Year
2012
Abstract
Use of bremsstrahlung is one of the most mature active interrogation technologies under development for searching for small quantities of special nuclear material (SNM). In this study, the scenario evaluated is that the SNM is highly enriched uranium (HEU), which is being smuggled into the country onboard a small ship in a maritime environment. A semi-empirical model has been developed that simulates a littoral combat ship (LCS) with a notional bremsstrahlung active interrogation system mounted on its deck. The inspection object (IO) is a small quantity of HEU stowed onboard a fishing trawler. Nearby to the trawler is an Unmanned Surface Vehicle (USV) containing neutron detectors that measure the delayed neutron response. This response is generated from photofissioning induced as the IO interacts with the bremsstrahlung. Results for bremsstrahlung energies of 60 MeV are provided. The semi-empirical model has been coded using the MATLAB© software package. This model consisted of combining fundamental radiation transport principles such as the inverse square law for distance from the IO to the neutron detectors along with published experimental and MCNPX data of the delayed neutron response per IO mass and bremsstrahlung beam coulombic charge. The model also considered the effects of air attenuation by bremsstrahlung and both lead and polyethylene shielding around the IO. This semi-empirical model has been compared with a MCNPX maritime model developed by the Defense Threat Reduction Agency (DTRA) that simulates the same littoral scenario as described above, but with a large converter and collimator that significantly reduced the intensity of the emitted bremsstrahlung. Consequently, the two models predicted different outcomes on finding the HEU.