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

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 10(12) Omega current amplifying system. This system improves the electronic sensitivity compared to the same Faraday cups coupled to 10(12) Omega 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 10(12) Omega current amplifier. The U-234/U-238 and U-235/U-238 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 U-234/U-238 and U-235/U-238 isotope ratios were studied by total evaporation using different configurations. First, U-234 or U-235 was measured with a Faraday cup coupled to a 10(12) Omega current amplifier and U-238 was measured with a Faraday cup coupled to a 10(12) Omega current amplifier. Then, 234U or U-235 was measured by discrete dynodes electron multiplier and U-238 was measured with a Faraday cup coupled to a 10(12) Omega current amplifier. In comparison to the configurations using the 10(12) Omega current amplifier, the analyzable quantity was reduced from 250 ng to 100 ng for the U-235/U-238 isotope ratio and from 50 ng to 3 ng for the U-234/U-238 isotope ratio with extended uncertainty below 0.28% for the U-235/U-238 isotope ratio, in compliance with the International Target Values (ITV 2010), and below 5% for the U-234/U-238 isotope ratio. (C) 2016 Elsevier B.V. All rights reserved.