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 (7–10) × 10−13 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 1 Bq/(kg Cl).

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.