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Simultaneous Pu and U Isotope Nuclear Forensics on an Environmentally Recovered Hot Particle

Bellucci, J. J., Whitehouse, M. J., Aleshin, M., Eriksson, M.
Analytical chemistry 2019 v.91 no.9 pp. 5599-5604
accidents, forensic sciences, image analysis, isotopes, mass spectrometry, mixing, scanning electron microscopy, Greenland
An environmentally recovered, mixed Pu–U hot particle from the Thule accident, Greenland has been analyzed by Scanning Electron Microscopy and a large-geometry Secondary Ion Mass Spectrometry based Scanning Ion Imaging (SII) method for simultaneous ²³⁵,²³⁶,²³⁸U and ²³⁹,²⁴⁰Pu isotope compositions. This SII technique permits the visual assessment of the spatial distribution of the isotopes of U and Pu and can be used to obtain quantitative isotope ratios in any user-defined square region up to a few 100 μm in size. The particle measured here has two resolvable U isotopic compositions with a single composition of weapons grade Pu. The bulk of the particle has enriched U and weapons-grade Pu with ²³⁵U/²³⁸U, ²³⁶U/²³⁸U, and ²⁴⁰Pu/²³⁹Pu of 1.12 ± 0.04, 0.006 ± 0.002, 0.054 ± 0.004, respectively (2σ). The Pu isotopic ratio was consistent across the sample but ²³⁹Pu/²³⁸Uᵣₐw decreased from 1.99 ± 0.07 to 0.11 ± 0.04 (2σ) corresponding to the area of the particle with a resolvably different U isotope composition. This portion of the particle has ²³⁵U/²³⁸U, ²³⁶U/²³⁸U, and ²⁴⁰Pu/²³⁹Pu ratios of 0.11 ± 0.04, 0.001 ± 0.002, and 0.05 ± 0.04, respectively (2σ). The origin of the less enriched U could be environmental that mixed with the particle or heterogeneously enriched U from the weapons. The heterogeneity of hot particles on a micrometer scale highlights the need for spatially resolved techniques to avoid mixing during a bulk or whole particle analysis, as the mixing end-members here would have been lost, and the measured ratios would have been inaccurate.