Single shot laser ablation MC-ICP-MS for depth profile analysis of U isotopes in UO2 single crystals

Abstract

An analytical procedure for determining the n(²³⁵U)/n(²³⁸U) amount ratio in consecutive layers of UO₂ single crystals was developed and validated. A 25 μm circular shaped laser beam with a fluence of only 0.24 J cm⁻² was employed for depth profiling ²³⁵U and ²³⁸U in UO₂ single crystals with U isotopes being detected via MC-ICP-MS. The time-resolved ²³⁵U and ²³⁸U MC-ICP-MS signals obtained from individual laser shots were processed automatically using software specifically developed for this purpose. Downhole fractionation of the n(²³⁵U)/n(²³⁸U) amount ratio was excluded by shooting 80 laser pulses on a reference UO₂ single crystal of known composition, revealing no measurable change in its U isotopic ratio during depth profiling. A linear relationship between the number of laser shots and the average depth of the laser ablation craters was established using confocal laser scanning profilometry. The ns-laser ablation system produced conical craters whose diameters were shown to increase with the number of laser pulses. The shape and roughness of the craters were studied as a function of both the number of pulses and focusing conditions. Using a dual beam focused ion beam (FIB), high resolution scanning electron microscopy (SEM) micrographs revealed the formation of rectangular “tiles” on the reference UO₂ single crystal after as little as five laser shots. The ordered, rectangular structure disappeared progressively with increasing number of laser pulses, while simultaneously a sub-micrometric porosity developed. The depth profiling capabilities of the laser ablation system were applied to two UO₂ single crystals produced under different experimental conditions involving solid state isotopic mixing of ²³⁵U and ²³⁸U in order to characterise U self-diffusion in UO_2+x. Both UO₂ single crystals featured a n(²³⁵U)/n(²³⁸U) ratio gradient, containing enriched U at the surface and depleted U in the bulk.