Previous discovery of antineutrinos made the existence of neutrinos certain, but there was no experimental proof yet. To remedy, Reines set up a tank containing 100,000 gallons of tetrachloryethylene in a mine in South Dakota, left it exposed for several months and came back with definitive proof of argon (and helium) from chlorine reacting with neutrinos.
A visual chronology of Asimov's ~1500 scientific inventions and discoveries.
