ALPHA PARTICLE

positively charged particle, identical to the nucleus of the helium-4 atom, spontaneously emitted by some radioactive substances, consisting of two protons and two neutrons bound together, thus having a mass of four units and a positive charge of two. Discovered and named (1899) by Ernest Rutherford, alpha particles were used by him and co-workers in experiments to probe the structure of atoms in thin metallic foils, work that resulted in the first nuclear conception of the atom (190911), and to bombard nitrogen, changing it to oxygen, in the first artificially produced nuclear transmutation (1919). Today, alpha particles are produced for use as projectiles in nuclear research by ionizationi.e., by stripping both electrons from helium atomsand then accelerating the now positively charged particle to high energies.