AlphaEmission

Alpha emission, also known as alpha decay, is a form of radioactive decay in which an unstable atomic nucleus releases an alpha particle, a helium-4 nucleus consisting of two protons and two neutrons. This process reduces the parent nucleus by four mass units and by two atomic numbers, producing a lighter daughter nuclide. For example, uranium-238 decays to thorium-234 via alpha emission: 238U → 234Th + 4He.

Mechanism and energetics: The alpha particle is pre-formed in the nucleus and must overcome the Coulomb barrier to escape. It does so primarily by quantum tunneling, a probabilistic process that depends on the barrier and the energy of the emitted particle. The decay rate varies widely among isotopes, giving half-lives from microseconds to many billions of years. The energy released, the Q-value, is typically a few MeV and is carried mainly by the alpha particle. The Geiger–Nuttall relation describes the empirical link between the alpha energy and the decay constant.

Penetration and safety: Alpha particles have high ionizing power but very short range in matter, being stopped by a sheet of paper or a few centimeters of air. They pose little external hazard but can cause significant internal damage if alpha-emitting materials are ingested or inhaled. Detection and measurement are common with specialized detectors; common alpha sources include radium, polonium, and americium-241, the latter used in some smoke detectors.

Applications and significance: Alpha decay explains the transmutation of heavy elements and contributes to radiometric dating methods, such as uranium–lead and related decay chains. Understanding alpha emission informs nuclear physics, radiation safety, and various industrial and scientific applications.