Iodine-129

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 by decay chain				Half-life range (y)	Fission products of 235U by yield
4n	4n+1	4n+2	4n+3
						4.5–7%	0.04–1.25%	<0.001%
228Ra№				4–6	†		155Euþ
244Cm	241Puƒ	250Cf	227Ac№	10–29		90Sr	85Kr	113mCdþ
232Uƒ		238Pu№	243Cmƒ	29–97		137Cs	151Smþ	121mSn
248Bk	249Cfƒ	242mAmƒ		141–351	No fission products have a half-life in the range of 100–210 k years ...
	241Amƒ		251Cfƒ	430–900
		226Ra№	247Bk	1.3 k – 1.6 k
240Pu	229Th№	246Cm	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		242Pu		327 k – 375 k			79Se₡
				1.53 M		93Zr
	237Np№			2.1 M – 6.5 M		135Cs₡	107Pd
236U№			247Cmƒ	15 M – 24 M			129I₡
244Pu№				80 M	... nor beyond 15.7 M years
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 m  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

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.

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

Fission product

Yield, % per fission

	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). 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 of 30 barns, 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 or by high-powered lasers.

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, as well as identifying younger groundwaters by the increased anthropogenic ¹²⁹I levels since the 1960s.

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 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.

See also

• Isotopes of iodine
• Iodine in biology
• The ¹²⁹I - ¹²⁹Xe chronometer

External links

• ANL factsheet
• Monitoring iodine-129 in air and milk samples collected near the Hanford Site: an investigation of historical iodine monitoring data
• Studies with natural and anthropogenic iodine isotopes: iodine distribution and cycling in the global environment
• Some Publications using ¹²⁹I Data from IsoTrace, 1997-2002

• 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.
• Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
• 
  "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."
• This is the heaviest isotope with a half-life of at least four years before the "Sea of Instability".
• 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.
• 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
• http://www.nndc.bnl.gov/chart/decaysearchdirect.jsp?nuc=129I&unc=nds, NNDC Chart of Nuclides, I-129 Decay Radiation, accessed 16-Dec-2012.
• http://www-nds.iaea.org/sgnucdat/c3.htm Cumulative Fission Yields, IAEA
• http://www-nds.iaea.org/sgnucdat/c3.htm Cumulative Fission Yields, IAEA
• http://www.nndc.bnl.gov/chart/reColor.jsp?newColor=sigg, NNDC Chart of Nuclides, I-129 Thermal neutron capture cross-section, accessed 16-Dec-2012.
• 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).
• J.Watson et al. (1965) "Iodine-129 as a Nonradioactive Tracer", Radiation Research, 26, 159-163.
• https://e-reports-ext.llnl.gov/pdf/234761.pdf P. Santschi et al. (1998) "129Iodine: A new tracer for surface water/groundwater interaction." Lawrence Livermore National Laboratory preprint UCRL-JC-132516. Livermore, USA.
• *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.