Imaging effects due to pixel distortions in CdZnTe detectors—results from the HREXI Calibration Facility

ProtoEXIST2 (P2) was a prototype imaging x-ray detector plane developed for wide-field time-domain astrophysics (TDA) in the 5 to 200 keV energy band. It was composed of an 8 × 8 array of 5 mm thick, 2 cm × 2 cm pixelated (32 × 32) CdZnTe (CZT) detectors with a 0.6-mm pitch that utilize the NuSTAR ASIC (NuASIC) for readout. During the initial detector development process leading up to postflight examination of the entire detector plane, distortions in expected pixel positions and shapes were observed in a significant fraction of the detectors. The High Resolution Energetic x-ray Imager (HREXI) calibration facility (HCF) was designed and commissioned to improve upon these early experiments and to rapidly map out and characterize pixel nonuniformities and defects within CZT detector planes at resolutions down to 50 μm. Using this facility, the subpixel level detector response of P2 was measured at 100 μm⁵ resolution and analyzed to extract and evaluate the area and profile of individual pixels, their morphology across the entire P2 detector plane for comparison with previous measurements and to provide additional characterization. In this article, we evaluate the imaging performance of a coded-aperture telescope using the observed pixel morphology for P2 detectors. This investigation will serve as an initial guide for detector selection in the development of HREXI detector planes, for the future implementation of the 4pi X-Ray Imaging Observatory (4piXIO)⁶ mission, which aims to provide simultaneous and continuous imaging of the full sky (4πsr) in the 3 to 200 keV energy band with 2 arcmin angular resolution and 10 arcsec source localization, as well as other, future coded-aperture instruments.