Presentation Accurate measurement 234U/238U activity ratio and 236U/238U isotope ratio in 10-8 range using thermal ionization mass spectrometry

S.K.Sahoo  ,  H.Arae  ,  T.Aono

Variations in the isotope ratios 234U/238U, 235U/238U, and U-236 abundance of natural uranium samples (e.g., uranium ores), are known to exist due to various physical, chemical, mass fractionation, redox transitions, radioactive decay, radioactive disequilibrium, alpha-recoil, and neutron capture. Precise measurement of uranium isotope ratios with sufficient accuracy is a challenge to resolve the range of natural variation in a representative set of samples. 234U belongs to the 238U natural radioactive decay series, and at equilibrium, the abundance ratio, 234U/238U, corresponds to their half-lives, i.e. 54.8 x 10-6. Uranium was chemically separated from uranium ore as well as some soil samples affected by Fukushima accident and its isotopic composition was measured using a thermal ionization mass spectrometer (TIMS). Chemical separation of U was carried out by two-column separation procedure using UTEVA resin and UTEVA resin. Since Japanese soil samples contain a large amounts of Fe and it makes U isotope ratio measurement difficult. Therefore, we separated U with UTEVA resin twice.The TIMS (Phoenix, IsotopX, UK) used has nine Faraday cups collectors and a Daly ion-counting system detector positioned behind axial Faraday and WARP (wide aperture retardation potential) filter. The WARP filter is designed to suppress the tailing effect. Abundance sensitivity of 236U using NBS 030a was 5.86ppm. The limit of detection for 236U/238U measurements using Daly ion counting system with WARP is about 2x10-9 to 4x10-9. This was evaluated using certified natural uranium reference materials from University of Vienna in house standard in the order of 1.01x10-8. This method for uranium isotopic composition has been shown to significantly improve the precision and accuracy in analysis of environmental samples. Results will be discussed in detail during presentation.
7th International Conference on Radionuclide Metrology Low-Level Radioactivity Measurement Techniques

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