The unstable nucleus of a radioisotope can occur naturally, or as a result of artificially altering the atom. In some cases a nuclear reactor is used to produce radioisotopes, in others, a cyclotron. Nuclear reactors are best-suited to producing neutron-rich radioisotopes, such as molybdenum-99, while cyclotrons are best-suited to producing proton-rich radioisotopes, such as fluorine-18.
The best known example of a naturally-occurring radioisotope is uranium. All but 0.7 per cent of naturally-occurring uranium is uranium-238; the rest is the less stable, or more radioactive, uranium-235, which has three fewer neutrons in its nucleus.Atoms with an unstable nucleus regain stability by shedding excess particles and energy in the form of radiation. The process of shedding the radiation is called radioactive decay. The radioactive decay process for each radioisotope is unique and is measured with a time period called a half-life. One half-life is the time it takes for half of the unstable atoms to undergo radioactive decay.
Some radioisotopes used in nuclear medicine have short half-lives, which means they decay quickly and are suitable for diagnostic purposes; others with longer half-lives take more time to decay, which makes them suitable for therapeutic purposes.
Industry uses radioisotopes in a variety of ways to improve productivity and gain information that cannot be obtained in any other way.They are also used on behalf of international nuclear safeguards agencies to detect clandestine nuclear activities from the distinctive radioisotopes produced by weapons programs.