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Natural radioactivity is exhibited by several elements, including radium, uranium, and other members of the actinide series, and by some isotopes of lighter elements, such as carbon-14, used in radioactive dating. Radioactivity may also be induced, or created artificially, by bombarding the nuclei of normally stable elements in a particle accelerator. Essentially there is no difference between these two manifestations of radioactivity.
The radiation produced during radioactivity is predominantly of three types, designated as alpha, beta, and gamma rays. These types differ in velocity, in the way in which they are affected by a magnetic field, and in their ability to penetrate or pass through matter. Other, less common, types of radioactivity are electron capture (capture of one of the orbiting atomic electrons by the unstable nucleus) and positron emission : both forms of beta decay and both resulting in the change of a proton to a neutron within the nucleus : an internal conversion, in which an excited nucleus transfers energy directly to one of the atom's orbiting electrons and ejects it from the atom.
Alpha Radiation
Alpha rays have the least penetrating power, move at a slower velocity than the other types, and are deflected slightly by a magnetic field in a direction that indicates a positive charge. Alpha rays are nuclei of ordinary helium atoms (see alpha particle). Alpha decay reduces the atomic weight, or mass number, of a nucleus, while beta and gamma decay leave the mass number unchanged. Thus, the net effect of alpha radioactivity is to produce nuclei lighter than those of the original radioactive substance. For example, in the disintegration, or decay, of uranium-238 by the emission of alpha particles, radioactive thorium (formerly called ionium) is produced. The alpha decay reduces the atomic number of the nucleus by 2 and the mass number by 4:
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