A NANOSIMS 50L INVESTIGATION INTO IMPROVING THE PRECISION AND ACCURACY OF THE 235U/238U RATIO DETERMINATION BY USING THE MOLECULAR 235U16O AND 238U16O SECONDARY IONS

A NanoSIMS 50 L was used to study the relationship between the ²³⁵U/²³⁸U atomic and ²³⁵U¹⁶O/²³⁸U¹⁶O molecular uranium isotope ratios determined from a variety of uranium compounds (UO₂, UO₂F₂, UO₃, UO₂(NO₃)₂·6(H₂O), and UF₄) and silicates (NIST-610 glass and the Plesovice zircon reference materials, both containing µg/g uranium). Because there is typically a greater abundance of ²³⁵U¹⁶O⁺ and ²³⁸U¹⁶O⁺ molecular secondary ions than ²³⁵U⁺ and ²³⁸U⁺ atomic ions when uranium-bearing materials are sputtered with an oxygen primary ion beam, the goal was to understand whether use of ²³⁵U¹⁶O/²³⁸U¹⁶O has the potential for improved accuracy and precision when compared to the ²³⁵U/²³⁸U ratio. The UO₂ and silicate reference materials showed the greatest potential for improved accuracy and precision through use of the ²³⁵U¹⁶O/²³⁸U¹⁶O ratio as compared to the ²³⁵U/²³⁸U ratio. For the UO₂, which was investigated at a variety of primary beam currents, and the silicate reference materials, which were only investigated using a single primary beam current, this improvement was especially pronounced at low ²³⁵U⁺ count rates. In contrast, comparison of the ²³⁵U¹⁶O/²³⁸U¹⁶O ratio versus the ²³⁵U/²³⁸U ratio from the other uranium compounds clearly indicates that the ²³⁵U¹⁶O/²³⁸U¹⁶O ratio results in worse precision and accuracy. This behavior is based on the observation that the atomic (²³⁵U⁺ and ²³⁸U⁺) to molecular (²³⁵U¹⁶O⁺ and ²³⁸U¹⁶O⁺) secondary ion production rates remain internally consistent within the UO₂ and silicate reference materials, whereas it is highly variable in the other uranium compounds. Efforts to understand the origin of this behavior suggest that irregular sample surface topography, and/or molecular interferences arising from the manner in which the UO₂F₂, UO₃, UO₂(NO₃)₂·6(H₂O), and UF₄ were prepared, may be a major contributing factor to the inconsistent relationship between the observed atomic and molecular secondary ion yields. Overall, the results suggest that for certain bulk compositions, use of the ²³⁵U¹⁶O/²³⁸U¹⁶O may be a viable approach to improving the precision and accuracy in situations where a relatively low ²³⁵U⁺ count rate is expected.

*The full published version of this work can be found in Minerals 2019, 9(5), 307; doi.org/10.2290/min9050307