$^{240}$Pu/$^{239}$Pu isotopic ratio measurements in micrometric Pu and MOX particles using Secondary Ion Mass Spectrometry

International audienceEvery accident affecting industrial or nuclear facilities emits micrometric fragments of material into the environment whose elemental and isotopic compositions are characteristic of the process or event. Particle analysis, mainly implemented in the framework of the Non Proliferation Treaty to detect clandestine nuclear activities, provides a powerful tool to identify the origin of the nuclear particulate matter and to assess the environmental impact of nuclear accidents. Initially, particle-scale isotopic analyses aimed at the determination of the U isotopic composition. Now, focus is increasingly given on Pu isotopic measurements to address its origin and potential use. Such measurements are more challenging because of isobaric interferences, including those induced by hydride ions, like $^{239}$PuH+ on $^{240}$Pu+ and $^{238}$UH+ on $^{239}$Pu+ in Mixed Oxide (MOX). Such ions are generated during ionization processes by Secondary Ion Mass Spectrometry. Based on a parametric study aiming at the measurement of uranium oxide, uranium carbide and uranium single and double hydride rates, we determined that Pu and U should be detected as elementary ions to limit the impact of such interferences, although mono-oxide ions are more abundant. Thus, we developed an analytical methodology to obtain accurate$^{240}$Pu/$^{239}$Pu atomic ratios both for weapon grade Pu and MOX materials. Hydride rate is first measured in U oxide particles and then applied to correct $^{240}$Pu+ and $^{239}$Pu+ signals. The relative difference of corrected $^{240}$Pu/$^{239}$Pu isotopic ratios with expected values is reduced by a factor of 4 when measuring weapon grade Pu particles and by a factor of 10-100 when measuring MOX particles containing 1 to 10 wt% of Pu. We also proposed a method to determine the Relative Sensitivity Factor (RSF) based on the decay of Pu in order to quantify the Pu content in MOX samples. The estimated lowest measurable $^{239}$Pu/$^{238}$U atomic ratio in MOX particles is ∼1.6 × 10$^{-3}