Contribution of the Faraday cup coupled to 1012 Ω current amplifier to uranium 235/238 and 234/238 isotope ratio measurements by thermal ionization mass spectrometry

Abstract

This work highlights the possibility of improving, for given deposited quantities, the accuracy of uranium isotope ratio determination by Thermal Ionization Mass Spectrometry (TIMS) using Faraday cups coupled to a 1012Ω current amplifying system. This system improves the electronic sensitivity compared to the same Faraday cups coupled to 1011Ω current amplifiers. The analytical procedure developed in a previous work (Quemet et al., 2014) was applied in order to study the improvements in accuracy using the Faraday cups equipped with 1012Ω current amplifier. The 234U/238U and 235U/238U isotope ratios were measured on the Certified Reference Material IRMM 052 (natural uranium). Results were evaluated relying on NF T 90–210 norm regarding method validation. The 234U/238U and 235U/238U isotope ratios were studied by total evaporation using different configurations. First, 234U or 235U was measured with a Faraday cup coupled to a 1012Ω current amplifier and 238U was measured with a Faraday cup coupled to a 1011Ω current amplifier. Then, 234U or 235U was measured by discrete dynodes electron multiplier and 238U was measured with a Faraday cup coupled to a 1012Ω current amplifier. In comparison to the configurations using the 1011Ω current amplifier, the analyzable quantity was reduced from 250ng to 100ng for the 235U/238U isotope ratio and from 50ng to 3ng for the 234U/238U isotope ratio with extended uncertainty below 0.28% for the 235U/238U isotope ratio, in compliance with the International Target Values (ITV 2010), and below 5% for the 234U/238U isotope ratio.