Radioactive decay processes
Beta decay is a type of radioactive decay in which a beta particle (an electron or a positron) is emitted. In the case of electron emission, it is referred to as "beta minus"(β- ), in the case of a positron, "beta plus"(β+ ).
The study of beta decay provided the first physical evidence of the neutrino. The energies of electrons emitted by beta decay were observed to be non-continuous (some being more energetic than others). A problem arose in trying to explain what happened to the missing energy if an electron was emitted with less than maximum energy - the Law of conservation of energy appeared to be violated. To solve this, Wolfgang Pauli proposed that the "missing" energy was carried away by another yet undiscovered particle - the neutrino. This was analysed in more detail by Enrico Fermi.