For an exposure of 41 minutes to the emanation the excited activity after removal rose to three times its initial value in about 3 hours and then fell again at about the normal rate to half value in 11 hours.
With a longer time of exposure to the emanation, the ratio of the increase after removal is much less marked. For a day’s exposure, the activity after removal begins at once to diminish. In this case, the increase of activity of the matter deposited in the last few hours does not compensate for the decrease of activity of the active matter as a whole, and consequently the activity at once commences to decay. This increase of activity with time explains the initial irregularity in the recovery curve, for the active matter deposited during the first few hours takes some time to reach its maximum activity, and the initial activity is, in consequence, smaller than would be expected from the equation.
The increase of activity on a rod exposed for a short interval in the presence of the thorium emanation has been further investigated by Miss Brooks. The curve C in [Fig. 65] shows the variation with time of the activity of a brass rod exposed for 10 minutes in the emanation vessel filled with dust-free air. The excited activity after removal increased in the course of 3·7 hours to five times its initial value, and afterwards decayed at the normal rate. The dotted line curve D represents the variation of activity to be expected if the activity decayed exponentially with the time. The explanation of this remarkable action is considered in detail in [section 207].
Fig. 65.
181. Effect of dust on the distribution of excited activity. Miss Brooks[[274]], working in the Cavendish Laboratory, observed that the excited activity due to the thorium emanation appeared in some cases on the anode in an electric field, and that the distribution of excited activity varied in an apparently capricious manner. This effect was finally traced to the presence of dust in the air of the emanation vessel. For example, with an exposure of 5 minutes the amount of excited activity to be observed on a rod depended on the time that the air had been allowed to remain undisturbed in the emanation vessel beforehand. The effect increased with the time of standing, and was a maximum after about 18 hours. The amount of excited activity obtained on the rod was then about 20 times as great as the amount observed for air freshly introduced. The activity of this rod did not increase after removal, but with fresh air, the excited activity, for an exposure of 5 minutes, increased to five or six times its initial value.
This anomalous behaviour was found to be due to the presence of dust particles in the air of the vessel, in which the bodies were made radio-active. These particles of dust, when shut up in the presence of the emanation, become radio-active. When a negatively charged rod is introduced into the vessel, a part of the radio-active dust is concentrated on the rod and its activity is added to the normal activity produced on the wire. After the air in the vessel has been left undisturbed for an interval sufficiently long to allow each of the particles of dust to reach a state of radio-active equilibrium, on the application of an electric field, all the positively charged dust particles will at once be carried to the negative electrode. The activity of the electrode at once commences to decay, since the decay of the activity of the dust particles on the wire quite masks the initial rise of the normal activity produced on the wire.
Part of the radio-active dust is also carried to the anode, and the proportion increases with the length of time during which the air has been undisturbed. The greatest amount obtained on the anode was about 60% of that on the cathode.
These anomalous effects were found to disappear if the air was made dust-free by passing through a plug of glass wool, or by application for some time of a strong electric field.
182. Decay of excited activity from radium. The excited activity produced on bodies by exposure to the radium emanation decays much more rapidly than the thorium excited activity. For short times of exposure[[275]] to the emanation the decay curve is very irregular. This is shown in [Fig. 66].