Radiation Damage Test of the X-ray CCDs for MAXI onboard the International Space Station

摘要

We have investigated the radiation damage effects on a CCD to be employed in the Japanese X-ray astronomy mission including the Monitor of All-sky X-ray Image (MAXI) onboard the International Space Station (ISS). The X-ray CCD camera, ACIS, onboard Chandra have been seriously damaged by low energy protons having energy of ～150 keV since low energy protons release their energy mainly at the charge transfer channel, resulting a decrease of the charge transfer efficiency. We thus focused on the low energy protons in our experiments. A 171 keV to 3.91 MeV proton beam was irradiated to a given device. We measured the degradation of the charge transfer inefficiency (CTI) and dark current as a function of incremental fluence. A 292 keV proton beam degraded the CTI most seriously. Taking into account the proton energy dependence of the CTI, we confirmed that the transfer channel has a lowest radiation tolerance. On the other hand, dark current increased after proton irradiation for all energies except 171 keV. We have also developed the different device architectures to reduce the radiation damage in orbit. We then investigated the spatial distribution of the low energy protons in the orbit of the ISS. We found that their density has a peak around l ～ 20° and b ～ ―5° independent of the attitude. The peak value is roughly two orders of magnitude larger than that at the South Atlantic Anomaly. Taking into account the new anomaly and orbit of the ISS, we estimated the charge transfer inefficiency of MAXI CCDs to be 1.1 x 10~(-5) per each transfer after two years of mission life in the worse case analysis if the highest radiation-tolerant device is employed. This value is well within the requirement and we have confirmed the high radiation-tolerance of MAXI CCDs.