Isotopes of iodine

There are 40 known isotopes of iodine (₅₃I) from ¹⁰⁸I to ¹⁴⁷I; all undergo radioactive decay except ¹²⁷I, which is stable. Iodine is thus a monoisotopic element.

Its longest-lived radioactive isotope, ¹²⁹I, has a half-life of 16.14 million years, which is 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 only 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 - ¹²³I, ¹²⁴I, ¹²⁵I, and ¹³¹I. All industrial use of radioactive iodine isotopes involves these four.

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.