Gamma-ray imaging and polarization measurement using three-dimensional position-sensitive cadmium zinc telluride detectors.

Abstract

The three-dimensional position-sensitive CdZnTe detector is a novel room temperature semiconductor gamma-ray spectrometer. It was initially developed to achieve good energy resolution even if the detector material is not uniform and has charge trapping problems. However, the capability of position sensing, as well as the excellent energy resolution, enables 3-D CdZnTe detector systems to perform gamma-ray Compton imaging and polarization measurements. Among various reconstruction algorithms, a direct algorithm such as filtered back-projection is preferable if computational time is critical. A real-time filtered back-projection algorithm based on spherical fast Fourier transforms was developed for Compton imaging devices. It was the first time that real-time imaging was realized on similar Compton imaging devices. A near field imaging technique was developed to image gamma-ray sources in the vicinity of the detector by taking advantage of the finite size of the detector. In this work, it was demonstrated by simulations that more powerful 3-D imaging capability can be achieved with detector arrays such as the Polaris system currently under development. In conventional Compton camera systems, the image reconstruction is performed in 2-D or 3-D spatial coordinates for a specific gamma-ray energy. A new energy-imaging integrated deconvolution method using the MLEM algorithm was developed to reconstruct he source distribution in a combined spatial and energy space. It was the first time that the spectral deconvolution method was applied in the spatial and spectral domain simultaneously. By employing this energy-imaging integrated deconvolution algorithm, a Compton camera can provide the image at any specific energy, as well as the spectrum at any specific direction. Since the ML solution estimates the true incident gamma-ray intensity, the deconvolved energy spectrum at the source direction is free of Compton continua. To overcome the difficulty of pre-storing the huge system response function in the memory, an analytical model was developed so that the system response function can be calculated event by event. A 3-D position-sensitive CdZnTe detector can also perform gamma-ray polarization measurements. When a polarized gamma-ray photon undergoes a Compton scatter, the probability for the photon to scatter at a fixed scattering angle depends on the azimuthal angle. The polarization of incident gamma rays can be measured using this effect by observing the angular distribution of scatters. We successfully demonstrated that a single 3-D position-sensitive CdZnTe detector can be an effective instrument to detect the polarization of incident gamma rays.