Argon-37 2026-11-05
Argon-37 is a radioactive isotope with protons and neutrons. It undergoes electron capture to chlorine-37, with an approximately -day half-life:
The mass number is unchanged while the nuclear charge decreases by one. Atomic vacancies left by the captured electron can produce detectable low-energy radiation during atomic relaxation.
Argon-37 made from chlorine was the counted product in the radiochemical solar-neutrino experiment. The method separated rare radioactive argon atoms from a much larger chlorine target, connecting nuclear counting to a very weak astronomical signal.[ref]
Raymond Davis Jr. Created 2026-10-18 Updated 2026-11-05
Raymond Davis Jr. developed a chemical method for observing solar neutrinos. In the chlorine experiment, rare interactions converted chlorine-37 into radioactive argon-37; extraction and counting of the argon atoms made a very weak interaction observable. The unexpectedly low measured rate became a central part of the solar neutrino problem.[ref]