Wiktionary
chlorine-37
n. (context isotope English) The minor stable isotope of chlorine, (nuclide 37 17 Cl), having seventeen protons and twenty neutrons; it amounts to about 24% of the element in nature
Wikipedia
Chlorine-37
Chlorine-37, or , 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 24.23% of natural chlorine, chlorine-35 accounting for 75.77%, giving chlorine atoms in bulk an apparent atomic weight of .
Remarkably, the Solar neutrinos were discovered by an experiment using a radiochemical method based on Chlorine-37 transmutation.