Progress in Precision and Accuracy for Isotope Ratio Measurements of Nuclear Samples using Thermal Ionization Mass Spectrometry

In July 1999 a new state of the art thermal ionization mass spectrometer (TIMS), the Finnigan MAT Triton, was installed at the New Brunswick Laboratory (NBL). The new instrument was purchased to improve the analytical capabilities for nuclear safeguard measurements and for the certification of nuclear reference materials. Significant progress has been made, particularly in the area of minor 234U/238U and 236U/238U isotope ratio measurements. As a new development, a multi-dynamic isotope ratio measurement technique has been applied to uranium. This method circumvents the need for frequent electronic amplifier gain calibration and cup efficiency testing of the Faraday multi-collector detection system of the mass spectrometer. It provides a higher degree of precision and accuracy of isotope ratio results by a factor of about 3 to 10 compared to previous instrumentation, which is especially important for certification measurements in the production of reference materials. Extensive effort was spent to validate and qualify this procedure for varying enrichments of uranium, requiring measurement procedures to accommodate a large range of isotope intensities. The qualification included a series of automated test sequences using the NBL U series certified reference materials. A comparison of results on the Triton with older Finnigan MAT261 instruments at NBL shows an improvement in the external reproducibility for the major 235U/238U ratio by a factor of about 3 and for the minor 234U/238U and 236U/238U ratios by factors of between 3 and 10. Further improvement is limited by the uncertainty of the major 235U/238U ratio in the certified reference materials, which are used for mass fractionation correction. The multi-dynamic measurement technique is suggested to become a standard method for nuclear safeguard laboratories. Furthermore, a study was performed to examine the relative intensities and isotope ratios of different uranium species, including U, UO, UO2 and UO3 that are evaporated from a sample filament in a TIMS measurement. Using the new Triton TIMS, which has a dispersion capability of 15%, it was possible to measure simultaneously the isotope ratios of U, UO and UO2 species for the first time. The results of this study indicate significant differences in the isotope ratios of different species during the course of these measurements. This may explain some accuracy problems commonly observed in total evaporation measurements.