Radon is highly radioactive and has a short half-life. The chief use of radon is in the treatment of cancer by radiotherapy. It has also found some use (mixed with beryllium) as a neutron source. All naturally occurring radon decays by the emission of alpha particles. The element is found in some spring waters, in streams, and to a very limited extent (about 1 part in 1021) in air. Radon is produced by the disintegration of its precursors in minerals, from which it diffuses in small amounts. In homes and other buildings in some areas of the United States, radon produced by the radioactive decay of uranium-238 present in soil and rock can reach levels regarded as dangerous, but the seriousness of the problem is unclear.
Twenty isotopes of radon are known, but only three occur naturally. Radon-222 (half-life 3.82 days) is produced by the decay of radium-226. Radon-220 (half-life 55 sec), also called thoron, is produced in the decay series of thorium-232. Radon-219 (half-life 4 sec), also called actinon, is produced in the decay series of uranium-235 (actinouranium). Ernest Rutherford discovered thoron in 1899. F. O. Dorn discovered radon-222 in 1900 and called it radium emanation. In about 1902, F. O. Giesel discovered actinon. In 1908 William Ramsay and R. W. Whytlaw-Gray isolated the element, which they called niton, and studied its physical properties. The name radon was adopted in the 1920s to refer to all the isotopes of the element, although the name emanation and symbol Em have been used.
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