Impact of drift time variation on the Compton image from large-volume CdZnTe crystals

In pixelated CdZnTe detectors, multiple interaction gamma-ray events are critical because they permit Compton imaging. However, in large-volume CdZnTe crystals, the depth of multiple interaction events is often poorly reconstructed due to significant nonuniformity of electron drift velocity while the depth of single interaction events can be accurately reconstructed using signal ratios. The nonuniformity of the drift velocity is likely due to nonuniformity in the electric field within the detector. This causes variation in the drift time even amongst events that occur at the same depth of interaction in the same pixel. The degradation in the depth reconstruction results in poorer imaging performance. The electron drift velocity at each depth is measured using 241Am alpha particles incident on the entire cathode surface of each detector. Cathode waveforms are recorded for events collected by each element in the 11×11 array of anode pixels. To illustrate the impact of electron drift time variation, two CdZnTe crystals fabricated by Redlen Technologies with similar spectral performance, but significantly different Compton imaging performance are selected. The detector with poorer imaging performance demonstrated significant nonuniformity of the electron drift velocity. This work clearly shows that even CdZnTe crystals with 1% FWHM energy resolution at 662 keV can have a nonuniform electron drift velocity, on the sub-pixel scale, which seriously degrades the Compton imaging performance of pixelated detectors.