Iodine-129 (¹²⁹I) is a long-lived radioisotope of

iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...

which occurs naturally, but also is of special interest in the monitoring and effects of man-made nuclear

fission product Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...

s, where it serves as both tracer and potential radiological contaminant.

Formation and decay

¹²⁹I is one of seven

long-lived fission product Long-lived fission products (LLFPs) are radioactive materials with a long half-life (more than 200,000 years) produced by nuclear fission of uranium and plutonium. Because of their persistent radiotoxicity it is necessary to isolate them from man ...

s. It is primarily formed from the

fission Fission, a splitting of something into two or more parts, may refer to: * Fission (biology), the division of a single entity into two or more parts and the regeneration of those parts into separate entities resembling the original * Nuclear fissio ...

of uranium and plutonium in

nuclear reactors A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nu ...

. Significant amounts were released into the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

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 Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 (e.g., iron-56); spontaneous breakdo ...

natural uranium Natural uranium (NU or Unat) refers to uranium with the same isotopic ratio as found in nature. It contains 0.711% uranium-235, 99.284% uranium-238, and a trace of uranium-234 by weight (0.0055%). Approximately 2.2% of its radioactivity comes fr ...

, by cosmic ray spallation of trace levels of xenon in the atmosphere, and by cosmic ray

muon A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wi ...

s striking tellurium-130. ¹²⁹I decays with a half-life of 15.7 million years, with low-energy

beta Beta (, ; uppercase , lowercase , or cursive ; grc, βῆτα, bē̂ta or ell, βήτα, víta) is the second letter of the Greek alphabet. In the system of Greek numerals, it has a value of 2. In Modern Greek, it represents the voiced labiod ...

and

gamma Gamma (uppercase , lowercase ; ''gámma'') is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter re ...

emissions, to stable

xenon-129 Naturally occurring xenon (54Xe) consists of seven stable isotopes and two very long-lived isotopes. Double electron capture has been observed in 124Xe (half-life ) and double beta decay in 136Xe (half-life ), which are among the longest measured ...

(¹²⁹Xe).

Fission product

¹²⁹I is one of the seven

s that are produced in significant amounts. Its yield is 0.706% per fission of ²³⁵U. 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 Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Ab ...

of 30 barns, and is relatively undiluted by other isotopes of the same element, it is being studied for disposal by

nuclear transmutation Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed. A transmutatio ...

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

meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...

s contained an isotopic anomaly in the form of an overabundance of ¹²⁹Xe. He inferred that this must be a

decay product In nuclear physics, a decay product (also known as a daughter product, daughter isotope, radio-daughter, or daughter nuclide) is the remaining nuclide left over from radioactive decay. Radioactive decay often proceeds via a sequence of steps ( ...

of long-decayed radioactive ¹²⁹I. This isotope is produced in quantity in nature only in

supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...

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.

References

External links

ANL factsheetMonitoring iodine-129 in air and milk samples collected near the Hanford Site: an investigation of historical iodine monitoring dataStudies with natural and anthropogenic iodine isotopes: iodine distribution and cycling in the global environment
{{Webarchive, url=https://web.archive.org/web/20140119053310/http://www.physics.utoronto.ca/~isotrace/isowhe.html , date=2014-01-19 Isotopes of iodine Fission products Radionuclides used in radiometric dating