X-ray interactions in a-Se based diagnostic and portal imagers

Research into medical X-ray detectors based on amorphous selenium (a-Se) has seen a resurgence lately due to the advent of flat-panel active matrix technology. There is presently no quantitative theory, however, to explain the signal formation due to the interaction of ionizing radiation with a-Se. One of the main obstacles in the characterization of this process is the lack of suitable interaction cross-sections. In this work the authors develop an expression for the calculation of inelastic electron cross-sections from a dipole oscillator strength distribution, which includes relativistic and exchange effects. They construct the oscillator strength distribution from known experimental results and calculate the total and cumulative differential cross-sections for incident electron energies ranging from a few eV to 23 MeV. The total cross-section agrees with that of isolated Se atoms at high energies, and provides the low energy cross-sections which depend on the dielectric properties of the amorphous state. From these cross-sections, Monte Carlo codes can be developed which simulate the creation of individual electron-hole pairs, which will lead to a better understanding of the signal-formation process in a-Se based X-ray detectors