What is beta minus (β⁻) decay?

Beta minus (β⁻) decay is a type of radioactive decay in which a neutron inside an unstable nucleus is converted into a proton, with the emission of an electron (the beta particle) and an antineutrino. As a result, the atomic number of the nucleus increases by one, while the mass number remains unchanged.

Beta minus decay occurs when a neutron converts into a proton, by emitting an electron and an antineutrino, and increasing the atomic number by one while preserving mass number.

This type of decay occurs in nuclei that are neutron-rich, meaning they have too many neutrons relative to protons for stability. By converting a neutron into a proton, the nucleus moves toward a more stable neutron-to-proton ratio.

Beta minus decay produces a continuous spectrum of electron energies and is commonly seen in radionuclides used for therapy and some diagnostic applications.

Understanding the physics

In neutron-rich nuclei, the balance between neutrons and protons lies outside the band of stability. Although neutrons help offset electrostatic repulsion between protons, an excess of neutrons can make the nucleus energetically unstable.

During β⁻ decay, one of the neutrons transforms into a proton via the weak nuclear force. This transformation produces:

A proton (which remains in the nucleus),
An electron (the beta particle), and
An antineutrino.

The emitted electron carries kinetic energy and escapes the nucleus. Because the available decay energy is shared between the electron and the antineutrino, the emitted electrons have a continuous range of energies, rather than a single fixed energy.

Since a neutron becomes a proton, the atomic number increases by one, meaning the element changes to the next element in the periodic table. However, the total number of nucleons remains the same.

Many β⁻ emitters also leave the daughter nucleus in an excited state, which may subsequently undergo gamma emission to release excess energy.

Where this matters clinically

Beta minus decay is important in nuclear medicine because it is the basis for several therapeutic radionuclides, where emitted electrons deposit energy locally in tissues. It also explains why certain diagnostic radionuclides produce gamma photons following beta decay. Understanding β⁻ decay helps clarify radiation dose distribution and tissue effects.