Page:Radio-activity.djvu/385

 The equation of decay after this time is therefore expressed by

I_{t}/I_{0} = (1 + Kλ_{2}/(λ_{2} - λ_{1})) e^{-λ_{1}t},

i.e. the activity decays according to an exponential law with the time.

209. Radiations from Thorium products. It has been shown in the last section that the activity of thorium, by successive precipitations with ammonia, is reduced to a limiting value of almost 25 per cent. of the initial activity. This "non-separable activity" consists of α rays, the β and γ rays being altogether absent. According to the disintegration theory, this is an expression of the fact that the initial break-up of the thorium atom is accompanied only by the expulsion of α particles. We have seen in section 156 that the thorium emanation also gives out only α rays. In the active deposit, thorium A gives out no rays, while thorium B emits all three types of rays.

Some hours after separation, Th X gives out α, β, and γ rays, but the appearance of β and γ rays is probably due to the thorium B associated with it. The β and γ ray activity of Th X is much reduced if a current of air is continuously aspirated through a solution of Th X to remove the emanation. It seems likely that if the emanation could be removed as fast as it was formed, so as to prevent the formation of thorium B in its mass, Th X itself would give out only α rays: but, on account of the rapid rate of change of the thorium emanation, it is difficult to realize this experimentally.

210. Transformation products of Thorium. The transformation products of thorium and the rays emitted by them are graphically shown below (Fig. 81).

Fig. 81.