Alpha particle

Alpha particle (α-, or alpha ray), one of the particles emitted in radioactive decay. It is identical with the nucleus of the helium atom, consisting of 2 protons plus 2 neutrons bound together. A moving alpha particle is strongly ionizing and so loses energy rapidly in traversing through matter. Natural alpha particles will traverse only a few centimeters of air before coming to rest.

See also: Radiation; Radioactivity; Nuclear energy.