Total ionizing dose effects on the SOI pixel sensor for X-ray astronomical use

We report on total ionizing dose effects on the X-ray SOI pixel sensor, XRPIX. XRPIX has been developed as an imaging spectrometer for X-ray astronomical use in space. Front- and back-illuminated (FI and BI) devices were irradiated with hard X-rays from an X-ray tube operated at 30 kV with a Molybdenum target. We found that the degradation rate of the readout noise of the BI device was approximately three times slower than that of the FI device as a function of radiation exposure. Those of both type of devices, however, were virtually identical when the readout noise was evaluated as a function of the absorbed dose at the buried oxide layer, DBOX. The pedestal and analog-to-digital conversion gain also displayed similar tendencies. These results demonstrate that BI type devices have a higher radiation tolerance as a focal plane sensor of an X-ray mirror and the radiation tolerance of XRPIX devices is governed by DBOX. The readout noise was stable up to about 1 krad in DBOX, increased by about 10% at 10 krad in DBOX, and continued to increase under further irradiation. If we employ an X-ray mirror with a half-power diameter of 10 arcsec and a focal length of 10 m, 10 krad in DBOX, a reasonable threshold of radiation tolerance in this experiment, is equivalent to more than three years in orbit, typically required of space-borne sensors.