First Successful Measurement of ¹²⁹I Levels in St. Joseph’s Lake Using Accelerator Mass Spectrometry at the University of Notre Dame

The isotope ¹²⁹I, with a half-life of 15.7 million years, serves critical roles ranging from an environmental tracer of fission products to a significant marker in nuclear astrophysics. Due to its low natural terrestrial abundance (approximately one part in 10¹³), accelerator mass spectrometry (AMS) offers a robust method for its detection, accurately distinguishing the ¹²⁹I signal from the stable isotope ¹²⁷I in aqueous samples. Recent advancements in our beamline configuration, time-of-flight detection systems, and aqueous sample processing at the University of Notre Dame’s Nuclear Science Laboratory (NSL) have significantly increased our measurement sensitivity for ¹²⁹I. This study presents the NSL’s first statistically significant measurement of the ¹²⁹I abundance in freshwater. The enhancements to the AMS system, improved time-of-flight resolution, modifications to sample processing, and implications for environmental and astrophysical applications will be discussed. In particular, we will also discuss our collaboration with the nuclear theory group at Notre Dame, especially regarding planned experiments to build upon advancements made by Wang et al. (2021, 2023) on the understanding of certain astrophysical processes (Wang, X., Clark, A. M., Ellis, J., et al. 2021, r-Process Radioisotopes from Near-Earth Supernovae and Kilonovae), (Wang, X., Clark, A. M., Ellis, J., et al. 2023, Proposed Lunar Measurements of r-process Radioisotopes to Distinguish the Origin of Deep-sea 244Pu).