Evaluation of gamma-ray response of the AstroPix4 HV-CMOS active pixel sensor

Abstract

AstroPix is a novel high-voltage CMOS active pixel sensor being developed for a next generation gamma-ray space telescope, AMEGO-X. To meet AMEGO-X instrument requirements, AstroPix must achieve full depletion of its $500\hat{\mathrm{I}}¼\mathrm{m}$ thick, $500\hat{\mathrm{I}}¼\mathrm{m}$-pitch pixels. It must be sensitive to gamma rays in the range of $25-700$ keV, with the energy resolution at 122 keV of $<10$%. Furthermore, given the space-based nature of AMEGO-X, the power consumption of AstroPix needs to be lower than $1.5mW/{\mathrm{cm}}^2$. We report the gamma-ray response of the latest version of AstroPix, AstroPix4. The chip contains $16\times 13$ array of $500\hat{\mathrm{I}}¼\mathrm{m}$-pitch pixels. The power consumption is estimated to be about $2mW/{\mathrm{cm}}^2$, which is approximately half the power of the previous AstroPix version. The input capacitance is reduced, allowing for the detection of the 14 keV photopeak from ${}^{57}\mathrm{Co}$ and a moderate energy resolution of 14% at 122 keV. The dynamic range is estimated to be in the range from 14 keV to $\sim 250$ keV. We found that the sensor depletion layer expands as expected and the measured depletion depth is approximately $90\hat{\mathrm{I}}¼\mathrm{m}$ when biased at $-240$ V.

4 pages, 7 figures, accepted for publication in Nucl. Instrum. Methods A