Event classification in 3D position sensitive pixelated CdZnTe detectors

Pixelated CdZnTe detectors with 3D position sensitivity have undergone significant development in the past decade. A CdZnTe detector with a pixelated anode is expected to provide excellent spectroscopy and to form a Compton image of the gamma-ray source distribution. However, there are additional event classification capabilities that are inherent to this device configuration, many of which are presented in this work. The majority of the classification effort has been directed toward gamma-rays. Seperating Compton escape from photoelectric absorption on an event-by-event basis is difficult to achieve but potentially very rewarding if achieved. This work discusses using interaction locations as well as number of interactions to increase the probability of observing the photoelectric effect relative to Compton scatter. Pair production events can be classified by the capture of 511 keV photons in coincidence with another energy deposition elsewhere in the detector. Also, the geometry of the interactions for one event can be used to identify pair production. Correctly identifying the number of interactions that occur for one event is critical for passing valid information to the Compton image reconstruction algorithm. This work illustrates the challenges involved in determining the number of interactions when there is little spatial separation. If a Compton scatter and a photoelectric event occur in the same pixel or in adjacent pixels then the shape of the waveforms induced on the preamplifier must be examined in order to determine the true number of interactions. This work also outlines several additional event identification capabilities. Muon tracks can be identified because they will induce a large charge on many pixels that reveal a trajectory through the detector. The cadmium content makes CdZnTe an excellent thermal neutron absorber. The prompt gamma-ray emission following neutron capture in cadmium-113 results in a peak 558.6 keV. While this device is not an i- eal neutron detector, as the only discrimination against gamma-rays is based on the energy deposited, the presence of a peak at 558.6 keV is a clear indicator of a neutron source. Finally, alpha and beta interactions can be eliminated for the purpose of low background experiments by excluding events that occur near the surface of the device. Experimental evidence of each classification capability is presented along with the limitations of the current state of the art detector systems.