Measurement of the intrinsic energy resolution of amorphous selenium for the next generation neutrinoless $\beta\beta$ decay detector

Imaging sensors made from an ionization target layer of amorphous selenium (aSe) coupled to a silicon complementary metal-oxide-semiconductor (CMOS) active pixel array for charge readout are a promising technology to search for the neutrinoless $\beta\beta$ decay of $^{82}$Se. We present results on the ionization response of aSe measured from the photoabsorption of 122keV $\gamma$ rays in a single-pixel device, and discuss its implications for a next-generation neutrinoless $\beta\beta$ decay detector based on this technology. We also report on the progress in the fabrication and testing of the first prototype imaging sensors based on the Topmetal-II pixelated CMOS charge readout chip.