Preparation and certification of synthetic uranium isotope mixtures with 236U/238U ratios of 10−6, 10−7, 10−8

Abstract

A set of 3 new synthetic uranium isotope mixtures with ²³⁶U/²³⁸U isotope ratios in the order of 10⁻⁶, 10⁻⁷and 10⁻⁸ has been produced and certified for the isotopic composition. For the preparation of these 3 mixtures a natural uranium starting material (²³⁵U/²³⁸U = 0.007 25) is mixed in 3 different proportions with a non-natural starting material, which is made up of a 1 ∶ 1 mixture of IRMM-187 (²³⁵U/²³⁸U = 0.0475) with a material highly enriched in ²³⁵U. The ²³⁶U/²³⁸U ratios of the 3 mixtures are calculated using the measured ²³⁶U/²³⁸U ratios of the natural (close to detection limit) and the non-natural starting materials. The ²³⁶U isotope intensity of the latter material was measured on a Faraday collector. The minor isotope abundances of the 3 mixtures were calculated from the minor isotope abundances of the starting materials and the measured ²³⁵U/²³⁸U ratios of the mixtures. Direct measurements of the ²³⁶U/²³⁸U ratios for all 3 mixtures were performed by TIMS using a secondary electron multiplier operated in ion counting mode for the ²³⁶U ion beam. The results agree well with the ratios obtained from the mixing calculations. This confirmation is considered a proof for the reliability of our TIMS procedure used to measure ²³⁶U/²³⁸U ratios within a wide dynamic range. Based on the confidence provided by the synthetic uranium isotope mixtures, the minor isotope ratios ²³⁴U/²³⁸U and ²³⁶U/²³⁸U were re-measured and re-certified by TIMS for the reference materials IRMM-183-187. On the new certificates for IRMM-183-187 the minor ratios ²³⁴U/²³⁸U and ²³⁶U/²³⁸U are certified with a circa ten-times lower uncertainty compared to previous values.