The Isotopic Composition of Natural Uranium Samples - Measurements Using the New n(233U)/n(236U) Double Spike IRMM-3636

Abstract

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 certified with an extended relative uncertainty of 0.016%. 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 the sample can be determined with the highest possible precision and accuracy. 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 recommended IUPAC value. The results clearly show that the historically used consensus value of 1/137.88 cannot be considered representative for natural uranium samples in general.