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Anatomy => Isotopic Tracer => Isotope
Isotope
INTRODUCTION
Isotope, one of two or more species of atom having the same atomic number, hence constituting the same element, but differing in mass number. As atomic number is equivalent to the number of protons in the nucleus, and mass number is the sum total of the protons plus the neutrons in the nucleus, isotopes of the same element differ from one another only in the number of neutrons in their nuclei.
RESEARCH
Experiments carried out early in the 20th century indicated that radioactive substances that were chemically inseparable might differ from each other only in the structure of their nuclei. The British physicist Sir Joseph Thomson demonstrated in 1912 the existence of stable isotopes by passing neon through a discharge tube and deflecting the neon ions by means of magnetic and electric fields; this showed that the stable element neon exists in more than one form. Thomson found two isotopes of neon, one of mass number 20 and another of mass 22. Later experiments showed that naturally occurring neon contains 90% of neon-20 (the isotope with mass 20), 9.73% neon-22, and 0.27% neon-21. Research on isotopes was continued by many scientists, notably the British physicist Francis William Aston; their work in detecting and studying isotopes was accelerated by the development of the mass spectrograph.
It is now known that most elements in the natural state consist of a mixture of two or more isotopes. Among the exceptions are beryllium, aluminum, phosphorus, and sodium. The chemical atomic weight (atomic wt.) of an element is the weighted average of the individual atomic weights, or mass numbers, of the isotopes. For example, chlorine, atomic wt. 35.457, is composed of chlorine-35 and chlorine-37, the former occurring with an abundance of 76% and the latter of 24%. All the isotopes of elements with atomic numbers higher than 83 (above bismuth in the periodic table) are radioactive, and a few of the lighter isotopes, such as potassium-40, are radioactive. About 280 naturally occurring stable isotopes (not radioactive) are known.
Artificial radioactive isotopes, known also as radioisotopes, were produced for the first time in 1933 by the French physicists Irène and Frédéric Joliot-Curie. Radioisotopes are prepared by the bombardment of naturally occurring atoms with nuclear particles, such as neutrons, electrons, protons, and alpha particles using particle accelerators.
SEPARATION
The separation of isotopes of the same element from each other is difficult. Full separation in one step by chemical methods is impossible, because isotopes of the same elements have the same chemical properties; physical methods are generally based on the extremely small differences in physical properties caused by the differences in mass of the isotopes. Electrolytic separation and various exchange procedures for isotope separation, however, depend on chemical rate or equilibrium differences that are based primarily on the difference in energy of chemical bonds, which are a function of isotope mass. The isotopes of hydrogen, deuterium (hydrogen-2) and ordinary hydrogen (hydrogen-1) were the first to be separated in appreciable quantities. This accomplishment is credited to the American chemist Harold Urey, who discovered deuterium in 1932.
Before 1940 many methods were used for the separation of small amounts of isotopes for research purposes. Some of the most successful were the centrifuge method, fractional distillation, thermal diffusion, electrolysis, gaseous diffusion, and electromagnetic separation. Each of these methods depends on the small difference in weight of the isotopes to be separated, and is most effective with the hydrogen isotopes, where the difference in mass between the two substances amounts to 100%; by contrast, the difference in mass between the carbon isotopes carbon-12 and carbon-13 or between the neon isotopes neon-20 and neon-22 amounts to only about 10%, and between the uranium isotopes uranium-235 and uranium-238 to only a little over 1%. This factor of 10 to 1 or 100 to 1 makes the separation far more than 10 or 100 times as difficult. In all processes except the electromagnetic, which is the sole one-stage procedure, isotope separation involves a series of production stages. The net result of any single stage is the separation of the original material into two fractions, one of which contains a slightly higher percentage of the heavy isotope than the original mixture and the other contains slightly more of the light isotope.
To obtain an appreciable concentration, or enrichment, in the desired isotope, it is necessary to separate further the enriched fraction. This process is usually carried out by means of a cascade, comprising a large number of stages. The enriched fraction from any stage becomes the raw material for the next stage, and the depleted fraction, which still contains a considerable percentage of the desired isotope, is mixed with the raw material for the preceding stage. Even the depleted material from the original stage is stripped in additional stages when the raw material (for example, uranium) is scarce. Suitable apparatus is designed to make the flow from stage to stage automatic and continuous.
Such a cascade is extremely flexible, and units can be shifted from one stage of the separation to another as desired. For example, in the separation of uranium, a large amount of material must be handled at the beginning, where the desired uranium-235 is mixed with about 140 times as much uranium-238; at the end of the process the uranium-235 is almost pure, and the volume of material is much smaller. Furthermore, by merely changing the piping, it is possible to shift stages to compensate for addition at an intermediate stage of material that results from preliminary enrichment by a different process.
° Centrifuge and Distillation
In the centrifuge method the apparatus is so arranged that vapor flows downward in the outer part of the rotating cylinder and upward in the central region of the cylinder. The centrifugal force acts more strongly on the heavy molecules than on the light ones, increasing the concentration of the heavy isotopes in the outer region. In separation by fractional distillation a mixture containing various isotopes is distilled. The molecules of the fraction having the lower boiling point (the lighter isotopes) tend to concentrate in the vapor stream and are collected.
° Thermal Diffusion
This method utilizes the tendency of lighter molecules of a liquid or gas to concentrate in a hot region and for heavier molecules to concentrate in a cold region. A simple form of thermal-diffusion apparatus consists of a tall vertical tube with a wire electrically heated to about 500° C (932° F) running down its center, producing a temperature gradient between the center and wall of the tube. The heavier isotopes tend to concentrate in the outer portions of the tube, and the lighter isotopes, to concentrate toward the center. At the same time, because of thermal convection, the gas or liquid near the wire tends to rise, and the cooler outer gas or liquid tends to fall. The overall effect is that the heavier isotopes collect at the bottom of the tube and the lighter at the top.
° Electrolysis
The electrolytic method of separation is of historical as well as current interest, because it was the first method used to separate practically pure deuterium. This method depends on the fact that when water undergoes electrolysis, the lighter hydrogen isotope tends to come off first, leaving behind a residue of water that is enriched in the heavier isotope.
° Gaseous Diffusion
This and the electromagnetic method of separating isotopes of uranium afforded the first large-scale separation ever achieved. The problem of separating uranium-235 from uranium-238 arose in 1940 after the demonstration of the susceptibility of the 235 isotope to fission by neutrons. Uranium-235 exists in naturally occurring uranium to the extent of 7 parts to 1000 of uranium-238. Under the auspices of the atomic bomb project, the various methods for separating isotopes were considered, and the gaseous-diffusion and electromagnetic methods were put into large-scale operation for the production of about 1 kg (2.2 lb) per day of uranium-235 to be used in atomic bombs.
The gaseous-diffusion method exploits the different rate of diffusion of gases of different molecular weight. The rate of diffusion of a gas is inversely proportional to the square root of the mass; light atoms diffuse through a porous barrier faster than heavier atoms. In the separation of uranium isotopes, the only gaseous compound of uranium, the fluoride of uranium, UF6, is used. The uranium hexafluoride is pumped continuously through porous barriers. The difference in weight between uranium-235 and uranium-238 is slightly greater than 1%, but the difference in weight between the fluorides is slightly less than 1%. The enrichment factor, which depends on the square root of the above difference, is theoretically 0.43% for an instantaneous process or 0.30% for a continuous process, but in practice an enrichment factor of only about 0.14% per stage has been achieved. To produce 99% uranium-235 from natural uranium, which contains about 0.7% uranium-235, 4000 such stages are required. The process requires the use of thousands of miles of pipe, thousands of pumps and motors, and intricate control mechanisms.
° Electromagnetism
Although the gaseous-diffusion method yields large amounts of uranium-235, the first comparatively large amounts of the isotope were produced by electromagnetic means at Oak Ridge, Tennessee. A series of separator units was built in which an ionic beam obtained from a uranium compound was passed through a magnetic field. Because the radius of the curvature of the path of the ions deflected by the beam depends on the mass of the ion, ions of different mass complete their path at different positions, and the uranium isotopes are appreciably separated. Only a small amount of material, however, can be treated in one operation. Because of this limitation on production, the use of the electromagnetic process for large-scale isotope separation was abandoned after the war in favor of the gaseous-diffusion process.
° Laser Beam
The concept of laser separation and enrichment of isotopes arose soon after the invention of the laser in 1960. It gained further incentive six years later with the development of the tunable dye laser, which provides photon beams in a selectably narrow range of infrared to ultraviolet wavelengths. According to this concept, if an element is first vaporized its atoms can then be selectively excited and ionized by an accurately tuned laser beam to separate out the desired isotope. Isotopes can also be separated in molecular form by selective laser-beam dissociation of those molecules of the compound that contain the desired isotope. Since 1972 such processes have been under development, particularly for uranium and plutonium enrichment-for nuclear power and nuclear weapons, respectively. Much of the work in the U.S. is classified, but a pilot plant may be operational by the later 1980s. The method is costly and technically difficult, but only a few stages are required for production of highly enriched material.
For the application of isotopes to biological, medical, chemical, and physical research, see Radiology.
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