ABSTRACT
Radioactivity, the spontaneous transformation of one element into another, produces a particles (positively charged helium nuclei), or β particles (electrons), or γ rays (high-energy electromagnetic radiation). Experimental work on the energy of the electrons emitted in β decay began in the early twentieth century, and the observations posed a problem. If β decay were a two-body process (for example, neutron decays to proton + electron, or n → p + e), then applying the laws of conservation of energy and conservation of momentum requires that the electron emitted be mono energetic. Thus the observation that electrons were emitted with all energies from zero up to a maximal energy that depended on the radioactive element—a continuous energy spectrum—cast doubt on both of these conservation laws. Physicists speculated that perhaps the electrons lost energy in escaping the substance, with different electrons losing different amounts of energy, thus accounting for the continuous energy spectrum. Careful experiments showed that this was not the case, so the problem remained. In the early 1930s Wolfgang Pauli suggested that a low-mass neutral particle, named by Enrico Fermi the neutrino, was also emitted in β decay. This solved the problem of the continuous energy spectrum because in a three-body decay (neutron → proton + electron + neutrino) the energy of the electron was no longer required to be unique. The electron could have a continuous energy spectrum, and the conservation laws were saved.
