Isotopes of iodine

Naturally occurring iodine (₅₃I) consists of one stable isotope, ¹²⁷I, and is a mononuclidic element for atomic weight. Radioisotopes of iodine are known from ¹⁰⁸I to ¹⁴⁷I.

The longest-lived of those, ¹²⁹I, has a half-life of 16.14 million years, which is too short for it to exist as a primordial nuclide. It is, however, found in nature as a trace isotope and universally distributed, produced naturally by cosmogenic sources in the atmosphere and by natural fission of the actinides. Today, however, most is artificial as fission product; like krypton-85 the contribution of past nuclear testing and of operating reactors are dwarfed by release from nuclear reprocessing.

All other iodine radioisotopes have half-lives less than 60 days, and four of these are used as tracers and therapeutic agents in medicine – ¹²³I, ¹²⁴I, ¹²⁵I, and ¹³¹I. All industrial use of radioactive iodine isotopes involves these four. In addition, one other isotope has a half-life in the same range – ¹²⁶I (12.93 days; decays almost equally to tellurium or to xenon).

The isotope ¹³⁵I has a half-life less than seven hours, which is inconveniently short for those purposes. However, the 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 one of the common radioactive fission products of nuclear fission, and thus occurs in 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, which is produced from the decay of ¹³²Te with a half-life of 3 days) is responsible for the most dangerous part of the short-term radioactive contamination after environmental release of the radioactive waste from a nuclear power plant. For that reason, iodine supplements (usually potassium iodide) are given to the populace after nuclear accidents or explosions (and in some cases prior to any such incident as a civil defense mechanism) to reduce the uptake of radioactive iodine compounds by the thyroid.