Caesium-137 (Cs-137) is a radioactive isotope of the element cesium. It has a half-life of about 30.1 years and is produced as a byproduct of nuclear fission in reactors and during the decay of certain isotopes, such as in the fallout from nuclear weapons testing or nuclear accidents. Cs-137 emits beta particles and gamma radiation, making it a source of ionizing radiation.
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