Impact of electric field non-uniformity on large CdZnTe crystals

In pixelated CdZnTe (CZT) detectors, multiple interaction gamma-ray events are critical because they permit Compton imaging. However, in large-volume CZT crystals, the depth of multiple interaction events are 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 electron drift velocity distribution was measured using alpha particles emitted from an ²⁴¹Am source, which were incident on the entire cathode surface of each detector. Cathode waveforms were recorded for events collected by each element in the 11×11 array of anode pixels. To illustrate the impact of nonuniform electron drift velocity, two CZT crystals fabricated by Redlen Technologies with similar spectral performance, but significantly different drift velocity uniformities were selected. The detector with significant nonuniformity of the electron drift velocity was correlated with much poorer imaging performance, which is expected when depth reconstruction is poor. This work clearly shows that even CZT crystals with 1% energy resolution at 662 keV can have a nonuniform electron drift velocity, on the sub-pixel scale, which seriously degrades the imaging performance of multiple-pixel events in pixelated detectors.