Chlorine-37 (), is one of the stable isotopes of chlorine, the other being chlorine-35 (). Its nucleus contains 17 protons and 20 neutrons for a total of 37 nucleons. Chlorine-37 accounts for 24.22% of natural chlorine, with chlorine-35 the remaining 75.78%, giving chlorine in bulk an apparent atomic weight of .
Remarkably, solar neutrinos were discovered by an experiment (Homestake Experiment) using a radiochemical method based on chlorine-37 transmutation.
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. Chlorine-37 transmutes into argon-37 via the reaction
Argon-37 then decays via electron capture (half-life 35.01 days) into chlorine-37 via the reaction
The detection of these electrons confirms that a neutrino event occurred. Detection methods involve several hundred thousand liters of carbon tetrachloride (CCl<sub>4</sub>) or tetrachloroethylene (C<sub>2</sub>Cl<sub>4</sub>) stored in underground tanks.