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Isotopes of caesium
Caesium (Cs) 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.〔(【引用サイトリンク】title=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.
Standard atomic mass: 132.9054519(2) u.
==Caesium-133==
Caesium-133 is the only naturally occurring and only stable isotope of caesium. It is also produced by nuclear fission in nuclear reactors. One specific quantum transition in the caesium-133 atom is used to define the second, a unit of time.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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