Long-term test of a stacked CdTe mini-HXI setup

This work presents a long-term operation of two stacked CdTe double sided strip detectors that are comparable with the CdTe detectors onboard Hitomi's HXI. The goal of this test is to study the evolution of the spectroscopic performance of the detectors during a one year operation cycle which resembles the in-orbit operation cycle of Hitomi HXI. Crystal defects inside CdTe cause a degradation of the spectroscopic performance (polarization effect) of the crystal which is becoming worse during detector operation. In order to prevent crystal polarization, the detectors are reset (switch-off of the depletion voltage) once a day. Our main investigation was to study if a long-term degradation can occur as a result of incomplete depolarization during the reset. We present the hardware setup and the analytical steps that were used to investigate the detector stability during each day and over the whole testing period. For the anode signals our results show at 60 keV: a daily line drift of ($-2.8\pm 0.7$) eV/ks while the long-term drift is ($-1.5\pm 1.2$) eV/day. The degradation of the energy resolution is measured to be ($+2.4\pm 0.3$) eV/ks FWHM and the loss of efficiency is ($-0.29\pm 0.02$) %/ks.