Developing analytical methods aimed at evaluating the elemental and isotopic composition of working reference materials analogs has become increasingly more desirable for advancing our knowledge of the mechanisms of nuclear fallout formation. Secondary ion mass spectrometry (SIMS) is one such technique that has risen to the top of the pack due to its ability to measure isotopic heterogeneities in nuclear materials on the micrometer scale. However, no technique is without challenges. SIMS suffers from the presence of molecular isobaric interferences at any given mass-to-charge ratio. These interferences complicate the interpretation of SIMS measurements and ultimately reduce the accuracy and precision of the measured isotope ratios. At the U.S. Naval Research Laboratory, we have developed a possible solution to the issue of molecular isobaric interferences called the Naval Ultra-Trace Isotope Laboratory’s Universal Spectrometer or NAUTILUS for short. The NAUTILUS combines a small geometry SIMS instrument, maintaining the ability to measure isotopic compositions with high accuracy and precision with micrometer spatial resolution, with a Single-stage accelerator mass spectrometer (SSAMS), adding the ability to dissociate molecular isobaric interferences within a stripper gas. In this work, we demonstrate the application of the NAUTILUS to the analysis of uranium doped silicate glasses as working reference materials analogs for nuclear fallout materials. These glasses range in uranium concentration from ~4 ppm to ~400 ppm and in uranium isotopic composition from 0.72% (natural) to 92.5% enriched in 235U. We found that using the NAUTILUS, not only could we measure the major uranium isotopes (e.g. 235U and 238U) directly, but also we could measure the minor uranium isotopes (e.g. 234U and 236U) from the glasses without the need for background subtraction or interference corrections. More recently, we have obtained preliminary data on glasses composed of both uranium and plutonium. These mixed actinides glasses are particularly problematic for SIMS due to the increased complexity of measuring both the uranium major and minor isotopes as well as the plutonium major (e.g. 239Pu and 240Pu) and minor (e.g. 241Pu and 242Pu) isotopes within the same sample matrix. Early measurements demonstrate that these analyses are possible with the NAUTILUS.

Direct, uncorrected, molecule-free analysis of actinides from glassy nuclear materials

2019 Spring National Meeting

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