SWAD: The effect of pixel geometry on the spatial uniformity of avalanche gain

Photon counting detectors (PCDs) have the potential to improve x-ray imaging, however current crystalline materials are still hindered by high production cost and performance limitations. We are developing a novel direct conversion amorphous Selenium (a-Se) based field-Shaping multi-Well Avalanche Detector (SWAD) for photon counting breast imaging applications. SWAD's multi-well Frisch grid design creates separate absorption and sensing regions capable of depth independent avalanche gain. The improved temporal response from unipolar time-differential (UTD) charge sensing combined with tunable avalanche gain within the well region attains the fast timing and energy resolution necessary for successful photon counting under clinical settings. The avalanche gain in a-Se sensors varies rapidly as a function of electric field, which may affect the overall energy resolution of the detector. The goal of this work is to investigate the uniformity of avalanche gain as a function carrier position within the a-Se bulk region for different SWAD design parameters. Our simulation results show that for the geometries modeled, the variation in avalanche gain along different field lines across the multi-well region can be kept below 4.2% by using multi-well pillars with a high aspect ratio. Additionally, the variation in avalanche gain was evaluated for charge clouds generated by incident x-rays with energies of 20, 40 and 60 keV. In all cases the variation in avalanche gain was found to decrease with increasing charge cloud size. For an optimized SWAD geometry, the variation in gain was negligible for each incident x-ray energy simulated.