Positron emission, beta plus decay, or β decay is a subtype of radioactive decay called beta decay, in which a proton inside a radionuclide nucleus is converted into a neutron while releasing a positron and an electron neutrino (). Positron emission is mediated by the weak force. The positron is a type of beta particle (β), the other beta particle being the electron (β) emitted from the β decay of a nucleus.
An example of positron emission (β decay) is shown with magnesium-23 decaying into sodium-23:
Because positron emission decreases proton number relative to neutron number, positron decay happens typically in large "proton-rich" radionuclides. Positron decay results in nuclear transmutation, changing an atom of one chemical element into an atom of an element with an atomic number that is less by one unit.
Positron emission occurs only very rarely naturally on earth, when induced by a cosmic ray or from one in a hundred thousand decays of potassium-40, a rare isotope, 0.012% of that element on earth.
Positron emission should not be confused with electron emission or beta minus decay (β decay), which occurs when a neutron turns into a proton and the nucleus emits an electron and an antineutrino.
Positron emission is different from proton decay, the hypothetical decay of protons, not necessarily those bound with neutrons, not necessarily through the emission of a positron, and not as part of nuclear physics, but rather of particle physics.