Chlorine-36

Chlorine-36 (³⁶Cl) is a radioactive isotope of chlorine whose half-life is 301,000 years; it decays primarily (98%) by beta-minus decay to ³⁶Ar, and the balance by electron capture to ³⁶S. This cosmogenic isotope occurs in natural chlorine alongside the two stable isotopes.

Trace amounts of radioactive ³⁶Cl exist in the environment, in a ratio of about to 1 with respect to the stable chlorine isotopes. This ³⁶Cl/Cl ratio is sometimes abbreviated as R³⁶Cl. This corresponds to a concentration of approximately .

³⁶Cl is produced in the atmosphere by spallation of ³⁶Ar by interactions with cosmic ray protons. In the top meter of the lithosphere, ³⁶Cl is generated primarily by thermal neutron activation of ³⁵Cl and spallation of ³⁹K and ⁴⁰Ca. In the subsurface environment, muon capture by ⁴⁰Ca becomes more important. The production rates from spallation in rocks at sea level are about 4200 atoms ³⁶Cl/yr/mole ³⁹K and 3000 atoms ³⁶Cl/yr/mole ⁴⁰Ca.

The half-life of this isotope makes it suitable for geologic dating in the range of 60,000 to 1 million years. Its properties make it useful as a proxy data source to characterize cosmic particle bombardment and solar activity of the past.

Additionally, large amounts of ³⁶Cl were produced by irradiation of seawater during atmospheric and underwater test detonations of nuclear weapons between 1952 and 1958. The residence time of ³⁶Cl in the atmosphere is about 2 years. It is as an event marker of 1950s water in soil and ground water. ³⁶Cl has seen use in other areas of the geological sciences, including dating ice and sediments.

See also

• Isotopes of chlorine

References