Analytical characterization of laser induced plasmas towards uranium isotopic analysis in gaseous uranium hexafluoride

To perform direct enrichment assay on gaseous uranium hexafluoride (UF₆) with laser induced breakdown spectroscopy (LIBS), the dominant spectral-line features, evolution of the signal and background of the U II 424.437 nm line, and its Stark width and shift, were studied as a function of UF₆ gas pressure and pulse energy of a nanosecond Nd:YAG laser. Vapor pressure of UF₆ was found to be the most important parameter for LIBS analysis of gaseous UF₆. Spectral congestion with numerous U lines of high excitation potential was observed and signal-to-background ratio (SBR) was low for measurements with 80 Torr UF₆. Only when both UF₆ vapor pressure and laser pulse energy were low, for example, less than 20 Torr pressure and 30 mJ pulse energy, the resultant LIBS spectra from gaseous UF₆ resembled those obtained from solid U samples. The experimental data also suggest that U and F atoms recombine back to UF₆ after the laser pulse. The U emission was found to decay fast with a persistent background signal, degrading SBR with delay time. Systematic positive biases were found for UF₆ enrichment assays performed with the ²³⁵U–²³⁸U line pair at 424.412–424.437 nm, which was confirmed to be caused by self-absorption. Even with optimization of experimental parameters and incorporation of a self-absorption term into the spectral-fitting algorithm, to reduce and compensate for self-absorption, self-absorption is still a main factor limiting accurate UF₆ enrichment assay. The use of another spectral window which contains no resonance lines is a prospective solution for the self-absorption issue.