Depth reconstruction validation in pixelated semiconductor detectors

The depth of interaction in pixelated CdZnTe and HgI 2 detectors is determined using either the cathode-to-anode signal ratio (CAR) or drift time information. The CAR can be used if a γ-ray interacts only once in the detector. The drift time must be used if the γ-ray interacts over multiple pixels. The accuracy of these reconstruction techniques is evaluated using a collimated fan beam of γ-rays that can scan across the entire depth of the detector, from the pixelated anode to the planar cathode. Signals are read-out using the VAS UM/TAT4 ASIC. First, the beam position associated with the anode and cathode of a detector is determined. To measure the anode position, the polarity of the applied bias must be reversed to ensure that electrons, rather than holes, drift through the bulk. Once the beam is properly calibrated to determine the actual cathode and anode locations, many reconstruction properties are studied. The discrepancy between the reconstructed depth and the true collimator position is found to be small for all detectors tested, with a minor reconstruction bias for interactions near the anode side. The thickness of the inactive layer near the anode has been measured to be less than 300μm for two 15mm thick CdZnTe detectors. The experiments have also been used to simultaneously evaluate the time-amplitude-walk calibrations and the reconstruction of multiple-pixel events based on the drift time.