radioisotope

(nuclear physics)

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 also 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 also Nuclear reaction; Nuclear reactor; Particle accelerator; Reactor physics; Units of measurement.

Radioisotope (biophysics)

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. See also Isotope; Radioisotope.

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. See also Autoradiography; Chromatography; Geiger-Müller counter; Particle detector; Spectroscopy.

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 also 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.