New Isotopic Techniques for Geoscience

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The determination of radionuclide concentrations is fundamental to many areas of science ranging from risk assessments for human health to the geochronologies needed to reconstruct records of climate. The global objectives of this Discovery Grant research program are to develop new accelerator mass spectrometry (AMS) techniques for radionuclide measurement and to apply these methods to study important environmental and geoscience issues.**** One key focus of our work is the development and application of on-line AMS reaction cell techniques to separate isobaric interferences (isotopes with the same mass as the rare radioisotope) that often prevent the accurate measurement of rare radionuclides. For example, Ba-135 is 1010 to 1015 times more abundant than Cs-135 in most environmental samples. The Ba-135 can be removed from the sample by a combination of rigorous trace chemical extraction and by passing the AMS anion beam of Cs and Ba ions through a gas filled reaction cell. This on-line reaction reduces the Ba-135 concentration by about 10,000 fold. Our work will resolve the remaining interference issues and complete the techniques to measure Cs-135 in a wide variety of matrices and concentrations found in environmental samples.**** We will use the knowledge that we gained from the studies of interference removal for Cs-135 to study several new isotope systems that have not previously been measured in environmental samples. We will investigate the interferences between Se-79/Br-79, Zr-73/Nb-73, Pb-107/Ag-107, and Sn-126/Te-126. Each of these rare radioisotopes is present in the environment as a result of nuclear weapon's test fallout and in older samples from the spontaneous fission of U-238. However their measurement using radioactive decay techniques has been impossible due to their weak beta emissions and their long radioactive half-life. The interference from the abundant isobar produced by the radioactive beta decay has, to date, prevented their measurement by AMS. We hypothesize that the isobar separation using an anion reaction cell will eliminate this interference.**** We will apply these new analytical techniques to a number of key scientific issues. This work also will open up possibilities to trace Cs and other long-lived isotopes on a large scale in many systems since the isotope ratio is different in different source material. For example: Oceanic systems have Cs-137 and Cs-135 from weapons' test fallout and from fuel reprocessing facilities and we will test the hypothesis that these sources can be distinguished using our analytical techniques.**** Training HQP in radiochemical, geochemical and isotopic techniques, is a very important objective of this work. The interdisciplinary approaches described in this proposal offer an excellent range of training opportunities for HQP in geoscience, radiochemistry, ecotoxicology and experimental physics.***