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During the beta decay of a nucleus, a neutron is converted to a proton and an electron is produced. In studying the electron’s velocity, it was clear that this decay violated energy-conversation and other laws. It was thus proposed that an additional particle—a neutrino—was formed during beta decay. In the early 1950s, Frederick Reines passed radiation from a nuclear reactor through a water tank and discovered reactions that proved the neutrino’s existence.