A preliminary study of 99Tc measurement using matrix-assisted low energy AMS

A renewed study of Tc and Ru fluoride anion formation in a Cs sputter source has confirmed an earlier observation that the relative yields of RuF_n⁻ are dependent on the sputter target matrix composition. The yield of RuF5− can be suppressed relative to TcF₅⁻ with the presence in a PbF₂-based sputter target of certain elements: some strongly as in the case of Nb and some modestly as in the case of Fe. This provides an opportunity for ⁹⁹Tc to be detected by low energy AMS using ⁹⁹TcF₅⁻ with the assistance of a carefully composed matrix to form the sputter target. Depending on the Ru content in a sample and the effort to reduce it during sample preparation, the best detection limit obtained so far was ≤5 in the unit of “fg 99Tc per mg FeOxHy precipitate”, using targets made of (⁹⁹Tc)FeO_xH_y + PbF₂ (∼1:10 by weight). In several preliminary linearity tests with the detection of +4 ions in the gas ionization detector, the determination of ⁹⁹Tc concentration within a FeO_xH_y precipitate was best shown with ∼15% uncertainty. The quantification was made by the average count rate of ⁹⁹Tc, subtracting that of ⁹⁹Ru (determined by multiplying the counts of the isobar-free ¹⁰¹Ru by the ratio of their natural abundances 0.748), over an hour long time under steady-sputtering conditions. This quantification method is required due to the lack of a stable Tc isotope. This method shows promise for analyzing ⁹⁹Tc in seawater samples using ≤2 L volumes.