Simultaneous U/Pu separation and direct isotope ratio measurements by using CO2 as gas in a collision/reaction cell based MC-ICPMS

Abstract : The measurement of U and Pu isotope ratios is of prime interest in nuclear research fields particularly for the management of radioactive waste. One of the major drawbacks in analysing Pu isotopes is the occurrence of 238U–238Pu isobaric interference. In this work we propose to suppress this interference by adding CO2 as a reactive gas in the collision-reaction cell of the MC-ICPMS. We have demonstrated that the different reactivities of Pu andUtowards CO2 gas allow accurate and precise Pu andUisotopic measurements on Pu+ and UO2 + forms, by using two different CO2 gas flow rates: 0.4 mL min1 for Pu+ and 1.5 mL min1 for UO2 +. Parameters like optimum CO2 gas flow in the cell, oxides isotope abundance, instrumental mass bias and memory blank, were carefully evaluated and corrected. The developed analytical method was validated using a transmutation 235U target sample irradiated in the Phenix French fast neutron reactor. A good agreement between isotope ratios of U and Pu measured with CO2 in the cell and U and Pu measured after chemical separation was obtained. The relative difference of the average isotope ratio values between the chemically purified fraction and the direct measurements using CO2 was found to be 0.38% and 0.11% for U and Pu, respectively.