1. 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.

2. Energy Resolution of the Double GEM Detector 5.4 keV collimated x-ray, Ar+20% CO2. FWHM ~ 17%

3. 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

4. Double GEM Gas Gain Uniformity Collimated 5.4keV x-ray (~1mm2), scanned the detector with a 1mmx1mm grid, over 9cmx9cm area. Pulse height histogram of all entries on the map Relative Amplitude

5. Ion Feedback in Single GEM Chamber Ion Feedback is defined as the current ratio between the window and the anode: fi = - Iw / Ia

6. Ion Feedback in a Double GEM Chamber • Other factors: • Induction field • GEM voltage • Transfer field • Asymmetry in the two GEM’s gains

7. 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)

8. Ion Feedback under 1%

9. “Line Response” of a Fine Zigzag Pattern 5.4 keV x-ray beam (0.1mmx3mm) stepped at 100µm intervals, center of gravity algorithm Overall rms position error: 93µm Including ~ 100µm fwhm x-ray photoelectron range, 100µm beam width, and alignment errors.

10. Intermediate Strip Patterns Other interpolating pad designs and their x-ray uniform irradiation responses Two Intermediate Strips Single Intermediate Zigzag

11. 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.

12. 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.