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.

In 1899 Rutherford determined some properties of the "Uranic rays" (thus named because the most common uranium isotope, ²³⁸U, is an α emitter and uranium salts were used as source of α-radiation), but at that time the nature of the radiation was an enigma. Ten years later Rutherford knew, he wrote a paper "On the Nature of the α-particle" together with Hans Geiger in which it was explained that α-particles are helium atoms that have lost their negative charge. Note that it was not known yet in 1908 that an atom consists of a nucleus plus orbiting electrons, but the existence of atoms had been firmly established at that time.