  We performed a measurement of the ionization response of 200 $\mu$m-thick
amorphous selenium (aSe) layers under drift electric fields of up to 50
V/$\mu$m. The aSe target was exposed to ionizing radiation from a $^{57}$Co
radioactive source and the ionization pulses were recorded with high
resolution. Using the spectral line from the photoabsorption of 122 keV
$\gamma$ rays, we measured 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 $\beta\beta$ decay of $^{82}$Se that aims to employ
imaging sensors with an active layer of aSe. We estimate the RMS line width of
the integrated ionization signal from neutrinoless $\beta\beta$ decay events
(of deposited energy 3 MeV) to be 2.0% for a drift field of 50 V/$\mu$m. The
energy resolution can be improved to 1% by correcting for the charge yield as a
function of ionization density along the imaged electron tracks.
