We performed a measurement of the ionization response of 200 μm-thick
amorphous selenium (aSe) layers under drift electric fields of up to 50
V/μm. The aSe target was exposed to ionizing radiation from a 57Co
radioactive source and the ionization pulses were recorded with high
resolution. Using the spectral line from the photoabsorption of 122keV γ
rays, we measure the charge yield in aSe and the line width as a function of
drift electric field. From a detailed microphysics simulation of charge
generation and recombination in aSe, we conclude that the strong dependence of
recombination on the ionization track density provides the dominant
contribution to the energy resolution in aSe. These results provide valuable
input to estimate the sensitivity of a proposed next-generation search for the
neutrinoless ββ decay of 82Se that aims to employ imaging
sensors with an active layer of aSe.Comment: 16 pages, 11 figures. Prepared for submission to JINS