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Atomic structure as a signature for nuclear forensics and archaeology

Cameron Tracy

Stanton Nuclear Security Fellow
Harvard Kennedy School

Nuclear forensics and nuclear archaeology involve the analysis of signatures that provide insight into past proliferation-relevant activities. For example, rare earth impurity concentrations in uranium ores can reveal the specific mines from which they were extracted, while radionuclide concentrations in graphite moderator components can constrain past plutonium production. Conventional approaches to nuclear forensics and archaeology rely primarily on compositional signatures, both elemental and isotopic. This talk will focus on new signatures related to atomic structure, or the spatial arrangement of atoms in a material. First, recent work has shown that the penetration of oxygen into UO2 surfaces can serve as a signature of atmospheric exposure during illicit trafficking. Characterization of surface oxidation using crystallographic probes can constrain the time, temperature, and humidity conditions of exposure. Second, preliminary experiments have demonstrated the potential of x-ray and neutron total scattering measurements, coupled with pair distribution function (PDF) analysis, to directly quantify defect concentrations in nuclear materials. Because radiation exposure tends to produce such defects, their presence serves as a signature of the proximity of materials to radiation sources. PDFs contain information about the positions of all atoms in a sampled volume, such that this approach allows for direct quantification of the immediate result of radiation exposure.