Alpha particle

An alpha particle is a positively charged particle with 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.^[1]

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.

[edit] History

In 1899^[2] Rutherford determined several properties of the "Uranium 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 cause and origin of the radiation emitted by uranium was an enigma. He discovered that there were two kinds of radiation involved, which he called α and β radiation. In retrospect, the nuclear reactions in the uranium salt likely to have been involved were:

Beta-radiation arises from the "daughter" element thorium (Th) that becomes protactinium (Pa) upon emission of a negative elementary charge,

Ten years later Rutherford knew the nature of the α-particle: Hans Geiger and he wrote a paper on its nature and charge,^[3] 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.

[edit] Reference

1. ↑ NIST CODATA Retrieved 12 June 2009
2. ↑ E. Rutherford, Uranium Radiation and the Electrical conduction Produced by it, Phil. Mag., vol. 47, p. 109 (1899)
3. ↑ E. Rutherford and H. Geiger, The Charge and Nature of the α-particle, Proc. Royal Soc. vol. A 81, p. 162–174 (1908). doi Subscription required

[edit] External link

Copy of Rutherford's 1899 paper Retrieved June 12, 2009.