In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta ray, and a respective neutrino are emitted from an atomic nucleus. By beta decay emission, a neutron is transformed into a proton by the emission of an electron, or conversely a proton is converted into a neutron by emission of a positron, thus changing the nuclide type. The beta particle and its associated neutrino do not exist within the nucleus prior to beta decay, but are created in the process. By the process of beta decay, an unstable atom obtains a more stable ratio of protons to neutrons. The stability of a nuclide to beta and other forms of decay is determined by its binding energy as a function of its numbers of protons and neutrons. The binding energies of all existing nuclides forms what is called the nuclear valley of stability. Beta decay is a consequence of the weak force, which is characterized by relatively lengthy decay times. Nucleons are composed of up or down quarks, and the weak force allows a quark to change type by the exchange of a W boson and the creation of an electron/antineutrino or positron/neutrino pair. For example: a neutron, composed of two down quarks and an up quark, decays to a proton composed of a down quark and two up quarks. Decay times for many nuclides that are subject to beta decay can be thousands of years.

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