Verification strategies and results for the new synthetic uranium isotopic mixtures at IRMM

Several new sets of synthetic isotope reference materials for instrument calibration have been developed at IRMM based on proven methods of purifying and mixing highly enriched oxides. In the first stage a set of 10 mixtures of 233U, 235U and 238U was made in which the 235U:238U ratios were kept at 1:1 and the 233U/235U ratios varied from 1.0 to 10-6 (IRMM- 074). This set is ideal for checking the linearity response of detectors used in isotope mass spectrometry. For an absolute calibration of 235U/238U ratio measurements two types of synthetic mixtures were prepared. First the IRMM-3020-3090 set of primary isotopic mixes with 20%, 35%, 50%, 75% and 90% 235U enrichment was made. Secondly also a much awaited double spike IRMM-3636 with a 233U/236U ratio of 1:1 was prepared which allows internal mass fractionation correction for 235U/238U ratio measurements. Furthermore, for accurate measurements of the 236U abundance in nuclear safeguards samples a set of calibration mixtures called IRMM-075 with 236U abundances from 10-5 to 10-9 was prepared and certified. For all synthetic isotope mixtures certification is done based on weights, purity and isotopic enrichment of the starting materials. However, a careful verification e.g. by mass spectrometry is necessary. This paper describes the strategy and results for the verification measurement program performed at IRMM based on high precision thermal ionization mass spectrometry.