Chlorine-36

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Chlorine-36 Full table
General
Name, symbol	Chlorine-36,36Cl
Neutrons	19
Protons	17
Nuclide data
Natural abundance	700×10-15
Half-life	301,000 ± 4,000 years

Chlorine-36 is an isotope of chlorine. Chlorine has two stable isotopes and one radioactive environmental isotope: the cosmogenic isotope ³⁶Cl. The ratio of ³⁶Cl to stable ³⁷Cl in the environment is ~700×10^-15. Its half-life is 301,000 ± 4,000 years. The long half-life of ³⁶Cl makes it useful to date groundwater up to one million years old.^[1] 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.

Trace amounts of radioactive ³⁶Cl exist in the environment, in a ratio of about 7x10⁻¹³ to 1 with stable isotopes. ³⁶Cl is produced in the atmosphere by spallation of ³⁶Ar by interactions with cosmic ray protons. In the subsurface environment, ³⁶Cl is generated primarily as a result of neutron capture by ³⁵Cl or muon capture by ⁴⁰Ca. ³⁶Cl decays to ³⁶S and to ³⁶Ar, with a combined half-life of 308,000 years. The half-life of this hydrophilic nonreactive isotope makes it suitable for geologic dating in the range of 60,000 to 1 million years. Additionally, large amounts of ³⁶Cl were produced by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958. The residence time of ³⁶Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in soil and ground water, ³⁶Cl is also useful for dating waters less than 50 years before the present. ³⁶Cl has seen use in other areas of the geological sciences, including dating ice and sediments.

References

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