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 RuFn- 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 Tc-99 to be detected by low energy AMS using (TcF5-)-Tc-99 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 Tc-99 per mg FeOxHy precipitate, using targets made of (Tc-99)FeOxHy + PbF2 (similar to 1:10 by weight). In several preliminary linearity tests with the detection of + 4 ions in the gas ionization detector, the determination of Tc-99 concentration within a FeOxHy precipitate was best shown with similar to 15% uncertainty. The quantification was made by the average count rate of Tc-99, subtracting that of Ru-99 (determined by multiplying the counts of the isobar-free Ru-101 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( 99)Tc in seawater samples using <= 2 L volumes.