Iodine-129

Iodine-129
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Iodine-129 (¹²⁹I) is a radioisotope of iodine.

content

1 Formation and decay
2 Fission Product
3 Applications
- 3.1 Groundwater age dating
- 3.2 Meteorite age dating
4 References
5 See also

Formation and decay

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 1950's and 1960's.

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.^[1]^[2]

¹²⁹I decays with a half-life of 15.7 million years, with low-energy beta and gamma emissions, to ¹²⁹Xe.

Long-lived fission products
Property: t^½ Unit: (Ma)		Yield (%)	Q * (KeV)	βγ *
⁹⁹Tc	.211	6.1385	294	β
¹²⁶Sn	.230	.1084	4050	βγ
⁷⁹Se	.295	.0447	151	β
⁹³Zr	1.53	5.4575	91	βγ
¹³⁵Cs	2.3	6.9110	269	β
¹⁰⁷Pd	6.5	1.2499	33	β
¹²⁹I	15.7	.8410	194	βγ

Fission Product

¹²⁹I is one of the 7 long-lived fission products that are produced in significant amounts. Its yield is 0.6576% per fission (U-235). 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 5/6 ¹²⁹I and 1/6 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, 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^[3] or by high-powered lasers.^[4]

Applications

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,^[5] as well as identifying younger groundwaters by the increased anthropogenic ¹²⁹I levels since the 1960's.^[6]

Meteorite age dating

In 1960 physicist John H. Reynolds discovered that certain meteorites contained an isotopic anomaly in the form of an overabundance of xenon-129. 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. ^[7]^[8]

References

^ R.Edwards. Iodine-129: Its Occurrence in Nature and Its Utility as a Tracer. Science, Vol 137 (1962) pp.851-853
^ Radioactives Missing From The Earth
^ 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).
^ J. Magill et al. Laser transmutation of iodine-129. Applied Physics B: Lasers and Optics. Vol. 77(4) (2003).
^ EXN.ca | Discovery Channel Canada's Web site
^ https://e-reports-ext.llnl.gov/pdf/234761.pdf
^ Clayton, Donald D. (1983). Principles of Stellar Evolution and Nucleosynthesis, 2nd edition, University of Chicago Press, p. 75. ISBN 0226109534.
^ Bolt, B. A.; Packard, R. E.; Price, P. B. (2007). "John H. Reynolds, Physics: Berkeley". The University of California, Berkeley. Retrieved on 2007-10-01.

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