Radiation damage test of the x-ray CCDs with low energy protons

Abstract

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. We measured the degradation of the charge transfer efficiency and the dark current as a function of incremental fluence. We have also developed the different device architectures to minimize the radiation damage in orbit. We thus compared the differences of performance after proton irradiation. 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 ∼ −55 • 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 × 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.