In beta decay a nucleus emits an electron. The electron isn’t in the nucleus originally, but results from the change of a neutron into a proton. Studies of the energy of the emitted electron showed that, instead of having a single energy like an alpha has, the energies of the electrons were spread from near zero to a maximum energy. Early investigators recognized that this suggested that energy was not conserved in beta decay. Austrian physicist Wolfgang Pauli (1900-1958) proposed in 1930 that a second particle was emitted along with the electron. This particle had to be neutral and zero, or extremely small mass, and he named it the neutrino, or “little neutral one.” The neutrino wasn’t detected experimentally until 1956. We now know that the neutrino emitted in beta decay is actually an anti-neutrino. The question of its mass will be discussed in the chapter on “Unanswered Questions.”

When a nucleus undergoes beta decay the number of protons increases by one as the number of neutrons decrease by one. So, for example, carbon-14 (6 protons and 8 neutrons) becomes nitrogen-14 (7 protons and 7 neutrons) with the emission of the beta (electron) and an anti-neutrino.