CHAPTER VII.. Radio-Active Emanations.

RADIO-ACTIVE EMANATIONS.

=138. Introduction.= A most important and striking property possessed by radium, thorium, and actinium, but not by uranium or polonium, is the power of continuously emitting into the surrounding space a material emanation, which has all the properties of a radio-active gas. This emanation is able to diffuse rapidly through gases and through porous substances, and may be separated from the gas with which it is mixed by condensation by the action of extreme cold. This emanation forms a connecting link between the activity of the radio-elements themselves and their power of exciting activity on surrounding objects, and has been studied more closely than the other active products on account of its existence in the gaseous state. The emanations from the three active bodies all possess similar radio-active properties, but the effects are more marked in the case of the emanation from radium, on account of the very great activity of that element.

Thorium Emanation.

=139. Discovery of the emanation.= In the course of examination of the radiations of thorium, several observers had noted that some of the thorium compounds, and especially the oxide, were very inconstant sources of radiation, when examined in open vessels by the electrical method. Owens found that this inconstancy was due to the presence of air currents. When a closed vessel was used, the current, immediately after the introduction of the active matter, increased with the time, and finally reached a constant value. By drawing a steady stream of air through the vessel the value of the current was much reduced. It was also observed that the radiations could apparently pass through large thicknesses of paper, which completely absorbed the ordinary α radiation.

In an investigation of these peculiar properties of thorium compounds, the writer found that the effects were due to an emission of radio-active particles of some kind from the thorium compounds. This “emanation,” as it was termed for convenience, possesses the properties of ionizing the gas and acting on a photographic plate, and is able to diffuse rapidly through porous substances like paper and thin metal foil.

The emanation, like a gas, is completely prevented from escaping by covering the active matter with a thin plate of mica. The emanation can be carried away by a current of air; it passes through a plug of cotton-wool and can be bubbled through solutions without any loss of activity. In these respects, it behaves very differently from the ions produced in the gas by the rays from active substances, for these give up their charges completely under the same conditions.

Since the emanation passes readily through large thicknesses of cardboard, and through filters of tightly packed cotton-wool, it does not seem likely that the emanation consists of particles of dust given off by the active matter. This point was tested still further by the method used by Aitken and Wilson, for detecting the presence of dust particles in the air. The oxide, enclosed in a paper cylinder, was placed in a glass vessel, and the dust was removed by repeated small expansions of the air over a water surface. The dust particles act as nuclei for the formation of small drops and are then removed from the air by the action of gravity. After repeated expansions, no cloud was formed, and the dust was considered to be removed. After waiting for some time to allow the thorium emanation to collect, further expansions were made but no cloud resulted, showing that for the small expansions used, the particles were too small to become centres of condensation. The emanation then could not be regarded as dust emitted from thorium.

Since the power of diffusing rapidly through porous substances, and acting on a photographic plate, is also possessed by a chemical substance like hydrogen peroxide, some experiments were made to see if the emanation could be an agent of that character. It was found, however, that hydrogen peroxide is not radio-active, and that its action on the plate is a purely chemical one, while it is the radiation from the emanation and not the emanation itself that produces ionizing and photographic effects.

=140. Experimental arrangements.= The emanation from thorium is given off in minute quantity. No appreciable lowering of the vacuum is observed when an emanating compound is placed in a vacuum tube and no new spectrum lines are observed.

For an examination of the emanation, an apparatus similar in principle to that shown in Fig. 51 is convenient.

The thorium compound, either bare or enclosed in a paper envelope, was placed in a glass tube C. A current of air from a gasometer, after passing through a tube containing cotton-wool to remove dust particles, bubbled through sulphuric acid in the vessel A. It then passed through a bulb containing tightly packed cotton-wool to prevent any spray being carried over. The emanation, mixed with air, was carried from the vessel C through a plug of cotton-wool D, which removed completely all the ions carried with the emanation. The latter then passed into a long brass cylinder, 75 cm. in length and 6 cm. in diameter. The insulated cylinder was connected with a battery in the usual way. Three insulated electrodes, E, F, H, of equal lengths, were placed along the axis of the cylinder, supported by brass rods passing through ebonite corks in the side of the cylinder. The current through the gas, due to the presence of the emanation, was measured by means of an electrometer. An insulating key was arranged so that any one of the electrodes E, F, H could be rapidly connected with one pair of quadrants of the electrometer, the other two being always connected with earth. The current observed in the testing cylinder vessel was due entirely to the ions produced by the emanation carried into the vessel by the current of air. On substituting a uranium compound for the thorium, not the slightest current was observed. After a constant flow has passed for about 10 minutes, the current due to the emanation reaches a constant value.

The variation of the ionization current with the voltage is similar to that observed for the gas ionized by the radiations from the active bodies. The current at first increases with the voltage, but finally reaches a saturation value.

=141. Duration of the activity of the emanation.= The emanation rapidly loses its activity with time. This is very readily shown with the apparatus of Fig. 51. The current is found to diminish progressively along the cylinder, and the variation from electrode to electrode depends on the velocity of the flow of air.

If the velocity of the air current is known, the decay of activity of the emanation with time can be deduced. If the flow of air is stopped, and the openings of the cylinder closed, the current steadily diminishes with time. The following numbers illustrate the variation with time of the saturation current, due to the emanation in a closed vessel. The observations were taken successively, and as rapidly as possible after the current of air was stopped.

Time in seconds Current 0 100 28 69 62 51 118 25 155 14 210 6·7 272 4·1 360 1·8

Curve A, Fig. 52, shows the relation existing between the current through the gas and the time. The current just before the flow of air was stopped is taken as unity. The current through the gas, which is a measure of the activity of the emanation, diminishes according to an exponential law with the time like the activity of the products Ur X and Th X. The rate of decay is, however, much more rapid, the activity of the emanation decreasing to half value in about one minute. According to the view developed in section 136, this implies that half of the emanation particles have undergone change in one minute. After an interval of 10 minutes the current due to the emanation is very small, showing that practically all the emanation particles present have undergone change.

The rate of decay has been more accurately determined by Rossignol and Gimingham who found that the activity fell to half value in about 51 seconds. Bronson, using the steady deflection method described in section 69, found the corresponding time 54 seconds.

The decrease of the current with the time is an actual measure of the decrease of the activity of the emanation, and is not in any way influenced by the time that the ions produced take to reach the electrodes. If the ions had been produced from a uranium compound the duration of the conductivity for a saturation voltage would only have been a fraction of a second.

The rate of decay of the activity of the emanation is independent of the electromotive force acting on the gas. This shows that the radio-active particles are not destroyed by the electric field. The current through the gas at any particular instant, after stoppage of the flow of air, was found to be the same whether the electromotive force had been acting the whole time or had been just applied for the time of the test.

The emanation itself is unaffected by a strong electric field and so cannot be charged. By testing its activity after passing it through long concentric cylinders, charged to a high potential, it was found that the emanation certainly did not move with a velocity greater than ·00001 cm. per second, for a gradient of 1 volt per cm., and there was no evidence to show that it moved at all. This conclusion has been confirmed by the experiments of McClelland.

The rate at which the emanation is produced is independent of the gas surrounding the active matter. If in the apparatus of Fig. 51 air is replaced by hydrogen, oxygen, or carbonic acid, similar results are obtained, though the current observed in the testing vessel varies for the different gases on account of the unequal absorption by them of the radiation from the emanation.

If a thorium compound, enclosed in paper to absorb the α radiation, is placed in a closed vessel, the saturation current due to the emanation is found to vary directly as the pressure. Since the rate of ionization is proportional to the pressure for a constant source of radiation, this experiment shows that the rate of emission of the emanation is independent of the pressure of the gas. The effect of pressure on the rate of production of the emanation is discussed in more detail later in