A radioisotope is a radioactive isotope of an element. A few elements, such as radium and uranium, have naturally occurring radioisotopes, but because of their usefulness in science and industry, a large number of radioisotopes are produced artificially. This is done by irradiation of stable isotopes with high-energy photons, or with particles such as neutrons in an accelerator or nuclear reactor. Radioisotopes with a wide range of half-lives and activities are available by these means. Because radioisotopes behave chemically and biologically in a very similar way to stable isotopes, and their radiation can easily be monitored, even in very small amounts, they are used to "label" particular atoms or groups in studying chemical reaction mechanisms and to "trace" the course of particular components in various physiological processes. The radiation emitted by radioisotopes may also be utilized directly for treating diseased areas of the body (see radiation therapy, sterilizing foodstuffs, or controlling insect pests.