Chlorine (17Cl) has two stable isotopes, 35Cl (75.8%) and 37Cl (24.2%), giving chlorine a standard atomic weight of 35.45. Artificical radioisotopes are known ranging from 28Cl to 52Cl, and there are also two isomers, 34mCl and 38mCl. The longest-lived radioactive isotope is 36Cl, which has a half-life of 301,000 years. All other isotopes and isomers have half-lives under an hour, and most under 10 seconds.
↑ ()– Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
↑ #– Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
1 2 #– Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
The representative terrestrial abundance of chlorine contains about is 24.2% of 37Cl, with a normal range of 23.9–24.5% of chlorine atoms.[2] When measuring deviations in isotopic composition, the usual reference point is "Standard Mean Ocean Chloride" (SMOC), although a NISTStandard Reference Material (975a) also exists. SMOC is known to have a 37Cl/35Cl ratio of 0.319627 ± 0.000199 (24.221% +/- 0.0015% chlorine-37)[9] and to have an atomic weight of 35.4525.
There is known variation in the isotopic abundance of chlorine. 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.[2]
Trace amounts of radioactive36Cl 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 36Ar by interactions with cosmic rayprotons. In the subsurface environment, 36Cl is generated primarily as a result of neutron capture by 35Cl or muon capture by 40Ca. 36Cl decays to either 36S (1.9%) or to 36Ar (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 36 Cl by neutron capture is an inevitable consequence of using natural isotope mixtures of chlorine (i.e. Those containing 35 Cl). This produces a long lived radioactive product which has to be stored or disposed off. Isotope separation to produce pure 37 Cl can vastly reduce 36 Cl production, but a small amount might still be produced by (n,2n) reactions involving fast neutrons.
Besides being a component of natural stable chlorine, the chief notability of this isotope is its use to detect solar neutrinos through inverse electron capture (producing the gas 37Ar). This was used in the first detection at the Homestake experiment. Subsequently gallium-71 was found more suitable for this purpose, and used in GALLEX/GNO and SAGE.
↑ Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3): 030003. doi:10.1088/1674-1137/abddaf.
↑ M. Sheppard and M. Herod (2012). "Variation in background concentrations and specific activities of 36Cl, 129I and U/Th-series radionuclides in surface waters". Journal of Environmental Radioactivity. 106: 27–34. doi:10.1016/j.jenvrad.2011.10.015. PMID22304997.
↑ Bhattacharya, Soumik; Tripathi, Vandana; Tabor, S. L.; Volya, A.; Bender, P. C.; Benetti, C.; Carpenter, M. P.; Carroll, J. J.; Chester, A.; Chiara, C. J.; Childers, K.; Clark, B. R.; Crider, B. P.; Harke, J. T.; Jain, R.; Liddick, S. N.; Lubna, R. S.; Luitel, S.; Longfellow, B.; Mogannam, M. J.; Ogunbeku, T. H.; Perello, J.; Richard, A. L.; Rubino, E.; Saha, S.; Shehu, O. A.; Unz, R.; Xiao, Y.; Zhu, Yiyi (2023-08-18). "β− decay of neutron-rich 45Cl located at the magic number N=28"(PDF). Physical Review C. 108 (2). American Physical Society (APS): 024312. doi:10.1103/physrevc.108.024312. ISSN2469-9985.
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