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Increased 14C AMS Efficiency from Reduced Competitive Ionization

Published online by Cambridge University Press:  29 June 2016

John S Vogel  and
Jason A Giacomo
John S Vogel*
Affiliation:
University of California (retired); 8300 Feliz Creek Dr., Ukiah, CA 95482, USA.
Jason A Giacomo
Affiliation:
Eckert Ziegler AG/Vitalea Science; 2121 Second St., Davis, CA 95618, USA.
*
*Corresponding author. Email: [email protected].
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Abstract

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.

Keywords

AMScarboncesiumanionionization
Type
Advances in Physical Measurement Techniques
Information
Radiocarbon , Volume 59 , Special Issue 3: Proceedings of the 22nd International Radiocarbon Conference (Part 2 of 2) , June 2017 , pp. 957 - 965
Copyright
© 2016 by the Arizona Board of Regents on behalf of the University of Arizona 

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