radioisotope

a radioactive form of an element, consisting of atoms with unstable nuclei, which undergo radioactive decay to stable forms, emitting characteristic alpha, beta, or gamma radiation. These may occur naturally, as in the cases of radium and uranium, or may be created artificially. Scientists create artificial radioisotopes by bombarding stable atoms of an element with subatomic particles in a nuclear reactor or in an atom smasher, or cyclotron. When the nucleus of a stable atom is charged by bombarding particles, the atom usually becomes unstable, or radioactive, and is said to be “labeled” or “tagged.” See also radiation therapy.