Isotopic Separation
As a result of his study of radioactive elements, Frederick Soddy concluded in 1913 that an element might occur in more than one form. The multiple forms of the element all have the same atomic number, but different atomic weights. Soddy referred to these forms as isotopes. Shortly after Soddy's work, Joseph J. Thomson and Francis Aston demonstrated that isotopes exist among stable elements as well as among radioactive ones.
The separation of isotopes has long presented a challenge to scientists. Because the isotopes of a single element all have the same electron structure, they have essentially the same chemical properties. The differences in their masses, due to varying numbers of neutrons in their nuclei, however, results in their having different physical properties. These differences in physical properties form the bases for the roughly half dozen different methods used for the separation of isotopes.
The mass spectrometer, invented by Aston in about 1919, was the first device used for isotopic separation. In a mass spectrometer, a mixture of isotopes is ionized, accelerated in an electric field, and then passed through a magnetic field. The magnetic field causes a deflection of isotopes that differ in the relative weights of the atoms of which they are composed. Lighter ions are bent through a smaller radius, and heavier ions through a larger radius in the machine. The mass spectrometer is still used widely in chemistry and physics as a research tool.
Isotopic separation became an important practical problem during the Manhattan Project of the 1940s. Of the two most abundant naturally occurring isotopes of uranium, only the relatively rare uranium-235 can be used for nuclear fission. Scientists working on the development of a fission bomb had to find a way to separate uranium-235 from the far more abundant isotope, uranium-238. Electromagnetic separation using mass spectrometers was attempted, but found to be too inefficient. That approach was abandoned in 1946.
A more efficient method of separation was developed in 1940 by the American physicist John Ray Dunning. Dunning reasoned that the gaseous forms of two or more isotopes would diffuse at different rates. In the case of uranium, the element is first converted therefore to a gaseous compound, uranium hexafluoride. The uranium hexafluoride is then allowed to diffuse through a porous barrier. Since uranium hexafluoride containing the uranium-235 isotope is less dense than that containing uranium-238, the former will diffuse slightly more quickly than the latter. Each time the uranium hexafluoride is allowed to diffuse, it becomes slightly more rich in uranium-235 hexafluoride. Although the diffusion process must occur several thousand times, it has turned out to be the most economical method for separating uranium isotopes from each other.
Other methods of separation were developed during the Manhattan Project, and most still find some limited use in research or production. For example, Philip Hauge Abelson recommended using thermal diffusion to separate uranium-235 hexafluoride from uranium-238 hexafluoride. During thermal diffusion, one isotope concentrates in the warmer part of a vessel and the second isotope concentrates in the cooler part. However, this method has since been abandoned because gaseous diffusion was found to be more efficient.
Other methods of separation sometimes work better with elements other than uranium. The most effective way to separate the isotopes of hydrogen, for example, is by electrolysis of water. During electrolysis, protium (hydrogen-1) is produced more rapidly than is deuterium (hydrogen-2), leaving a liquid product that is enriched in the latter. This method is, therefore, a common, if somewhat uneconomical, method of obtaining heavy water (2H2O).
Enrichment by distillation works on much the same principle as does electrolysis. However, distillation is much too expensive and is no longer used commercially.
One of the most recent methods for separation involves the use of a laser. The laser beam is used to excite differentially one of the two isotopes to be separated, but not the other. Removal of the excited isotope by standard mass spectrometric techniques proceeds more efficiently than it does with conventional methods.
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