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

An analytical procedure for the determination of the n(235U)/n(238U) ratio in consecutive layers of UO2 single crystals was developed and validated. A circular 25 µm laser beam and a fluence of only 0.2 J cm-2 were employed for depth profiling of 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 evaluated automatically with a home-made software. Downhole fractionation of the n(235U)/n(238U) ratio was excluded by shooting 80 laser pulses on a reference UO2 single crystal of known composition (N=5), 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 generated laser ablation craters was established employing confocal laser scanning profilometry. After ~50 laser shots this relationship flattened and surface roughness of the crater bottoms increased noticeably at the same time. The employed ns-laser ablation system produced conical craters with their corresponding crater diameters expanding with increasing number of laser pulses. 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 already after five laser shots. The ordered, rectangular structure was disappearing progressively with increasing number of laser pulses, while simultaneously the sub-micrometric porosity amplified, leading to a sombrero-like crater profile. The depth profiling capabilities of the laser ablation system were applied to two UO2 single crystals produced under different experimental conditions. Both featured a n(235U)/n(238U) ratio gradient, containing enriched U at the surface and depleted U in the bulk. As such, the variable diffusion behaviour of 235U into the UO2 single crystals was studied systematically. The laser ablation rate varied distinctly between the two UO2 single crystals, i.e. ~7.5 nm and ~13 nm, requiring individual depth calibration for each material.

KRACHLER Michael;  BULGHERONI Antonio;  MARTINEZ FERRI Ana;  MA Yue;  MIARD Audrey;  GARCIA Phillipe; 

2019-10-18

ROYAL SOC CHEMISTRY

JRC116169

0267-9477 (online),   

https://publications.jrc.ec.europa.eu/repository/handle/JRC116169,   

10.1039/c9ja00212j (online),