The isotopic composition of natural uranium samples—Measurements using the new n(233U)/n(236U) double spike IRMM-3636

The new double spike isotope reference material IRMM-3636 was prepared gravimetrically using highly enriched 233U and 236U isotopic material. The n(233U)/n(236U) ratio was adjusted to about 1:1 and certified with an extended relative uncertainty of 0.016% (coverage factor k = 2). By mixing the n(233U)/n(236U) double spike with a sample and applying internal mass fractionation correction, the n(235U)/n(238U) ratio of a given sample can be determined with the highest possible precision and accuracy. Due to the low relative abundance of 234U for the IRMM-3636 double spike (<0.00037%) measurements for n(234U)/n(236U) of samples close to natural isotopic composition (n(234U)/n(236U) ≅ 5.4 × 10−5) are also possible with a rather small correction and without significant increase in the uncertainty.The IRMM-3636 double spike was applied for measurements of natural uranium ore samples. The results for n(235U)/n(238U) ratios were compared to earlier data obtained using UF6 gas sources mass spectrometry and to the most recent IUPAC recommended value, published in 2003. The results show that the historically used “consensus value” of 1/137.88 (earlier IUPAC value) cannot be considered representative for natural uranium samples.