1 The GEM Readout Alternative for XENON Uwe

1 The GEM Readout Alternative for XENON Uwe Oberlack Rice University PMT Readout ionization charge GEM Readout down drift t=z/v ionization charge scintillation light drift down up conversion to charge on Cs. I photocathode scintillation light drift t=z/v conversion to charge on Cs. I photocathode drift tmax charge extraction from liquid to gas conversion to UV light and proportional multiplication conversion to charge signal and amplification in PMTs up tmax charge extraction from liquid to gas charge multiplication in multi-GEM structure charge readout on 2 D strip anodes conversion to charge on Cs. I coating of lowest GEM

2 The Gas Electron Multiplier GEM A GEM (F. Sauli, 1997) is a thin metal-insulatormetal structure, densely perforated with small holes. A voltage across the metal layers generates a sufficiently strong field within the holes to focus the electrons and multiply them. Drift Copper Amplification Polymer foil The GEM is technically realized at CERN through copper-coating on 50 mm thick kapton (polymer) foil, with chemically etched holes of conical profile. A Transfer standard GEM has a hexagonal pattern of 70 mm diameter holes in the metal, 55 mm in the foil, with a pitch of 140 mm. A 2 D readout of strip anodes on the transfer side of the GEM can provide ~ 1 mm spatial resolution.

3 Advantages of a GEM Readout 1 Compactness. 1 Large area devices. 1 Low intrinisic radioactivity. 1 Low cost and good availability (CERN). 1 High efficiency. 1 High (~1 mm) 2 D spatial resolution for added background suppression. 1 Tested performance in various largescale applications: COMPASS, HERA-B, . . . 1 Tested performance with pure xenon gas. 1 Expertise: GEM inventor F. Sauli is a XENON collaborator.

4 Multi-GEM Structures and Cs. I Coating The charge gain can be increased by employing several GEMs in a sequence. This is particularly important for pure noble gases, where the gain of a single GEM is limited to ~100. Recent tests with a triple-GEM structure, operated with pure xenon gas at room temperature, demonstrated achievable gains of several 1000 at pressures of 1 -2 atm. (A. Bondar et al. , prepr. physics/0103082) For the conversion of upwards orientated primary scintillation photons that escape through the liquid/gas surface, the lowest GEM can be coated with Cs. I. Recent results on Cs. I-coated multi-GEM structures showed excellent performance as gas avalanche photomultipliers. (D. Mörmann et al. , NIMA 471, 333 [2001] and prepr. WIS/12/01 -jun-DPP Weizmann Inst. , Israel)

5 GEM Implementation in the XENON Detector 1 Triple-GEM structure with Cs. I coating. 1 Mesh stearing electrode to tune field for optimum charge transmission and photoelectron extraction from the Cs. I. Replace 1 Double-sided PC board with X/Y strips for fine spatial resolution. 1 Low-noise electronics for optimum thresholds.

6 Studies of a GEM Readout for XENON at Goals: 1. Explore the feasibility of a combined charge and photon readout with a Cs. I coated multi-GEM structure for XENON. 2. Develop a design for integration in the 10 kg module. Ways: 1 Set up a small test chamber and gas purification system. 1 Assemble a Triple-GEM structure with a simple readout, procuring the GEMs from CERN. 1 Evaluate the performance of a 3 -GEM structure at the operating conditions of a 2 -phase xenon detector, wrt. maximum gain, impact on Xe purity, stability with time, . . . 1 Evaluate the detection of primary scintillation light with a Cs. I coated GEM structure and stearing electrode. 1 Convert the experience into a design for the 10 kg module.