Signal formation in amorphous-Se-based x-ray detectors

The present paper addresses the problem of charge creation by x rays in amorphous selenium $(a-\mathrm{Se})$ and the subsequent transport and recombination of these charges. X-ray detectors based on $a-\mathrm{Se}$ are under study in medical imaging for diagnostic purposes (keV energy range) and for the verification of radiotherapy treatments (MeV energy range). A quantitative theory is developed that includes collective and single electron-hole pair excitations by the passing electron. This theory is incorporated into a Monte Carlo code to calculate track structures in $a-\mathrm{Se}.$ The initial positions of the electron-hole pairs along the track structures are used to study the kinetics of recombination versus incident x-ray energy and applied electric field. The experimentally observed energy dependence of recombination is attributed to a spur size that is dependent on the velocity of the ionizing electrons. Our theory and simulations agree with available experimental data in the energy range from 20 keV to 10 MeV.