Tuning of nuclear spectroscopic telescope array application specific integrated circuits to improve low energy threshold of future hard x-ray imaging detectors

Detector commanding, processing and readout of spaceborne instrumentation is often accomplished with application specific integrated circuits (ASICs). The ASIC designed for the nuclear spectroscopic telescope array (NuSTAR) mission enables future tiled CdZnTe (CZT) detector array readout for x-ray detectors, such as the high resolution energetic x-ray imager (HREXI). Modified NuSTAR ASIC (NuASIC) gain settings have been implemented for HREXI's broader targeted imaging energy range (3 to 300 keV) compared with NuSTAR (2 to 79 keV), which may require updated NuASIC internal parameters for optimal energy resolution. To reach HREXI's targeted low energy threshold, we have also enabled the NuASIC's "charge pump mode," which introduces an additional tuning parameter. We describe the mechanics of the NuASIC's adjustable parameters and use our recently developed ASIC test stand to probe a "bare" NuASIC using its internal test pulser. We record the effects of parameter tuning on the device's electronics noise and low energy threshold and report the optimal set of parameters for HREXI's updated gain setting. We detail a semiautomated procedure to derive the optimal parameters for each of HREXI's large area closely tiled NuASIC/CZT detectors to expedite instrument integration.