We report on a continuing program of improvements to NEC's MC-SNICS ion source at the KCCAMS laboratory at UC Irvine. Earlier changes aimed primarily at improving source serviceability and reliability are summarized in Southon and Santos (2004). Recent work has focused on increasing the source output and efficiency. We replaced the extractor and preacceleration assemblies for a threefold improvement in pumping conductance, leading to lower source pressures and quicker startup after sample wheel changes. We have also replaced the standard conical ionizer and Cs focus electrode with a new assembly based on a Spectramat spherical ionizer and a negative ion immersion lens at cathode potential. This new geometry routinely delivers up to 150ľA of C- at 7.5kV cathode voltage, and source efficiencies of over 20% have been determined for 1mg carbon samples run to extinction. Efficiencies on 5-10 microgram samples are sufficiently high (>10%) that 4-5% of the radiocarbon atoms in such samples can be measured.
J R Southon and G M Santos, 2004, Ion source development at KKCAMS, University of California Irvine, Radiocarbon 46: 33-40.
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