Measurement of uranium samples for nuclear forensics by laser ablation multi-collector inductively coupled plasma mass spectrometry using a pre-cell mass filter for collision/reaction cell (MC-ICP-MS/MS)

Abstract

In this paper we present the performance of the newly developed Neoma™ multi-collector inductively coupled plasma mass spectrometry equipped with a pre-cell mass filter for collision/reaction cell (MC-ICP-MS/MS) coupled to laser ablation for nuclear forensics for the first time. The characteristics of the recently introduced MC-ICP-MS/MS instrument are assessed by measuring the U isotopic composition and U isotope ratio spatial distribution. We found that the achieved uranium isotope ratios agree well with the certified values. Furthermore, precisions using the current instrument have considerably improved due to the use of merely Faraday detectors for the analysis, their extended range (up to 100 V) and shorter dwell times (i.e. more data points) besides the multiple collection property of the MC-ICP-MS. The achieved ²³⁵U/²³⁸U relative combined uncertainty of 0.053 % for low-enriched uranium material using laser ablation is more than one order of magnitude better than using a single collector ICP-MS and approximately 5 times better than the previously reported MC-ICP-MS values. This ²³⁵U/²³⁸U amount ratio uncertainty is also much better than the recommended International Target Values for low-enriched uranium, which is generally used in nuclear safeguards. The use of higher mass resolution (R = ∼7100) compared to the standard measurement at R = ∼2100 does not worsen significantly the achieved isotope ratio uncertainty. This feature is particularly important for laser ablation studies of confiscated scrap samples or impure nuclear materials in nuclear forensics, where the matrix constituents can highly affect the obtained results. By the coupling the instrument to laser ablation, the overall analysis can be performed in a quasi-non-destructive way in a very short time (within a few hours) and producing practically no radioactive waste in addition. The developed method was applied for two types of tangible illicit nuclear materials (2 highly enriched U metal samples with ∼90 % ²³⁵U abundance and 3 low-enriched UO₂ samples) for characterization. Due to the improved precision an observable inhomogeneity at micron level was found in UO₂ materials, which can further refine the origin assessment. However, this finding has to be taken into account during the reporting of the U isotope abundances. In conclusion, besides its potential in other scientific disciplines, where high precision isotope analysis is required, the established technique using the novel MC-ICP-MS/MS instrument is essential in the security toolkit in nuclear forensic response without considerable alteration of the legal evidence.