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

• Determination of the 234U/238U and 235U/238U isotopic ratio by TIMS with electron multipliers detectors and Faraday cups coupled to 1012 Ω current amplifying system.
• Faraday cup equipped with 1012 Ω current amplifier were used in order to reduce the amounts of samples deposited on the filament.
• Different detector configurations were performed to obtain accurate isotope results.

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 1012 Ω current amplifying system. This system improves the electronic sensitivity compared to the same Faraday cups coupled to 1011 Ω 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 1012 Ω 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 1012 Ω current amplifier and 238U was measured with a Faraday cup coupled to a 1011 Ω current amplifier. Then, 234U or 235U was measured by discrete dynodes electron multiplier and 238U was measured with a Faraday cup coupled to a 1012 Ω current amplifier. In comparison to the configurations using the 1011 Ω current amplifier, the analyzable quantity was reduced from 250 ng to 100 ng for the 235U/238U isotope ratio and from 50 ng to 3 ng 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.

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