Iodine-131

Iodine-131 (131I), also called radioiodine, is a radioisotope of iodine.131I decays with a half-life of 8.0197 days with beta and gamma emissions. This nuclide of iodine atom has 78 neutrons in nucleus, the stable nuclide 127I has 74 neutrons. On decaying, 131I transforms into ¹³¹Xe: 131I is a fission product with a yield of 2.8336%, and was released in nuclear weapons tests and the Chernobyl accident. However, the short half-life means it is not present in cooled spent nuclear fuel, unlike iodine-129.
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Noun
1. heavy radioactive isotope of iodine with a half-life of 8 days; used in a sodium salt to diagnose thyroid disease and to treat goiter
(hypernym) iodine, iodin, I, atomic number 53