A number of the iodine isotopes are used as radioisotopes in nuclear medicine: iodine-123 and iodine-131 is used for single photon emission computed tomography (SPECT) imaging and iodine-124 for positron emission tomography. They may result in different image quality.[1]
There are 37 isotopes of iodine (I) and only one, 127I, is stable.
In many ways, 129I is similar to 36Cl. It is a soluble halogen, fairly non-reactive, exists mainly as a non-sorbing anion, and is produced by cosmogenic, thermonuclear, and in-situ reactions. In hydrologic studies, 129I concentrations are usually reported as the ratio of 129I to total I (which is virtually all 127I). As is the case with 36Cl/Cl, 129I/I ratios in nature are quite small, 10?14 to 10?10 (peak thermonuclear 129I/I during the 1960s and 1970s reached about 10?7). 129I differs from 36Cl in that its half-life is longer (15.7 vs. 0.301 million years), it is highly biophilic, and occurs in multiple ionic forms (commonly, I? and IO3?) which have different chemical behaviors. This makes it fairly easy for 129I to enter the biosphere as it becomes incorporated into vegetation, soil, milk, animal tissue, etc.
Excesses of stable 129Xe in meteorites have been shown to result from decay of “primordial” 129I produced newly by the supernovas which created the dust and gas from which the solar system formed. 129I was the first extinct radionuclide to be identified as present in the early solar system. Its decay is the basis of the I-Xe radiometric dating scheme, which covers the first 83 million years of solar system evolution.
Standard atomic mass.
Patients receiving radioiodine treatment are warned not to have sexual intercourse for one month (or shorter, depending on dose given), and women are told not to become pregnant for six months afterwards. “This is because a theoretical risk to a developing fetus exists, even though the amount of radioactivity retained may be small and there is no medical proof of an actual risk from radioiodine treatment. Such a precaution would essentially eliminate direct fetal exposure to radioactivity and markedly reduce the possibility of conception with sperm that might theoretically have been damaged by exposure to radioiodine.”[6] These guidelines vary from hospital to hospital and will depend also on the dose of radiation given. One also advises not to hug or hold children when the radiation is still high, and a one or two metre distance to others may be recommended[citation needed].
Many airports now have radiation detectors in order to detect the smuggling of radioactive materials that may be used in nuclear weapons manufacture. Patients should be warned that if they choose to travel by air, they may set off radiation detectors at airports up to 12 weeks after their treatment with 131I.[citation needed]
It is used in nuclear medicine both diagnostically and therapeutically. Examples of its use in radiation therapy include the treatment of thyrotoxicosis and thyroid cancer. Diagnostic tests exploit the mechanism of absorption of iodine by the normal cells of the thyroid gland. As an example iodine-131 is one of the radioactive isotopes of iodine that can be used to test how well the thyroid gland is functioning.
131I is also used as a radioactive label for radiopharmaceuticals that can be used for imaging and therapy e.g. 131I-metaiodobenzylguanidine (131I-MIBG) for imaging and treating pheochromocytoma and neuroblastoma.
Iodine in food is absorbed by the body and preferentially concentrated in the thyroid where it is needed for the functioning of that gland. When 131I is present in high levels in the environment from radioactive fallout, it can be absorbed through contaminated food, and will also accumulate in the thyroid. As it decays, it may cause damage to the thyroid. The primary risk from exposure to high levels of 131I is the chance occurrence of radiogenic thyroid cancer in later life. Other risks include the possibility of non-cancerous growths and thyroiditis.
The risk of thyroid cancer in later life appears to diminish with increasing age at time of exposure. Most risk estimates are based on studies in which radiation exposures occurred in children or teenagers. When adults are exposed, it has been difficult for epidemiologists to detect a statistically significant difference in the rates of thyroid disease above that of a similar but otherwise unexposed group[citation needed].
The risk can be mitigated by taking iodine supplements, raising the total amount of iodine in the body and therefore reducing uptake and retention in tissues and lowering the relative proportion of radioactive iodine. Such supplements were distributed to the population living nearest to the Chernobyl nuclear power plant after the disaster.
Within the USA, the highest 131I fallout doses occurred during the 1950s and early 1960s to children who consumed fresh sources of milk contaminated as the result of above ground testing of nuclear weapons.[3] The National Cancer Institute provides additional information on the health effects from exposure to 131I in fallout,[4] as well as individualized estimates, for those born before 1971, for each of the 3070 counties in the USA from the nuclear weapons tests conducted at the Nevada Test Site.[5]
The primary emissions of 131I decay are 364 keV gamma rays (81% abundance) and 606 keV beta particles (89% abundance).[1]
131I is a fission product with a yield of 2.8336% from uranium-235, and was released in nuclear weapons tests and the Chernobyl accident. It was also emitted in large quantities at the Hanford Site in Washington State[2]. However, the short half-life means it is not present in cooled spent nuclear fuel, unlike iodine-129 whose halflife is nearly a billion times that of I-131.
Iodine-131 (131I), also called radioiodine, is a radioisotope of iodine which has medical and pharmaceutical uses.
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]
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]
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]