The actinium X is produced at a constant rate from the parent matter actinium, and is transformed according to an exponential law with the time. The constant of change λ = ·068 (day)^-1, and this value is characteristic of the product actinium X. As in the case of thorium, the above experiments show that the emanation does not arise from actinium itself but from actinium X. The emanation in turn breaks up and gives rise to an active deposit on the surface of bodies.

212. Analysis of the active deposit from the emanation. Debierne^[312] observed that the excited activity produced by actinium decayed to half value in about 41 minutes. Miss Brooks^[313] showed that the curves of decay of the excited activity after removal depended upon the duration of exposure to the emanation. The curves for different times of exposure have already been shown in Fig. 69.

Bronson, using the direct deflection method described in section 69, accurately determined the activity curve corresponding to a short exposure to the actinium emanation. The curve obtained is shown in Fig. 83.

This curve is similar in shape to the corresponding curve obtained for the active deposit from thorium, and is explained in a similar way. The activity I_t at any time t is given by

where λ₁ and λ₂ are two constants, and I_T the maximum activity reached after an interval T. After 20 minutes the activity decreased exponentially with the time, falling to half value in 35·7 minutes. This gives the value λ₁ = ·0194 (min.)^-1. By comparison with the curve, the value of λ₂ was found to be ·317 (min.)^-1. This corresponds to a change in which half the matter is transformed in 2·15 minutes. Exactly as in the analogous curve for thorium, it can be shown that the matter initially deposited undergoes two changes, the first of which is a rayless one. The same difficulty arises in fixing which of the values of λ refers to the first change. An experiment made by Miss Brooks (loc. cit.) shows that the rayless product has the slower period of transformation. The active deposit of actinium was dissolved off a platinum wire and then electrolysed. The anode was found to be active, and the activity fell off exponentially with the time, decreasing to half value in about 1·5 minutes. Allowing for the difficulty of accurately measuring such a rapid rate of decay, this result indicates that the product which gives out rays has the rapid period of 2·15 minutes. The analysis of the active deposit of actinium thus leads to the following conclusions:

(1) The matter initially deposited from the emanation, called actinium A, does not give out rays, and is half transformed in 35·7 minutes.

(2) A changes into B, which is half transformed in 2·15 minutes, and gives out both α and β (and probably γ) rays.

Godlewski found that the active deposit of actinium was very easily volatilized. Heating for several minutes at a temperature of 100° C. was sufficient to drive off most of the active matter. The active deposit is readily soluble in ammonia and in strong acids.

213. Radiations from actinium and its products. Actinium in radio-active equilibrium gives out α, β, and γ rays. Godlewski found several points of distinction between the β and γ rays of actinium and of radium. The β rays of actinium appear to be homogeneous, for the activity measured by an electroscope was found to fall off accurately according to an exponential law with the thickness of matter traversed. The β rays were half absorbed in a thickness of 0·21 mm. of aluminium. This indicates that the β particles are all projected from actinium with the same velocity. In this respect actinium behaves very differently from radium, for the latter gives out β particles whose velocities vary over a wide range.

After the β rays were absorbed, another type of more penetrating rays was observed, which probably corresponds to the γ rays from the other radio-elements. The γ rays of actinium were, however, far less penetrating than those from radium. The activity due to these rays was reduced to one-half after passing through 1·9 mms. of lead, while the thickness of lead required in order to absorb half the γ rays of radium is about 9 mms.

The active deposit gave out α and β (and probably γ) rays. It was difficult to decide definitely whether actinium X gave out β as well as α rays. When the actinium X was heated to a red heat, the β activity was temporarily reduced to about half its initial value. This decrease was probably due to the removal of the active deposit, which, we have seen, is readily volatilized by heat. If the β ray activity cannot be further reduced, this would point to the conclusion that actinium X, as well as actinium B, gives out β rays, but the evidence so far obtained is not conclusive.

The ease with which the active deposit is volatilized by heat offers a very simple explanation of the initial peculiarities of the decay and recovery curves (Fig. 82) of actinium X. The activity of actinium X rises at first, but there is no corresponding decrease in the activity of the actinium left behind. It has been shown that the active deposit is soluble in ammonia, and, in consequence, is removed with the actinium X. The products actinium A and B and actinium X, immediately after separation, are in radio-active equilibrium and we should not therefore expect to find any increase of activity after removal, such as is observed in the case of thorium, where thorium A and B are not removed with thorium X. However, in heating the actinium X to drive off the ammonium salts, some of the active deposit is volatilized. After cooling, the amount of the active deposit increases to nearly its old value and there is a corresponding increase of the activity.

214. Products of Actinium. There is one very interesting point of distinction between the radio-active behaviour of thorium and actinium. The latter after removal of actinium X, shows only about 5 per cent. of the original activity, while thorium, after removal of Th X, always shows a residual activity of about 25 per cent. of the maximum value. This very small residual activity indicates that actinium, if completely freed from all its products, would not give out rays at all, in other words, the first change in actinium is a rayless one.

The radio-active products of actinium are shown graphically in Fig. 84. Some of their chemical and physical properties are tabulated below.