Uranium isotope abundance ratios in natural uranium metal certified reference material 112-A

Certified reference material (CRM) 112-A, uranium metal assay standard, with "natural" U isotopic composition and CRM 145, uranium assay standard solution made by dissolving CRM 112-A metal were analyzed using TRITON Thermal Ionization Mass Spectrometer (TIMS) to characterize the uranium isotopic abundances. The certified ²³⁵U/ ²³⁸U "major" ratio of 0.0072543 (40) in CRM C112-A and CRM 145 is determined using the total evaporation (TE) and modified total evaporation (MTE) methods. In the MTE method, the total evaporation process is interrupted on a regular basis to allow correction of background from peak tailing, internal calibration of the secondary electron multiplier (SEM) detector versus the Faraday cups, peak-centering, and ion source re-focusing. For the "minor" ²³⁴U/ ²³⁸U and ²³⁶U/ ²³⁸U ratio measurements using MTE, better precision and accuracy are achieved compared to the TE analyses. The certified ²³⁴U/ ²³⁸U ratio of 0.000052841 (82) in CRM C112-A and CRM 145 is determined using a conventional analysis technique that incorporates an internal mass bias correction utilizing the measured ²³⁵U/ ²³⁸U ratio and correction for peak tailing from ²³⁵U and ²³⁸U. The ²³⁶U/ ²³⁸U isotope abundance ratio in CRM C112-A and CRM 145 is estimated to be <5 × 10 ^-9. The CRM 112-A and CRM 145 materials show no evidence for any statistically significant unit-to-unit variations in uranium isotope abundance ratios. The homogeneity of both CRMs is established. The measurements leading to the certification of uranium isotopic abundances are discussed.