Isotopes of iodine 1 Isotopes of iodine There are 37 known isotopes of iodine (I) from 108I to 144I, but only one, 127I, is stable. Iodine is thus a monoisotopic element. Its longest-lived radioactive isotope, 129I, has a half-life of 15.7 million years, which is far too short for it to exist as a primordial nuclide. Cosmogenic sources of 129I produce very tiny quantities of it that are too small to affect atomic weight measurements; iodine is thus also a mononuclidic element—one that is found in nature essentially as a single nuclide. Most 129I derived radioactivity on Earth is man-made: an unwanted long-lived byproduct of early nuclear tests and nuclear fission accidents. All other iodine radioisotopes have half-lives less than 60 days, and four of these are used as tracers and therapeutic agents in medicine. These are 123I, 124I, 125I, and 131I. Essentially all industrial production of radioactive iodine isotopes A Pheochromocytoma is seen as a involves these four useful radionuclides. dark sphere in the center of the body The isotope 135I has a half-life less than seven hours, which is too short to be (it is in the left adrenal gland). Image is by MIBG scintigraphy, with used in biology. Unavoidable in situ production of this isotope is important in radiation from radioiodine in the 135 nuclear reactor control, as it decays to Xe, the most powerful known neutron MIBG. Two images are seen of the absorber, and the nuclide responsible for the so-called iodine pit phenomenon. same patient from front and back. Note the dark image of the thyroid 131 In addition to commercial production, I (half life 8 days) is the most common due to unwanted uptake of radioactive fission-product of nuclear fission, and is thus produced inadvertently radioiodine from the medication by in very large amounts inside nuclear reactors. Due to its volatility, short half life, the thyroid gland in the neck. Accumulation at the sides of the head and high abundance in fission products, 131I, (along with the short-lived iodine is from salivary gland uptake of 132 132 isotope I from the longer-lived Te with a half life of 3 days) is responsible iodide. Radioactivity is also seen in for the largest part of radioactive contamination during the first week after the bladder. accidental environmental contamination from the radioactive waste from a nuclear power plant. The standard atomic mass for iodine is 126.90447(3) u. Isotopes of iodine 2 Notable radioisotopes Iodine-129 as an extinct radionuclide Excesses of stable 129Xe in meteorites have been shown to result from decay of "primordial" iodine-129 produced newly by the supernovas which created the dust and gas from which the solar system formed. This isotope has long decayed and is thus referred to as "extinct." Historically, 129I was the first extinct radionuclide to be The portion of the total radiation activity (in air) contributed by each isotope versus identified as present in the early solar time after the Chernobyl disaster, at the site. Note the prominence of radiation from system. Its decay is the basis of the I-Xe I-131 and Te-132/I-132 for the first week. (Image using data from the OECD iodine-xenon radiometric dating scheme, report, and the second edition of 'The radiochemical manual'.) which covers the first 85 million years of solar system evolution. Iodine-129 as a long-lived marker for nuclear fission contamination Iodine-129 (129I; half-life 15.7 million years) is a product of cosmic ray spallation on various isotopes of xenon in the atmosphere, in cosmic ray muon interaction with tellurium-130, and also uranium and plutonium fission, both in subsurface rocks and nuclear reactors. Artificial nuclear processes, in particular nuclear fuel reprocessing and atmospheric nuclear weapons tests, have now swamped the natural signal for this isotope. Nevertheless, it now serves as a groundwater tracer as indicator of nuclear waste dispersion into the natural environment. In a similar fashion, 129I was used in rainwater studies to track fission products following the Chernobyl disaster. In some 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 IO −) which have different chemical behaviors. This 3 makes it fairly easy for 129I to enter the biosphere as it becomes incorporated into vegetation, soil, milk, animal tissue, etc. Radioiodines I-123, I-124, I-125, and I-131 in medicine and biology Due to preferential uptake of iodine by the thyroid, radioiodine isotopes are extensively used in imaging and (in the case of I-131) destroying dysfunctional thyroid tissues, and other types of tissue that selectively take up certain iodine-131-containing tissue-targeting and killing radiopharmaceutical agents (such as MIBG). Iodine-125 is the only other iodine radioisotope used in radiation therapy, but only as an implanted capsule in brachytherapy, where the isotope never has a chance to be released for chemical interaction with the body's tissues. Isotopes of iodine 3 Iodine-131 Iodine-131 (I131) is a beta-emitting isotope with a half-life of eight days, and comparatively energetic (190 KeV average and 606 KeV maximum energy) beta radiation, which penetrates 0.6 to 2.0 mm from the site of uptake. This beta radiation can be used in high dose for destruction of thyroid nodules and for elimination of remaining thyroid tissue after surgery for the treatment of Graves' disease. Especially in Graves' disease, a thyroidectomy is often performed before the radiotherapy, to avoid side effects of epilation and radiation toxicity. The purpose of this therapy, which was first explored by Dr. Saul Hertz in 1941,[1] is to destroy thyroid tissue that could not be removed surgically. In this procedure, I131 is administered either intravenously or orally following a diagnostic scan. This procedure may also be used to treat patients with thyroid cancer or hyperfunctioning thyroid tissue. The I131 is taken up into thyroid tissue and concentrated there. The beta particles emitted by the radioisotope destroys the associated thyroid tissue with little damage to surrounding tissues (more than 2.0 mm from the tissues absorbing the iodine). Due to similar destruction, I131 is the iodine radioisotope used in other water-soluble iodine-labeled radiopharmaceuticals (such as MIBG) which are used therapeutically to destroy tissues. The high energy beta radiation from I131 causes it to be the most carcinogenic of the iodine isotopes. It is thought to cause the majority of excess thyroid cancers seen after nuclear fission contamination (such as bomb fallout or severe nuclear reactor accidents like the Chernobyl disaster). Iodine-123 and iodine-125 The gamma-emitting isotopes iodine-123 (half-life 13 hours), and (less commonly) the longer-lived and less energetic iodine-125 (half-life 59 days) are used as nuclear imaging tracers to evaluate the anatomic and physiologic function of the thyroid. Abnormal results may be caused by disorders such as Graves' disease or Hashimoto's thyroiditis. Both isotopes decay by electron capture (EC) to the corresponding tellurium nuclides, but in neither case are these the metastable nuclides Te-123m and Te125m (which are of higher energy, and are not produced from radioiodine). Instead, the excited tellurium nuclides decay immediately (half-life too short to detect). Following EC, the excited Te-123 from I-123 emits a high-speed 127 keV internal conversion electron (not a beta ray) about 13% of the time, but this does little cellular damage due to the nuclide's short half-life and the relatively small fraction of such events. In the remainder of cases, a 159 keV gamma ray is emitted, which is well-suited for gamma imaging. Excited Te-125 from EC decay of I-125 also emits a much lower-energy internal conversion electron (35.5 keV) which does relatively little damage due to its low energy, even though its emission is more common. The relatively low-energy gamma from I-125/Te-125 decay is poorly suited for imaging, but can still be seen, and this longer-lived isotope is necessary in tests which require several days of imaging, for example fibrinogen scan imaging to detect blood clots. Both I-123 and I-125 emit copious low energy Auger electrons after their decay, but these do not cause serious damage (double-stranded DNA breaks) in cells, unless the nuclide is incorporated into a medication that accumulates in the nucleus, or into DNA (this is never the case is clinical medicine, but it has been seen in experimental animal models). Iodine-125 is also commonly used by radiation oncologists in low dose rate brachytherapy in the treatment of cancer at sites other than the thyroid, especially in prostate cancer. When I-125 is used therapeutically, it is encapsulated in titanium seeds and implanted in the area of the tumor, where it remains. The low energy of the gamma spectrum in this case limits radiation damage to tissues far from the implanted capsule. Iodine-125, due to its suitable longer half-life and less penetrating gamma spectrum, is also often preferred for laboratory tests that rely on iodine as a tracer that is counted by a gamma counter, such as in radioimmunoassaying.