GEM Chambers at BNL

The detector from CERN, can be configured with up to 4 GEMs

The detector for pad readout and drift studies, 2 GEM maximum.

Energy Resolution of the Double GEM Detector

5.4 keV collimated x-ray, Ar+20% CO2. FWHM ~ 17%

Gas Gain vs. Photon Flux

Ar+20% CO2, 5.4 keV x-rays (~1mm2), Ed=1kV/cm, Et=4kV/cm, Ei=5kV/cm, Qa~0.2pC

Double GEM Gas Gain Uniformity

Collimated 5.4keV x-ray (~1mm2), scanned the detector with a 1mmx1mm grid, over 9cmx9cm area.

90 100 110 120 130 140 150

Relative Amplitude

Pulse height histogram of all entries on the map

Ion Feedback in Single GEM Chamber

Ion Feedback is defined as the current ratio between the window and the anode: fi = - Iw / Ia

7.0)(i

d

a

w

E

E

I

I

Ion Feedback in a Double GEM Chamber

Other factors:

•Induction field

•GEM voltage

•Transfer field

•Asymmetry in the two GEM’s gains

Ion Feedback in a Triple GEM Chamber

The reduced second transfer field also results in large reduction in the effective gain (~ a factor of 5)

Ion Feedback under 1%

“Line Response” of a Fine Zigzag Pattern

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Reconstructed Position [µm]

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5.4 keV x-ray beam (0.1mmx3mm) stepped at 100µm intervals, center of gravity algorithm

Overall rms position error: 93µmIncluding ~ 100µm fwhm x-ray photoelectron range,

100µm beam width, and alignment errors.

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Reconstructed Position [µm]

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Intermediate Strip Patterns

Single Intermediate Zigzag

Two Intermediate Strips

Other interpolating pad designs and their x-ray uniform irradiation responses

Summary

• Double GEM demonstrated very good energy resolution with collimated x-

ray beam. It also exhibits somewhat a large gain variation over 9cmx9cm

area.

• The Double GEM detector’s gain has a dependence on photon flux, even

down at kHz/mm2 range. The amount of gain change varies over a GEM

detector.

• With a reasonable drift field (~1kV/cm), it is difficult to keep the ion

feedback rate under 10% for double GEM, 2% for triple GEM. To reduce

the ion feedback to below 1%, 5 GEM planes are needed.

• Interpolating pad readout for GEM with better than 100µm resolution in

one direction possible (@ 2mm pitch) with 5.4keV x-rays, with minimal

diffusion.

Further R&D Topics

• Detailed simulations to determine the acceptable ion feedback

• Spatial variation of the GEM gain – If the variation is stable over time, it can be corrected by calibration.

• Degradation of energy resolution with intermediate strip readout

• Dependence of gas gain on flux– Difficult to correct

• Join multiple GEM foils– TPC’s active area is larger than CERN’s GEM foil capacity

• Drift properties of the TPC gas

• GEM Operation in pure CF4

– Reached a gas gain of 600 on a triple GEM before HV instability

• Aging Study– Should we do our own study or rely on others’ results?

• Integration of TPC & HBD: – Design of the field cage, its optical transparency.