Chlorine-36

Chlorine-36 (36Cl) is an <a href="/facts/Isotope_of_chlorine/fdtzH9YF">isotope of chlorine</a>. <a href="/facts/Chlorine/y27UwYBM">Chlorine</a> has two <a href="/facts/Stable_isotope/vgcdxgjA">stable isotopes</a> and one naturally occurring <a href="/facts/Radioactive/Ya6FVjt4">radioactive</a> isotope, the <a href="/facts/Cosmogenic/TlJwc4wV">cosmogenic</a> isotope 36Cl. Its half-life is 301,300 ± 1,500 years. 36Cl decays primarily (98%) by beta-minus decay to 36<a href="/facts/Argon/QX6NRH6d">Ar</a>, and the balance to 36<a href="/facts/Sulfur/IK0amTfM">S</a>.
Trace amounts of <a href="/facts/Radioactive/Ya6FVjt4">radioactive</a> 36Cl exist in the environment, in a ratio of about (7–10) × 10−13 to 1 with respect to the stable chlorine isotopes. This 36Cl/Cl ratio is sometimes abbreviated as R36Cl. This corresponds to a concentration of approximately 1 Bq/(kg Cl).
36Cl is produced in the atmosphere by <a href="/facts/Cosmic_ray_spallation/gSlq9VEP">spallation</a> of 36<a href="/facts/Argon/QX6NRH6d">Ar</a> by interactions with <a href="/facts/Cosmic_ray/uxCVdTKd">cosmic ray</a> <a href="/facts/Proton/clCjepXP">protons</a>. In the top meter of the <a href="/facts/Lithosphere/1mqaxASd">lithosphere</a>, 36Cl is generated primarily by <a href="/facts/Thermal_neutron/UMKR4XgJ">thermal neutron</a> activation of 35Cl and spallation of 39<a href="/facts/Potassium/FWPwS7Rj">K</a> and 40<a href="/facts/Calcium/hf252CC0">Ca</a>. In the subsurface environment, <a href="/facts/Muon_capture/BEdvDAeL">muon capture</a> by 40Ca becomes more important. The production rates are about 4200 atoms 36Cl/yr/mole 39K and 3000 atoms 36Cl/yr/mole 40Ca, due to spallation in rocks at sea level.
The half-life of this isotope makes it suitable for <a href="/facts/Geologic_dating/U8hRTGwL">geologic dating</a> in the range of 60,000 to 1 million years. Its properties make it useful as a <a href="/facts/Proxy_(statistics)/W5V2eqaT">proxy</a> data source to characterize cosmic particle bombardment and solar activity of the past.
Additionally, large amounts of 36Cl were produced by irradiation of <a href="/facts/Seawater/fRBJjbws">seawater</a> during atmospheric and <a href="/facts/Underwater_explosion/NppW55Fw">underwater</a> <a href="/facts/Nuclear_weapons_testing/s2srGDFP">test detonations</a> of <a href="/facts/Nuclear_weapon/QVdBcMdm">nuclear weapons</a> between 1952 and 1958. The residence time of 36Cl in the atmosphere is about 2 years. Thus, as an event marker of 1950s water in <a href="/facts/Soil/5Ddpr3QM">soil</a> and <a href="/facts/Ground_water/JYM2IDS2">ground water</a>, 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, including dating ice and sediments.

Chlorine-36 open-in-new

Chlorine-36