Optimization of the LLNL/CAMS gas-accepting ion source and 1 MV compact AMS for natural abundance radiocarbon analysis.
Taylor Broek1, Ted Ognibene1, Karis McFarlane1, Kimber Moreland1, Thomas Brown1
1Lawrence Livermore National Laboratory-Center for Accelerator Mass Spectrometry, Livermore, United States
The Lawrence Livermore National Laboratory-Center for Accelerator Mass Spectrometry (LLNL/CAMS) 1 MV AMS system was recently repurposed from a biomedical AMS instrument to a natural abundance 14C spectrometer. The system is equipped with a gas-accepting hybrid ion source capable of measuring both solid (graphite) and gaseous (CO2) samples, however, routine radiocarbon (14C) measurements are currently only conducted with graphite. Here we describe ongoing experiments intended to establish and optimize 14CO2 measurement capabilities at natural abundance levels. This optimization encompasses two simultaneous efforts to: 1) maximize the production of C- ions from CO2 gas and 2) limit the influence of scattered ions in the AMS beamline, with the ultimate objective of producing radiocarbon data comparable in both accuracy and precision to those generated from solid samples. We optimized ionization efficiency by adjusting CO2 concentration, flow rates, and target design and reduced angular scattering through modifications to the beamline and changes to operational parameters. As configured, we currently achieve >8% conversion of CO2 to C-, resulting in approximately 250 ± 50 14C counts/ µgC from modern (OX I) gas samples. We validate our current capabilities using background materials and standards of known 14C abundance. Finally, we compare results from solid and gaseous samples derived from splits of CO2 generated from a soil incubation experiment.
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