I decays with a half-life of 8.02 days with beta minus and gamma emissions. This isotope of iodine has 78 neutrons in its nucleus, while the only stable nuclide, I, has 74. On decaying, I most often (89% of the time) expends its 971 keV of decay energy by transforming into stable xenon-131 in two steps, with gamma decay following rapidly after beta decay:
The primary emissions of I decay are thus electrons with a maximal energy of 606 keV (89% abundance, others 248–807 keV) and 364 keV gamma rays (81% abundance, others 723 keV). Beta decay also produces an antineutrino, which carries off variable amounts of the beta decay energy. The electrons, due to their high mean energy (190 keV, with typical beta-decay spectra present) have a tissue penetration of .