It is also not produced by nuclear weapons because 133Cs is created by beta decay of original fission products only long after the nuclear explosion is over. Caesium-134 is not produced via beta decay of other fission product nuclides of mass 134 since beta decay stops at stable 134Xe. It is produced both directly (at a very small yield because 134Xe is stable) as a fission product and via neutron capture from nonradioactive 133Cs (neutron capture cross section 29 barns), which is a common fission product. The second, symbol s, is defined by taking the fixed numerical value of the caesium frequency, Δ ν Cs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s −1.Ĭaesium-134 has a half-life of 2.0652 years. Since 1967, the official definition of a second is: The SI base unit of time, the second, is defined by a specific caesium-133 transition.
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