Isotopes of iodine

There are 37 known isotopes of iodine (I) and 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 involves these four useful radionuclides.

The isotope 135I has a half-life less than seven hours, which is too short to be used in biology. Unavoidable in situ production of this isotope is important in nuclear reactor control, as it decays to 135Xe, the most powerful known neutron absorber, and the nuclide responsible for the so-called iodine pit phenomenon.

In addition to commercial production, 131I (half life 8 days) is the most common radioactive fission-product of nuclear fission, and is thus produced inadvertently in very large amounts inside nuclear reactors. Due to its volatility, short half life, and high abundance in fission products, 131I, (along with the short-lived iodine isotope 132I from the longer-lived 132Te with a half life of 3 days) is responsible for the largest part of radioactive contamination during the first week after accidental environmental contamination from the radioactive waste from a nuclear power plant.

The standard atomic mass for iodine is 126.90447(3) u.

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 identified as present in the early solar system. Its decay is the basis of the I-Xe iodine-xenon radiometric dating scheme, 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 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.

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.

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 Grave's disease. Especially in Grave's disease, often a thyroidectomy is performed before the radiotherapy, in order to avoid side effects of epilation and radiation toxicity. The purpose of this therapy, which was first explored by Dr. Saul Hertz in 1941, is to destroy the remaining thyroid tissue that was impossible to be removed by surgery. 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.

After the intake, the beta particles emitted by the high dose of 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, iodine-131 is the iodine radioisotope used in other water-soluble iodine-labeled radiopharmaceuticals (such as MIBG) which are intended to be used therapeutically to destroy tissues.

The high energy beta radiation from I-131 causes it to be the most carcinogenic of the iodine isotopes, and it is thought to cause the majority of the excess in 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.

Most medical imaging with iodine is done with a standard gamma camera. However, the gamma rays from iodine-123 and iodine-131 can also be seen by single photon emission computed tomography (SPECT) imaging.

Iodine-124
Iodine-124 is a proton-rich isotope of iodine with a half-life of 4.18 days. Its modes of decay are: 74.4% electron capture, 25.6% positron emission. 124I decays to 124Te. Iodine-124 can be made by numereous nuclear reactions via a cyclotron. The most common starting material used is 124Te.

Iodine-124 as the iodide salt can be used to directly image the thyroid using positron emission tomography (PET). Iodine-124 can also be used as a PET radiotracer with a usefully longer half-life compared with fluorine-18. In this use, the nuclide is chemically bonded to a pharmaceutical to form a positron-emitting radiopharmaceutical, and injected into the body, where again it is imaged by PET scan.

Iodine-135 and nuclear reactor control
Iodine-135 is an isotope of iodine with a half-life of 6.6 hours. It is an important isotope from the viewpoint of nuclear reactor physics. It is produced in relatively large amounts as a fission product, and decays to xenon-135, which is a nuclear poison with a very large slow neutron cross section, which is a cause of multiple complications in the control of nuclear reactors. The process of buildup of xenon-135 from an accumulated iodine-135 can temporarily preclude a shut-down reactor from restarting. This is known as xenon-poisoning or "falling into an iodine pit."

Iodine-128 and other isotopes
Iodine fission-produced isotopes not discussed above (iodine-128, iodine-130, iodine-132, and iodine-133) have a life lives of a couple of hours or minutes, rendering them almost useless in other applicable areas. Those mentioned are neutron-rich and so go through beta decay to their xenon counterparts. Iodine-128 (25 min half-life) can decay to either tellurium-128 by electron capture, or to xenon-128 by beta decay. It has a specific radioactivity of 2.177 x 106 TBq/g.

Non-radioactive iodide as protection from unwanted radioiodine uptake by the thyroid
Thyroid iodine uptake blockade with potassium iodide is used in nuclear medicine scintigraphy and therapy with some radioiodinated compounds that are not targeted to the thyroid, such as iobenguane (MIBG), which used to image or treat neural tissue tumors, or iodinated fibrinognen, which is used in fibrinogen scans to investigate clotting. These compounds contain iodine, but not in the iodide form. However, since they may be ultimately metabolized or break down to radioactive iodide, it is common to administer non-radioactive potassium iodide to insure that iodide from these radiopharmaceuticals is not sequestered by the normal affinity of the thryoid for iodide.

Potassium iodide has been distributed to populations exposed to nuclear fission accidents such as the Chernobyl disaster. The iodide solution SSKI, a saturated solution of potassium (K) iodide in water, has been used to block absorption of the radioiodine (it has no effect on other radioisotopes from fission). Tablets containing potassium iodide are now also manufactured and stocked in central disaster sites by some governments for this purpose. In theory, many harmful late-cancer effects of nuclear fallout might be prevented in this way, since an excess of thyroid cancers, presumably due to radioiodine uptake, is the only proven radioisotope contamination effect after a fission accident, or from contamination by fallout from an atomic bomb (prompt radiation from the bomb also cases other cancers, such as leukemias, directly). Taking large amounts of iodide saturates thyroid receptors and prevents uptake of most radioactive iodine-131 that may be present from fission product exposure (although it does not protect from other radioisotopes, nor from any other form of direct radiation). The protective effect of KI lasts approximately 24 hours, so must be dosed daily until a risk of significant exposure to radioiodines from fission products no longer exists. Iodine-131 (the most common radioiodine contaminant in fallout) also decays relatively rapidly with a half-life of eight days, so that 99.95% of the original radioiodine has vanished after three months.