Backgrounds and blanks in Iodine-129 measurements at the Australian National University

The molecular hydride negative ions of mass 129 amu, ¹²⁷IH₂⁻, ¹²⁷ID⁻ and ¹²⁸TeH⁻, have been studied in order better to understand potential backgrounds in AMS measurements of ¹²⁹I. All three of these ions appear to be stable or, if metastable, to have lifetimes that are sufficiently long to allow at least some of them to reach the high voltage terminal of the accelerator. It is found, however, that these make only a minor contribution to the combined rate of ¹²⁷I⁷⁺ and ¹²⁸Te⁷⁺ ions that pass around the analysing magnet when it is set for ¹²⁹I⁷⁺ detection. The principal contribution comes instead from the injection of ¹²⁷I⁻ ions into the accelerator along with the ¹²⁹I⁻ ions, despite the use of a low Cs sputtering energy in the ion source that was intended to eliminate them. This contribution is, however, small and is effectively removed by a Wien filter. In addition, a sample of Woodward iodine, obtained from the University of Arizona, was found to have an ¹²⁹I/¹²⁷I ratio of (13.3 ± 0.9) × 10⁻¹⁵. This is the lowest ratio so far reported. In contrast, the batch of Woodward iodine that was being used as an iodine carrier in our laboratory was found to have a much higher ratio of (78.2 ± 2.4) × 10⁻¹⁵. A search for a lower ratio blank failed to find a suitable material.