Evaluation of the Compton camera method for spectroscopic imaging with ambient-temperature detector technology

ABSTRACT A prototype Compton camera using ambient-temperature semiconductor detectors is developed for gamma ray spectroscopicimaging. Two camera configurations are evaluated, one using an intrinsic silicon detector for the front plane detector and theother using a CdZnTe detector for the front plane. Both configurations use a large-volume coplanar grid CdZnTe detector forthe back plane. The effect of detector noise, energy resolution, and timing resolution on camera performance is described.Technical issues underlying the development of Compton cameras for spectroscopic imaging are presented and imaging ofradioactive sources is demonstrated.Keywords: Gamma ray imaging, Compton camera, nuclear material safeguards 1. INTRODUCTION Recent advances in ambient-temperature detector technology have led to the development of portable gamma ray measurement systems that combine spectroscopy and imaging.' A variety of nuclear material measurement applications couldbenefit from capabilities provided by portable spectroscopic imaging systems. For example, nondestructive assay of depositsof special nuclear material (SNM) can be improved because the location of deposits and the distribution of SNM within canbe determined directly. Consequently, the effort required to implement nuclear material holdup measurements relative toestablished techniques is reduced. We are investigating a number of portable imaging concepts that may be suitable fornuclear material measurement applications. Compton camera systems are a particularly interesting technology because theyhave a large field-of-view (approaching 2ic) and spatial resolution comparable to conventional imaging systems.The goal of spectroscopic imaging is to determine the spatial distribution and identity of radioactive nuclides in a scene. Thiscan be accomplished remotely by the detection of gamma-ray radiation produced from decay of the radioactive isotopes.Specific applications of spectroscopic imaging systems include nondestructive assay of nuclear materials,"2 nuclearmedicine,3'4'5'6 and astrophysics.7'8'9 The first two techniques cover energy ranges from about 100 keV to 1 MeV, while theupper limit for astrophysics approaches 30 MeV for high energy cosmic radiation sources.Important parameters for a spectroscopic imaging system are spatial resolution, field of view, detection efficiency, andenergy resolution. Spatial resolution defines the ability of a system to resolve two distinct, spatially separated radioactivesources. Field of view defines the solid angle accepted by the system. The amount of time required to examine a given areadepends on the system's field of view and detection efficiency. Energy resolution defines the ability of an imaging system todistinguish gamma rays from different isotopes.Technical issues that must be addressed in designing a Compton camera include the optimization of detector characteristics,system timing resolution, and spatial resolution. Detector parameters of particular importance are energy resolution, noise,and timing. These factors significantly influence system performance in the 100 keV to 1 MeV range. Detector energyresolution determines the ability of the camera to discriminate between closely separated energy peaks. This is especially