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http://en.wikipedia.org/wiki/Iodine-131


Iodine-131
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Iodine-131
Full table
General
Name, symbol Radioiodine,131I
Neutrons 78
Protons 53
Nuclide data
Half-life 8.0197 days
Isotope mass h u

Iodine-131 (131I), also called radioiodine (though many other radioactive isotopes of this element are known), is an important radioisotope of iodine. Its uses are mostly medical and pharmaceutical. It also plays a role as a major radioactive hazard present in nuclear fission products, and was a significant contributor to the health effects from open-air atomic bomb testing in the 1950s, and from the Chernobyl disaster. This is because I-131 is a major uranium fission product, comprising nearly 3% of the total products of fission.

Due to its mode of beta decay, iodine-131 is notable for causing mutation and death in cells which it penetrates, and other cells up to several millimeters away. For this reason, high doses of the isotope are sometimes paradoxically less dangerous than low doses, since they tend to kill thyroid tissues which would otherwise become cancerous as a result of the radiation. Thus, iodine-131 is increasingly not used in small doses in medical use, but increasingly only in large and maximal treatment doses, as a way of killing targeted tissues. This is known as "therapeutic use."

Although the isotope can be "seen" by nuclear medicine imaging techniques (i.e., gamma cameras) whenever given for therapeutic use, other safer radioisotopes of Iodine are coming to be preferred in situations when only nuclear imaging is required. I-131 is still occasionally used for diagnostic work, due to its low expense compared to other isotopes. This availability, in turn, is due to the relative ease of creating the nuclide by neutron bombardment of natural tellurium in a nuclear reactor, and then separating it by various simple methods. (Other Iodine radioisotopes are usually created by far more expensive techniques starting with reactor radiation of xenon gas).

Much smaller incidental doses of iodine-131 than are used in medical treatment, are thought to be the major cause of increased thyroid cancers after accidental nuclear contamination.