Design and Test of a Calibration System for Avalanche Photodiodes Used in X-Ray Compton Polarimeters for Space

The development and calibration of a measurement system designed for assessing the performance of the avalanche photodiodes (APDs) used in the Compton scattering polarimeter of the CUSP project is discussed in this work. The designed system is able to characterize the APD gain GAPD and energy resolution across a wide range of temperatures T (from -20 °C to +60 °C) and bias voltages Vbias (from 260 V to 410 V). The primary goal was to experimentally determine the GAPD dependence on the T and Vbias in order to establish a strategy for stabilizing GAPD by compensating for T fluctuations, acting on Vbias. The results demonstrate the system capability to accurately characterize APD behavior and develop feedback mechanisms to ensure its stable operation. This work provides a robust framework for calibrating APDs for space environments. It is essential for the successful implementation of spaceborne polarimeters such as the Compton scattering polarimeter foreseen aboard the CUbeSat Solar Polarimeter (CUSP) mission under development to perform solar flare X-ray polarimetry.