Proton Irradiation Experiment for X-ray Charge-Coupled Devices of the Monitor of All-Sky X-ray Image Mission Onboard the International Space Station: I. Experimental Setup and Measurement of the Charge Transfer Inefficiency

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). 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) 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 the lowest radiation tolerance. We have also developed the different device architectures to reduce the radiation damage in orbit. Among them, the "notch" CCD, in which the buried channel implant concentration is increased, resulting in a deeper potential well than outside, has three times higher radiation tolerance than that of the normal CCD. We then estimated the charge transfer inefficiency of the CCD in the orbit of ISS, considering the proton energy spectrum. The CTI value is estimated 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 acceptable limit and we have confirmed the high radiation-tolerance of CCDs for the MAXI mission.

Proton radiation damage experiment on P-Channel CCD for an X-ray CCD camera onboard the ASTRO-H satellite

“…However, this difference does not appear to affect the degree of the damage at least in terms of the CTI, considering the relatively smooth connection between the large-CCD and the mini-CCD data. The same conclusion was deduced for N-channel CCDs in a similar proton radiation experiment [19].…”

Development of the X-ray CCD in Japan

“…The energy of the proton beam is between 173 keV and 3.9 MeV. We observed the degradation of the CCD performance as a function of the fluence of protons [29,30]. The greatest damage is observed using about 300 keV protons.…”

Application of a mesh experiment for a proton beam onto the charge-coupled device

“…By adding a thin foil of aluminum with a thickness of 5 m, the lowest effective energy was reduced to 170 keV. The characteristics of the proton beam employed have been described by Miyata et al [6] We controlled the energy of the proton beam such that the proton beam induced serious damage near the charge transfer channel inside the CCD.…”

“…If the CCD is used in an LEO, like ASCA, the notch structure is expected to slow down the degradation of the CTI by a factor of 2 3 over 10 y. [6]. Protons deposit much energy just before they terminate in Si.…”

“…Miyata et al [6] measured the CTI for CCDs manufactured by Hamamatsu Photonics K.K. to study the effect of the notch structure.…”