Accelerator Mass Spectrometry of 129I towards its lower limits

We present the performance of Accelerator Mass Spectrometry (AMS) of 129I using the low energy facility TANDY of the Laboratory of Ion Beam Physics at ETH Zurich, Switzerland. Running the tandem accelerator at 300 kV in combination with helium as a stripper gas we obtain high transmission of >50% trough the accelerator for 129I ions in charge state 2+, molecules at mass 129 are sufficiently suppressed at appropriate stripper-gas pressures. While the high-energy spectrometer provides excellent suppression of the stable isotope 127I, mass-to-charge state (m/q) interferences are significantly reduced in 2+, allowing for measurements essentially free of background from other masses (isotopes and m/q interferences). The main challenge in the AMS of 129I comes from cross talk between samples in the ion source. With sufficient care low-level samples (129I/127I < 10−13) can be well measured, e.g. Woodward iodine was measured to 129I/127I = (3.4 ± 0.3) × 10−14, demonstrating that low-energy AMS of 129I provides both high overall efficiency and very low background.