  The CUbesat Solar Polarimeter (CUSP) project is a CubeSat mission orbiting
the Earth aimed to measure the linear polarization of solar flares in the hard
X-ray band by means of a Compton scattering polarimeter. CUSP will allow the
study of the magnetic reconnection and particle acceleration in the flaring
magnetic structures of our star. CUSP is a project in the framework of the
Alcor Program of the Italian Space Agency aimed at developing new CubeSat
missions. It is approved for a Phase B study. In this work, 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 \textsuperscript{55}Fe radioactive source. Moreover, the APDs
were also characterized as being coupled to a GAGG scintillator.
