Until the early part of the 20th century neutrinos were unknown. Only the study of nuclear decays revealed the existence of a yet unknown particle. Nuclear physicists studied three types of radioactivity: alpha (charged helium), beta (electrons), and gamma (high-energy light) radiation. They found that both the alpha and gamma spectrum emitted in nuclear decays were discrete. In 1930, however, James Chadwick [6] observed that the energy spectrum of electrons (the beta particle) emitted in nuclear beta-decay was continuous. This observation could not be explained at the time. In an attempt not to abandon the fundamental conservation law of energy Wolfgang Pauli [7] postulated a new particle to explain Chadwick’s observation. Three years later, Enrico Fermi [8] called this new particle the neutrino and incorporated it in his theory of weak interaction.
Continuous energy spectrum of electrons (beta particles) emitted in nuclear beta decay.