Studies of anions from sputtering III: The 41K background in 41CaF3− measurement by AMS

The anion KF3− is found to be produced from a Cs+ sputter source with a relative yield of ∼10−4 of KF2−, the superhalogen [1] anion of K in the target. Therefore, from CaF2 samples, 41KF3− is probably the dominant molecular ion by which 41K background is introduced in AMS measurements of 41Ca using CaF3−. Evidence is found that a source of K is the impurities in the Cs used for sputtering. The next most important molecule generating 41K background is probably 41K57Fe− due to the presence of iron in the parts of the ion source. The use of collision induced dissociation (CID) for separating CaF3− and KF3− has been investigated. The anion 41KF3− has been shown to be reduced with respect to 41CaF3−, by three orders of magnitude using CID at eV energies using the Isobar Separator for Anions (ISA) described in Ref. [2]. This is a degree of isobar discrimination sufficient for making very-low level 41Ca measurements using small accelerators without the need of additional dE/dx measurement.