Isotopes of barium
Naturally occurring barium is a mix of six stable isotopes and one very long-lived radioactive primordial isotope, barium-130, identified as being unstable by geochemical means in 2001, presumably decaying by double electron capture with a half-life of ×1021 years. The two measurements are discordant; the above reflects the total range, the value in the table below is a crude average.
With the total range of mass numbers known 114 to 154, there are thirty-three known radioisotopes in addition to 130Ba. The longest-lived of these is 133Ba, which has a half-life of 10.538 years; all others have half-lives shorter than two weeks. The longest-lived isomers are 133mBa at 38.90 hours and 135m1Ba at 28.11 hours. The analogous 137m1Ba occurs in the decay of the common fission product caesium-137.
Barium-114 is theorized to undergo cluster decay, emitting a nucleus of stable 12C to produce 102Sn. This decay has not been observed, with only an upper limit on the branching ratio of such decay.