Material uniformity of cadmium zinc telluride in gamma-ray imaging detectors

The material uniformity of cadmium zinc telluride (CZT) crystals in gamma-ray imaging detectors is examined using several existing techniques and a new technique called thermally stimulated current (TSC) imaging that has been developed for this dissertation. The TSC imaging model, simulations, and experimental demonstrations are presented here for the first time. CZT radiation detectors are used in nuclear medicine as well as other medical, industrial, national security, and scientific applications; however, the scarcity and cost of high-quality CZT materials have hindered the use of CZT in these applications. Understanding CZT's material properties and their effects on detector performance should be helpful in developing crystal growth methods that have improved yield of useful detector material. Data obtained from CZT samples using infrared transmission, electron microprobe, X-ray diffraction, and electron backscatter diffraction (EBSD) mapping methods are used to understand their crystal structure. Data obtained from these samples using both TSC imaging and conventional leakage current measurements while these samples were operated as pixelated detector arrays are used to understand their charge transport properties. Collimated gamma-ray mapping was used to understand the detector performance properties of these samples. Correlations among these spatially mapped data are investigated. Contrary to the suggestions of other researchers, it is found that leakage current is not inversely correlated with detector performance. Detector performance in these samples is well correlated with their crystal structure. High-angle grain boundaries are shown to trap charge carriers, and estimates of the locations of these boundaries are derived from the gamma-ray mapping data. EBSD distinguishes itself from X-ray diffraction methods in identifying the locations and types of grain boundaries intersecting the sample surface. Using the new TSC imaging method, evidence is obtained showing a higher density of a particular trap near incommensurate boundaries in a CZT sample. Other researchers have indicated that an electron trap associated with dislocations is present in CZT. Their observation is consistent with a conclusion drawn from these TSC imaging data that due to higher densities of dislocations near incommensurate grain boundaries these boundaries host electron traps while {111} twin boundaries do not