Determination of ¹²⁹I using tandem accelerator mass spectrometry

Abstract

Iodine-129, with a half life of 15.7Myr (ref. 1), is one of the longest lived of the cosmogenic radionuclides. Although the primordial supply of ¹²⁹I is now extinct, ¹²⁹I is continuously being produced in the atmosphere primarily by cosmic ray reactions on xenon², in the Earth primarily by spontaneous fission of ²³⁸U (ref. 3), and in meteorites and the Moon primarily by proton and neutron induced reactions on Te and Ba (ref. 4). The relatively large quantity of ¹²⁹I introduced into the environment from the nuclear age may be useful as a tracer in groundwater hydrology⁵. Fissiogenic ¹²⁹I in large granite formations (batholiths) could be used to establish the suitability of these sites for long term nuclear waste storage, because groundwater movement would carry away the more soluble iodides and disturb the equilibrium ¹²⁹I/²³⁸U ratio. The concentration of ¹²⁹I in meteorites can be combined with results for shorter-lived radionuclides to provide information on the constancy of the galactic cosmic ray flux over longer time scales than previously possible and also on the preterrestrial history of meteorites. We have applied the new atom counting technique to ¹²⁹I analysis with a sensitivity (the minimum number of atoms in the sample required to obtain a quantitative result) of less than 10⁷ atoms of ¹²⁹I in a 1-mg sample. An AgI standard with a known ¹²⁹I/¹²⁷I ratio of 10⁻¹¹ was determined to ±10%, and the background contribution from a reagent grade AgI sample gave an upper limit to the ratio of about 3×10⁻¹³. We used a time-of-flight measurement to distinguish ¹²⁹I from the stable ¹²⁷I ions that were not separated by the mass analysis system. Our results for the meteorites Bruderheim and Dhajala represent the first direct (that is, not inferred⁶ from radiogenic Xe) determination of ¹²⁹I in meteorites. A Xe molecular negative ion was discovered but we show that it is not a problem for ¹²⁹I analysis.