Optimizing production of Pb beams for 205,210Pb analysis by Accelerator Mass Spectrometry

The measurement of rare radioactive lead isotopes (205Pb or 210Pb) by AMS requires the production of strong Pb negative molecular anion beams from the ion source. This paper summarizes the results of tests of different target composition on the strength and stability of 208PbF3− currents and 210Pb counts. In an 834 SIMS-type Cs+ sputter source, the superhalogen, PbF3− had the largest current or ionization efficiency from a survey of Pb molecular anions. The target matrix that produced the largest current of PbF3− was composed of PbF2, AgF2 and CsF. The ratio of AgF2 and CsF does not affect the ionization efficiency of PbF3−. Chemically refluxed targets of PbF2, AgF2 and CsF increased the ionization efficiency of PbF3−. The count rate of the rare isotope, 210Pb, was increased with the addition of microgram quantities of stable PbF2 to the targets. In an SO-110 type Cs+ sputter source the ionization efficiency of PbF3− was increased with lower rather than higher Cs+ fluence.