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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

Nuclide t 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

Actinides and fission products by half-life
Actinides[6] by decay chain Half-life
range (y)
Fission products of 235U by yield[7]
4n 4n+1 4n+2 4n+3
4.5-7% 0.04-1.25% <0.001%
228RaNo 4-6 + 155Euþ
244Cm? 241Pu? 250Cf 227AcNo 10-29 90Sr 85Kr 113mCdþ
232U? 238Pu? 243Cm? 29-97 137Cs 151Smþ 121mSn
248Bk[8] 249Cf? 242mAm? 141-351

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

241Am? 251Cf?[9] 430-900
226RaNo 247Bk 1.3 k - 1.6 k
240Pu 229Th 246Cm? 243Am? 4.7 k - 7.4 k
245Cm? 250Cm 8.3 k - 8.5 k
239Pu? 24.1 k
230ThNo 231PaNo 32 k - 76 k
236Np? 233U? 234UNo ? 99Tc? 126Sn
248Cm 242Pu 327 k - 375 k 79Se?
1.53 M 93Zr
237Np? 2.1 M - 6.5 M 135Cs? 107Pd
236U 247Cm? 15 M - 24 M 129I?
244Pu 80 M

... nor beyond 15.7 M years[10]

232ThNo 238UNo 235U?No 0.7 G - 14.1 G

Legend for superscript symbols
?  has thermal neutron capture cross section in the range of 8-50 barns
metastable isomer
No  primarily a 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

Yield, % per fission[11]
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 seven long-lived fission products that are produced in significant amounts. Its yield is 0.706% per fission of 235U.[11] 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 ​ 129I and ​ 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,[12] 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[13] or by high-powered lasers.[14]


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.[15][16][17]

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.[18][19]

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. ^ R. Edwards. Iodine-129: Its Occurrence in Nature and Its Utility as a Tracer. Science, Vol 137 (1962) pp. 851-853.
  4. ^ "Radioactives Missing From The Earth".
  5. ^, NNDC Chart of Nuclides, I-129 Decay Radiation, accessed 16-Dec-2012.
  6. ^ 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 nuclides have half-lives of at least four years (the longest-lived nuclide 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.
  7. ^ Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
  8. ^ 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. Bibcode:1965NucPh..71..299M. 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 ?- 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."
  9. ^ This is the heaviest nuclide with a half-life of at least four years before the "Sea of Instability".
  10. ^ Excluding those "classically stable" nuclides 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.
  11. ^ a b Cumulative Fission Yields, IAEA
  12. ^, NNDC Chart of Nuclides, I-129 Thermal neutron capture cross-section, accessed 16-Dec-2012.
  13. ^ 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).
  14. ^ J. Magill et al. "Laser transmutation of iodine-129". Applied Physics B: Lasers and Optics. Vol. 77(4) (2003).
  15. ^ Watson, J. Throck; Roe, David K.; Selenkow, Herbert A. (1 January 1965). "Iodine-129 as a "Nonradioactive" Tracer". Radiation Research. 26 (1): 159-163. doi:10.2307/3571805. JSTOR 3571805.
  16. ^ P. Santschi et al. (1998) "129Iodine: A new tracer for surface water/groundwater interaction." Lawrence Livermore National Laboratory preprint UCRL-JC-132516. Livermore, USA.
  17. ^ *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.
  18. ^ Clayton, Donald D. (1983). Principles of Stellar Evolution and Nucleosynthesis (2nd ed.). University of Chicago Press. pp. 75. ISBN 978-0226109534.
  19. ^ 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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