Isotopes of chlorine

List of isotopes

|-id=Chlorine-28 | 28Cl | 28.03035(54)# | | p | 27S | 1+# | | |-id=Chlorine-29 | 29Cl | 29.01505(20)# | 5.4(19) zs | p | 28S | (1/2+) | | |-id=Chlorine-30 | 30Cl | 30.005018(26) | | p | 29S | 3+# | | |-id=Chlorine-31 | β+ (97.6%)

| | |-id=Chlorine-34 | 34Cl | 33.973762490(52) | 1.5267(4) s | β+ | 34S | 0+ | | |-id=Chlorine-34m | β+ (55.4%)

| |- | β− (98.1%)

| |-id=Chlorine-38 | 38Cl | 37.96801041(11) | 37.230(14) min | β− | 38Ar | 2− | | |-id=Chlorine-38m | 715(3) ms | IT | 38Cl | 5− | | |-id=Chlorine-39 | 39Cl | 38.9680082(19) | 56.2(6) min | β− | 39Ar | 3/2+ | | |-id=Chlorine-40 | 40Cl | 39.970415(34) | 1.35(3) min | β− | 40Ar | 2− | | |-id=Chlorine-41 | 41Cl | 40.970685(74) | 38.4(8) s | β− | 41Ar | (1/2+) | | |-id=Chlorine-42 | β−

Stable isotope analysis

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. 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 have a 37Cl/35Cl ratio of 0.319627 ± 0.000199 (24.221% ± 0.0015% chlorine-37) 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.

Chlorine-36

Main article: Chlorine-36

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 of 36Ar by interactions with cosmic ray protons. 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 by neutron capture is an inevitable consequence of using natural isotope mixtures of chlorine (i.e. Those containing ). This produces a long lived radioactive product which has to be stored or disposed off. Isotope separation to produce pure can vastly reduce production, but a small amount might still be produced by (n,2n) reactions involving fast neutrons.

Chlorine-37

Main article: Chlorine-37

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.

References

References

1. Mukha, I.. (2018). "Deep excursion beyond the proton dripline. I. Argon and chlorine isotope chains". Physical Review C.
2. M. Zreda. (1991). "Cosmogenic chlorine-36 production rates in terrestrial rocks". Earth and Planetary Science Letters.
3. 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.
4. (2023-08-18). "β⁻ decay of neutron-rich ⁴⁵Cl located at the magic number N=28". American Physical Society (APS).
5. [https://academic.oup.com/mnras/article/508/1/1020/6308835 Chlorine-bearing species and the 37Cl/35Cl isotope ratio ...] - Figure 8. This is the only source I find giving the actual composition of SMOC (as the isotope ratio). There surely should be a better one.