The Separation of Isotopes by Thermal Diffusion

Abstract

The phenomenon of thermal diffusion in gases consists in the fact that a temperature gradient in a mixture of two gases or isotopic forms of the same gas gives rise to a concentration gradient of the two gases. The effect in general is to concentrate the lighter molecule in the hot reservoir and the heavier molecule in the cold reservoir. The phenomenon was predicted by Enskog (1) in 1911 and independently by Chapman (2) in 1917. It was experimentally verified in 1917 by Chapman and Dootson (3). In the case of isotopes the separation was small compared to other separation methods and did not appear to be of any practical use (4). In 1938 Clasius and Dickel (5) developed a method of greatly enhancing the thermal diffusion effect. Their apparatus consisted of a log vertical tube closed at both ends and containing a gas at approximately atmospheric pressure; along the axis of the tube was suspended a platinum wire which could electrically heated to be a high temperature. The lighter molecules being concentrated near the hot wall were carried upward by convection currents, while the heavier molecules were concentrated at the bottom. Equilibrium was established when the thermal siphoning effect is balanced by self diffusion.</p> <p> The separate stable isotopes of carbon, nitrogen, oxygen, and sulphur are of considerable importance in isotope exchange and tracer work. The method of chemical exchange is particularly suited to the production of relatively large quantities of material of medium enrichment. Where high dilution of the tagged material is necessary, or in experiments designed to measure the physical and chemical properties of the enriched isotope, a higher concentration is desirable. Thermal diffusion provides a convenient method for the production of small quantities of highly enriched material. Considerable quantities of sulphur and oxygen enriched in S34 and O18 respectively were produced at McMaster several years ago and it seemed desirable to secure further enrichment of these isotopes by the thermal diffusion method. Thermal diffusion columns set up previously (38) have been testes and used in the separation of the isotopes of nitrogen, oxygen, and sulphur using N2, O2 and SO2 gases respectively. Nitrogen gas was used in the separation of the nitrogen isotope due to its availability and its thermal stability. The simplest gaseous sulphur compound which is thermally stable in the temperature range necessary is SO2. Calculations from viscosity data (7,10) indicate that SO2 would be a particularly poor gas to use. However, since several factors influence the separation it was decided to try SO2 before discarding the data. In the separation of the oxygen isotopes the starting material was water whose O18 content was 7.5 normal. Of the common gaseous compounds of oxygen: O2, CO and CO2, the gas predicted to give the largest separation is CO with O2 next. Oxygen was used due to its ease of preparation and the fact that its predicted separation is only slightly less than that of CO2.