4-pi Compton imaging using a single three-dimensional position sensitive cadmium zinc telluride detector.

Abstract

Compton imagers are promising tools for nuclear non-proliferation, astronomy, and medical applications, but typically have a limited angular field-of-view and poor efficiency resulting from requiring that gamma rays interact in two or more detectors. This work is the first successful demonstration of Compton imaging with a 4pi field-of-view using a single CdZnTe detector. There are several challenges involved in using a single detector for Compton imaging. First, the energies and three-dimensional positions of gamma-ray interactions must be determined. The detector used in this work has a pixellated anode to provide position sensitivity in two dimensions, and the third dimension is obtained from signal timing. Then, the sequence of interactions in the detector must be determined. Two such methods are attempted and compared. Finally, image reconstruction must be performed. Both simple backprojection and maximum likelihood reconstruction algorithms are demonstrated. Several factors that can degrade the performance of the 4pi imager are examined. Physical processes, such as Doppler broadening, coherent scatter, and pair production, affect the resolution of the imager. Doppler broadening has the most significant effect, limiting the resolution to at least several degrees in CdZnTe. The anode thresholds in the detector and the dynamic range of the ASICs limit the observable events in the detector, contributing to a major loss in efficiency for both low- and high-energy gamma rays. Also, the size of the anode pixels determines both the amount of charge sharing and fraction of sequences that involve more than one interaction under one pixel. Finally, the sequence-ordering algorithm creates artifacts in the image and degrades resolution. Intrinsic imaging efficiency is defined as the fraction of incident gamma rays in which the total energy was deposited in two or more observed events, no pair production occurred, and the determined sequence order is kinematically possible. With a detector volume of only 2.25 cm<super>3</super> the measured intrinsic imaging efficiency of the imager is 1.86% at 662 keV, an improvement by three orders of magnitude from the previous design. Using a new weighting method for maximum likelihood reconstruction yields a measured imaging resolution of 17&deg; full-width at half-maximum at the same energy. Two point sources separated by 18&deg; can be resolved, confirming the measured resolution. Imaging of high-energy (2.6 MeV) and extended sources is also demonstrated.