Characterization and Optimization of a Spectral Window for Direct Gaseous Uranium Hexafluoride Enrichment Assay Using Laser-Induced Breakdown Spectroscopy

Through a systematic scanning of ²³⁵U and ²³⁸U emission lines between 280 nm and 745 nm, the optimal emission line for direct gaseous uranium hexafluoride (UF₆) enrichment assay using laser-induced breakdown spectroscopy (LIBS) was found. Screening for spectral features that are potentially useful for U isotopic analysis was gauged from the magnitude of the ²³⁵U–²³⁸U isotopic shift and the signal-to-background ratio of the emission line through a parameter termed ΔSBR _235U–238U. The ΔSBR spectrum shows peaks at wavelength positions where there are strong lines with significant ²³⁵U–²³⁸U shifts. The screening identified 13 spectral-window candidates, which were down selected based on their overall accuracy in predicting the ²³⁵U enrichment of three UF₆ samples of natural (0.720 atom% ²³⁵U) and low-enriched (4.675 atom% and 9.157 atom% ²³⁵U) grades. The U(I) 646.498 nm emission line, with a determined ²³⁵U–²³⁸U isotopic shift of −17.7 pm, was found to be the optimal spectral window for direct UF₆ enrichment assay. The root mean square error for enrichment assays on the three natural and low-enriched UF₆ samples, with each sample measured in six replicates, was 0.31% in absolute ²³⁵U content. Each measurement comprised LIBS signals accumulated from 3000 laser shots. The analytical bias and precision were better than 0.5% and 0.3%, respectively, in absolute [²³⁵U/(²³⁵U + ²³⁸U)] ratios. Specific for the two low-enriched UF₆ samples, the relative standard deviations from six replicated measurements were around 2%.