Total Gamma-ray Counting In Support Of Arms Control

The verification of the absence of special nuclear material may be a key requirement in potential future warhead treaty verification. An absence verification system could use detailed gamma-ray spectral features to discriminate between benign radioactive sources, such as depleted uranium (DU), and special nuclear materials, such as plutonium (Pu) or highly enriched uranium (HEU), but that approach could reveal more information than the host may be willing to allow. Previous work has suggested that the ratio of total gamma-ray counts using a hand-held sodium iodide detector acquired with and without a thin tungsten attenuator can differentiate DU from HEU and Pu for the limited range of geometries considered, which included engineered shielding. Current work is expanding that study to include ratios obtained with different detector materials (polyvinyl toluene, Geiger-Mueller tube), varying attenuator types and thicknesses, and lower energy discriminator thresholds to find an ideal procedure with maximum discriminating power without using spectral analysis. The approach works well for bare materials but as with all passive measurements can be defeated by adequate shielding materials. The shielding materials may consist of container walls or engineered shielding inside those walls. Transmission measurements through the outer container will also be conducted to determine the impact of potential shielding hidden within the container. The transmission measurements will be performed off-axis to avoid passing through the material being surveyed. The transmission measurements will produce a sensitivity threshold that can be combined with the total gamma-ray counting results to give confidence that if SNM in a certain amount were present, it would be detected, and the lack of gamma-ray detection is due to the absence of the material and not due to additional shielding. The modeling study is being verified through laboratory benchmark measurements. This work is supported by the US Dept. of Energy, National Nuclear Security Administration, Office of Defense Nuclear Nonproliferation Research and Development. This is PNNL document number PNNL-SA-159850.