Positively charged particle emitted by various radioactive materials during decay. It consists of two neutrons and two protons, and is thus identical to the nucleus of a helium atom. The rest mass of the alpha particle amounts to 6.64424ยท10-27 kg, or 3.7273ยท109 eV. Alpha radiation is the radiation with the lowest penetration potential of the three radiation types (alpha, beta, gamma radiation). Alpha radiation can already be stopped by a sheet of paper and is only dangerous for living creatures if the substance emitting alpha rays is inhaled or ingested with food or enters wounds.