Uranium hexafluoride (UF6) gas source mass spectrometry for certification of reference materials and nuclear safeguard measurements at IRMM

Abstract

Uranium hexafluoride gas source mass spectrometry at IRMM is based on two foundations, firstly the operation of a UF₆ gas source mass spectrometer (GSMS) and secondly the preparation of primary UF₆ reference materials, which were converted from gravimetrically prepared mixtures of highly enriched oxides of ²³⁵U and ²³⁸U. Recently a new GSMS for uranium isotopic measurements using UF₆ gas, the “URANUS” from Thermo Fisher, was installed at IRMM, which also allows measurements of the so-called “minor” isotope ratios n(²³⁴U)/n(²³⁸U) and n(²³⁶U)/n(²³⁸U). In this paper the design and the implementation of measurement techniques for the new URANUS GSMS are described. This includes the “single standard” and the “double standard” (DS) method as well as the newly developed “memory corrected double standard” method (MCDS). This required a detailed investigation of memory effects within the GSMS instrument, in particular regarding the dependence of memory effects on the isotope ratios of samples and standards. The results of this study led to new recommendations for the selection of the standards for a given sample and for suitable measurement procedures. The measurement performance for the “major” isotope ratio n(²³⁵U)/n(²³⁸U) as well as the “minor” isotope ratios n(²³⁴U)/n(²³⁸U) and n(²³⁶U)/n(²³⁸U) is presented and compared with other mass spectrometric techniques. With the installation and validation of the new URANUS GSMS instrument IRMM has established two new complementary techniques for measuring the full isotopic composition of uranium samples. UF₆ GSMS in combination with the MCDS method is considered the preferred technique for samples in the UF₆ form and for smaller uncertainties for measurements of the major ratio n(²³⁵U)/n(²³⁸U), while thermal ionization mass spectrometry (TIMS), in combination with the “modified total evaporation” (MTE) method as well as ion counting and high abundance sensitivity for the detection of ²³⁶U, provides a superior measurement performance for the minor isotope ratios n(²³⁴U)/n(²³⁸U) and n(²³⁶U)/n(²³⁸U).