Proton irradiation experiment for X-ray charge-coupled devices of the monitor of all-sky X-ray image mission onboard the international space station: II. Degradation of dark current and identification of electron trap level

Abstract

We have investigated the radiation damage effects on a charge-coupled device (CCD) to be used for the Japanese X-ray mission, the monitor of all-sky X-ray image (MAXI), onboard the international space station (ISS). A temperature dependence of the dark current as a function of incremental dose is studied. We found that the protons having energy of >292 keV seriously increased the dark current of the devices. In order to improve the radiation tolerance of the devices, we have developed various device architectures to minimize the radiation damage in orbit. Among them, nitride oxide enables us to reduce the dark current significantly and therefore we adopted nitride oxide for the flight devices. We also compared the dark current of a device in operation and that out of operation during the proton irradiation. The dark current of the device in operation became twofold that out of operation, and we thus determined that devices would be turned off during the passage of the radiation belt. The temperature dependence of the dark current enables us to determine the electron trap level that generates the dark current. We fitted dark current as a function of temperature by the thoretical models and found that the dark current increase after proton irradiations is caused by, at least, two kinds of electron trap levels. The shallow trap level (Ec - Et < 0.2 eV where Ec and Et are the energy at the bottom of the conduction band and the energy level of electron trap) might be associated with oxygen which is dominant at the operating temprature of >210K. On the other hand, another trap level is located roughly at the center of the silicon bandgap which might be associated with divacancies or P-V traps. We finally investigated the spatial distribution of the low-energy protons in the orbit of the ISS. Their density has a peak around / ∼ 20 and b ∼ -55° independent of the altitude. The peak value is roughly two orders of magnitude higher than that at the South Atlantic Anomaly.