Chlorine-36

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Chlorine-36 is an isotope of chlorine. Chlorine has two stable isotopes and one radioactive environmental isotope: the cosmogenic isotope 36Cl. Chlorine-36 has a long half-life, making it useful to date groundwater up to one million years old. There also are observed natural variations in 37Cl. Most such variations in 37Cl values of hydrologic systems are related to diffusion processes.

Chlorine-36 is produced in the upper atmosphere through spallation reactions and in solid materials on the surface of the earth in three ways: spallation reactions, muon reactions and thermal neutron adsorption. Spallation reactions also occur when gamma rays interact with minerals in the top several meters of the Earth's surface. It can also be produced through muon reactions and thermal neutron absorption. 36Cl was also produced during nuclear bomb testing in the middle of the 20th century.

The ratio of 36Cl to stable 37Cl in the environment is ~700×10-15. Its half-life is 301,000 ± 4,000 years. For hydrological applications see [1].