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Caesium-137 has a half-life of about 30.05 years. [1] About 94.6% decays by beta emission to a metastable nuclear isomer of barium: barium-137m (137m Ba, Ba-137m). The remainder directly populates the ground state of 137 Ba, which is stable.
The low decay energy, lack of gamma radiation, and long half-life of 135 Cs make this isotope much less hazardous than 137 Cs or 134 Cs. Its precursor 135 Xe has a high fission product yield (e.g., 6.3333% for 235 U and thermal neutrons ) but also has the highest known thermal neutron capture cross section of any nuclide.
137 lead-196m3: 140 lead-190m1: ... caesium-137: 30.17 952 10 9 seconds ... Radioactive isotope table "lists ALL radioactive nuclei with a half-life greater than ...
Caesium-137, with a half-life of 30 years, is the main medium-lived fission product, along with Sr-90. Cs-137 is the primary source of penetrating gamma radiation from spent fuel from 10 years to about 300 years after discharge.
The longest-lived isomer is 133m Ba, which has a half-life of 38.9 hours. The shorter-lived 137m Ba (half-life 2.55 minutes) arises as the decay product of the common fission product caesium-137 . Barium-114 is predicted to undergo cluster decay , emitting a nucleus of stable 12 C to produce 102 Sn.
The radioactive 135 Cs has a very long half-life of about 2.3 million years, the longest of all radioactive isotopes of caesium. 137 Cs and 134 Cs have half-lives of 30 and two years, respectively. 137 Cs decomposes to a short-lived 137m Ba by beta decay , and then to nonradioactive barium, while 134 Cs transforms into 134 Ba directly.
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Caesium-137 is one such radionuclide. It has a half-life of 30 years, and decays by beta decay without gamma ray emission to a metastable state of barium-137 (137m Ba). Barium-137m has a half-life of a 2.6 minutes and is responsible for all of the gamma ray emission in this decay sequence. The ground state of barium-137 is stable.