Uranium isotopic assay instrument

Full text: The isotopic assay instrument under development at Pacific Northwest National Laboratory (PNNL) is capable of rapid prescreening to detect small and rare particles containing high concentrations of uranium in a heterogeneous sample. The isotopic measurement concept is based on laser vaporization of solid samples followed by sensitive isotope-specific detection using either uranium atomic fluorescence emission or uranium atomic absorbance. Both isotopes are measured concurrently, following a single ablation laser pulse using two external-cavity violet diode lasers. The simultaneous measurement of both isotopes enables the correlation of the fluorescence and absorbance signals on a shot-to-shot basis. This measurement approach demonstrated negligible channel crosstalk between isotopes. Scanning the heterogeneous samples provides high-resolution imagery of sample isotopic fluorescence and absorbance. Isotopically selective excitation of uranium vapor also was the basis of the Atomic Vapor Laser Isotope Separation (AVLIS) program at Lawrence Livermore National Laboratory for uranium enrichment. Laser ablation combined with measurements of laser-induced fluorescence (LALIF) and through-plume laser absorbance (LAPLA) have been used on a variety of samples that include various steels and glasses. Reported elemental detection limits typically range between 1 and 100 ppm, with 1 to 1000 ng of material ablated and mass detection limits of 0.1 pg for single-shot measurements. The availability of violet diode lasers from Nichia and other manufacturers provides significant flexibility in isotopic assay measurements. With external-cavity diode lasers, 235U can be excited at 404.3 nm with fluorescence at 394.4 nm, while 238U can be excited at 415.4 nm with fluorescence at 493.3 nm. The approximately 100-nm difference in fluorescence wavelengths is trivial to separate spectrally using optical bandpass filters. Likewise, for LAPLA measurements, the two-laser absorption channels can be separated easily with optical filters or diffraction gratings. While these lasers are not as small and low-powered as those in CD-ROM drives, they are small enough to be integrated suitably into a transportable instrument. Diode laser LALIF and LAPLA spectrometry was applied to measure gadolinium isotope ratios in solid samples by laser ablation. Gadolinium has excitation wavelengths very close to the transitions of interest in uranium. Gadolinium has several isotopes, and the natural 152Gd/160Gd ratio of 0.009 is in the range of what will be encountered for 235U/238U isotopic ratios. LAPLA measurements were demonstrated clearly using 152Gd (0.2% isotopic abundance) with a good signal-to-noise ratio. The ability to measure gadolinium abundances at this level indicates that measurements of 235U/238U isotopic ratios for natural (0.72%), depleted (0.25%), and low-enriched uranium samples will be feasible