Development and applications of the NSF-Arizona AMS iodine-129 program

Abstract

The use of the long-lived radioisotope ¹²⁹I as an environmental tracer is increasing in popularity. An application of nuclear physics that is known as Accelerator Mass Spectrometry (AMS) can be used to measure ¹²⁹I in various sample matrices at a level of a few parts in 10¹⁴. The system employs a 3 MV electrostatic accelerator as one component of a mass spectrometer. As accelerated ions have kinetic energies of several million electron volts, nuclear physics techniques can be used to remove interferences from the ions of interest. Techniques for the extraction of iodine from a variety of samples including calcium carbonate, seawater, plants and animals have been developed. Results from many of these samples have been submitted as part of a laboratory inter-comparison exercise. As an application of oceanographic tracer studies, ¹²⁹I has been measured in a banded coral that lived in the southern Pacific Ocean for more than 50 years. The results of this study confirm that iodine is fixed within the coral skeleton upon deposition, and that these measurements record the history of iodine levels in the surface waters of the ocean over time. ¹²⁹I may also be a good indicator of past changes in the galactic cosmic ray flux and the magnitude and orientation of Earth's magnetic field. A 40 kyr iodine record extracted from a stalagmite in the Bahama Islands shows varying ¹²⁹I concentrations that may be linked to such changes. This record is in agreement with other published records of ¹⁰Be and ³⁶Cl. As ¹²⁹I has a much longer half-life than other cosmogenically produced isotopes, there is a potential for extending these records much further back in time.