PositronEmission

Positron emission, also known as beta-plus decay, is a radioactive decay process in which a proton in an unstable nucleus is transformed into a neutron, releasing a positron and an electron neutrino. As a result, the atomic number decreases by one while the mass number remains the same. The reaction is energetically allowed when the mass difference between parent and daughter exceeds 1.022 MeV, the rest mass of the emitted positron.

After emission, the fast positron loses energy in surrounding matter and soon encounters an electron, annihilating to produce two gamma photons of 511 keV emitted in nearly opposite directions. This annihilation radiation is exploited in positron emission tomography (PET) to produce images of radiotracer distribution and tissue metabolism.

Positron emitters are typically proton-rich nuclides. Common medically relevant isotopes include fluorine-18, carbon-11, nitrogen-13, oxygen-15, and rubidium-82. These are used to label biologically active molecules (radiopharmaceuticals); the best known example is fluorine-18 labeled fluorodeoxyglucose (FDG), used to image glucose metabolism.

Production and use: Isotopes are produced in cyclotrons or on-site generators; their relatively short half-lives require rapid synthesis, delivery, and imaging. In clinical PET, detectors measure coincident 511 keV photons to reconstruct quantitative images of tracer concentration.

Safety and history: Positron emission was discovered in the 1930s. As a form of radioactivity, it involves ionizing radiation; radiopharmaceuticals are chosen to balance diagnostic benefit with radiation dose and are subject to regulatory controls.