RAPID DETERMINATION OF URANIUM ISOTOPICS USING A MANPORTABLE LIBS INSTRUMENT AND A HIGH-RESOLUTION SPECTROMETER

Field-deployable instruments based on the technique of laser-induced breakdown spectroscopy (LIBS) are being developed for real-time detection of nuclear and other threat materials. The capability to quickly determine the enrichment of special nuclear materials (SNM) is now being added to the current detection and reporting of elemental uranium (U), U compounds, explosives, tri-butyl phosphate (TBP), and many other materials. Rapid determination of SNM isotopics is of interest to pre- and post-detonation forensics and to certain arms-control inspections. High- resolution optical spectroscopy is required to resolve the mass-split uranium or plutonium isotopic atomic lines. Here we describe the use of LIBS to detect 235U and 238U atomic emission lines directly in air using laser plasmas generated by a manportable LIBS instrument of low pulse energy. The isotope shift of the 424.437 nm U(II) emission line was monitored using two high-resolution spectrometers: the DEMON (LTB Berlin) and the more compact echelle EMU-65 (Catalina Scientific). The ability to clearly resolve the isotope shift in air was demonstrated using a series of enriched and natural uranium samples. (Past work suggested that adverse line-broadening effects may require the SNM sample be covered with a low-pressure and/or special gas.) In addition, the accuracy of enrichment measurements was determined along with optimum LIBS detection parameters. A survey was also made of other candidate emission lines suitable for isotopic analysis. The importance of this work is (1) the demonstration that LIBS can determine uranium enrichments under ambient atmospheric conditions; (2) that enrichments can be determined in less than 30 seconds; (3) that a small manportable LIBS instrument can be used to observe enrichment and to collect high-quality spectra; and (4) that an optical spectrometer (EMU-65) adequate for resolving isotopically split uranium atomic lines can be of reasonably small size and weight, while providing broad spectral coverage (200-1000 nm) to monitor, simultaneously, other materials of interest.