alpha particle

An alpha particle is a type of particle emitted during nuclear reactions: radioactive decay emitting such a particle is termed alpha decay. It specifically consists of two protons and two neutrons, i.e., it is a helium-4 nucleus. When produced by such decay, it has considerable velocity relative to the source, thus considerable kinetic energy (on the order of 5 MeV).

This is in contrast to a beta particle, similarly emitted, but consisting of an electron or positron. Collisions of alpha or beta particles with a nucleus can also create nuclear reactions, e.g., alpha capture. Cosmic rays include alpha and beta particles.