Radiation-Induced Degradation of Si-CMOS Detector Aboard HIBARI Satellite in Low Earth Orbit

Degradation of Si-CMOS detectors of STTs in low earth orbit is presented. The 50kg microsatellite HIBARI was launched on November 2021, and designed for demonstrating a new attitude control method, named "Variable Shape Attitude Control (VSAC)". The attitude determination is therefore the key issue to evaluate the performance of VSAC. For this requirement, HIBARI possesses two star-trackers (STTs) equipped with Si-CMOS detectors. The space radiation environment in low Earth orbit poses significant challenges to Si-devices, which are susceptible to critical damage from exposure to high-energy protons. We have monitored the degradation of Si-CMOS of STTs aboard HIBARI for 2.5 years since its launch. Due to the limited bandwidth of RF-communication, we have only recorded the number of hot-pixels on the Si-CMOS detector, but with detector temperatures and the aim-point directions of STTs. First, we found that the number of hot-pixels strongly depends on the detector temperature, however it is inconvenient to evaluate the degradation because the temperature of STTs are not actively managed and varies by about ±7K every orbit. To evaluate the degradation tendency, we took into account the physical mechanism of activation of hot-pixels by radiations. For hot-pixel activation, defection formation due to non-ionizing interactions can be dominant effect. We proposed an empirical model describing the temperature-dependence of hot-pixel numbers based on the simple physical models. By making use of this model function, we calculated the number of hot-pixels at isothermal conditions of T=25˚C. This clearly shows that the two STTs change its characteristics with exactly the same behavior in spite of different pointing directions, temperatures and parameter settings. Furthermore, we discuss the correlation between these damages and solar activity, as well as the correlation with imaging parameters. These results provide useful insight into the various Si-devices on board spacecraft.