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The term radioisotope is shorthand for radioactive isotope . Isotopes are forms of an element whose atoms differ from each other in the number of neutrons contained in their nuclei and, hence, in their atomic masses. Hydrogen-1, hydrogen-2, and hydrogen-3 are all isotopes of each other.

Isotopes may be stable or radioactive. That is, they may exist essentially unchanged forever (stable), or they may spontaneously emit an alpha particle or beta particle and/or a gamma ray , changing in the process into a new substance. Hydrogen-1 and hydrogen-2 are stable isotopes, but hydrogen-3 is radioactive.

The first naturally occurring radioisotopes were discovered in the late 1890s. Scientists found that all isotopes of the heaviest elementsuranium, radium, radon , thorium, and protactinium, for exampleare radioactive. This discovery raised the question as to whether stable isotopes of other elements could be converted to radioactive forms.

By the 1930s, the techniques for doing so were well established, and scientists routinely produced hundreds of radioisotopes that do not occur in nature . As an example, when the stable isotope carbon-12 is bombarded with neutrons, it may be converted to a radioactive cousin, carbon-13. This method can be used to manufacture radioisotopes of nearly every element.

Naturally occurring radioisotopes are responsible for the existence of background radiation . Background radiation consists of alpha and beta particles and gamma rays emitted by these isotopes. In addition to the heavy isotopes mentioned above, the most important contributors to background radiation are carbon-14 and potassium-40.

Synthetic radioisotopes have now become ubiquitous in human society. They occur commonly in every part of nuclear power plant operations. They are also used extensively in the health sciences, industry, and scientific research. A single example of their medical application is cancer therapy. Gamma rays emitted by the radioisotope cobalt-60 have been found to be very effective in treating some forms of cancer. Other gamma-emitting radioisotopes can also be used in this procedure.

The potential for the release of radioisotopes to the environment is great. For example, medical wastes might very well contain radioisotopes that still emit measurable amounts of radiation. Stringent efforts are made, therefore, to isolate and store radioisotopes until their radiation has reached a safe level.

These efforts have two aspects. Some radioisotopes have short half-lives. The level of radiation they emit drops to less than 1% of the original amount in a matter of hours or days. These isotopes need only be stored in a safe place for a short time before they can be safely discarded with other solid wastes.

Other radioisotopes have half-lives of centuries or millennia. They will continue to emit harmful radiation for thousands of years. Safe disposal of such wastes may require burying them deep in the earth, a procedure that still has not been satisfactorily demonstrated. In spite of the potential environmental hazard posed by radioisotopes, they do not presently pose a serious threat to plants, animals, or humans. The best estimates place the level of radiation from artificial sources at less than five percent of that from natural sources.

See also Half-life; Radioactive decay; Radioactive waste management; Radioactivity

[David E. Newton ]



Baker, P., et al. Radioisotopes in Industry. Washington, DC: U.S. Atomic Energy Commission, 1965.

Corless, W. R., and R. L. Mead. Power from Radioisotopes. Washington, DC: U.S. Atomic Energy Commission, 1971.

Kisieleski, W. E., and R. Baserga. Radioisotopes and Life Processes. Washington, DC: U.S. Atomic Energy Commission, 1967.

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ra·di·o·i·so·tope / ˌrādēōˈīsəˌtōp/ • n. Chem. a radioactive isotope. DERIVATIVES: ra·di·o·i·so·top·ic / -ˌīsəˈtäpik/ adj.

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radioisotopeaslope, cope, dope, elope, grope, hope, interlope, lope, mope, nope, ope, pope, rope, scope, slope, soap, taupe, tope, trope •myope • telescope • periscope •stereoscope • bioscope • stroboscope •kaleidoscope • CinemaScope •gyroscope • microscope • horoscope •stethoscope • antelope • envelope •zoetrope • skipping-rope • tightrope •towrope • heliotrope • lycanthrope •philanthrope • thaumatrope •misanthrope •isotope, radioisotope

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radioisotope (ray-di-oh-I-sŏ-tohp) n. an isotope of an element that emits alpha, beta, or gamma radiation during its decay into another element. Artificial radioisotopes, such as iodine-131 and cobalt-60, are produced by bombarding elements with beams of neutrons. They are widely used in medicine as tracers and as sources of radiation for the different techniques of radiotherapy.

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radioisotope (radioactive isotope) An isotope of an element that is radioactive. See labelling.