Portal Monitor Characterization for Internally and Externally Deposited Radionuclides

In this evaluation, both the internal and external radionuclide detection efficiencies for a portal monitor were evaluated as a function of photon energy using an anthropomorphic phantom. Pass-through and static measurements were completed using Am-241, Co-57, Ba-133, Cs-137, Co-60, Cd-109, and Mn-54 in various locations both external and internal to the phantom. Other parameters, such as single detector uniformity, total detector uniformity, background linearity, and activity linearity have been analyzed. It was found that the minimum detectable activity for internally deposited Cs-137 in the abdomen was approximately ten times higher for pass-through versus static measurements. Additionally, it was found that the minimum detectable activity for Cs-137 in the abdominal region for both internal and external pass-through scenarios are nearly equivalent. In general, if the expected radionuclide source term is primarily non-transuranic, the pass-through mode offers sufficient sensitivity to identify potential overexposures while providing much greater personnel throughput. However, minimum detectable committed effective doses for transuranics such as Am-241, show potential for personnel over exposure if the radionuclide mixture contains a significant fraction of transuranics. It is therefore recommended that nuclear facilities evaluate their radionuclide source term in order to bound potential personnel doses.