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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.

Actinides and fission products by half-life
Actinides[1] by decay chain Half-life
range (y)
Fission products of 235U by yield[2]
4n 4n+1 4n+2 4n+3
4.5–7% 0.04–1.25% <0.001%
228Ra 4–6 155Euþ
244Cmƒ 241Puƒ 250Cf 227Ac 10–29 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ№ 243Cmƒ 29–97 137Cs 151Smþ 121mSn
248Bk[3] 249Cfƒ 242mAmƒ 141–351

No fission products
have a half-life
in the range of
100–210 k years ...

241Amƒ 251Cfƒ[4] 430–900
226Ra 247Bk 1.3 k – 1.6 k
240Puƒ№ 229Th 246Cmƒ 243Amƒ 4.7 k – 7.4 k
245Cmƒ 250Cm 8.3 k – 8.5 k
239Puƒ№ 24.1 k
230Th 231Pa 32 k – 76 k
236Npƒ 233Uƒ№ 234U 150 k – 250 k 99Tc 126Sn
248Cm 242Puƒ 327 k – 375 k 79Se
1.53 M 93Zr
237Npƒ№ 2.1 M – 6.5 M 135Cs 107Pd
236U 247Cmƒ 15 M – 24 M 129I
244Pu 80 M

... nor beyond 15.7 M years[5]

232Th 238U 235Uƒ№ 0.7 G – 14.1 G

Legend for superscript symbols
₡  has thermal neutron capture cross section in the range of 8–50 barns
ƒ  fissile
metastable isomer
№  naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
†  range 4–97 y: Medium-lived fission product
‡  over 200,000 y: Long-lived fission product

Formation and decay[edit]

129I 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 some by cosmic ray muons striking tellurium-130.[6][7]

129I decays with a half-life of 15.7 million years, with low-energy beta and gamma emissions, to xenon-129 (129Xe).[8]

Fission product[edit]

Yield, % per fission[9]
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  ?

129I is one of the 7 long-lived fission products that are produced in significant amounts. Its yield is 0.706% per fission (U-235).[10] Larger proportions of other iodine isotopes like 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,[11] 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[12] or by high-powered lasers.[13]


Groundwater age dating[edit]

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.[14][15][16]

Meteorite age dating[edit]

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 iodine-129. 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.[17][18]

See also[edit]


  1. ^ Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no isotopes have half-lives of at least four years (the longest-lived isotope in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
  2. ^ Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
  3. ^ Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. doi:10.1016/0029-5582(65)90719-4. 
    "The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk248 with a half-life greater than 9 y. No growth of Cf248 was detected, and a lower limit for the β half-life can be set at about 104 y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y."
  4. ^ This is the heaviest isotope with a half-life of at least four years before the "Sea of Instability".
  5. ^ Excluding those "classically stable" isotopes with half-lives significantly in excess of 232Th; e.g., while 113mCd has a half-life of only fourteen years, that of 113Cd is nearly eight quadrillion years.
  6. ^ R. Edwards. Iodine-129: Its Occurrence in Nature and Its Utility as a Tracer. Science, Vol 137 (1962) pp. 851–853.
  7. ^ Radioactives Missing From The Earth
  8. ^, NNDC Chart of Nuclides, I-129 Decay Radiation, accessed 16-Dec-2012.
  9. ^ Cumulative Fission Yields, IAEA
  10. ^ Cumulative Fission Yields, IAEA
  11. ^, NNDC Chart of Nuclides, I-129 Thermal neutron capture cross-section, accessed 16-Dec-2012.
  12. ^ J.A. Rawlins et al. "Partitioning and transmutation of long-lived fission products". Proceedings International High-Level Radioactive Waste Management Conference. Las Vegas, USA (1992).
  13. ^ J. Magill et al. "Laser transmutation of iodine-129". Applied Physics B: Lasers and Optics. Vol. 77(4) (2003).
  14. ^ J.Watson et al. (1965) "Iodine-129 as a Nonradioactive Tracer", Radiation Research, 26, 159-163.
  15. ^ P. Santschi et al. (1998) "129Iodine: A new tracer for surface water/groundwater interaction." Lawrence Livermore National Laboratory preprint UCRL-JC-132516. Livermore, USA.
  16. ^ *G. Snyder and J. Fabryka-Martin. (2007). I-129 and Cl-36 in dilute hydrocarbon waters: Marine-cosmogenic,in situ, and anthropogenic sources." Applied Geochemistry, 22(3) 692-714.
  17. ^ Clayton, Donald D. (1983). Principles of Stellar Evolution and Nucleosynthesis (2nd ed.). University of Chicago Press. p. 75. ISBN 0226109534. 
  18. ^ Bolt, B. A.; Packard, R. E.; Price, P. B. (2007). "John H. Reynolds, Physics: Berkeley". The University of California, Berkeley. Retrieved 2007-10-01. 

Further reading[edit]

  • Snyder, G. T.; Fabryka-Martin, J. T. (2007). "129I and 36Cl in dilute hydrocarbon waters: Marine-cosmogenic, in situ, and anthropogenic sources". Applied Geochemistry. 22 (3): 692. doi:10.1016/j.apgeochem.2006.12.011. 
  • Snyder, G.; Fehn, U. (2004). "Global distribution of 129I in rivers and lakes: Implications for iodine cycling in surface reservoirs". Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms. 223–224: 579. doi:10.1016/j.nimb.2004.04.107. 

External links[edit]