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Increased 14C AMS Efficiency from Reduced Competitive Ionization

  • John S Vogel (a1) and Jason A Giacomo (a2)

Observation of 80 µA/mm2 C current from a 0.5-mm-diameter sample compared to 20 µA/mm2 from a 1-mm-diameter sample contradicts the long-held surface ionization hypothesis of cesium sputter ion source operation. Resonant ionization occurs in neutral Cs plasma above a sample in a sputtered pit or well. A collision-radiation model of that plasma followed electronic excitation and radiation relaxation up to the Cs(7d) state. The Cs(5d) metastable state dominates plasma in a 1-mm-diameter well, but high electron densities in narrow wells drive a majority to Cs(7d) and higher. Competitive ionization by Al2 dimers from the sample holder reduces Cs(7s,p) states resonant in ionization energy with C. Al anions from states above Cs(7p) in narrow wells also diminish radiation cascades to the Cs(7s,p), reducing C. We tested sample wells of non-ionizing Zn to maintain high ionization efficiency for small samples in narrow wells.

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