Caesium-137 has a half-life of about 30.05 years.
About 94.6% decays by beta emission to a metastable nuclear isomer of barium: barium-137m (Ba, Ba-137m). The remainder directly populates the ground state of Ba, which is stable. Barium-137m has a half-life of about 153 seconds, and is responsible for all of the gamma ray emissions in samples of Cs. Barium-137m decays to the ground state by emission of photons having energy 0.6617 MeV. A total of 85.1% of Cs decay generates gamma ray emission in this manner. One gram of Cs has an activity of 3.215 terabecquerel (TBq).