After his defection to the Soviet Union in 1950, he worked at the Joint Institute for Nuclear Research (JINR) in Dubna. He had proposed using chlorine to detect neutrinos. In a 1959 paper, he argued that the electron neutrino () and the muon neutrino () were different particles. Solar neutrinos were detected by the Homestake Experiment, but only between one third and one half of the predicted number were found. In response to this solar neutrino problem, he proposed a phenomenon known as neutrino oscillation, whereby electron neutrinos became muon neutrinos. The existence of the oscillations was finally established by the Super-Kamiokande experiment in 1998. He also predicted in 1958 that supernovae would produce intense bursts of neutrinos, which was confirmed in 1987 when Supernova SN1987A was detected by neutrino detectors.