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Iodine-129 (129I) is a long-lived radioisotope of iodine which occurs naturally, but also is of special interest in the monitoring and effects of man-made nuclear fission decay products, where it serves as both tracer and potential radiological contaminant.

Formation and decay

fission products
Nuclide t12 Yield Decay
[a 1]
(Ma) (%)[a 2] (keV)
99Tc 0.211 6.1385 294 ?
126Sn 0.230 0.1084 4050[a 3] ??
79Se 0.327 0.0447 151 ?
93Zr 1.53 5.4575 91
135Cs 2.3 6.9110[a 4] 269 ?
107Pd 6.5 1.2499 33 ?
129I 15.7 0.8410 194
  1. ^ Decay energy is split among ?, neutrino, and ? if any.
  2. ^ Per 65 thermal-neutron fissions of U-235 and 35 of Pu-239.
  3. ^ Has decay energy 380 keV,
    but decay product Sb-126 has decay energy 3.67 MeV.
  4. ^ Lower in thermal reactor because predecessor absorbs neutrons.

129I is one of seven long-lived fission products. It is primarily formed from the fission of uranium and plutonium in nuclear reactors. Significant amounts were released into the atmosphere as a result of nuclear weapons testing in the 1950s and 1960s.

It is also naturally produced in small quantities, due to the spontaneous fission of natural uranium, by cosmic ray spallation of trace levels of xenon in the atmosphere, and by cosmic ray muons striking tellurium-130.[3][4]

129I decays with a half-life of 15.7 million years, with low-energy beta and gamma emissions, to xenon-129 (129Xe).[5]

Fission product

129I is one of the seven long-lived fission products that are produced in significant amounts. Its yield is 0.706% per fission of 235U.[6] Larger proportions of other iodine isotopes such as 131I are produced, but because these all have short half-lives, iodine in cooled spent nuclear fuel consists of about 56 129I and 16 the only stable iodine isotope, 127I.

Because 129I is long-lived and relatively mobile in the environment, it is of particular importance in long-term management of spent nuclear fuel. In a deep geological repository for unreprocessed used fuel, 129I is likely to be the radionuclide of most potential impact at long times.

Since 129I has a modest neutron absorption cross-section of 30 barns,[7] and is relatively undiluted by other isotopes of the same element, it is being studied for disposal by nuclear transmutation by re-irradiation with neutrons[8] or by high-powered lasers.[9]

Yield, % per fission[6]
Thermal Fast 14 MeV
232Th not fissile 0.431 ± 0.089 1.68 ± 0.33
233U 1.63 ± 0.26 1.73 ± 0.24 3.01 ± 0.43
235U 0.706 ± 0.032 1.03 ± 0.26 1.59 ± 0.18
238U not fissile 0.622 ± 0.034 1.66 ± 0.19
239Pu 1.407 ± 0.086 1.31 ± 0.13 ?
241Pu 1.28 ± 0.36 1.67 ± 0.36 ?


Groundwater age dating

129I is not deliberately produced for any practical purposes. However, its long half-life and its relative mobility in the environment have made it useful for a variety of dating applications. These include identifying very old waters based on the amount of natural 129I or its 129Xe decay product, as well as identifying younger groundwaters by the increased anthropogenic 129I levels since the 1960s.[10][11][12]

Meteorite age dating

In 1960 physicist John H. Reynolds discovered that certain meteorites contained an isotopic anomaly in the form of an overabundance of 129Xe. He inferred that this must be a decay product of long-decayed radioactive 129I. This isotope is produced in quantity in nature only in supernova explosions. As the half-life of 129I is comparatively short in astronomical terms, this demonstrated that only a short time had passed between the supernova and the time the meteorites had solidified and trapped the 129I. These two events (supernova and solidification of gas cloud) were inferred to have happened during the early history of the Solar System, as the 129I isotope was likely generated before the Solar System was formed, but not long before, and seeded the solar gas cloud isotopes with isotopes from a second source. This supernova source may also have caused collapse of the solar gas cloud.[13][14]

See also


  1. ^ Audi, G.; Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S. (2017). "The NUBASE2016 evaluation of nuclear properties" (PDF). Chinese Physics C. 41 (3): 030001. Bibcode:2017ChPhC..41c0001A. doi:10.1088/1674-1137/41/3/030001.
  2. ^ Wang, M.; Audi, G.; Kondev, F. G.; Huang, W. J.; Naimi, S.; Xu, X. (2017). "The AME2016 atomic mass evaluation (II). Tables, graphs, and references" (PDF). Chinese Physics C. 41 (3): 030003-1-030003-442. doi:10.1088/1674-1137/41/3/030003.
  3. ^ Edwards, R. R. (1962). "Iodine-129: Its Occurrenice in Nature and Its Utility as a Tracer". Science. 137 (3533): 851-853. Bibcode:1962Sci...137..851E. doi:10.1126/science.137.3533.851. PMID 13889314. S2CID 38276819.
  4. ^ "Radioactives Missing From The Earth".
  5. ^, NNDC Chart of Nuclides, I-129 Decay Radiation, accessed 7 May 2021.
  6. ^ a b Cumulative Fission Yields, IAEA
  7. ^, NNDC Chart of Nuclides, I-129 Thermal neutron capture cross-section, accessed 16-Dec-2012.
  8. ^ Rawlins, J. A.; et al. (1992). "Partitioning and transmutation of long-lived fission products". Proceedings International High-Level Radioactive Waste Management Conference. Las Vegas, USA. OSTI 5788189.
  9. ^ Magill, J.; Schwoerer, H.; Ewald, F.; Galy, J.; Schenkel, R.; Sauerbrey, R. (2003). "Laser transmutation of iodine-129". Applied Physics B. 77 (4): 387-390. Bibcode:2003ApPhB..77..387M. doi:10.1007/s00340-003-1306-4. S2CID 121743855.
  10. ^ Watson, J. Throck; Roe, David K.; Selenkow, Herbert A. (1 January 1965). "Iodine-129 as a "Nonradioactive" Tracer". Radiation Research. 26 (1): 159-163. Bibcode:1965RadR...26..159W. doi:10.2307/3571805. JSTOR 3571805. PMID 4157487.
  11. ^ Santschi, P.; et al. (1998). "129Iodine: A new tracer for surface water/groundwater interaction" (PDF). Lawrence Livermore National Laboratory. OSTI 7280.
  12. ^ Snyder, G.; Fabryka-Martin, J. (2007). "I-129 and Cl-36 in dilute hydrocarbon waters: Marine-cosmogenic,in situ, and anthropogenic sources". Applied Geochemistry. 22 (3): 692-714. Bibcode:2007ApGC...22..692S. doi:10.1016/j.apgeochem.2006.12.011.
  13. ^ Clayton, Donald D. (1983). Principles of Stellar Evolution and Nucleosynthesis (2nd ed.). University of Chicago Press. pp. 75. ISBN 978-0226109534.
  14. ^ Bolt, B. A.; Packard, R. E.; Price, P. B. (2007). "John H. Reynolds, Physics: Berkeley". The University of California, Berkeley. Retrieved .

Further reading

External links

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