Experimental limitations of coded aperture imaging using thick 3D-position-sensitive CdZnTe detectors

Abstract

3D-position-sensitive detectors have demonstrated Compton imaging and excellent energy resolution at room-temperature operation. Unfortunately, Compton imaging is not practical at gamma-ray energies below 300keV in CdZnTe detectors, due to the higher probability of photoelectric absorption. To extend our imaging capabilities to lower energies, coded aperture masks have been applied to the Polaris gamma-ray detection system composed of eighteen 20mm × 20mm × 15mm CdZnTe detectors provided by Redlen Technologies. Coded aperture images formed by 15mm thick pixelated CdZnTe detectors will be distorted if electron clouds are created over one pixel and are collected by another, an effect known as pixel jumping. Often, when the entire volume of the detector is irradiated with <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs or <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">57</sup> Co gamma rays, the count rate in each pixel is not uniform, especially for depths near the cathode side of the detector. Since the total efficiency of the detector volume matches that of simulation, this indicates that the non-uniformities are caused by cloud migration between pixels rather than complete count loss. As expected, the detectors with the most uniform count rates also provide the best coded aperture images. The focus of this research is to quantify the limitations of coded aperture imaging based on the severity of the detector count rate non-uniformity, and to examine the strength of the correlation between pixel jumping and coded aperture image quality.