What is beta plus (β⁺) decay?
Beta plus (β⁺) decay, also known as positron emission, is a type of radioactive decay in which a proton inside an unstable nucleus is converted into a neutron, with the emission of a positron (the beta plus particle) and a neutrino. As a result, the atomic number decreases by one, while the mass number remains unchanged.
Beta plus decay converts a proton into a neutron, emitting a neutrino and a positron that later annihilates to produce two 511 keV photons used in PET imaging.
This process occurs in proton-rich nuclei, where there are too many protons relative to neutrons for stability. By converting a proton into a neutron, the nucleus moves toward a more stable neutron-to-proton ratio.
Beta plus decay is the fundamental process underlying positron emission tomography (PET), as the emitted positron subsequently undergoes annihilation, producing two detectable gamma photons.
Understanding the physics
In proton-rich nuclei, electrostatic repulsion between protons becomes energetically unfavourable. To reduce this imbalance, one proton can transform into a neutron through the weak nuclear force.
During β⁺ decay, the proton converts into:
A neutron (which remains in the nucleus),
A positron (the antimatter counterpart of the electron), and
A neutrino.
Because mass must be conserved, β⁺ decay can only occur if the parent nucleus has sufficient excess energy. Specifically, at least 1.022 MeV of energy is required to create the positron–electron pair (twice the rest mass of an electron). For this reason, not all proton-rich nuclei are capable of positron emission; some instead undergo electron capture.
The emitted positron travels a short distance in tissue before encountering an electron. When they meet, they annihilate, producing two gamma photons of 511 keV, emitted in nearly opposite directions. These photons form the basis of PET imaging.
As with beta minus decay, the emitted positron has a continuous energy spectrum because the available decay energy is shared between the positron and the neutrino.
Where this matters clinically
Beta plus decay enables PET imaging. The annihilation photons produced following positron emission are detected in coincidence by PET scanners, allowing tomographic imaging of metabolic and molecular processes. The physical properties of β⁺ decay directly influence PET spatial resolution and radiation dose.