Events reconstruction in 3-D position sensitive cadmium zinc telluride gamma ray spectrometers.

The wide band gap semiconductor material CdZnTe has long been of interest for gamma-ray spectroscopy at room temperature. Despite the resolution improvement using single-polarity charge sensing techniques, the variations in detector response, such as the peripheral asymmetry in coplanar grids and the material non-uniformity can still degrade the energy resolution of large volume CdZnTe detectors. A 3-D CdZnTe spectrometer employs a pixellated anode and a conventional planar cathode. The pixel position provides the interaction's lateral coordinates, while the interaction depths for single-pixel events are derived from the cathode to anode signal ratio. For multiple-pixel events, individual electron drift times are recorded to obtain the depth for each interaction. The 3-D position sensitivity enables corrections of non-uniform detector response down to the limit of the position resolution over the whole detector volume. This work is the first successful demonstration of 3-D position sensing for multiple-pixel events, together with single-pixel events, all done through application specific integrated circuits (ASIC). Significant improvements in energy resolutions for both single-pixel and multiple-pixel events have been achieved. Various calibrations are carried out in order to reconstruct single-pixel and multiple-pixel events. Variations in the readout electronics must be corrected first, such as the channel-to-channel gain variation, the baseline offset, the non-linearity, the baseline drift and the gain drift in each channel. The position-dependent charge collection due to the variations in detector response can be corrected using the 3-D position sensing. For multiple-pixel events, additional calibrations are needed, such as the correlation between the electron drift time and the interaction depth, the timing-amplitude-walk, and the weighting potential cross-talk between pixels. Two 2.25 cm3 volume 3-D position-sensitive CdZnTe gamma-ray spectrometers employing VAS/TAT ASIC readouts were developed. Both detectors produced excellent energy resolution. One detector achieved 0.78% FWHM and 1.24% FWHM resolution at 662 keV for single-pixel events and two-pixel events, respectively. Possible reasons for energy resolution degradation of multiple-pixel events are discussed. Spectroscopy for multiple gamma-ray energies and high-energy (>2 MeV) source is demonstrated. The electron mobility-lifetime products of both detectors were measured and compared. Possible factors affecting the detector response are also analyzed.Ph.D.Applied SciencesNuclear engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124987/2/3163978.pd