Characterization of avalanche photodiodes (APDs) for the CUbesat Solar Polarimeter (CUSP) mission

The CUbesat Solar Polarimeter (CUSP) is a CubeSat mission to measure the linear polarization of solar flares in the hard X-rays band by means of a Compton scattering polarimeter. CUSP is a project in the framework of the Alcor Program of the Italian Space Agency aimed to develop new CubeSat missions. CUSP is approved for a Phase B study that will last for 12 month starting from the beginning of 2024. We report on the characterization of the Avalanche Photodiodes (APDs) that will be used as readout sensors of the absorption stage of the Compton polarimeter. We assessed the APDs gain and energy resolution as a function of temperature by irradiating the sensor with a Fe55 radioactive source. Moreover, the APDs were characterized also coupled to a GAGG scintillator.