Determination of 129I using tandem accelerator mass spectrometry

Details Determination of 129I using tandem accelerator mass spectrometry Determination of 129I using tandem accelerator mass spectrometry

Jul 1980

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980natur.286..138e&link_type=abstract

Nature, Volume 286, Issue 5769, pp. 138-140 (1980).

Physics

8

Scientific paper

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.

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