Improved techniques for high accuracy isotope ratio measurements of nuclear materials using thermal ionization mass spectrometry

A set of new techniques with significantly improved accuracy and precision has been developed and qualified for isotope ratio measurements of nuclear material using thermal ionization mass spectrometry (TIMS). First, we have modified the classical total evaporation technique to provide both high precision and high accuracy for a broader range of isotope ratios. This NBL-modified total evaporation technique acquires isotope ratio data until the entire sample is consumed, similar to the classical total evaporation technique, but also corrects the usually low abundances of isotopes such as U-234 and U-236 for peak tailing contributions originating from the isotope intensities of U-235 and U-238. Using the NBL-modified total evaporation technique, we have analyzed a series of NIST/NBL certified reference materials (CRMs) and report new results. For the U-235/U-238 isotope ratio, the relative standard deviation (RSD) between sample turrets is below 0.01%, based on the analysis of 5-10 samples per sample turret. As a second new development, the NBL-modified total evaporation technique is combined with a multi-dynamic measurement routine that further improves the accuracy of the U-234/U-238 and U-236/U-238 isotope ratio measurements. Multi-dynamic isotope ratio measurements combine the simultaneous detection of multiple isotopes with dynamic cycling of mass in two or more steps. This technique is applied on a new mass spectrometer equipped with a dispersion quadrupole, which for the first time allows the necessary fast switching of the ion beam dispersion during multi-dynamic measurements. The advantage of multi-dynamic compared to static measurements is that the dependence of the isotope ratio results on both the cup efficiencies and the current amplifier gains is eliminated. The result is a higher degree of accuracy for nuclear materials analyses. The impact of the NBL-modified total evaporation and the multi-dynamic measurement techniques on several recent certification projects at NBL is demonstrated for uranium. These techniques have significantly improved reproducibility and accuracy for nuclear safeguards measurements and certification of nuclear materials. They have the potential to improve precision and accuracy for other mass spectrometric applications as well. (C) 2003 Elsevier B.V. All rights reserved.