Development of the ProtoEXIST2 advanced CZT detector plane

The ProtoEXIST program was conceived for the development of a highly scalable detector plane architecture utilizing pixilated CdZnTe (CZT) detectors for eventual deployment in a large scale (1-4 m² active area) coded aperture X-ray telescope. Development is now underway for ProtoEXIST2, which ultimately will be comprised of a closely tiled 8×8 array of 19.9 mm × 19.9 mm, 5 mm thick Redlen CZT crystals, analogous to ProtoEXIST1, but will now utilize the NuASIC which accommodates the direct bonding of CZT detectors with a 32×32 pixilated anode with a 604.8 μm pixel pitch and a guard ring surrounding the perimeter of the anode pattern. Currently 6 prototype detectors have been fabricated and tested. An energy threshold of approximately 6 keV and energy resolutions of 2.5 keV FWHM at 122.0 keV and 2.1 keV at 59.6 keV have been achieved with individual detectors. The performance and energy resolution of the individual prototype detectors is discussed in detail. In addition, during preliminary testing of the prototype ProtoEXIST2 detectors reduced photopeak efficiency and pixel distortions were observed for pixels within approximately 3 mm of the CZT edge. These distortions have been largely corrected with two independent techniques: the application of an 73 mm × 73 mm extended cathode centered directly above the detector and the use of a grounded, 6mm tall, electrostatic side shield surrounding the perimeter of the detector. The effects of the side shield and extended cathode are covered in detail.