Alpha particles are relatively slow and heavy compared with other forms of nuclear radiation.
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Alpha particles are composite particles consisting of two protons and two neutrons tightly bound together (Figure 1). They are emitted from the nucleus of some radionuclides during a form of radioactive decay, called alpha-decay. An alpha-particle is identical to the nucleus of a normal (atomic mass 4) helium atom or a doubly ionised helium atom.
Alpha particles are relatively slow and heavy compared with other forms of nuclear radiation. The particles travels at 5 to 7 percent of the speed of light or 20,000,000 metres per second and has a mass approximately equivalent to 4 protons.
Ernest Rutherford, a New Zealand scientist, distinguished and named alpha rays in 1909. By measuring the charge and mass of alpha particles he discovered that they are the same as the nuclei of ordinary helium atoms. Rutherford's studies contributed to our understanding of the atom and its nucleus through the Rutherford-Bohr planetary model of the atom.
Alpha-decay occurs mainly in the radioactive decay of the heavier elements, particularly in those members of the natural decay series heavier than lead (atomic number 82), such as uranium and thorium. Alpha-particles are emitted with one of a few discrete energies characteristic of the radionuclide from which they were emitted. These energies can be used to identify the radionuclide involved.
Alpha-decay occurs when the ratio of neutrons to protons in the nucleus is low. For example: Polonium-210 has 126 neutrons and 84 protons, a ratio of 1.50 to 1. Following radioactive decay by the emission of an alpha particle, the ratio becomes 124 neutrons to 82 protons, or 1.51 to 1 (Figure 2).
Alpha particles are highly ionising because of their double positive charge, large mass (compared to a beta particle)and because they are relatively slow. They can cause multiple ionisations within a very small distance. This gives them the potential to do much more biological damage for the same amount of deposited energy. Alpha-articles, because they are highly ionising, are unable to penetrate very far through matter and are brought to rest by a few centimetres of air or less than a tenth of a millimetre of living tissue (Figure 3).
Many alpha emitters occur naturally in the environment. For example, alpha particles are given off by uranium-238, radium-226, and other members of the uranium decay series which are present in varying amounts in nearly all rocks, soils, and water.
Alpha particles can't penetrate the normal layer of dead cells on the outside of our skin but could damage the cornea of the eye. Alpha-particle radiation is normally only a safety concern if the radioactive decay occurs in an atom that is already inside the body or inside a cell. Alpha-particle emitters are particularly dangerous if inhaled, ingested, or if they enter a wound.