Chlorine-37
Encyclopedia
Chlorine-37, or , is one of the stable isotope
Stable isotope
Stable isotopes are chemical isotopes that may or may not be radioactive, but if radioactive, have half-lives too long to be measured.Only 90 nuclides from the first 40 elements are energetically stable to any kind of decay save proton decay, in theory...

s of chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

, the other being chlorine-35 . Its nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...

 contains 17 protons and 20 neutrons for a total of 37 nucleon
Nucleon
In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...

s. Chlorine-37 accounts for 24.23% of natural chlorine, chlorine-35 accounting for 75.77%, giving chlorine atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s in bulk an apparent atomic weight of .

Remarkably, the Solar neutrinos were discovered by an experiment using a radiochemical method based on Chlorine-37 transmutation.

Neutrino detection

One of the historically important radiochemical methods of solar neutrino detection
Neutrino detector
A neutrino detector is a physics apparatus designed to study neutrinos. Because neutrinos are only weakly interacting with other particles of matter, neutrino detectors must be very large in order to detect a significant number of neutrinos. Neutrino detectors are often built underground to isolate...

 is based on inverse electron capture
Electron capture
Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...

  triggered by the absorption of an electron neutrino
Electron neutrino
The electron neutrino is a subatomic lepton elementary particle which has no net electric charge. Together with the electron it forms the first generation of leptons, hence its name electron neutrino...

. Chlorine-37 transmutes into Argon-37 via the reaction
+ → +


Argon-37 then de-excites itself via electron capture
Electron capture
Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...

 (half-life = 35 d) into chlorine-37 via the reaction
+ → +


These last reactions involve Auger electron
Auger electron
The Auger effect is a physical phenomenon in which the transition of an electron in an atom filling in an inner-shell vacancy causes the emission of another electron. When a core electron is removed, leaving a vacancy, an electron from a higher energy level may fall into the vacancy, resulting in...

s of specific energies. The detection of these electrons confirms that a neutrino event took place. Detection methods usually involve several thousand liters of carbon tetrachloride
Carbon tetrachloride
Carbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent...

 (CCl4) or tetrachloroethylene
Tetrachloroethylene
Tetrachloroethylene, also known under its systematic name tetrachloroethene and many other names, is a chlorocarbon with the formula Cl2C=CCl2. It is a colourless liquid widely used for dry cleaning of fabrics, hence it is sometimes called "dry-cleaning fluid." It has a sweet odor detectable by...

 (C2Cl4) stored in underground tanks.

Occurrence

The representative terrestrial abundance of chlorine-37 is 24.22(4)% of chlorine atoms, with a normal range of 24.14–24.36% of chlorine atoms. When measuring deviations in isotopic composition, the usual reference point is "Standard Mean Ocean Chloride" (SMOC), although a NIST Standard Reference Material (975a) also exists. SMOC is known to be around 24.219% chlorine-37 and to have an atomic weight of around 35.4525

There is a known variation in the isotopic abundance of chlorine-37. This heavier isotope tends to be more prevalent in chloride minerals than in aqueous solutions such as sea water, although the isotopic composition of organochlorine compounds can vary in either direction from the SMOC standard in the range of several parts per thousand.
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