Performance verification of next-generation Si CMOS soft X-ray detector for space applications

All-sky surveys of X-ray transient objects in the soft X-ray band are essential for observing high-redshift events to probe unexplored physical conditions in the early universe and discovering mysterious electromagnetic counterparts of gravitational-wave sources. HiZ-GUNDAM is a future satellite mission that intends to perform a wide-field survey in the soft X-ray band (0.4–4 keV). The HiZ-GUNDAM X-ray detector has Si pixel sensors that require fine positioning accuracy (e.g. tens of micrometers), high detection efficiency, a large detection area (tens of cm2) and, a relatively high frame rate of ∼10 frames per second that can be achieved by a back-illuminated Si CMOS image sensor. One of the most promising candidates is GSENSE6060BSI with 6144 × 6144 pixels fabricated by Gpixel Inc. In this paper, for the initial performance test, we used a smaller-sized CMOS sensor, GSENSE400BSI-TVISB, which is similar to GSENSE6060BSI in terms of pixel characteristics, such as pixel size, epilayer thickness, and resistivity, except for the number of pixels (2048 × 2048). We conducted a detailed study on the spectroscopic performance of GSENSE400BSI-TVISB in the soft X-ray band and its radiation tolerance for space applications. For the spectroscopic performance, low-energy X-ray lines such as Al-Kα (1.5 keV), Mn-Kα (5.9 keV), and Mn-Kβ (6.5 keV) were clearly detected. Furthermore, using the obtained X-ray events, we estimated >50% detection efficiency in the 0.4–4 keV band, corresponding to a ∼10-μm Si thickness of the detectable layer. We also performed radiation tolerance tests against gamma rays from 60Co, 100-MeV protons and 5.5-MeV alpha rays (5–30 krad, corresponding to 6–30 year operation in orbit). After the irradiation tests, we observed that GSENSE400BSI-TVISB has strong radiation tolerance for our purpose: the lower detectable energy of ∼0.4 keV can be obtained if the CMOS is operated with an exposure of 0.1 s at −20 °C.