Synthetic 239Pu/242Pu isotope mixtures as basis for absolute Pu isotopic measurements at CBNM

In view of the characterisation of reference materials for isotopic measurements of Plutonium, a series of 20 synthetic isotope mixtures have been made. Highly enriched 239Pu and 242Pu materials were mixed gravimetrically as Pu oxides in ratios ranging from 10-1 to 10+1. Taking into account the different uncertainty sources: chemical impurities in the starting materials, isotopic composition of the starting materials, hygroscopicity, stoichiometry and weighing, the total uncertainty for any of the mixtures is ≤0.02%. This means that the correction factor for isotope fractionation of two neighbouring masses (e.g. 239Pu/240Pu) can be determined to ±0.01% for all practical purposes. These mixtures are stored at CBNM in units of 1mg and will form the basis of absolute Pu isotopic measurements at CBNM for the next 20 years