A compact scintillator-based coded aperture imager for localizing illicit nuclear materials

Efficient detection, localization, and identification of illicit nuclear materials is a critical need for national security. Gamma-ray signatures can be exploited for these purposes, but very high detection efficiency is needed in addition to a practical design that can be effective in the field. Modern, dense scintillators with low intrinsic background provide the highest detection efficiency (albeit at somewhat lower energy resolution than semiconductor detectors), and solid-state photodetector readout of the scintillation light can enable a compact and robust design compared to conventional photomultiplier tubes. We explore the feasibility of a coded aperture gamma-ray imager which combines gadolinium oxyorthosilicate scintillator (GSO) and avalanche photodiode (APD) readout. Each GSO crystal is 5 times 5 mm in cross-section and 25 mm long, and is coupled to a Hamamatsu S8664-55 APD with matching active area. 225 detectors are arrayed in a 15 times 15 matrix within a tungsten shield box. Readout is effected with a custom PC board which includes multiple 32-channel ASICs to handle preamplification, shaping, and energy and time discrimination. The output is fed to a compact data acquisition system originally designed for medical imaging with the RatCAP PET system. Various coded aperture designs are being evaluated for optimal imaging performance. Detector design and performance are presented.