Gamma Response in a Synthetic Aperture System for Roadside Detection of Special Nuclear Material

The detection and identification of Special Nuclear Material (SNM) sources has become of great concern. This research focused on the development, optimization and assessment of a Synthetic Aperture SNM Detection System (T-SADS system) utilizing 3-D forward and adjoint deterministic Discrete Ordinate (Sn) transport with the PENTRAN code system coupled with Monte Carlo transport simulations using MCNP5. The T-SADS system’s multiple sets of specially moderated neutron and gamma detection blocks placed in series tangential to the travel path of a vehicle, and can detect and rapidly allow for the identification of sources present in a vehicle traveling past the detectors. This paper discusses gamma detection capabilities, and the methodology behind the computational assessment of the T-SADS gamma detection block. Deterministic adjoint transport calculations were performed in order to determine the importance of photons, efficiency, and relative to the Field of View (FOV) of the detector. Background contributions assumed to appear from surrounding sources, cosmic background, and ground scatter were established to determine a minimum Currie-limit necessary to achieve a 95% Probability of Detection and a 5% Probability of False Alarm. Results confirmed the T-SADS system’s ability to detect gamma signatures with 95% confidence for HEU and WGPu sources traveling at speeds of up to 60 miles per hour.