Chlorine-36 (³⁶Cl) is an

isotope of chlorine Chlorine (17Cl) has 25 isotopes with mass numbers ranging from 28Cl to 52Cl and 2 isomers (34mCl and 38mCl). There are two stable isotopes, 35Cl (75.77%) and 37Cl (24.23%), giving chlorine a standard atomic weight of 35.45. The longest-lived rad ...

Chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...

has two stable isotopes and one naturally occurring

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...

isotope, the cosmogenic isotope ³⁶Cl. Its half-life is 301,300 ± 1,500 years. ³⁶Cl decays primarily (98%) by beta-minus decay to ³⁶ Ar, and the balance to ³⁶ S. Trace amounts of

³⁶Cl exist in the environment, in a ratio of about (7–10) × 10⁻¹³ to 1 with stable chlorine isotopes. This corresponds to a concentration of approximately 1 Bq/(kg Cl). ³⁶Cl is produced in the atmosphere by

spallation Spallation is a process in which fragments of material (spall) are ejected from a body due to impact or stress. In the context of impact mechanics it describes ejection of material from a target during impact by a projectile. In planetary p ...

of ³⁶ Ar by interactions with

cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

s. In the top meter of the lithosphere, ³⁶Cl is generated primarily by

thermal neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium wi ...

activation of ³⁵Cl and spallation of ³⁹ K and ⁴⁰ Ca. In the subsurface environment, muon capture by ⁴⁰ Ca becomes more important. The production rates are about 4200 atoms ³⁶Cl/yr/mole ³⁹K and 3000 atoms ³⁶Cl/yr/mole ⁴⁰Ca, due to spallation in rocks at sea level. 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 Proxy may refer to: * Proxy or agent (law), a substitute authorized to act for another entity or a document which authorizes the agent so to act * Proxy (climate), a measured variable used to infer the value of a variable of interest in climate re ...

data source to characterize cosmic particle bombardment and solar activity of the past. Additionally, large amounts of ³⁶Cl were produced by irradiation of

seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...

during atmospheric and underwater test detonations of

nuclear weapon A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bomb ...

s between 1952 and 1958. The residence time of ³⁶Cl in the atmosphere is about 2 years. Thus, as an event marker of 1950s water in

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

and

ground water Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...

, 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

{{reflist, 2 Isotopes of chlorine Environmental isotopes Radionuclides used in radiometric dating

See also

References