Analytical considerations in the determination of uranium isotope ratios in solid uranium materials using laser...

Validated analytical measurement protocols for the fast and accurate determination of the uranium (U) isotopic composition (²³⁴U, ²³⁵U, ²³⁶U, ²³⁸U) of solid nuclear materials were developed employing ns-laser ablation (LA) coupled to multi-collector ICP-MS. The accuracy of the analytical procedure was assured by frequent (n = 65) analysis of a pressed pellet of certified isotopic reference material CRM U-030 (∼3 wt% ²³⁵U). The expanded uncertainty (k = 2) for the n(²³⁵U)/n(²³⁸U) ratio was as low as 0.05%, rising to 0.62% and 1.09% for n(²³⁴U)/n(²³⁸U) and n(²³⁶U)/n(²³⁸U) ratios, respectively. LA-MC-ICP-MS measurements of a pressed pellet of certified isotopic reference material CRM U-020 (∼2 wt% ²³⁵U) before analysis of each sample allowed calculation of the ion counter gains and mass bias correction. Both individual spot analysis and line scan analysis were used to measure n(²³⁴U)/n(²³⁸U), n(²³⁵U)/n(²³⁸U), and n(²³⁶U)/n(²³⁸U) ratios in two low-enriched UO₂ pellets from the fourth Collaborative Materials Exercise (CMX-4), four seized low-enriched UO₂ pellets intercepted from illicit trafficking and one metal sample consisting of depleted U. LA-MC-ICP-MS results of all investigated samples matched well with U isotope ratios obtained by thermal ionisation mass spectrometry (TIMS). This independent confirmation of the LA-MC-ICP-MS results by TIMS underpinned the high quality of generated analytical data. Acquisition of several thousand data points within a couple of minutes during line scan analysis yielded detailed information on the spatial distribution of the U isotopic composition of selected UO₂ pellets, revealing straightforwardly their (in-)homogeneity on the μm-scale. Calculating skewness and half width of the frequency distributions of the n(²³⁵U)/n(²³⁸U) amount ratio allowed the quantitative assessment of the (in-)homogeneity of the investigated samples. This information allows drawing conclusions on the starting materials used for the production of the pellets. From a nuclear forensics perspective, LA-MC-ICP-MS provides quick, accurate results on the spatial distribution of major and minor U isotopes while preserving the sample i.e. piece of evidence, essentially intact.