Time encoded imaging of extended gamma-ray sources in 3D using the OrionUM pixelated CdZnTe with subpixel estimation and depth of interaction corrections

Abstract

Nuclear holdup, or the accumulation of material in processing equipment, is important to localize, identify, and quantify for economic reasons and criticality safety. As a part of the quantification process is to estimate the geometric distribution of the material, high-resolution 3D images are desired. To image gamma-ray material in 3D, we propose a time-encoded imaging technique that uses a mobile coded aperture to modulate the gamma-ray signal spatially and temporally. In the process, we investigate means to improve upon the image resolution. Naturally, the quality of the reconstructed image is dependent on the size of the aperture and the fidelity of the recorded projection, among other factors. The major degradation in the recorded projection originates from the poor position reconstruction of the gamma-ray interaction location within the detector. By utilizing the subpixel capabilities of the OrionUM pixelated CdZnTe system, the position of each gamma interaction can be estimated with a resolution of 500 μm full-width-at-half-maximum (FWHM) for a 120 keV gamma ray. With subpixel estimation, the average FWHM of a double-Gaussian fit improves by almost 10% when imaging two 57Co sources placed 1 cm apart. Next, by applying a collimator verified depth of interaction correction scheme, the image resolution improves by 7.5% and 12.3% for the FWHM and the full-width-at-tenth-maximum respectively for the studied source configuration. Finally, the estimation of the 3D distribution of a gamma-ray source is demonstrated via a depth refocusing technique. This technique is shown on spatially extended special nuclear material measured at Idaho National Laboratory and estimates the material's out of plane angle to within 20% of the true angle.