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 36
Ar 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 40
Ca. 36Cl decays to either 36
S (1.9%) or to 36
Ar (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 by
neutron capture is an inevitable consequence of using natural isotope mixtures of chlorine (i.e. Those containing ). This produces a long lived radioactive product which has to be stored or disposed of.
Isotope separation to produce pure can vastly reduce production, but a small amount might still be produced by (n,2n) reactions involving
fast neutrons. ==Chlorine-37==