Spatial pileup considerations for pixellated gamma-ray detectors

High-spatial-resolution solid-state detectors being developed for gamma-ray applications benefit from having pixel dimensions substantially smaller th an detector slab thickness. This leads to an enhanced possibility of charge partially spreading to neighboring pixels as a result of diffusion (and se condary photon emission) transverse to the drift direction. An undesirable consequence is the effective magnification of the event "size" and the spat ial overlap issues which result when two photons are absorbed in close prox imity within the integration time of the detector/readout system. In this work, we develop the general statistics of spatial pileup in imagin g systems and apply the results to detectors we are developing based on pix ellated cadmium zinc telluride (CdZnTe) and a multiplexing application-spec ific integrated circuit (ASIC) readout. We consider the limitations imposed on total count rate capacity and explore in detail the consequences for th e LISTMODE data-acquisition strategy. Algorithms are proposed for identifyi ng and, where possible, resolving overlapping events by maximum-likelihood estimation. The efficacy and noise tolerance of these algorithms will be te sted with a combination of simulated and experimental data in future work.