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 UF 6 gas source mass spectrometer (GSMS) and secondly the preparation of primary UF 6 reference materials, which were converted from gravimetrically prepared mixtures of highly enriched oxides of 235 U and 238 U. Recently a new GSMS for uranium isotopic measurements using UF 6 gas, the "URANUS" from Thermo Fisher, was installed at IRMM, which also allows measurements of the so-called "minor" isotope ratios n( 234 U)/n( 238 U) and n( 236 U)/n( 238 U). 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( 235 U)/n( 238 U) as well as the "minor" isotope ratios n( 234 U)/ n( 238 U) and n( 236 U)/n( 238 U) 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. UF 6 GSMS in combination with the MCDS method is considered the preferred technique for samples in the UF 6 form and for smaller uncertainties for measurements of the major ratio n( 235 U)/n( 238 U), 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 236 U, provides a superior measurement performance for the minor isotope ratios n( 234 U)/n( 238 U) and n( 236 U)/n( 238 U).

Gas Source Isotope Ratio Mass Spectrometry (IRMS)

“…As a consequence, ion scattering and crosstalk effects between the three cups had to be investigated carefully. 131 …”

Certification of uranium hexafluoride reference materials for isotopic composition

“…It is noted that the IRMM-019 to IRMM-029 series were at the time all characterized and certified using the ''simple'' double standard method without memory correction, which was found to provide significantly lower results compared to the memory corrected data (MCDS method, for details see Ref. [16]). This might explain why some of the new values are significantly higher than the old ones.…”

“…The uncertainties on the deviations varied a lot depending on the material and the isotope amount ratios. This is depending on the ion beam sizes encountered during the measurements, the calibrants, the instrument sensitivity but also on the selected analysis method (MCDS or single standard method [16]), which is chosen depending on the isotope abundance difference between the calibrants and the sample.…”

“…Several correction methods can be applied to evaluate the result: the 'memory corrected double standard' (MCDS) method or the single standard memory correction(s) using one or the other of the two reference materials as calibrant and a combination of both for memory correction. All those methods are thoroughly described in [16]. For each material to be certified, the most appropriate method was selected following the isotopic distance between the available standards and the material.…”

“…The agreement with the new certified values was confirmed individually for each ampoule containing UF 6 . The GSMS measurements were performed using the memory corrected double standard (MCDS) method or single standard method following the isotopic distance [16], using reference materials from the EC-171/031-445 series as much as possible. Figure 5 shows the relative deviations of the confirmation measurement for each material from the certified isotope amount ratios (Fig.…”