Electron and Antineutrino
Early studies of beta decay revealed a continuous energy spectrum up to a maximum, unlike the predictable energy of alpha particles. Another anomaly was the fact that the nuclear recoil was not in the the direction opposite the momentum of the electron. The emission of another particle was a probable explanation of this behavior, but searches found no evidence of either mass or charge. The interesting history has Wolfgang Pauli in 1930 proposing an as yet unobserved particle to explain the continuous distribution of energy of the emitted electrons. Then Enrico Fermi called this particle a neutrino and developed a theory of beta decay in which the neutrino carried away the missing energy and momentum. With no charge and almost no mass, it was hard to detect, and not until 1956 was experimental detection of the neutrino achieved. For symmetry reasons, the particle emitted along with the electron from nuclei is called an antineutrino. The emission of a positron is accompanied by a neutrino.
Beta decay concepts