James Chadwick’s 1932 nuclear experiment discovered the neutron.
Chadwick bombarded beryllium with alpha particles from polonium. The resulting radiation knocked protons out of a paraffin-wax target with unexpectedly great energy. Chadwick concluded that the radiation consisted of electrically neutral particles with roughly the mass of protons.
The discovery solved a major problem in nuclear physics. Atomic nuclei were known to contain positive charge, but their measured masses could not be explained by protons alone without creating contradictions. Neutrons supplied mass without adding electric charge.
A neutron is not the same as a neutrino: neutrons are heavy particles found in atomic nuclei, whereas neutrinos are extremely light leptons. Chadwick received the 1935 Nobel Prize in Physics for the discovery.