Coded Aperture Imaging Applied to Pixelated CdZnTe Detectors.

Abstract

In the past decade, there has been a significant increase in demand for radiation detectors to detect, identify, and locate potentially threatening nuclear materials. The Polaris system was developed to be used for such applications. This portable, room-temperature operated detector system is composed of 18 thick CdZnTe detectors, and has the ability to detect gamma rays of energies between 30 keV and 3 MeV with an energy resolution <1% FWHM at 662 keV. Detection is extended to source directionality using Compton imaging to map out gamma-ray distributions in 4-pi space. This modality is most effective at imaging gamma-ray energies greater than 300 keV. Due to the low Compton-interaction probability in CdZnTe at lower energies, an alternate imaging technique, coded aperture imaging (CAI), was implemented to extend gamma-ray imaging to the energy range where photoelectric absorption is most probable. The purpose of this work is to describe the evolution of the CAI modality as applied to the Polaris system.

During the course of this study, for the first time, CAI is applied to thick 3D position sensitive CdZnTe detectors to image lower-energy gamma rays. With the knowledge of 3D positions of gamma interactions, masks are applied to five of the six sides of a single CdZnTe crystal, extending the field-of-view (FOV) to near 4-pi through simulation and measurement. Material properties such as “pixel jumping” that are caused by non-uniform electric fields within the detector that result in degradation of image quality are also studied. Next, a single mask is applied to a 3 x 3 array of detectors showing improved image quality when combining images from multiple detectors. Finally, CAI is combined with Compton imaging and applied to the 18-detector Polaris system allowing for the extension of gamma-ray imaging capabilities across the entire dynamic range of the electronic readout system. This work was funded by the US Department of Homeland Security Domestic Nuclear Detection Office and National Science Foundation Academic Research Initiative.