Alpha particle (alpha radiation, alpha ray)
Nuclear Physics
A positively charged particle (a Helium-4 nucleus) made up of two neutrons and two protons. It is the least penetrating of the three common forms of radiation, being stopped by a sheet of paper. It is not dangerous to living things unless the alpha-emitting substance is inhaled or ingested or comes into contact with the lens of the eye.
- — Glossary of Nuclear Science Terms retrieved 2014-05-20