Improvement of Sub-Pixel Position Sensing in Nonuniform Large-Volume Pixelated CdZnTe Crystals

In pixelated 3-D position sensitive CdZnTe detectors, the Compton imaging performance highly depends on the accuracy of the reconstructed 3-D interaction positions. The lateral position resolution is traditionally limited by the anode pixel pitch. The use of transient signals from the pixels that do not collect charge can give a more precise lateral interaction position estimate. Material defects and electric field nonuniformity are present in many large-volume CdZnTe crystals. These detector nonuniformities can severely degrade the accuracy of the sub-pixel position algorithm. It is observed that the rise time of the collecting anode signal tends to be longer for the pixels with nonuniform electron drift velocities. This is likely due to diffusion or trapping and detrapping of the electron cloud as it passes through a region with very low drift velocity. In a nonuniform detector, the sub-pixel position distribution was significantly improved by removing the signals with rise time greater than 500 ns. It is shown that rise time is a good indicator of material nonuniformity and imaging performance. When only events with appropriate rise time of anode signals are used, even poor detector material can yield excellent sub-pixel position information, improving the gamma-ray imaging performance. The sub-pixel position and C/A ratio were calculated based on the digitized preamplifier pulse waveforms from the collecting and non-collecting anode pixels.