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An alpha particle (also termed alpha radiation or alpha rays) is a helium nucleus stripped of its orbital electrons and which is emitted from a radioactive atom with a velocity of about 1/20 that of the speed of light and with energies ranging from 4 to 9 MeV. The alpha particle was the first nuclear radiation to be discovered; beta particles and gamma rays were identified soon thereafter.

Alpha particles cause ionizations in matter when they are deflected by the positive charge of a nucleus and pull the orbital electrons (attracted by the alpha’s positive charge) along with them. Alpha particles also cause excitation along their path by pulling inner orbital electrons to outer orbits. Energy is then given off by the atom as fluorescent radiation (low energy x-rays) when the electrons drop back down to the inner orbital vacancies.

Because of its relatively large mass (2 neutrons and 2 protons), high electrical charge (2+) and low velocity, the specific ionization of an alpha particle is high and, as a result, it creates many ion pairs in a very short path length. Because of this, it loses all of its energy in a short distance. The range in air is only several centimeters even for the most energetic alpha particles.

Alpha particles are highly ionizing because of the double positive charge, large mass (compared to a beta particle), and because they are relatively slow. They can cause multiple ionizations within a short distance which gives alpha particles the potential to do much more biological damage for the same amount of deposited energy. Alpha particles cannot penetrate the normal layer of dead cells on the outside of our skin but can damage the cornea of the eye. Alpha-particle radiation is normally only a safety concern if the radioactive decay occurs from an atom that is already inside the body or a cell. Alpha-particle emitters are particularly dangerous if inhaled, ingested, or if they enter a wound. Once inside the body, surrounded by living tissue, damage will be to the local area in which the alpha emitter is deposited. Thus, alpha emitters are an internal hazard and intake to the body must be prevented.

See also: Beta Decay, Beta Particle, Gamma Decay, Nuclear Energy.