radionuclide

A radionuclide is a nucleus of an atom that is radioactive, meaning it is unstable and undergoes radioactive decay to reach a more stable configuration. The term refers to the specific nuclide, defined by its atomic number and mass number, such as carbon-14, uranium-238, or radon-222. Radionuclides may be natural or artificial.

Decay leads to emission of particles or electromagnetic radiation: alpha particles, beta particles, gamma rays, or neutrons. Each radionuclide has a characteristic half-life, the time required for half of a sample to decay, ranging from fractions of a second to billions of years. The activity is measured in becquerels (disintegrations per second) or curies.

Natural radionuclides occur in the Earth's crust and atmosphere, including uranium-238, uranium-235, thorium-232, potassium-40, and radon-222. Artificial radionuclides are produced in nuclear reactors or accelerators, including technetium-99m, iodine-131, cesium-137, and lutetium-177.

Applications include medicine, where radionuclides are used for diagnosis with gamma emitters and PET isotopes and for therapy with beta-emitters. In industry they serve as tracers and for non-destructive testing, in research to study chemical pathways, and in power generation and environmental studies.

Safety and regulation: handling requires shielding, containment, monitoring for contamination, and limits on exposure. Regulatory frameworks govern licensing, waste management, and disposal, with radiation protection practices emphasizing time, distance, and shielding to minimize risk.