X-ray Spectral Performance of a Sony CMOS near Fano Limit after Per-pixel Gain Calibration

Charge-coupled devices (CCDs) have been successfully used over the three past decades for X-ray astronomy and have demonstrated their excellent performance including a very low noise and excellent uniformity. Next generation of X-ray missions requires a higher readout speed, smaller pixels, lower power consumption and a better radiation tolerance while maintaining spectral performance and pushing imaging capability to the diffraction limit. Recent improvements of complementary metal oxide semiconductor (CMOS) detectors and the use of backside illuminated CMOS offer a promising alternative to CCDs for future X-ray instruments. CMOS sensors provide a high readout speed, low readout noise, low power consumption and can be operated at room temperature. In addition, the active pixel design of CMOS prevents the inherent and increasing charge transfer inefficiency affecting CCD performance over time due to space radiation. However, many challenges remain, including a better understanding of their performance in terms of energy resolution compared to CCDs. We investigated the capacity of the Sony STARVIS CMOS sensors with small pixels (<3 µm) to detect X-ray photons. Using the MIT polarimetry beamline, we determined the soft X-ray performance and demonstrated their capacity to detect soft X-ray photons down to 183.3 eV (Boron line) with an excellent spectral resolution (35 eV), close to the achievable Fano limit with this technology. We have also investigated possible fast methods for performing pixel-by-pixel gain calibration to bring the energy resolution of these devices closer to that of CCDs.