Adjacent 2-pixel event discrimination in 3-D position sensitive imaging CdZnTe detectors using the UM VAD ASIC

In a pixelated 3-D position sensitive CdZnTe gamma-ray imaging detector, 2-pixel side-neighbor events make up about 50 percent of 2-pixel events. These side-neighbor events can originate from either Compton scattering or charge sharing. Distinguishing between these two categories of events will be of interest for advanced data processing, such as event classification or improvement for Compton imaging efficiency. A digitizer UM_VAD ASIC developed in collaboration with Gamma-Medica IDEAS has been used for pixelated 3-D position sensitive CdZnTe detectors. Because this ASIC digitizes pulse waveforms outputted by the preamplifier, it has achieved better depth separation uncertainty, about 0.3mm compared to Imm FWHM from the GMI VAS 2.3/TAT4 ASIC. It is also possible to use the signals from the pixels neighboring the collecting pixel to determine the subpixel position of an electron cloud interaction. This work focuses on the development of a method to distinguish between 2-pixel side-neighbor events caused by Compton scattering and those caused by charge-sharing. The sub-pixel position information is used in addition to the depth separation of the two interactions to provide better discrimination capability. The experimental results and the algorithm have been verified through Geant4 simulations.