Animation showing the penetrating power of the three different types of radiation. Uppermost is alpha radiation, the emission of alpha particles helium nuclei. At centre is beta radiation, the emission of high_energy electrons or positrons. Bottom is gamma radiation, the emission of very high energy gamma ray photons. Alpha radiation travels only a few centimetres through air, and is stopped by thin paper. Beta radiation travels a few metres, and can be stopped by a thin sheet of metal. Gamma radiation travels indefinitely through air, and is only weakened attenuated by a slab of concrete or metal.