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This editable Main Article is under development and subject to a disclaimer.

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An alpha particle (α-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: mα = 4.001 506 179 127 u.[1]

Discovered and named in 1899 by Ernest Rutherford, alpha radiation (α-radiation) was used by him and co-workers 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 with α-particles, changing it to oxygen, producing in 1919 the first artificial nuclear transmutation.

## History[edit]

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 a 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:

92 238 U -> 90 234 T h + 2 4 H e + γ {\displaystyle {}_{\,\;92}^{238}\mathrm {U} \rightarrow {}_{\,\;90}^{234}\mathrm {Th} +{}_{2}^{4}\mathrm {He} +\gamma }

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

90 234 T h -> 91 234 P a + β − {\displaystyle {}_{\,\;90}^{234}\mathrm {Th} \rightarrow {}_{\,\;91}^{234}\mathrm {Pa} +\beta ^{-}}

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. In 1908 it was not yet known that an atom consists of a nucleus plus orbiting electrons, though the existence of atoms had already been firmly established by then.

## Reference[edit]

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

## External link[edit]

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