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

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 235,236,238 U and 239,240 Pu 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 235 U/ 238 U, 236 U/ 238 U, and 240 Pu/ 239 Pu of 1.12 ± 0.04, 0.006 ± 0.002, 0.054 ± 0.004, respectively (2σ). The Pu isotopic ratio was consistent across the sample bu 239 Pu/ 238 U raw 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 235 U/ 238 U, 236 U/ 238 U, and 240 Pu/ 239 Pu 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.