Alpha radiation
Alpha radiation is a form of ionising radiation emitted during the so-called alpha decay of certain radionuclides (also known as "alpha emitters").
Alpha radiation consists of helium-4 atomic nuclei (the so-called "alpha particles"). Examples of alpha emitters are thorium-232 or the uranium isotopes U-235 and U-238. Due to their electrical charge and relatively large mass, alpha particles have only a very shallow penetration depth into matter and can be easily shielded - a sheet of paper is usually sufficient.