Chlorine-37

Chlorine-37 (³⁷
Cl
), is one of the stable isotopes of chlorine, the other being chlorine-35 (³⁵
Cl
). Its nucleus contains 17 protons and 20 neutrons for a total of 37 nucleons. Chlorine-37 accounts for 24.23% of natural chlorine, chlorine-35 accounting for 75.77%, giving chlorine atoms in bulk an apparent atomic weight of 35.45(1) g/mol.^[1]

Chlorine-37, ³⁷Cl

General
Symbol	37Cl
Names	chlorine-37, 37Cl, Cl-37
Protons (Z)	17
Neutrons (N)	20
Nuclide data
Natural abundance	24.23%
Isotope mass	36.965903 Da
Isotopes of chlorine Complete table of nuclides

Remarkably, solar neutrinos were discovered by an experiment (Homestake Experiment) using a radiochemical method based on chlorine-37 transmutation.^[2]

Neutrino detection

Main article: Homestake experiment

One of the historically important radiochemical methods of solar neutrino detection is based on inverse electron capture triggered by the absorption of an electron neutrino.^[3] Chlorine-37 transmutes into argon-37 via the reaction^[4]

³⁷
Cl
+
ν
_e → ³⁷
Ar
+
e⁻
.

Argon-37 then decays via electron capture (half-life 35 d) into chlorine-37 via the reaction

³⁷
Ar
+
e⁻
→ ³⁷
Cl
+
ν
_e.

These last reactions involve Auger electrons of specific energies.^[3][5] The detection of these electrons confirms that a neutrino event occurred. Detection methods involve several hundred thousand liters of carbon tetrachloride (CCl₄) or tetrachloroethylene (C₂Cl₄) stored in underground tanks.^[2][3][6]

Occurrence

The representative terrestrial abundance of chlorine-37 is 24.22(4)% of chlorine atoms,^[7] 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.^[8]

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 seawater, although the isotopic composition of organochlorine compounds can vary in either direction from the SMOC standard in the range of several parts per thousand.^[8]

See also

• Beta decay
• Neutrino detection
• Isotopic tracer
• Isotopes of chlorine

References

1. Prohaska, Thomas; et al. (2022-05-25), "Standard atomic weights of the elements 2021 (IUPAC Technical Report)", Pure and Applied Chemistry, vol. 94, no. 5, pp. 573–600, doi:10.1515/pac-2019-0603, ISSN 0033-4545, retrieved 2024-06-07{{citation}}: CS1 maint: date and year (link)
2. J.N. Bahcall (1969). "Neutrinos from the Sun". Scientific American. 221 (1): 28–37. Bibcode:1969SciAm.221a..28B. doi:10.1038/scientificamerican0769-28.
3. Sutton, Christine (1992). Spaceship Neutrino. Cambridge University Press. pp. 151–152. ISBN 978-0-521-36404-1. OCLC 25246163. chlorine-37 neutrino.
4. F.H. Shu (1982). The Physical Universe: An Introduction to Astronomy. University Science Books. p. 122. ISBN 978-0-935702-05-7. chlorine 37 neutrino.
5. A.H. Snell, F. Pleasonton (1955). "Spectrometry of the Neutrino Recoils of Argon-37". Physical Review. 100 (5): 1396–1403. Bibcode:1955PhRv..100.1396S. doi:10.1103/PhysRev.100.1396.
6. A. Bhatnagar, W. Livingston (2005). Fundamental of Solar Astronomy. World Scientific. pp. 87–89. ISBN 978-981-238-244-3.
7. Rosman, K. J. R.; Taylor, P. D. P. (1998). "Isotopic Compositions of the Elements 1997" (PDF). Pure and Applied Chemistry. 70 (1): 217–235. doi:10.1351/pac199870010217.
8. de Laeter, J. R.; et al. (2003), "Atomic Weights of the Elements: Review 2000", Pure and Applied Chemistry, 75 (6): 683–800, doi:10.1351/pac200375060683