Positronemissio

Positronemissio, in English often called beta-plus decay, is a radioactive decay process in which a proton in an atomic nucleus is transformed into a neutron, emitting a positron and an electron neutrino. The result is a daughter nucleus with atomic number Z−1 and the same mass number A. This decay occurs in proton-rich nuclei that lie above the valley of stability.

The process requires that the mass difference between the parent and daughter nuclei provide at least the energy needed to create the positron. In practice, the Q-value for positron emission must exceed about 1.022 MeV (two electron masses) for the decay to be energetically allowed. The emitted positron carries away kinetic energy and the neutrino carries away remaining energy and momentum. If the nucleus is sufficiently energetic or has a favorable configuration, positron emission competes with other decay modes such as electron capture and beta-minus decay.

After emission, the positron quickly loses energy through interactions with matter and eventually encounters an electron, where it annihilates. The annihilation typically produces two gamma photons of 511 keV emitted in nearly opposite directions, a signature exploited in imaging techniques. The range of the positron before annihilation depends on the surrounding material and the positron’s initial energy.

Positronemissio occurs in several medically relevant isotopes, including fluorine-18, nitrogen-13, carbon-11, and oxygen-15, which are commonly used in positron emission tomography (PET). These isotopes have half-lives suitable for medical procedures and are produced in cyclotrons or generator systems. The method provides functional imaging by mapping metabolic processes in living tissue.