Because of the large neutron capture cross sections for 149Sm, 155Gd, and 157Gd, accurate and sensitive isotopic analyses of samarium and gadolinium can be used to investigate natural neutron irradiation exposure. In this paper, chemical separation procedures and mass spectrometric methods for the precise determination of samarium and gadolinium isotopic compositions are described. Isotopic ratios other than 144Sm/152Sm, 152Gd/160Gd, and 154Gd/160Gd for both samarium and gadolinium in commercial reagents, artificially neutron-irradiated reagents, and geological reference rocks are quantified within 0.001% precision at the 95% confidence level. The present method can detect effects caused by timeintegrated neutron flux of 1.3 × 1014 neutrons/cm2 in the environmental samples.
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