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

摘要

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 U-235-U-2(38) 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.