Beta decay

Mechanism and particles involved

Beta decay proceeds by the transformation of a nucleon within the nucleus. In beta minus (β−) decay a neutron becomes a proton while emitting an electron and an antineutrino. In beta plus (β+) decay a proton converts to a neutron, emitting a positron and a neutrino. Microscopically, these transformations are described by the exchange of W bosons in the weak interaction; the emitted neutrino or antineutrino carries away a portion of the decay energy and ensures conservation of lepton number.

Energy and spectra

Unlike the nearly discrete energies of alpha or gamma emission, the kinetic energy of beta particles follows a continuous spectrum because the available decay energy is shared between the beta particle and the neutrino. The shape of the spectrum and the maximum endpoint energy provide information about the nuclear transition and are used to identify isotopes and to test weak-interaction theory.

Types, selection rules and competing processes

Beta transitions are classified by changes in nuclear angular momentum and parity (allowed or forbidden), which affect the decay rate and spectrum shape. For proton-rich nuclei, positron emission may compete with electron capture, in which an orbital electron is captured by the nucleus and combines with a proton to form a neutron plus a neutrino. The relative likelihood of these channels depends on the available energy and nuclear structure.

History and significance

Measurements of continuous beta spectra led to the postulation of the neutrino to account for missing energy; this particle and the theory of beta decay were central to the development of modern weak-interaction physics. Today beta decay plays a major role in nuclear astrophysics (for example in nucleosynthesis pathways), radiometric dating techniques, and particle-physics experiments that probe neutrino properties.

Applications and examples

• Radiocarbon dating uses beta decay of isotopes produced in the atmosphere to date organic materials.
• Medical imaging and therapy utilize beta emitters: positron emitters are the basis of positron emission tomography (PET), while electron emitters are used in certain radiotherapies.
• Laboratory measurements of beta spectra and decay rates inform nuclear models and support searches for physics beyond the Standard Model.

For further background in the broader subjects that touch beta decay, see introductions to nuclear physics, fundamental reviews of radioactive decay, and resources on elementary particles such as the beta particle, electrons, positrons, neutrons, and protons.