Isotopes of caesium

Caesium (55Cs) has 40 known isotopes, making it, along with barium and mercury, one of the elements with the most isotopes.[5] 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, 137
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.

Isotopes of caesium (55Cs)
Main isotopes[1] Decay
abun­dance half-life (t1/2) mode pro­duct
131Cs synth 9.7 d ε 131Xe
133Cs 100% stable
134Cs synth 2.0648 y ε 134Xe
β 134Ba
135Cs trace 2.3×106 y β 135Ba
137Cs synth 30.17 y[2] β 137Ba
Standard atomic weight Ar°(Cs)
  • 132.90545196±0.00000006
  • 132.91±0.01 (abridged)[3][4]

Beginning in 1945 with the commencement of nuclear testing, caesium radioisotopes were released into the atmosphere where caesium 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.

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