Coded-Aperture Imaging with High-Resolution Large-Volume CZT

Large-volume 3D-position-sensitive CZT offers several advantages for coded-aperture imaging, including high photoelectric fraction, sufficient depth to capture even high-energy gamma rays, and good energy resolution. Sub-pixel resolution can be used to determine interaction positions to within 0.3 mm. All of these factors enable coded-aperture imaging to be extended to higher energies. However, because of the low gamma-ray modulation achieved from the mask and gaps between crystals, significant artifacts can be introduced into the image. These artifacts are reduced in a new instrument variant, H420, which uses an automated rotating mask to perform mask and anti-mask measurements alternately, eliminating the unmodulated background contribution in the image reconstruction. A second instrument, Polaris-2.2, implements a new crystal positioning method to virtually eliminate gaps between crystals and is also capable of performing mask and anti-mask measurements. The design and the imaging results from both instruments will be presented for both point and distributed sources.