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Chlorine-36
 
 
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Chlorine-36 is an isotope
Isotope

Isotopes are any of the different types of atoms of the same chemical element, each having a different atomic mass . Isotopes of an element have atomic nucleus with the same number of protons but different numbers of neutron....
 of chlorine
Chlorine

Chlorine...
. Chlorine has two stable isotope
Stable isotope

Stable isotopes are chemical Isotope that are not radioactive . By this definition, there are 256 known stable isotopes of the 80 elements which have one or more stable isotopes....
s and one radioactive environmental isotope
Environmental isotopes

The environmental isotopes are a subset of the isotopes, both Stable isotopes and Radioactive isotopes, which are the object of Isotope geochemistry....
: the cosmogenic isotope 36Cl. The ratio of 36Cl to stable 37Cl in the environment is ~700 × 10-15. Its half-life is 301,000 ± 4,000 years. The long half-life
Half-life

The half-life of a quantity whose value decreases with time is the interval required for the quantity to decay to half of its initial value. The concept originated in describing how long it takes atoms to undergo radioactive decay but also applies in a wide variety of other situations....
 of 36Cl makes it useful to date groundwater
Groundwater

Groundwater is water located beneath the ground surface in soil porosity spaces and in the fractures of lithologic formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water....
 up to one million years old. There also are observed natural variations in 37Cl.






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Chlorine-36 is an isotope
Isotope

Isotopes are any of the different types of atoms of the same chemical element, each having a different atomic mass . Isotopes of an element have atomic nucleus with the same number of protons but different numbers of neutron....
 of chlorine
Chlorine

Chlorine...
. Chlorine has two stable isotope
Stable isotope

Stable isotopes are chemical Isotope that are not radioactive . By this definition, there are 256 known stable isotopes of the 80 elements which have one or more stable isotopes....
s and one radioactive environmental isotope
Environmental isotopes

The environmental isotopes are a subset of the isotopes, both Stable isotopes and Radioactive isotopes, which are the object of Isotope geochemistry....
: the cosmogenic isotope 36Cl. The ratio of 36Cl to stable 37Cl in the environment is ~700 × 10-15. Its half-life is 301,000 ± 4,000 years. The long half-life
Half-life

The half-life of a quantity whose value decreases with time is the interval required for the quantity to decay to half of its initial value. The concept originated in describing how long it takes atoms to undergo radioactive decay but also applies in a wide variety of other situations....
 of 36Cl makes it useful to date groundwater
Groundwater

Groundwater is water located beneath the ground surface in soil porosity spaces and in the fractures of lithologic formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water....
 up to one million years old. There also are observed natural variations in 37Cl. Most such variations in 37Cl values of hydrologic systems are related to diffusion processes.

Chlorine-36 is produced in the upper atmosphere through spallation
Spallation

In general, spallation is a process in which fragments of material are ejected from a body due to impact or stress. In nuclear physics, it is the process in which a heavy nucleus emits a large number of nucleons as a result of being hit by a high-energy Elementary_particle, thus greatly reducing its atomic weight....
 reactions and in solid materials on the surface of the earth in three ways: spallation reactions, muon
Muon

The muon is an elementary particle similar to the electron, with negative electric charge and a spin of . Together with the electron, the tau lepton, and the three neutrinos, it is classified as a lepton....
 reactions and thermal neutron absorption
Neutron capture

Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus....
. Spallation reactions also occur when gamma ray
Gamma ray

Gamma rays are a form of electromagnetic radiation produced by atom particle interactions, such as electron-positron annihilation or radioactive decay....
s interact with minerals in the top several meters of the Earth's surface. It can also be produced through muon reactions and thermal neutron absorption. 36Cl was also produced during nuclear bomb testing in the middle of the 20th century.

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
Spallation

In general, spallation is a process in which fragments of material are ejected from a body due to impact or stress. In nuclear physics, it is the process in which a heavy nucleus emits a large number of nucleons as a result of being hit by a high-energy Elementary_particle, thus greatly reducing its atomic weight....
 of 36Ar
Argon

Argon is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table ....
 by interactions with cosmic ray
Cosmic ray

Cosmic rays are energetic particles originating from space that impinge on Earth's atmosphere. Almost 90% of all the incoming cosmic ray particles are protons, about 9% are helium nuclei and about 1% are electrons ....
 proton
Proton

The proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H+....
s. In the subsurface environment, 36Cl is generated primarily as a result of neutron capture
Neutron capture

Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus....
 by 35Cl or muon capture
Muon capture

Muon capture is the capture of a negative muon by a proton, usually resulting in production of a neutron and a neutrino, and sometimes a gamma ray photon....
 by 40Ca
Calcium

Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the earth's Crust ....
. 36Cl decays to 36S
Sulfur

Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant Valence non-metal....
 and to 36Ar
Argon

Argon is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table ....
, with a combined half-life
Half-life

The half-life of a quantity whose value decreases with time is the interval required for the quantity to decay to half of its initial value. The concept originated in describing how long it takes atoms to undergo radioactive decay but also applies in a wide variety of other situations....
 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 irradiation of seawater
Seawater

Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5%, or 35 parts per thousand . This means that every 1 kg of seawater has approximately 35 grams of sea salt ....
 during atmospheric detonations of nuclear weapon
Nuclear weapon

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either nuclear fission or a combination of fission and nuclear fusion....
s 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
Soil

Soil is the naturally occurring, unconsolidated or loose covering on the Earth's surface. Soil is composed of particles of broken rock that have been altered by chemical and environmental processes including weathering and erosion....
 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, including dating ice and sediments.