D. Elmore, H.E. Gove, R. Ferraro, University of Rochester

L.R. Kilius, H.W. Lee, K.H. Chang, R.P. Beukens, A.E. Litherland, Dept. of Physics, Toronto, Canada

C.J. Russo, K.H. Purser, General Ionex Corporation, Newburyport, Massachusetts

M.T. Murrell, University of California, San Diego

R.C. Finkel, University of California, San Diego

Nature 286 (1980) 138-140

Iodine-129 with a half life of 15.7 Myr (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 nenon2 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 quanitiy of 129I introduced into the environment the nuclear age may be useful as a tracer in groundwater hydrology. Fissiogenic 129I in large granite formations (batholiths) could be used to establish the suitability of these sites fo 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 meterorites can be combined with results for shorter-lived radiunuclides 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 ,he sample required to obtain a quantitative result) of less than 10^7 atoms of 129I in 1-mg sample. An Agl standard with a known 129I/129I ratio of 10-11 was determined to +- 10% and the background contribution from a reagent grade Agl sample gave and upper limit to the ratio of about 3 x 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 meterorites. A Xe molecular negative ion was discovered but we show that it is not a problem for 129I analysis.