AMS of I-129: cross contamination and its correction

Low-energy AMS is well suited for measurements of the long-lived nuclide ¹²⁹I because the interfering stable isobar ¹²⁹Xe does not form negative ions, thus high ion energies are not required for discrimination in the final detector. Furthermore, low-energy AMS has the advantage that in combination with helium stripping the most probable charge state can be selected; in our case at the TANDY running at 300 kV we select charge state 2+ with a transmission of >50%. With a proper spectrometer at the high-energy side interferences of the stable isotope ¹²⁷I can be completely eliminated. Contrary to many AMS nuclides ¹²⁹I readily forms negative ions and the overall efficiency is high. The challenges lie more in the ion source where cross contamination can be quite severe due to the volatile nature of iodine. This is particularly of importance when analyzing samples that are influenced from anthropogenic sources because the isotopic ratios can span several orders of magnitude. On the other hand special care must be taken when analyzing samples with low isotopic ratios (¹²⁹I/¹²⁷I < 10^-13) or samples with very low iodine content (carrier free samples) due to the very same reason. This talk discusses the advantages and challenges of low-energy AMS of I-129 with the focus on the issues with cross contamination and its correction.