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 129I. 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 239Pu and 240Pu are equivalent to <106 atoms and measurements of known materials indicate that our 239Pu and 240Pu measurements are accurate and precise for samples containing from ∼1012 atoms down to the Bq level (∼106 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 236U. Our current 236U background level is equivalent to ∼106 atoms and the linear measurement range is 5-6 orders of magnitude. We have also utilized our Heavy Isotope AMS system for the measurement of 129I. Initial measurements of available low level samples show that background contributions for 1 mg I samples are below 129I/127I levels of ∼10-14, and measurements of prepared standard samples demonstrates linear measurement response to 129I/127I levels greater than 10-10. 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