Radioactive Decay
I-131 decays with a half-life of 8.02 days with beta and gamma emissions. This nuclide of iodine has 78 neutrons in nucleus, while the stable nuclide 127I has 74 neutrons. On decaying, 131I most often (89% of the time) expends its 971 keV of decay energy by transforming into the stable 131Xe (Xenon) in two steps, with gamma decay following rapidly after beta decay:
+ 606 keV
+ 364 keV
The primary emissions of 131I decay are thus beta particles 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, as always in this process, also produces an antineutrino, which carries off variable amounts of the beta decay energy.
The beta particles, due to their high mean energy (190 keV, with typical beta-decay spectra present) have a tissue penetration of 0.6 to 2 mm.
131I is a fission product with a yield of 2.878% from uranium-235, and can be released in nuclear weapons tests and nuclear accidents. However, the short half-life means it is not present in significant quantities in cooled spent nuclear fuel, unlike iodine-129 whose half-life is nearly a billion times that of I-131.
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