The application of pinhole and coded aperture imaging in the nuclear environment

A gamma-ray imaging system optimised for use in the nuclear environment has been developed. The system allows the simple selection of either pinhole or coded aperture image formation techniques depending on the nature of the scene. The detector portion of the instrument is the same for both options and comprises a 3′′ diameter PSPMT coupled to an array of CsI(Tl) crystals each with dimensions 3.5×3.5×25 mm on a 3.8 mm pitch. The detector is mounted within a tungsten shell, which provides at least 35 mm of shielding to sources outside of the FOV. The field of view for both imaging techniques is 12.8°. The pinhole diameter of 6 mm provides an angular resolution of ∼2° at 662 keV. The coded aperture comprises a 127 hexagonal-URA produced by machining 3 mm holes on a 3.5 mm pitch in a 3 mm deep tungsten sheet and provides an angular resolution of ∼1° at 662 keV. Both imaging techniques have been used to view a range of radiation scenes to determine their relative performance. As expected the coded aperture showed better sensitivity in high background environments however for scenes with low and moderate background rates with several sources in the FOV the pinhole system performed better. The results suggest that a lightweight system, which allows both coded aperture and pinhole imaging, can provide excellent sensitivity and dynamic range coverage for a wide range of radiation scenes.