, Taiyuan 030006, ChinaUniversity of Science and Technology of China, Hefei 230026, ChinaAbstract A kind of thick GEM-like gaseous electron multiplier(THEM), which is mechanically an expansionof the GEM with its various dimensions being enlarged, is studied. The leak current of THGEM plates ismeasured. The effective gain and energy resolution of a single THGEM are studied with a source of SFe,andhe effective gain of the single THGEM versus the electric field strength in the induction region is investigatedThe results show that the leak current of THGEM plates is less than 200 pA. In an atmospheric-pressurestandard gas mixture, 8x10 effective gain and about 32% energy resolution can be reached for the single-THGEM detectorKey words gas electron multipliers, hole multiplier, effective gain, energy resolutionPAcs29.40.-n,29.40.C8,29.40Gx1 Introductionand 70 um holes arranged in a triangular pattern with140 Hm between centers. Generally the configura-Hole multipliers, because they are especially at- tion of the micron holes of GEM is double-conical.tractive, have been the subject of numerous studies THGEM is fabricated with a standard printed-circuitand widely applied in the field of particle physics ex- board(PCB )technique. The electrode of THGEMperiments. The famous and extensively studied hole made by drilling millimetric holes in a 0.5-2 mm thickmultiplier is the gas electron multiplier (GEm)1. Cu-plated G-10 PCB around each hole chemicallyWhat we describe here is a thick GEM-like gaseous etched the rim by about 0.1 mm [4 This similari-electron multiplier(THGEM)(2 which is mechani- ty of structure and difference in the dimension scalescally an expansion of GEM, operating at atmospheric give a similar operation principle and different experpressure. THGEM's structure is similar to that of iment results in THGEM and GEMGEM, but it has a geometrical dimension expansionThe THGEM operation principle is similar to thatIn comparison with a standard GEM, the spatial res- of standard GEM. Upon application of a high volt-olution of THGEM (sub-millimeter)is not as good as age difference across the electrodes of THGEM,aGEM's(a few tens of microns)[3, but THGEM is strong electric field(Ehole) is established within themore robust, cheaper and can reach higher gain than holes. Calculating the strength of the electric field ofGEMTHGEM by MAXWELL 2D 5], the Hole within theMost of the GEMs used are of a standard design: hole can reach a maximum of 25 kv/cm. Electrons50 um thick kapton foil with 5 um copper electrodedinami- he drift region drift中國煤化工Received 19 January 2009, Revised 10 February 2009CNMHGSupported by National Natural Science Foundation of China(10675146, 10775151, 10775181)1Email: anzh@ihep.accn@2009 Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Sciences and the Instituteof Modern Physics of the Chinese Academy of Sciences and IOP Publishing LtdC(HEP NP)ol.34towards the THGEM under the drift field(Ep)and and pitch 1 mm. Then the copper was etched at athen are focused into the dipole field within the 60 um distance around the hole's rim(Fig. 1).Themicron-hole. The electrons are multiplied within effective area of THGEM is 50 mmx70 mm, and therethe hole under a strong electric field Hole. In an are 3500 holes on the plateatmospheric-pressure standard gas mixture, the ef-Because it is a special new product for the PCBfective gain can reach 10-10 for a single-THGEM industry, the THGEM plate production process is im-GEMs 6]. 10@ effective gain of a double THGEM groups according to the detector's operata ng gelement which is about 50 times larger than a single mature. The created holes are classified into threFor the study on the performance of THGEMn effective area of 50 mmx70 mm ThGeM boardwas produced. The multiplier property and the energy resolution of single THGEM were studied witha source of 56Fe 5.9 ke V X-rays2 The structure of the Them elec-The THGEM plates were designed by us andthen were produced in the PCB industry 7. FR-4(S1141), which is a type of material commonly usedfor making PCBs in industry, is used for the PCBinsulator, because of its good electrical quality es-Fig. 1. A photograph of a 1 mm thick THGEMpecially in terms of fame resistance and dielectricwith 0.3 mm hole diameter, 1 mm pitch andwithstanding and excellent mechanical process abil-60 Hm etched copper rim, preventing dis-ity. The THGEM electrodes are made of double-sidedtarge at high potentialscopper-clad plates. The thickness of the insulatoris 1 mm(=0.04 mm) and the variation of the plate(a) Good holes: The structure of the hole is idealthickness is lower than 3%. The thickness of the cop(Fig. 2(a)). The electric field concentrates on the cen-per electrode is 25 um. The insulator was first drilled ter of the hole. These holes can tolerate higher volwith a hexagonal pattern of holes of diameter 0.3 mmI tages and run stablyabLE natFig. 2.(a)a photo of a typical good hole;(b)a photo of a typical bad hole(b)Flawed holes: Flawed holes are due to techni- Tocal error. The center of the drilled hole and etched shotL中國煤化 Itie tence errohole was not precisely centered, there is more thanC MH Due to the etching10 um displacement between the two centers. Some- process. Some parts of the rim around the hole aretimes this situation appears in all holes of the plate. not etched at all and there are some protrusions onAN Zheng-Hua et al: Experimental study on the performance of a single- THGEM gas detectorthe rim of the etched copper. As shown in Fig. 2 (b), 3.1 The measurement of leak current ofthese holes will induce discharge when high voltageTHGEM platesis supplied. The detector will not operate properlyat all. In all the area, the ratio of the bad holes isIn air, with relative air humidity lower than 30%small, and this is not a serious problem for the imag-and temperature 24" C, we measured the leak currenting of X-rays or spacial resolution for experimentsof three THGEM plates. The leak current of theSo when bad holes are found in experiments, we deal THGEM plates was measured from 100 V to 3000 Vwith these holes to prevent them from discharging in at 200V intervals. The results of the measurementare shown in Fig. 4. The leak currents of the threeoperation.THGEM plates are all lower than 200 pA, when theFigure 3 is a schematic diagram of the single- high voltage across the THGEM electrodes is underTHGEM detector. The gap of the drift region is3 kv. That means that the insulation resistance of5 mm. The distance between the induction electroand the THGEM plate is 3 mm. The signal was readthe three THGEM plates is larger than 15 T Ohmout by an induction electrode using a standard PCBboard with small pads5 mm drift EpTHGEM富〓¨富〓富邵富z富富3 mm induction E,15Fig. 3. Single-THGEM detector set-upig. 4. The leak current of three THGEM plates3.2 THGEM effective gain and multiplierTo independently vary the electric field of differentregions, individual high voltage power(CEAN N47IAThe effective gain of the single THGEM has beenand N126) is supplied to every electrode. Also there tested. The effective gain as a function of electricis a 110 M Ohm serial resistor added to limit the field strength of induction(E1)has also been studeventual discharge current. The operation gas of the ied. In measurement, we use a 130 um transparentTHGEM detector is Ar/Co2(70: 30)at atmospheric- electrode as the drift electrode and a standard PCBpressure, and the style of the gas supply is flowboard with a small pad of 3 cmx3 cm area as theinduction electrode3 Result and discussion(1)The effective gain of THGEMFigure 5(a)shows the effective gain of the single THGEM as a function of high voltage acrossLeak current measurements of THGEM plates the THGEM electrodes(VTHgem), in Ar/CO2(70: 30)were obtained when high voltage was added across with Ep= 2 kV/cm and E1=5 kv/cm. The effectivethe THGEM electrodes by a picoammeter(KEITH- gain of THGEM increased with VTHGem by a good ex-LEY 6485). When studying the multiplier property of ponential way. When VTHGEM =2625 V, the effectivethe single THGEM detector, a preamplifier(ORTEC. gain of the single ThGEM detector can reach 8x10142AH), connected with a linear amplifier(ORTEC This result is just 2 times less than the triple-GEm's570), coupled with a readout using a multichannel gain中國煤化工analyzer(ORTEC 916)was applied. The electronic(2duction fieldchain sensitivity and linearity were calibrated for ef-CNMH gain of the sinfective gain and energy resolution measurements, us. gle THGEM as a function of El, in Ar/CO2(70: 30)ing a calibrated capacitor directly connected to the with Ep 1.5 kv/cm and VTHGEM= 2550 V.Thepreamplifier input and to a precision pulse generator. result shows that the effective gain of THGEM canChinese Physics C(HEP NPVol 34reach a saturation state and the avalanched electrons Fe source was collimated by a 2 mm thick lead withare almost fully collected by the induction electrode a 2 mm diameter hole The result shows that the es-when the electric field in the transfer region is over cape peak of Ar is obvious, and the energy resolution4.5 kV/cm.of the 5Fe 5.9 kev X-ray full energy photo-electronpeak is 31% FWHM20F THGEM85.9 keV X-ray31%86422402452.55200300400500Rory/kvchannel50503050.10.203Ep/(kv/cm)Fig. 5.(a) Effective gain of the single-THGEMmultiplier, measured in Ar/CO2(70:30)withEp=2 kv/cm and Et=5 kV/cm;(b)Ef-fective gain of single-THGEM as a function of0505El, in Ar/ CO2(70: 30)with ED= 1.5 kv/cmand VTHGEM= 2550 v.3. 3 X-ray energy resolution of the THGEMdetector2302.352402452502552.60The energy resolution of the THGEM detectorwas measured with a 5Fe 5.9 keV X-ray. Fig. 6(a)Fig. 6.(a) An energy spectrum with a 5.9 kevshows the energy spectrum of 5Fe In measurementFe x-ray in a single-THGEM, Ar/ CO2we used a 20 um thick aluminum polyester foil as the(70: 30)mixture, effective gain 1.2x10drift electrode and a standard PCB board with a pad(b) Energy resolution as a function of Ep, inof 7 cmx8 cm area as the induction electrode. A driftAr/CO2(70 30 )mixture with Et =4 kv/cmgas gap of 5 mm was added in front of the THGEMand VTHGEM=2535 V;(c) Energy resolutionelectrode, with a drift field of Ed =0.33 kV/cm,a function of VTHGEM, in Ar/CO2(70-30)中國煤化工cm and E=VTHGEM=2525 V and E 4 kv/cm. The de-tector was operated at an effective gain of 1. 2x103CNMHGPulses were recorded via a charge-sensitive preampli-Figure o(b) presents tne energy resolution as afier(ORTEC 142AH)and a linear amplifier(ORTEC function of Ep. With the variation of Ep, the en-570), on a multi-channel analyzer (ORTEC 916). The ergy resolution changes dramatically. When EpNo. 1AN Zheng-Hua et al: Experimental study on the performance of a single- THGEM gas detector0. 35 kV/cm the best energy resolution of the sin- detector will have wide application in the field of par-gle THGEM detector can be obtained. The rea- ticle physics experiments for large area sub-millimeterson for this is that the pitch between two holes is localization resolution at atmospheric pressure. Iflarge(1 mm)compared with the diameter of the hole coupled to a neutron converter or other gaseous or(0.3 mm), and the drift direction of ionized electrons solid radiation converters(e.g. a photocathode), thewill be changed by a strong Ep. That will mean THGEM detectors will have more fields of applicarthat the electrons cannot be focused into the holes tionof THGEM by the dipole field of the hole. So the en-From the results of the experiment, the THGEMergy resolution becomes worse. Fig. 6(c)shows the detector has the following propertiesenergy resolution as a function of VTHGEM. with the(1)The voltage across the THGEM electrodesincrease of VThGEM, the effective gain of the THGEM is much higher than GEM. In an atmospheric pres-increases and the energy resolution of THGEM be- sure standard gas mixture, the voltage of the sincomes better. When Vthgem is over 2.5 kv, the en- gle THGEM with 1 mm thickness is 2.2 kV for theergy resolution is 32% and it is stableavalanche process. For a double-THGEM multiplierThe reason for unsatisfactory energy resolution the total high voltages will go up to more than 9 kVmay be caused by two reasons: 1. The effective gain This requires a high quality, high voltage system.of the single THGEM is not large enough and the(2)The effective gain of THGEM as a function ofratio of signal to noise is small. This will make the En is simple. The detector can work at a strong Erenergy resolution worse. 2. The immature produc3)The energy resolution of THGEM is dramati-tion process of the tHeM plate leads to the non- cally influenced by Ep. Too strong an Ep will makeuniformity of the electric field of THGEM holes. The the energy resolution worse. In order to get a goodifference of the electric field of the holes makes the energy resolution, a suitable Ep is determined by theenergy resolution worse.pitch of the THGEM holes and the strength of thedipole field of the holes. The condition of getting a4 Conclusiongood energy resolution is that all the ionized eleare focused into the hole to avalanche(4) When THGEM is operated in high gain mode,THGEM is a robust and stable gaseous detectorscharge eventually occurs. But generally smallDue to the large hole size, the THGEM detector has number of discharges will not damage the THGEMa lower requirement for environment of operation and electrode plate. Investigation of the long-term gainhas little chance to discharge, even if there is a little stability of THGEM needs more timedust on it The leak current of Them is smallIn summary, the THGEM is a new gaseous detec.generally lower than 200 pA when the high voltage tor with wide potential applications in many fieldsacross the THGEM electrodes is under 3.0 kV. This The optimization of THGEM needs a systematicmeans the insulation resistance of THGEM is larger study. The spatial resolution of THGEM as a func-than 15 T Ohm. An effective gain of 103-104 and tion of size and pitch of the hole is the subject of ourfast pulses with rise time in the ten nanosecond scale subsequent research. The rim width versus effectiveare reached in the single-THGEM detector. Higher gain is also an important subjectgains of single and double THGEM have been ob-served [4], after optimization of the insulator widtHWe wish to express our thanks to Prof. Wang Pei-of the rim and other geometrical dimensions. Because Liang for the useful information about calibration asthe THEGM electrodes can be produced by an eco- well as to Prof Heng Yue-Kun and Dong Jing fonomic and simple method, the THGEM multiplier- their suggestions and materialReferences5 MAXWELL 2D Student Version, ANSOFT Co. Pittsburg1 Sauli F Nucl. Instrum. Methods A, 1997, 386: 531-534PA. USA2 Chechik R. Breskin A. Shalem C. Nucl. Instrum. Methods6中國煤化工 natrumA,2004,535:303-3083 Ledermann B, Kaminski J, Kappler S et al. Nucl. InstrumHCNMHGology Co, Shenzhen,Methods A. 2007. 5814 Shalem C, Chechik R,B30a et al. Nucl. Instrum. 8 DONG Jing, XIE YI-Gang et al. HEP NP, 2007, 31(8)ethods A, 2006, 558664-668(in Chinese)