Connected to the Linear Collider project TESLA at DESY, studies
						on the readout of TPCs based on the GEM-technology are ongoing.
						For particle identication via dE/dx - measurement, a good
						energy resolution is indispensable, and therefore losses of
						primary electrons have to be avoided. It turned out, that in the
						GEM transverse diffusion inside or close to the holes is a not
						negligible reason for these losses. For Ar-CH4 90:10 and
						TPC-like field configurations it was found, that when operated
						in normal amplification mode, the Standard Geometry GEM should
						not lose primaries, whereas for low gains, also when operated in
						magnetic fields up to 5T, a GEM with larger pitch and hole
					diameter would be necessary

Kaminski, Jochen

Kappler, Steffen

Ketzer, B.

Ledermann, B.

Mueller, Thomas

Ropelewski, L.

Sauli, F.

English

KITopen

CHARGE CARRIER TRANSFER IN THEGAS ELECTRON MULTIPLIER AT LOW GAS GAINSJ. Kaminskia, S. Kapplerab, B. Ketzerb, B. Ledermanna,T. Mullera, L. Ropelewskib, F. SaulibaInstitut fur Experimentelle Kernphysik, Universitat Karlsruhe, GermanybCERN, EP-Division, CH-1211 Geneva 23, SwitzerlandAbstractConnected to the Linear Collider project TESLA at DESY, studies on the readoutof TPCs based on the GEM-technology are ongoing. For particle identication viadE=dx - measurement, a good energy resolution is indispensable, and therefore lossesof primary electrons have to be avoided. It turned out, that in the GEM transversediusion inside or close to the holes is a not negligible reason for these losses. ForAr-CH4 90:10 and TPC-like eld congurations it was found, that when operated innormal amplication mode, the Standard Geometry GEM should not lose primaries,whereas for low gains, also when operated in magnetic elds up to 5T, a GEM withlarger pitch and hole diameter would be necessary.1 IntroductionThe Gas Electron Multiplier (GEM) [1] is a two-side metal-clad Kapton foil, perforated witha high density of photolithographically etched holes. In the so called Standard Geometryholes of typically 70m diameter are placed in a triangular pattern with a pitch of 140m.On application of a potential dierence (usually 300-500V) between upper and lower metallayer, a strong electric eld is built up inside the holes. Inserted in the drift gap of a gasdetector, drifting electrons are guided into these holes, where they undergo proportionalgas amplication. A large fraction of the electrons then is released into the volume belowthe foil, where they can be collected by readout electronics or { in order to achieve evenCorresponding author: Steen.Kappler@cern.ch12 International Workshop on Linear Collidershigher total gains { transferred to another GEM. Large size triple-GEM detectors with atwo-dimensional microstrip readout are successfully operated in the small area tracker of theCOMPASS experiment at CERN [2]. Among other applications, extensive R&D is ongoingon an improved readout of Time Projection Chambers (TPCs) connected to the LinearCollider project TESLA at DESY [3]. Compared to a conventional TPC readout of wiresplus pads, the GEM technology oers two major advantages: higher spatial resolution andfaster signals improve the multi-track resolution, and the conguration of the electric eldlines intrinsically suppresses the ion feedback. For the eÆcient readout of a TPC, eectivegains of 103 to 104 are required. For a double-GEM structure, this would mean eectivegains between 30 and 100 per GEM.2 Charge carrier transfer in the GEMTo exploit the particle identication capability of the TPC via dE=dx - measurement, a goodenergy resolution is indispensable, and therefore losses of primary electrons have to be keptas small as possible. At GEM voltages up to 100V, the electric eld inside the holes is smallenough, that no gas amplication takes place. In this regime the transmission of electrons(and ions) through the GEM can be measured directly and possible losses ascertained [4].For this purpose, a single-GEM detector of 10 10cm2 active area, with a 10mm gap to thecathode and a 2.5mm gap to the anode was built. The drift region between cathode andGEM was irradiated sideways 1 by a high ux of 5.9keV X-rays and all resulting currentson cathode, anode, and the two GEM electrodes were measured. Figure 1 shows the resultsfor a Standard Geometry GEM in dierent gas mixtures at a eld conguration, which isthinkable for the rst GEM in a TPC, facing a drift eld of a few hundred V/cm. It wasfound, that the electron transmission t depends strongly on the transverse diusion t insideand close to the GEM hole. For pure CO2, where t ranges from 150 to 165m after 1cm,we nd almost full transmission (t = 0:98 at UGEM = 100V ), whereas operated in Ar-CO290:10 with t between 240 and 290m after 1cm, the transmission is only t = 0:67. In acomplementary measurement, the electron transmission of two dierent GEM geometriesin the same gas (Ar-CO2 90:10) was compared: the Standard Geometry and a Double-SizeGeometry. For the latter, where pitch and hole diameter are twice as big, the transmissionwas raised to t = 0:95. Nevertheless, the price to pay for the improved transmission in thisgeometry is a much smaller gas gain.Recapitulating these results, we nd, that in order to achieve a maximum transmission,the electron cloud must be suÆciently focused in top of the GEM hole, and the trans-verse diusion has to be small compared to the diameter of the region of extracting eldlines. Furthermore, it is possible to extrapolate the case without gas gain to the normal1sideways means parallel to the GEM surface; the authors want to point out that an irradiation perpen-dicular to the GEM would create electron-ion pairs inside the GEM hole, therefore modify the charging upprocess of the Kapton, and bias the transmission behaviour [4].LCWS(2002), Jeju, Korea 3Figure 1: a) Electron transmission times gain versus GEM voltage UGEM at constant driftand induction elds (ED = 150V/cm and EI = 2:5kV/cm) in dierent gas mixtures; theshaded area corresponds to the voltages where gas amplication starts to occur. b) Trans-verse diusion versus electric eld strength; the shaded area corresponds to eld strengthrange inside the GEM holes at UGEM = 100V. Computed with the MAGBOLTZ softwarepackage.Figure 2: Diusion versus electric eld strength; the light shaded area corresponds to theeld strength range inside the GEM holes at UGEM = 100V, the dark shaded area toUGEM  300V. Computed with the MAGBOLTZ software package. a) Comparison of thetransverse diusion of pure CO2 and Ar-CH4 90:10. b) Diusion in Ar-CH4 90:10 at dierentmagnetic elds parallel to the electric eld.4 International Workshop on Linear Collidersamplication mode, as illustrated in gure 2.a). For the Standard Geometry GEM at volt-ages UGEM  300V , the transverse diusion in Ar-CH4 90:10 inside the hole drops to below170m after 1cm. Since this is comparable to what was found for pure CO2 at UGEM = 100Vwith almost full transmission, one can assume that no electrons should get lost before multi-plication. Figure 2.b) shows, that in presence of a 5T magnetic eld the transverse diusionin Ar-CH4 90:10 for electric elds greater than 10kV/cm is not lowered suÆciently: the re-duction by 15% down to t  250m after 1cm (for UGEM = 100V) would not be suÆcientto get full transmission. Therefore, in a scenario where the rst GEM has to be operatedat very low gas gain, pitch and hole diameter have to be adapted, and e.g. the Double-SizeGeometry would nd application.3 ConclusionIt could be stressed, that apart from the conguration of the electric elds, which gives theessential condition for charge carrier transfer in the GEM, transverse diusion is a dominantand not negligible reason for losses of primary electrons. Depending on the operation voltageof the rst GEM, and thus its eective gain, diameter and pitch of the GEM holes mighthave to be optimized.AcknowledgementsThis work would not have been possible without the contribution of Rui de Oliveira andMiranda van Stenis. The authors would also like to thank Michael Hauschild for presentingthis paper at the International Workshop on Linear Colliders in Jeju Island, Korea.References[1] F. Sauli, Nucl. Instrum. Methods A 386, 531 (1997).[2] B. Ketzer et al., IEEE Trans. Nucl. Sci., Volume 48, Nr. 4 (2001).[3] The TESLA Collaboration, TESLA Technical Design Report (2001).[4] J. Kaminski et al., Status of GEM-TPC studies, International Workshop on Physicsand Detectors for a Linear Collider, St Malo/France (2002).

Charge carrier transfer in the gas electron multiplier at low gas gains

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Charge carrier transfer in the gas electron multiplier at low gas gains

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