The LLNL-CAMS modified high-intensity Cs-sputter source has several key characteristics, including very high ion current output, that make it useful for ¹⁴C AMS applications. Within the AMS community there have been suggestions that operation a sputter ion source at very high output levels could result in low ionization efficiency and the likelihood of consuming (burning through) small samples without acquiring adequate statistics. We have recently re-determined the ionization efficiency of the CAMS sputter sources using graphite targets and demonstrated relatively high ionization efficiency at high output levels.

Under routine ion source operating conditions (~11.5kV cathode potential, 40kV extraction potential, ~130W Cs boiler power) and 250-280µA ¹²C^– equivalent output, we continuously acquired 1.03*10^{7 14}C counts in our detector without completely consuming an 83µmole (~1mg carbon) Oxalic-I graphite target.  Since an 83µmole OX-1 target contains ~6.21*10^{7 14}C atoms, this corresponds to an overall efficiency of ~16% for the ionization and transport of ¹⁴C atoms from sample to detector. Accounting for the ~50% stripping efficiency (C^–1 to C⁺⁴; 4µg/cm² carbon foil) and a small (~10%) loss on the grid lens at the accelerator entrance, the above overall efficiency implies an ionization efficiency of 35-40%. Because the operation of ion sources at such high output levels results in higher emittance ion beams, acceptance issues in the low-energy transport system and/or at the stripper canal in some AMS systems may not allow exploitation of high output/high efficiency ion sources for precise and accurate ¹⁴C AMS measurements.

This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.