A NanoSIMS 50 L Investigation into Improving the Precision and Accuracy of the 235U/238U Ratio Determination by Using the Molecular 235U16O and 238U16O Secondary Ions

Abstract

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.

Isotopic Heterogeneity Imaged in a Uranium Fuel Pellet with Extreme Ultraviolet Laser Ablation and Ionization Time-of-Flight Mass Spectrometry

“…The average U and UO ratios measured with EUV TOF are similar, where 235 U/ 238 U = 0.0231 ± 0.0007 (2σ) and 235 UO/ 238 UO = 0.0233 ± 0.0008 (2σ). For NanoSIMS, average U and UO ratios measured are different, , where 235 U/ 238 U = 0.0213 ± 0.0004 (2σ) and 235 UO/ 238 UO = 0.0233 ± 0.0002 (2σ). This could be from the low overall U ion counts from NanoSIMS preferentially producing UO species from uranium oxides because of the lower ionization potential of the O – adducts compared to the atomic ions, that is, 5.7 eV for UO + versus 6.2 eV for U + .…”

“…For all maps, three EUV laser shots were taken at each pixel location. 26 The integrated number of counts for the 235,238 U and 235,238 U 16 O isotopes collected at each location were summed to a single pixel 16,34 (indicated hereafter as "summed U"). The pixels of the resulting EUV TOF ion maps were not smoothed to show the isotopic information in each pixel.…”

Imaging isotopic content at the nanoscale using extreme ultraviolet laser ablation and ionization mass spectrometry

“…14 For both EUV TOF and NanoSIMS analyses, the 235 U and 235 UO ion counts and 238 U and 238 UO ions collected at each pixel (i.e., the background-subtracted peak area) were subjected to a threshold and summed to a single pixel. 13,24 The threshold was used to remove insufficient 235 U, 235 UO signals (i.e., 0 ion counts of the minor isotope for EUV TOF and NanoSIMS analysis) or saturated 238 U, 238 UO signals (i.e., counts of the major isotope above the ADC's upper dynamic range for EUV TOF analysis). Three laser shots were taken at each pixel location for EUV TOF analyses 13,25 and eight passes per pixel were collected with NanoSIMS.…”

A preliminary investigation into the use of molecular oxide and hydride secondary ion relationships for improvement of the 236U/238U determination on a NanoSIMS 50L

“…From other studies (e.g. 5 ) it is known that the 23x U 16 O molecular secondary ions are typically detected in greater abundances as compared to the 23x U elemental secondary ions. Therefore, it is possible that the increased count rate of the 236 U 16 O relative to the 236 U results in improved precision and accuracy.…”