Photon counting performance of amorphous selenium and its dependence on detector structure

Photon counting detectors (PCD) have the potential to improve x-ray imaging, however they are still hindered by high production costs and performance limitations. By using amorphous Selenium (a-Se) the cost of PCDs can be significantly reduced compared to currently used crystalline semiconductors and enable large area deposition. To overcome the limitation of low carrier mobility and low charge conversion gain in a-Se, we are developing a novel direct conversion a-Se field-Shaping multi-Well Avalanche Detector (SWAD). SWADs multi-well, dual grid design creates separate non-avalanche interaction (bulk) and avalanche sensing (well) regions, achieving depth-independent avalanche gain. Unipolar time differential (UTD) charge sensing, combined with tunable avalanche gain in the well region allows for fast timing and comparable charge conversion gain to crystalline semiconductors. In the present work we developed a probability based numerical simulation to model the charge generation, transport and signal collection of three different a-Se detector configurations and systematically show the improvements in energy resolution attributed to UTD charge sensing and avalanche gain. Pulse height spectra (PHS) for each detector structure, exposed to a filtered 241Am source, are simulated and compared against previously published PHS measurements of a conventional a-Se detector. We observed excellent agreement between our simulation of planar a-Se and the measured results. The energy resolution of each generated PHS was estimated by the full-width-at-half-maximum (FWHM) of the primary photo-peak. The energy resolution significantly improved from ~33 keV for the planar a-Se detector to ~7 keV for SWAD utilizing UTD charge sensing and avalanche gain.