Isotope

I

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. See Atom.

II

Research

In 1912, the British physicist Sir Joseph Thomson demonstrated the existence of isotopes by passing neon through a discharge tube and deflecting the neon ions by means of magnetic and electric fields. The neon atoms were separated into two groups, deflected by different amounts because their masses differed. This showed that the 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 is 90 per cent neon-20 (the isotope with mass 20), 9.73 per cent neon-22, and 0.27 per cent neon-21. Research on isotopes was continued by many scientists, notably the British physicist Francis William Aston. Work in detecting and studying isotopes was accelerated by the development of the mass spectrometer.

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, aluminium, phosphorus, and sodium. The chemical atomic weight of an element is the weighted average of the individual atomic weights, or mass numbers, of the isotopes. For example, chlorine, atomic weight 35.457, is composed of chlorine-35 and chlorine-37, the former occurring with an abundance of 76 per cent and the latter of 24 per cent. 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.

III

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 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 per cent; 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 per cent, and between the uranium isotopes uranium-235 and uranium-238 to only a little over 1 per cent. 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.

A

Centrifuge and Distillation

In the centrifuge method the apparatus is arranged so that vapour flows downwards in the outer part of the rotating cylinder and upwards in the central region of the cylinder. The centrifugal effect produces an increased 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 vapour stream and are collected.