Measurement of uranium-236 in particles by secondary ion mass spectrometry

The determination of the relative isotopic abundance by secondary ion mass spectrometry of ²³⁶U in uranium-containing material is complicated by the presence of ²³⁵U¹H⁺ ions at the same nominal mass as the uranium isotopic peak. The net intensity of the ²³⁶U signal is usually determined by a peak-stripping procedure, whereby the ²³⁵U¹H⁺ contribution is obtained by applying the ²³⁸U¹H⁺/²³⁸U⁺ ratio to the ²³⁵U⁺ signal. The subtraction of one signal from another has consequences for the uncertainty of the final ²³⁶U abundance determination that may be especially significant when the amount of sample is limited, as is the case with small uranium particles that are of great interest for nuclear safeguards. We have developed a model based on Poisson counting statistics to determine the effects of various parameters on the uncertainty of the ²³⁶U abundance, including uranium enrichment level, hydride-to-parent ratio, uranium mass consumed during analysis, single versus multichannel counting, and sample substrate composition. The model predictions have been successfully tested against experimental measurements of uranium oxide particles of both 3% and 90% enrichment in ²³⁵U.