DEVELOPMENTS IN THE MEASUREMENT OF ACTINIDES AND129I AT LLNL BY ACCELERATOR MASS SPECTROMETRY

The application of ultra-sensitive heavy isotope measurements continues to expand in a variety of fields relevant to the management of nuclear materials, including nuclear isotope forensics and radiobioassay. We have developed a heavy isotope accelerator mass spectrometry (AMS) system at Lawrence Livermore National Laboratory's (LLNL) Center for Accelerator Mass Spectrometry (CAMS). The system was designed particularly for the measurement of actinide concentrations and isotopic ratios, but also allows the measurement of other heavy isotopes such as {sup 129}I. The system includes a fast isotope switching capability that allows flexibility in isotope selection and for the quasi-continuous normalization to a reference isotope spike. Current background levels for {sup 239}Pu and {sup 240}Pu are equivalent to <10{sup 6} atoms and measurements of known materials indicate that our {sup 239}Pu and {sup 240}Pu measurements are accurate and precise for samples containing from {approx}10{sup 12} atoms down to the Bq level ({approx}10{sup 6} atoms). Recent exploitation of the fast isotope switching capability has allowed the quasi-simultaneous measurement of several Pu isotopes in individual samples. Our AMS measurement capability has been extended to U isotopes, with particular emphasis on {sup 236}U. Our current {sup 236}U background level is equivalent to {approx}10{sup 6} atoms and themore » linear measurement range is 5-6 orders of magnitude. We have also utilized our Heavy Isotope AMS system for the measurement of {sup 129}I. Initial measurements of available low level samples show that background contributions for 1 mg I samples are below {sup 129}I/{sup 127}I levels of {approx}10{sup -14}, and measurements of prepared standard samples demonstrates linear measurement response to {sup 129}I/{sup 127}I levels greater than 10{sup -10}. The AMS technique provides high rejection of interferences, including molecular interferences, and low susceptibility to matrix components, both of which are of particular relevance to the measurement of complex sample matrices. The attendant significant reductions of demands on sample preparation chemistry allow relatively simple, cost-effective procedures to be employed. When such sample preparation improvements are combined with the high sample throughput capabilities of our AMS system, the result is a rapid and cost-effective measurement technique for heavy isotopes in a wide range of studies.« less