There are several sources of radioactive isotopes. Some radioactive isotopes are present as terrestrial radiation. Radioactive isotopes of radiumthorium, and uranium, for example, are found naturally in rocks and soil. Uranium and thorium also occur in trace amounts in water. Radon, generated by the radioactive decay of radium, is present in air. Organic materials typically contain small amounts of radioactive carbon and potassium. Cosmic radiation from the Sun and other stars is a source of background radiation on Earth. Other radioactive isotopes are produced by humans via nuclear reactions, which result in unstable combinations of neutrons and protons. One way of artificially inducing nuclear transmutation is by bombarding stable isotopes with alpha particles.

 

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