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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 ~-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.

The Chandra ACIS has seriously damaged by the low energy proton in orbit. Radiation hardness is an important characteristics of a device in space use. Therefore we applied the mesh experiment to a relatively low energy proton beam employing the CCD developed for the MAXI~(Monitor of All-sky X-ray Image) CCD camera named SSC that is scheduled to be onboard the International Space Station~(ISS) in 2005. It consists of 1024×1024 pixels with 24μm square. It is equipped with a `notch' structure in the middle of the pixel so that it is designed to be radiation hard. A mesh experiment is, so far, the only practical technique to study the CCD response with subpixel resolution. The mesh is made of gold with 10μm thick with many holes of 2μm in diameter. The proton beam is controlled so that the proton passing through the mesh hole can penetrate into the CCD chip with leaving damages near the buried channel inside.

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.