184. The excited activity from actinium. The emanation of actinium, like that of thorium and radium, produces excited activity on bodies, which is concentrated on the negative electrode in an electric field. Debierne10 found that the excited activity decays approximately according to an exponential law, falling to half value in 41 minutes. Giesel11 examined the rate of decay of the excited activity of “emanium”—which, we have seen, probably contains the same radio-active constituents as actinium—and found that it decayed to half value in 34 minutes. Miss Brooks12 found that the curves of decay of the excited activity from Giesel’s emanium varied with the time of exposure to the emanation. The results are shown graphically in Fig. 69, for time exposures of 1, 2, 5, 10 and 30 minutes, and also for a long exposure of 21 hours. After 10 minutes the curves have approximately the same rate of decay. For convenience, the ordinates of the curves are adjusted to pass through a common point. For a very short exposure, the activity is small at first, but reaches a maximum about 9 minutes later and finally decays exponentially to zero.
The curve of variation of activity for a very short exposure has been determined accurately by Bronson; it is shown later in Fig. 83. He found that the decay of activity is finally exponential, falling to half value in 36 minutes.