Caesium-135

Main isotopes of caesium

iso NA half-life DM DE (MeV) DP 133Cs 100% is stable with 78 neutrons 134Cs syn 2.0648 y ε 1.229 134Xe β− 2.059 134Ba 135Cs trace 2.3×106 y β− 0.269 135Ba 137Cs syn 30.17 y β− 1.174 137Ba
Standard atomic weight (Ar)	7002132905451960000♠132.90545196(6)

Caesium (₅₅Cs) or cesium as it is spelled in the United States has 40 known isotopes, making it, along with barium and mercury, the element with the most isotopes. The atomic masses of these isotopes range from 112 to 151. Only one isotope, ¹³³Cs, is stable. The longest-lived radioisotopes are ¹³⁵Cs with a half-life of 2.3 million years, ¹³⁷Cs with a half-life of 30.1671 years and ¹³⁴Cs with a half-life of 2.0652 years. All other isotopes have half-lives less than 2 weeks, most under an hour.

Beginning in 1945 with the commencement of nuclear testing, caesium isotopes were released into the atmosphere where it is absorbed readily into solution and is returned to the surface of the earth as a component of radioactive fallout. Once caesium enters the ground water, it is deposited on soil surfaces and removed from the landscape primarily by particle transport. As a result, the input function of these isotopes can be estimated as a function of time.

Caesium-133 is the only stable isotope of caesium. One specific quantum transition in the caesium-133 atom is used to define the second, a unit of time.

Caesium-134 has a half-life of 2.0652 years. It is produced both directly (at a very small yield because ¹³⁴Xe is stable) as a fission product and via neutron capture from nonradioactive Cs-133 (neutron capture cross section 29 barns), which is a common fission product. 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 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.

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