The Road to the Neutrino

The usual potted history of the neutrino hypothesis that we find in textbooks goes something like this: Radioactivity—the spontaneous transformation of one element into another—produces α particles, or β particles, or γ rays. Experimental work on the energy of the electrons emitted in β decay began early in the 20th century, and the observations posed a problem: If there were only two bodies (the daughter nucleus and an electron) in the final state of a β decay, the conservation of energy and momentum would require that the spectrum of decay electrons must be monoenergetic. Thus, the observation of a continuous spectrum—electrons emitted with all energies from zero up to a maximum that depended on the radioactive element—cast doubt on both of these conservation laws. Or perhaps the electrons lost varying amounts of energy in escaping the radioactive substance, thus accounting for the continuous energy spectrum. But careful experiments showed that this was not the case. So the problem persisted. In the early 1930s, Wolfgang Pauli suggested that an undetected neutral particle of low mass was also emitted in β decay. Enrico Fermi dubbed this putative particle the “neutrino.” That solved the problem of the continuous spectrum, because, in a three‐body decay, the energy of the electron is no longer required to be unique. The energy and momentum conservation laws were saved.