Portable nuclear safeguard equipment using pinhole gamma camera

The collimator design for nuclear survey system should be differently taken into consideration from the collimator for medical environments because it has always been used in the high-energy radiation environments. The purpose of this study is to research the optimum pinhole design for accomplishing the improved image at high-energy radiation field under condition. The relative resolution and sensitivity advantages of pinhole collimators were investigated using analytic formulations and Monte Carlo simulations. Simulations using GATE (Geant4 Application for Tomographic Emission) were performed to model the pinhole gamma camera system. A gamma camera consists of a cone-shaped pinhole collimator with a tungsten aperture and a CsI(Tl) scintillation crystal 6.0 mm thick and 50.0 mm × 50.0 mm in area. The focal length and the acceptance angle of the pinhole collimator were set to 60.35 mm and 45°, respectively. The intrinsic spatial resolution and sensitivity were simulated by changing the pinhole diameter and channel height. The point source was located 60.35 mm above the center of the pinhole, and the projection data was estimated for pinhole diameter values from 2.0 mm to 4.0 mm while the channel heights were fixed between 2.0 mm and 6.0 mm. The optimal ranges of channel height and pinhole diameter were determined through evaluation of the intrinsic resolution and sensitivity tradeoff curves. The channel type is selected by these analyses and we also verify these simulation results through experimental test of three types of collimators. We allowed to determine the optimal values of pinhole diameter and channel height to be 2, 3, 4 mm and 6, 4, 6 mm, respectively. The simulated and experimental results agreed with 6% and 14% discrepancies in sensitivities and spatial resolution, respectively. The results demonstrated that the pinhole collimator designed in this study could be utilized to perform radiation monitoring system.