Caesium-137

Caesium-137 is a radioactive fission product with a half-life of about 30 years. Its decay produces barium-137m, whose gamma rays make it a major source of radioactive contamination and an industrial radiation source.

Beta decay: A radioactive decay process in which a nucleus emits an electron or positron and changes its atomic number. Caesium-137 decays by emitting an electron, becoming metastable barium-137.
Radiation therapy: The use of ionizing radiation to damage cancer cells and control tumors. Caesium-137 sources have been used in external-beam cancer treatment.
Chernobyl disaster: The 1986 accident at the Chernobyl Nuclear Power Plant that released radioactive material across Europe. Caesium-137 was a major long-lived contaminant released by the reactor fire.
Cobalt-60: A radioactive cobalt isotope with a half-life of about 5.3 years that emits energetic gamma rays. It is a common alternative gamma source with a shorter half-life and higher-energy emissions.
Radiation exposure: The receipt of ionizing radiation by a person or material from an external source or internal contamination. Caesium-137 can irradiate people externally or after entering the body.
Barium-137m: A metastable nuclear state of barium-137 that emits a gamma ray as it settles into a lower-energy state. This short-lived daughter produces the prominent gamma radiation associated with caesium-137.
Brachytherapy: A form of radiation therapy that places radioactive sources in or near a tumor. Caesium-137 has served in sealed sources for treating cancers, especially gynecological tumors.
Fukushima Daiichi nuclear disaster: The 2011 nuclear accident in Japan following an earthquake and tsunami. Releases included caesium-137, which contaminated land and marine environments.
Strontium-90: A radioactive fission product with a half-life of about 29 years that decays to yttrium-90. Its similar persistence invites comparison, but its chemistry and beta-emitting decay chain differ.
Biological half-life: The time an organism takes to eliminate half of a substance through biological processes. It determines how long absorbed caesium remains in the body, apart from radioactive decay.

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