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  • B y G. E. H a r r iso n , P h .D.

    Physics Department, University of Birmingham

    (Communicated by S. W. J . Smith, F.R.S.Received 22 February 1937)

    I n tro du ctio n

    A special theoretical treatment of thermal diffusion in mixtures of two gases, in which one component is rare, has been given by Chapman (1929). An interesting result which emerges from this theory is that this phenomenon may produce a large change in the quantity of the rare constituent on the cold and hot sides.

    Let Mx be the mass of the molecules of the rare and heavy constituent, M2 that of the light constituent, and r the ratio of the proportion by volume of the rare constituent on the cold to that on the hot side, after thermal diffusion. When M2/M1 is small, the formulae expressing the relationship between r and s12, the repulsive force index between unlike molecules, can be expressed in a simple form. In this case, the experimental value of the ratio, r, together with the coefficient of ordinary diffusion of the two gases and the coefficient of viscosity of the lighter gas, suffices to determine s12.

    The purpose of the present experiments has been to determine the ratio, r, in radon-hydrogen and radon-helium gas mixtures. The value of s12 for these two pairs of gases was then calculated.

    Information on the very massive monoatomic molecule of radon is of special interest. Other methods of obtaining information regarding the molecular field, from viscosity measurements, or the equation of state, could not be applied owing to the extremely small quantity of gas available.

    The Thermal Diffusion of Radon Gas Mixtures

    E x per im en ta l

    (a) General Principles of the MethodThe gas apparatus used is shown in the figure. This was filled with a

    uniform gas mixture of radon and hydrogen or radon and helium. A y-ray method was employed to measure the quantity of radon which alone was estimated in these experiments, the other gas having no direct effect upon the measuring instrument. The apparatus containing the gas mixture was

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  • The Thermal Diffusion of Radon Gas Mixtures 81

    mounted in front of a rectangular block of lead 30 cm. long and 15cm. in width. This is also seen in plan in the figure.

    Two rectangular apertures A and B were cut in this block, the aperture B being adjustable in width by means of a screw Two ionization chambers Ix and I2, of the Compton type, were mounted opposite the apertures A and B as shown in the figure. These were charged to equal and opposite potentials

    Ice bath lectric healer

    Slate screen

    F ig . 1 D iagram of A ppara tus.

    of 50 V, so that a balance could be obtained between the ionization currents produced by the radioactive material in front of these apertures. The measuring instrument was a Compton electrometer, E.

    The quantities of radon in front of the apertures A and B were compared by means of the y-rays emitted by the solid decay products, radium B and C, which were deposited on the interior walls of the gas apparatus in the process of radioactive change. The radiation was filtered by 3 mm. of lead. Thus, 4 hr. after any change in the distribution of radon in the gas

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  • 82 G. E. Harrison

    apparatus, the y-ray ionization in each of the chambers Ix and / 2 was proportional to the quantity of radon opposite the respective aperture. The lead block served to limit the y-rays received by the chambers to that from the active deposits in two uniform portions of the long, wide bore tubing opposite the apertures.

    With taps Sx and S2 closed, one side of the gas apparatus, H, was heated to a constant temperature of about 100 C., while the other side, C, was cooled in an ice bath. The pressures on the hot and cold sides were thus proportional to the absolute temperatures. These pressures were equalized by opening the tap S2 in the long, narrow bore, side tube P for a short time. The proportion by volume of radon in the gas mixture remained unaltered, any thermal separation being avoided by making this pressure equalization through the long, narrow bore tube. The aperture B was now increased in width so that the ionizations produced in the two chambers and / 2 were equal. This means, with suitable precautions to be described later, that the two chambers were exposed to ionization produced by equal quantities of radon. Provided the temperatures of the hot and cold sides remain constant, any subsequent change in this balance must be due to an alteration in the proportion by volume of radon in the gas mixture on the hot and cold sides, brought about by thermal separation.

    The 1 cm. bore tap Sx was now opened for 1 hr. to give full time for any thermal separation to take place. The ionizations received by the two chambers were then found to be different, due to the radon molecules passing over to one side, and an equal number of the lighter gas molecules passing over to the other side. The resulting rate of drift of the electrometer needle was timed, and by adding radium needles of known content opposite one of the apertures, the difference in the quantity of radon opposite the apertures was determined. By employing this differential method, in which everything was balanced before thermal diffusion commenced in the gas mixture, the thermal separation of the radon was measured directly.

    (6) The Gas MixturesSince the temperature gradient across the tap Sx was considerable, the gas

    apparatus was constructed from pyrex glass. The total volume of the vessel was 25*8 c.c. and the overall length 30 cm., while the diameter of the wide bore tubing was 1 cm. A narrow, closed, capillary tube containing a known amount of radon, at a given time, was pushed into the bore of the tap S3 and a plate D was afterwards waxed on to the end. The apparatus was exhausted through a tap $4, not shown in the diagram, and then hydrogen or helium

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  • from a cylinder introduced, so that the final pressure was about one atmosphere. With the tap $4 closed, and the taps Sx and S2 open, the radon capillary was broken in the apparatus by turning the tap Diffusion was allowed to take place overnight to produce a uniform gas mixture, after which, the taps Sv $ 2 and S3 were all closed. The proportion by volume of radon in the mixtures was one in several hundred thousands, and any impurities introduced into the mixture with the radon had a negligible effect upon the results obtained.

    A cylindrically shaped electric heater was fitted over the hot side, H, of the apparatus, so that the gas mixture within the heater could be brought to a constant temperature of about 100 C. The cold side of the vessel, G, was immersed in an ice bath. A copper constantan thermo junction was used to measure the temperature difference between the hot and cold sides, by placing the hot junction within the electric heater and the cold junction in the ice bath. This temperature difference was maintained throughout the experiment.

    The Thermal Diffusion of Radon Gas Mixtures 83

    (c) The Electrical ApparatusThe electrical arrangement was similar to that used by Ahmad (1924) to

    measure the absorption of hard y-rays. The ionization chambers, Ix and /2, were of the Compton type, each consisting of a cubical box of sheet brass of 6 cm. side and 2 mm. wall thickness, divided internally by brass plates into a number of compartments with a central electrode. The electrodes were carried by a central horizontal rod, supported by a second rod at right angles, passing through an amberoid plug. The leads were all shielded by earthed brass tubes, which were closed at either end by an ebonite plug so that they could be exhausted. A large, slate slab minimized cross scattering between the two chambers. A uranium oxide resistance was also included in the circuit to balance the small natural leak of the system. The Compton electrometer was adjusted so that with 50 V on the needle the sensitivity was about 5000 mm. scale divisions per volt.

    In the absence of radioactive material, the natural leak of the electrical system was balanced to a few scale divisions per minute by the use of the uranium oxide resistance. The apparatus containing the gas mixture was now aligned in front of the apertures A and B, and the heating furnace and ice bath placed in position. Compensation was made for the difference in the absorption of y-rays by the ice bath and electric heater. The width of aperture B was adjusted until the difference between the two y-ray ionization currents was only a few scale divisions per minute, the gas mixture

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  • 84 G. E. Harrison

    still being at room temperature. It was found that the widths of the apertures A and B were equal when this balance was obtained, so that equal volumes of the gas mixture gave equal ionization intensities. The gas mixture was therefore of uniform composition.

    The heating current was now applied to the furnace and the cold end of the apparatus immersed in ice. After a steady temperature difference of 100 C. had been obtained between the two sides, the tap S2 was opened for a minute or so, so that the gas pressures on the cold and hot sides could equalize without any thermal diffusion taking place.

    After 3 | hr. the ionization currents were again balanced by altering the width of the aperture B. The change in widt