Chlorine-40

Main isotopes of chlorine

Iso­tope Decay abun­dance half-life (t1/2) mode pro­duct 35Cl 76% stable 36Cl trace 3.01×105 y β− 36Ar ε 36S 37Cl 24% stable
Standard atomic weight (Ar, standard)	[35.446, 35.457] Conventional: 35.45

Chlorine (₁₇Cl) has 24 isotopes with mass numbers ranging from ²⁸Cl to ⁵¹Cl and 2 isomers (^34mCl and ^38mCl). There are two principal stable isotopes, ³⁵Cl (75.78%) and ³⁷Cl (24.22%), giving chlorine a standard atomic weight of 35.45. The longest-lived radioactive isotope is ³⁶Cl, which has a half-life of 301,000 years. All other isotopes have half-lives under 1 hour, many less than one second. The shortest-lived are ²⁹Cl and ³⁰Cl, with half-lives less than 20 and 30 nanoseconds, respectively—the half-life of ²⁸Cl is unknown.

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 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.

...
Wikipedia