Alpha particle

An alpha particle is a positively charged particle with of absolute value of charge 2e, where e is the elementary charge. An α-particle is in fact the nucleus of the helium-4 isotope, consisting of two protons and two neutrons, thus having a mass close to 4 u (u stands for unified atomic mass unit). More precisely: m_α = 4.001 506 179 127 u.

Discovered and named (1899) by Ernest Rutherford, α-radiation was used by him and coworkers in experiments that probed the structure of atoms in thin metallic foils, work that resulted in the first conception of the atom as a heavy nucleus with light electrons orbiting the nucleus (1909–1911). Later Rutherford and collaborators bombarded nitrogen by α-particles, changing it to oxygen, producing in 1919 the first artificially nuclear transmutation.