1The GEM Readout Alternative for XENON

Uwe OberlackRice University

PMT Readout

conversion to UV light andproportional multiplication

conversion to charge signal and amplification in PMTs

ionization charge scintillation light

up

conversion tocharge on CsIphotocathode

t=z/vdrift

charge extraction from liquid to gas

down

drift tmax

GEM Readoutionization charge scintillation light

t=z/vdrift

charge extraction from liquid to gas

down

conversion tocharge on CsIphotocathode

drift tmax

up

charge multiplication in multi-GEM structure

conversion to charge on CsI coating of lowest GEM

charge readout on 2D strip anodes

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The Gas Electron Multiplier GEMA GEM (F. Sauli, 1997) is a thin metal-insulator-metal 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. 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 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.

~140 mm

~70 mm

Drift

Amplification

Transfer

A 2D readout of strip anodes on the transfer side of the GEM can provide ~ 1 mm spatial resolution.

Copper

Polymer foil

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Advantages of a GEM Readout

Compactness.Large area devices.Low intrinisic radioactivity.Low cost and good availability (CERN).High efficiency.High (~1 mm) 2D spatial resolution for

added background suppression.Tested performance in various large-

scale applications: COMPASS, HERA-B, ...Tested performance with pure xenon gas.Expertise: GEM inventor F. Sauli is a XENON collaborator.

A triple-GEM module being assembled for the Small Area Tracker of the COMPASS experiment at the SPS in CERN.

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Multi-GEM Structures and CsI CoatingThe 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 CsI. Recent results on CsI-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)

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GEM Implementation in the XENON Detector

ReplaceReplace

Triple-GEM structure with CsI coating.Mesh stearing electrode to tune field for optimum charge transmission and photoelectron extraction from the CsI.

Double-sided PC board with X/Y strips forfine spatial resolution.

Low-noise electronics for optimum thresholds.

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Studies of a GEM Readout for XENON at

Goals: 1. Explore the feasibility of a combined charge and photon

readout with a CsI coated multi-GEM structure for XENON.

2. Develop a design for integration in the 10 kg module.

Ways:Set up a small test chamber and gas purification system.Assemble a Triple-GEM structure with a simple readout,

procuring the GEMs from CERN.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, ...

Evaluate the detection of primary scintillation light with a CsI coated GEM structure and stearing electrode.

Convert the experience into a design for the 10 kg module.