Resolution degradation of multiple-pixel event in pixellated CZT detectors

The signals from 3-D position-sensitive pixellated CdZnTe (CZT) detectors have been modeled for the VAS_UM/TAT4 ASIC readout electronics. Each detector has dimensions of 20 mm × 20 mm × 15 mm, with an 11 × 11 square pixel anodes fabricated on the anode surface having a pixel pitch of 1.72 mm. Gamma-ray interactions with detector material, charge drift, charge trapping and diffusion, signal induction, system noise, pulse shaping, as well as ASIC triggering processes have all been simulated. This model is used to study energy resolution degradation of multiple-pixel events in pixellated CZT detectors. While full charge collection is achieved when interactions occur directly under the pixel, the decreased weighting potential under the gap reduces the total pulse amplitude of events that occur between pixels. Furthermore, weighting potential cross-talk induced by charge sharing between neighboring pixels will alter the total induced charge on each pixel. Through signal modeling, these specific effects on multiple-pixel-event energy resolution can be quantified and compared to the overall measured degradation.