Four isotopes of cerium occur naturally. These are 140Ce, which makes up 88.5% of cerium on earth; 142Ce, which makes up 11% of cerium on earth; 138Ce, which makes up 0.3% of cerium on earth; and 136Ce, which makes up 0.2% of cerium on Earth. However, only 140Ce is stable. 142Ce, 138Ce, and 136Ce have half-lives of 5×1016 years, 1.5×1015 years, and 70 trillion years respectively. Additionally, some isotopes of cerium occur naturally in trace amounts as the result of fission of uranium.
39 isotopes of cerium have been discovered. They range from 119Ce to 157Ce. Cerium also has 10 known nuclear isomers. Aside from the nearly-stable isotopes, the longest-lived isotopes of cerium are 139Ce and 144Ce, which have half-lives on the order of 107 seconds (115.74 days). An additional eight isotopes have half-lives greater than 1000 seconds (16 minutes and 40 seconds). All but 16 isotopes of cerium have half-lives greater than 1 second. The least stable isotopes whose half-lives are known are 153Ce, 154Ce and 155Ce. They have half-lives on the order of 10−7 seconds. All the isotopes lighter than 140Ce decay by electron capture or β+ decay. All the isotopes heavier than 140Ce decay by β− decay.
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