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

Abstract

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.