A type of ionising radiation. The nucleus of a helium atom.
One of the three types of ionising radiation (along with beta and gamma). An alpha particle is two protons and two neutrons (or the nuclues of a helium atom). It's the most ionising form of radiation and is most dangerous when inside the body. It is harmless outside of the body as it is blocked very easily by 5cm of air, a sheet of paper or the outside of your skin. Alpha particles have a charge of +2 and a mass of 4. Alpha radiation is used in smoke detectors.