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

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 UO2+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.