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see radioactive isotoperadioactive isotope
or radioisotope,
natural or artificially created isotope of a chemical element having an unstable nucleus that decays, emitting alpha, beta, or gamma rays until stability is reached.
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Radioisotope (biology)

A radioactive isotope used in studying living systems, such as in the investigation of metabolic processes. The usefulness of radioisotopes as tracers arises chiefly from three properties: (1) At the molecular level the physical and chemical behavior of a radioisotope is practically identical with that of the stable isotopes of the same element. (2) Radioisotopes are detectable in extremely minute concentrations. (3) Analysis for radioisotopic content often can be achieved without alteration of the sample or system. In some applications, principally those in which reaction rates and transfer rates are studied, isotopes, particularly radioisotopes, have unique advantages as tracers.

The amount of isotope to be used and the path by which it is introduced into the system are governed by many factors. Sufficient tracer to be detectable must be used, but the amounts of material which are introduced must be small enough not to disturb the system by their mass, pharmacological effects, or the effects of radiation. The mass of 1 curie, the unit of disintegration rate, depends inversely upon the half-life and directly upon the atomic weight of the particular radioisotope; it is 1 gram for 226Ra (half-life 1620 years), but only 8 micrograms for 131I (half-life 8.0 days). In tracer experiments with small animals, microcurie quantities are usually adequate.

There are many methods for detecting the presence of radioactive material. The Geiger counter has largely been displaced by thallium-activated sodium iodide scintillation crystals for counting gamma rays, but Geiger counters and proportional counters are still useful for counting alpha and beta particles. In histological and cytological studies the method of autoradiography, in which photographic film is exposed through contact with the specimen, is very useful. The autoradiographic method is also used extensively in conjunction with paper or column chromatography, particularly in studies of metabolic pathways.

One of the outstanding achievements in which radioisotopes have played a role has been the use of carbon-14 in the elucidation of the metabolic path of carbon in photosynthesis. The products produced in the first few seconds following exposure to light have been identified by combinations of paper chromatography and autoradiography. The extrathyroidal metabolism of iodine, the path of iodine in the thyroid gland, and other problems of intermediary metabolism have been studied intensively. The concept of the dynamic state of cell constituents is largely attributable to discoveries made with isotopic tracers. At one time it was thought that concentration gradients across cell membranes depended upon their being impermeable, but the use of isotopes has refuted this hypothesis by proving that in many such cases the substances involved are normally transported in both directions across the membrane. In physiology, radioisotopes have been used in a wide variety of permeability, absorption, and distribution studies. See Absorption (biology), Cell membrane, Photosynthesis

The kinetics of cellular proliferation has provided a rich vein for application of radioisotopic methods. For example, the lifetime of human red blood cells (about 120 days) was established with the use of 59Fe-labeled cells. Some applications, such as the intake of 131I by the thyroid, the measurement of the red-cell mass with 51Cr-labeled red cells, and the absorption of 60Co-labeled vitamin B12, are of practical clinical importance in the diagnosis and treatment of disease, and knowledge of the rates of distribution and disposal of a wide variety of radioactive substances is basic to the problem of evaluating the hazard from fallout radiation.

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.


A radioactive isotope (as distinguished from a stable isotope) of an element. Atomic nuclei are of two types, unstable and stable. Those in the former category are said to be radioactive and eventually are transformed, by radioactive decay, into the latter. One of the three types of particles or radiation (alpha particles, beta particles, and gamma rays) is emitted during each stage of the decay. See Isotope, Radioactivity

The term radioisotope is also loosely used to refer to any radioactive atomic species. Whereas approximately a dozen radioisotopes are found in nature in appreciable amounts, hundreds of different radioisotopes have been artificially produced by bombarding stable nuclei with various atomic projectiles.

A very wide variety of radioisotopes are produced in particle accelerators, such as the cyclotron. Charged particles, such as deuterons (D+) and protons (H+), are accelerated to great speeds by high-voltage electrical fields and allowed to strike targets in which nuclear reactions take place; for example, proton in, neutron out (p,n), increasing the target-atom atomic number by one without changing the atomic mass; and deuteron in, proton out (d,p), increasing the atomic mass by one without changing the atomic number. The target elements become radioactive because the nuclei of the atoms are unbalanced, having an excess or deficit of neutrons or protons. Although the particle-accelerating machines are most versatile in producing radioisotopes, the amount of radioactive material that can be produced is relatively smaller than that made in a nuclear reactor [less than curie amounts; a curie (abbreviated Ci) is that quantity of a radioisotope required to supply 3.7 × 1010 disintegrations per second or 3.7 × 1010 becquerels (Bq)]. For large-scale production, nuclear reactors with neutron fluxes of 1 × 1010 to 5 × 1015 neutrons per square centimeter per second are required. See Nuclear reaction, Particle accelerator, Units of measurement

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.


(ray-dee-oh-ÿ -sŏ-tohp) See radioactive decay.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006


(nuclear physics)
An isotope which exhibits radioactivity. Also known as radioactive isotope; unstable isotope.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


an isotope that is radioactive
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
Founded in 2005 to commercialize superconducting accelerators, Niowave manufactures medical radioisotopes to cure cancer and save lives.
The RadioGenix System is a high tech system that is approved for processing non-uranium/non highly enriched uranium molybdenum-99 (Mo-99) for the production of the important medical radioisotope, technetium-99m (Tc-99m).
Studies performed during the learning curve for the procedure usually show a higher identification rate using both blue dye and radioisotope in combination [16,17].
AnazaoHealth Corporation was the supplier of the colloidal [sup.32]P; this was imported to our country because we currently do not produce our radioisotopes.
In all, 27 past US space missions have used this radioisotope power for their electricity and heat.
Technetium-99m is the most commonly used medical radioisotope for imaging and scanning of various internal organs such as the brain, lungs, kidneys, liver and thyroid as well as bone for diagnostic purposes.
In this method, certain radioisotope after administration or injection into the body, this article will be focusing on the bones of course in pathological areas, attracting more radioisotope material can be visible [4,10].
Watch for our continuing series on radioisotopes in our April and June issues for more of the story of how these groups and others are finding new methods tot producing radioisotopes and developing new ways to do medical amiging in the face of Canada's radioisotope shortage.
Besides, contributing in power generation, the Atomic Energy Commission is also focusing its attention on application of ionizing radiation and radioisotope in the fields of health, agriculture and industry.
However, there has been comparatively little research on the long-term health effects of radioisotope exposure on the respiratory system.
The High Performance Radioisotope Identifier (HPRID) unit was developed to discriminate threats from benign radioactivity easily.