We have developed SOIPIXs based on the CMOS SOI technology for the future X-ray astronomical satellite. SOIPIXs has the event trigger output function implemented in each pixel offers microsecond time resolution and its event trigger function enables to separate celestial X-rays and non-X-ray background by combining the anticoincidence system and to reduce the non-X-ray background that dominates the high X-ray energy band above 5-10 keV. A fully depleted SOIPIXs with a 300-500 um thick depletion layer and back illumination offers wide band coverage of 0.3-40 keV.
In order to use XRPIXs in space environment, to investigate the radiation hardness of XRPIXs is important because semiconductor detectors such as XRPIXs and CCDs are damaged by interacting with many cosmic rays which are composed primarily energy protons in orbit. The damage causes the increase of dark current and the degradation of the performance such as the energy resolution of XRPIXs.
To evaluate the radiation hardness of XRPIXs, we have carried out the radiation damage test at the heavy ion medical accelerator (HIMAC) in Japan. For this experiment, we used the XRPIX2b-FZ (Takeda et al, 2015) which was the front illuminated XRPIX with 300um thick depletion layer. XRPIX2b-FZ has 144 x 144 pixels and the pixel size is 30um x 30 um. We installed XRPIX2b-FZ in the vacuum chamber and cooled it around -80 C degree. The proton beam flux was much strong for our purpose of this experiment, we set the 3 um thick Au film as a scatterers in the cubic flange in front of vacuum chamber in order to reduce the beam flux. We introduced the scattered proton beam to the two direction of the downstream of the beam line, and one was irradiated to XRPIX2b-FZ in the vacuum chamber and the other was irradiated to the faraday cup connected to the cubic flange to monitor the scattered beam flux. We also obtained the total doze of proton beam using the faraday cup.
We irradiated the proton beam to XRPIX2b-FZ until the total irradiation dose reached 10 krad while increasing the irradiation dose and evaluated the performance such as leak current, gain and energy resolution using X-ray from 109 Cd after the proton irradiation of 1 rad, 400 rad, 1 k rad, 4 k rad, and 10 krad.
From above experimental results, we found that the gain and the energy resolution was degraded by 0.2 % and 10 % respectively with 400 rad whose equivalent time in orbit was 3.5 years, and the gain and energy resolution became worse by 0.8 % and 32 % respectively after irradiation of 4k rad. We investigated the reason of the degradation of the energy resolution and found the degradation was mainly caused by the increasing the read out noise. We also found the number of bad pixels clearly increased by about 10 times after the irradiation of 10 krad.