Alpha particles or alpha rays are a form of particle radiation which are highly ionizing and have low penetration. They consist of two protons and two neutrons bound together into a particle that is identical to a helium nucleus, and can be written as He²⁺.

Alpha particles are emitted by radioactive nuclei such as uranium or radium in a process known as alpha decay. This sometimes leaves the nucleus in an excited state, with the emission of a gamma ray removing the excess energy. In contrast to beta decay, alpha decay is mediated by the strong nuclear force.

Alpha rays are easily absorbed by materials and can travel only a few centimeters in air. They can be absorbed by tissue paper or the outer layers of human skin and so are not generally dangerous to life unless the source is ingested or inhaled. If alpha radiation does enter the body, however, they are the most dangerous form of ionizing radiation. They are the most strongly ionizing, and with large enough doses can cause any or all of the symptoms of radiation poisoning.

Most smoke detectors contain a small amount of the alpha emitter Americium-241. This isotope is extremely dangerous if inhaled or ingested, but the danger is minimal if the source is kept sealed.

See also: radioactivity, cosmic rays, nuclear physics, radioactive isotopes, radioactive decay