Isotopes of chlorine

Chlorine-36

Trace amounts of radioactive ³⁶Cl exist in the environment, in a ratio of about 7×10⁻¹³ 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 either ³⁶S (1.9%) or to ³⁶Ar (98.1%), 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 neutron 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, forecasts, and elements. In chloride-based molten salt reactors the production of ³⁶
Cl by neutron capture is an inevitable consequence of using natural isotope mixtures of chlorine (i.e. Those containing ³⁵
Cl). This produces a long lived radioactive product which has to be stored or disposed off. Isotope separation to produce pure ³⁷
Cl can vastly reduce ³⁶
Cl production, but a small amount might still be produced by (n,2n) reactions involving fast neutrons.

Chlorine-37

Stable chlorine-37 makes up about 24.23% of the naturally occurring chlorine on earth. Variation occurs as chloride mineral deposits have a slightly elevated chlorine-37 balance over the average found in sea water and halite deposits.