Major, minor, and trace uranium isotopes were measured at Lawrence Livermore National Laboratory in environmentally acquired samples using different instruments to span large variations in concentrations. Multi-collector inductively-coupled plasma mass spectrometry (ICPMS) can be used to measure major and minor isotopes: 238U, 235U, 234U and 236U. Accelerator mass spectrometry (AMS) can be used to measure minor and trace isotopes: 234U, 236U, and 233U. Natural uranium contains relatively consistent concentrations of 238U (99.2745%), 235U (0.720%), and 234U (0.0055%), and essentially no 233U. Due to neutron capture from spontaneous fission of 235U in ores and cosmic radiation, there exists a range of natural concentrations of 236U but no widespread survey has been completed to date because levels are too low for routine detection by techniques other than AMS. In general, the existence of 236U indicates exposure of uranium to a neutron source. ICPMS measures 236U/235U to ~ 2e-6 while the current AMS system at LLNL achieves 236U/235U ~ 1e-7, with the expectation of 2-3 orders of magnitude improvement in sensitivity with the addition of another high energy filter. Samples analyzed by AMS for 236U and 234U were normalized relative to a 233U spike of 3e9 atoms in 400 ng of uranium samples. Comparing 236U/234U from ICPMS and AMS produced agreement within 10% for samples at 236U levels high enough to be measurable by both techniques. Concentrations of 236U/235U ranged from below detection limits to 2e-4.
This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.
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See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)