A set of 3 new synthetic uranium isotope mixtures with 236U/238U isotope ratios in the order of 10-6, 10-7 and 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 = 0.00725) is mixed in 3 different proportions with a non-natural starting material, which is for its part characterized by 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 to detect 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 to 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 ten times lower uncertainty compared to previous values.

RICHTER Stephan;  ALONSO MUNOZ Adolfo;  DE BOLLE Willy;  KUEHN Heinz;  VERBRUGGEN Andre;  WELLUM Roger;  TAYLOR Philip; 

2006-02-13

ROYAL SOC CHEMISTRY

JRC31029

https://publications.jrc.ec.europa.eu/repository/handle/JRC31029,