Performance evaluation of the high-voltage CMOS active pixel sensor AstroPix for gamma-ray space telescopes

AstroPix is a novel monolithic high-voltage CMOS active pixel sensor proposed for next generation medium-energy gamma-ray observatories like the All-sky Medium Energy Gamma-ray Observatory eXplorer (AMEGO-X). For AMEGO-X AstroPix must maintain a power consumption of less than 1.5mW/cm² while having a pixel pitch of up to 500μm. We developed the second and third versions of AstroPix, namely AstroPix2 and AstroPix3. AstroPix2 and AstroPix3 exhibit power consumptions of 3.4mW/cm² and 4.1mW/cm², respectively. While AstroPix2 has a pixel pitch of 250 μm, AstroPix3 achieves the desired size for AMEGO-X with a pixel pitch of 500 μm. Performance evaluation of a single pixel in an AstroPix2 chip revealed a dynamic range from 13.9 keV to 59.5 keV, with the energy resolution meeting the AMEGO-X target value (<10% (FWHM) at 60 keV). We performed energy calibration on most of the pixels in an AstroPix3 chip, yielding a mean energy resolution of 6.2 keV (FWHM) at 59.5 keV, with 44.4% of the pixels satisfying the target value. The dynamic range of AstroPix3 was assessed to span from 22.2 keV to 122.1 keV. The expansion of the depletion layer aligns with expectations in both AstroPix2 and AstroPix3. Furthermore, radiation tolerance testing was conducted on AstroPix. An AstroPix2 chip was subjected to an equivalent exposure of approximately 10 Gy from a high-intensity ⁶⁰Co source. The chip was fully operational after irradiation although a decrease in gain by approximately 4% was observed.