Promises and pitfalls of ns-laser ablation for depth profiling of UO2 single crystals

Abstract

A total number of 960 laser ablation (LA) craters produced by a ns-LA system were characterized systematically for their depth profiling capability of UO2 single crystals. For gaining improved understanding of laser-UO2 interaction, a variable number of circular laser pulses, having a nominal diameter of 25 µm, were shot repeatedly (N = 5) on the surface of a UO2 single crystal. To this end, performance at both diverse fluence settings and different number of individual laser pulses was evaluated at laser repetition rates of 1 Hz and 2 Hz. As such, the impact of cumulative fluence of 50, 150, 250, 350, 450, 550, and 650 J cm−2 on the integrity of a UO2 single crystal was investigated. Confocal laser scanning profilometry and a dual beam focused ion beam microscope were employed to assess essential parameters such as LA rate, surface roughness, silhouette and depth of LA craters. While the LA rate decreased with increasing single shot fluence, surface roughness continuously intensified with expanded cumulative fluence. The average depth of LA craters was higher, the higher was the cumulative fluence. The conical geometry and the variability of the bumpy bottom of LA craters limited the experimentally achievable depth resolution to ~ 1 µm. Identical LA experiments carried out weeks apart, produced very similar data, proving high reproducibility of the laser-UO2 interaction.