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.
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 detectors gain has a dependence on photon flux, even down at kHz/mm 2 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 –TPCs active area is larger than CERNs GEM foil capacity Drift properties of the TPC gas GEM Operation in pure CF 4 –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.