Radio-activity/Chapter 10

by Ernest Rutherford

 

CHAPTER X.

TRANSFORMATION PRODUCTS OF URANIUM, THORIUM, AND ACTINIUM.

205. In the last chapter the mathematical theory of successive changes has been considered. The results there obtained will now be applied to explain the radio-active phenomena observed with uranium, thorium, actinium, radium, and their products. Transformation products of Uranium.

It has been shown in sections 127 and 129 that a radio-active constituent Ur X can be separated from uranium by several different processes. The activity of the separated Ur X decays with the time, falling to half value in about 22 days. At the same time the uranium, from which the Ur X has been separated, gradually regains its lost activity. The laws of decay of Ur X and of the recovery of the lost activity of the uranium are expressed by the equations

I_{t}/I_{0} = e^{-λt}, and I_{t}/I_{0} = 1 - e^{-λt},

where λ is the radio-active constant of Ur X. The substance Ur X is produced from uranium at a constant rate, and the constant radio-activity observed in uranium represents a state of equilibrium, where the rate of production of new active matter is balanced by the rate of change of the Ur X already produced.

The radio-active processes occurring in uranium present several points of difference from the processes occurring in thorium and radium. In the first place, uranium does not give off an emanation, and in consequence does not produce any excited activity on bodies. So far only one active product Ur X has been observed in uranium. This active product Ur X differs from Th X and the emanations, inasmuch as the radiation from it consists almost entirely of β rays. This peculiarity of the radiations from Ur X initially led to some confusion in the interpretation of observations on Ur X and the uranium from which it had been separated. When examined by the photographic method, the uranium freed from Ur X showed no activity, while the Ur X possessed it to an intense degree. With the electric method, on the other hand, the results obtained were exactly the reverse. The uranium freed from Ur X showed very little loss of activity, while the activity of the Ur X was very small. The explanation of these results was given by Soddy^[1] and by Rutherford and Grier^[2]. The α rays of uranium are photographically almost inactive, but produce most of the ionization in the gas. The β rays, on the other hand, produce a strong photographic action, but very little ionization compared with the α rays. When the Ur X is separated from the uranium, the uranium does not at first give out any β rays. In the course of time fresh Ur X is produced from the uranium, and β rays begin to appear, gradually increasing in intensity until they reach the original value shown before the separation of the Ur X.

In order to determine the recovery curves of uranium after the separation of Ur X, it was thus necessary to measure the rate of increase of the β rays. This was done by covering the uranium with a layer of aluminium of sufficient thickness to absorb all the α rays, and then measuring the ionization due to the rays in an apparatus similar to Fig. 17.

Uranium has not yet been obtained inactive when tested by the electric method. Becquerel^[3] has stated that he was able to obtain inactive uranium, but in his experiments the uranium was covered with a layer of black paper, which would entirely absorb the α rays. There is no evidence that the α radiation of uranium has been altered either in character or amount by any chemical treatment. The α rays appear to be inseparable from the uranium, and it will be shown later that thorium and radium as well as uranium also possess a non-separable activity consisting entirely of α rays. The changes occurring in uranium must then be considered to be of two kinds, (1) the change which gives rise to the α rays and the product Ur X, (2) the change which gives rise to the β rays from Ur X.

The possibility of separating the Ur X, which gives rise to the β rays of uranium, shows that the α and β rays are produced quite independently of one another, and by matter of different chemical properties.

Following the general considerations discussed in section 136 we may suppose that every second some of the atoms of uranium—a very minute fraction of the total number present will suffice—become unstable and break up, expelling an α particle with great velocity. The uranium atom, minus one α particle, becomes the atom of the new substance, Ur X. This in turn is unstable and breaks up with the expulsion of the β particle and the appearance of a γ ray.

The changes occurring in uranium are graphically shown in Fig. 77.

Fig. 77.

On this view the α ray activity of uranium should be an inherent property of the uranium, and should be non-separable from it by physical or chemical means. The β and γ ray activity of uranium is a property of Ur X, which differs in chemical properties from the parent substance and can at any time be completely removed from it. The final product, after the decay of Ur X, is so slightly active that its activity has not yet been observed. We shall see later (chapter XIII.) that there is some reason to believe that the changes in uranium do not end at this point but continue through one or more stages, finally giving rise to radium, or in other words that radium is a product of the disintegration of the uranium atom.

Meyer and Schweidler^[4], in a recent paper, state that the activity due to uranium preparations increases somewhat in a closed vessel. On removing the uranium no residual activity, however, was observed. They consider that this effect may be due to a very short-lived emanation emitted by uranium.

206. Effect of crystallization on the activity of uranium. Meyer and Schweidler^[5] recently observed that uranium nitrate, after certain methods of treatment, showed remarkable variations of its activity, measured by the β rays. The α ray activity, on the other hand, was unaltered. Some uranium nitrate was dissolved in water and then shaken up with ether, and the ether fraction drawn off. The early experiments of Crookes showed that, by this method, the uranium in the ether portion was photographically inactive. This is simply explained by supposing that the uranium X is insoluble in ether, and consequently remained behind in the water fraction. The ether fraction gradually regained its β ray activity at the normal rate to be expected if Ur X was produced by the uranium at a constant rate, for it recovered half its final activity in about 22 days. Some of the uranium in the water fraction was crystallized and placed under an electroscope. The β ray activity fell rapidly at first to half its value in the course of four days. The activity then remained constant, and no further change was observed over an interval of one month. Other experiments were made with crystals of uranium nitrate, which had not been treated with ether. The nitrate was dissolved in water and a layer of crystals separated. The β ray activity of these crystals fell rapidly at first, the rate varying somewhat in different experiments, but reached a minimum value after about five days. The β ray activity then rose again at a slow rate for several months.

The rapid drop of activity of the crystals seemed, at first sight, to indicate that crystallization was able in some way to alter the activity of uranium.

Dr Godlewski, working in the laboratory of the writer, repeated the work of Meyer and Schweidler, and obtained results of a similar character, but the initial drop of activity was found to vary both in rate and amount in different experiments. These results were at first very puzzling and difficult to explain, for the mother liquor, left behind after removal of the crystals, did not show the corresponding initial rise, which would be expected if the variation of activity were due to the partial separation of some new product of uranium.

The cause of this effect was, however, rendered very evident by a few well-considered experiments made by Godlewski. The uranium nitrate was dissolved in hot water in a flat dish, and allowed to crystallize under the electroscope. Up to the moment of crystallization the β ray activity remained constant, but as soon as the crystals commenced to form at the bottom of the solution the β ray activity rapidly rose in the course of a few minutes to five times the initial value. After reaching a maximum, the activity very gradually decreased again to the normal value. If, however, the plate of crystals was reversed, the β ray activity was found at first to be much smaller than the normal, but increased as fast as that of the other side diminished.

The explanation of this effect is simple. Ur X is very soluble in water and, at first, does not crystallize with the uranium, but remains in the solution, and, consequently, when the crystallization commences at the bottom of the vessel the upper layer of liquid becomes richer in uranium X. Since the β rays arise only from the product Ur X and not from the uranium itself, and the Ur X is mostly confined to the upper layer, a much greater proportion of the β rays escape than if the Ur X were uniformly distributed throughout the thick layer of uranium. When the amount of water added is just sufficient to supply the water of crystallization, the Ur X in the upper layer of crystals gradually diffuses back through the mass and, in consequence, the activity of the upper surface diminishes and of the lower surface rises. A similar explanation applies to the effects observed by Meyer and Schweidler. The water fraction, left behind after treatment with ether, contained all the Ur X. The first layer of crystals formed in it contained some Ur X, and this was for the most part confined to the top layer of crystals. The amount of β rays at first diminished owing to the gradual diffusion of the Ur X from the surface. In the first experiment, the amount of Ur X present was in radio-active equilibrium with the uranium, and, after the initial drop, the β ray activity remained constant. In the second experiment, the gradual rise is due to the fact that the crystals of uranium first formed contained less than the equilibrium amount of Ur X. After falling to a minimum, the β ray activity, in consequence, slowly rose again to the equilibrium value.

These effects exhibited by uranium are of great interest, and illustrate in a striking manner the difference in properties of Ur X and the uranium. The gradual diffusion of the Ur X throughout the mass of crystals is noteworthy. By measurements of the variation with time of the β ray activity, it should be possible to deduce its rate of diffusion into the crystallized mass.

Transformation products of Thorium.

207. Analysis of the active deposit. The radio-active processes occurring in thorium are far more complicated than those in uranium. It has already been shown in chapter vi that a radio-active product Th X is continuously produced from the thorium. This Th X breaks up, giving rise to the radio-active emanation. The emanation produces from itself a type of active matter which is deposited on the surface of bodies, where it gives rise to the phenomena of excited or induced activity. This active deposit possesses some distinctive chemical and physical properties which distinguish it from the emanation and the Th X. We have seen (section 180) that the rate at which the active deposit loses its activity depends upon the time of exposure of the body made active to the emanation. The explanation of the activity curves for different time of exposure will now be considered.

The curve of variation of activity for a short exposure of 10 minutes has already been given in Fig. 65. The activity is small at first but increases rapidly with the time; it passes through a maximum about 4 hours later, and finally decays exponentially with the time, falling to half value in 11 hours.

This remarkable effect can be explained completely^[6] if it be supposed that the active deposit consists of two distinct substances. The matter initially deposited from the emanation, which will be called thorium A, is supposed to be changed into thorium B. Thorium A is transformed according to the ordinary exponential law, but the change is not accompanied by any ionizing rays. In other words, the change from A to B is a "rayless" change. On the other hand, B breaks up into C with the accompaniment of all three kinds of rays. On this view the activity of the active deposit at any time represents the amount of the substance B present, since C is inactive or active to a very minute extent.

If the variation of the activity imparted to a body exposed for a short interval in the presence of the thorium emanation, is due to the fact that there are two successive changes in the deposited matter A, the first of which is a "rayless" change, the activity I_{t} at any time t after removal should be proportional to the number Q_{t} of particles of the matter B present at that time. Now, from equation (4) section 197, it has been shown that

Q_{t} = (λ_{1}n/(λ_{1} - λ_{2}))(e^{-λ_{2}t} - e^{-λ_{1}t}).

The value of Q_{t} passes through a maximum Q_{T} at the time T when

λ_{2}/λ_{1} = e^{-(λ_{1}-λ_{2})T}.

The maximum activity I_{T} is proportional to Q_{T} and

I_{t}/I_{T} = Q_{t}/Q_{T} = (e^{-λ_{2}t} - e^{-λ_{1}t})/(e^{-λ_{2}T} - e^{-λ_{1}T}).

It will be shown later that the variation with time of the activity, imparted to a body by a short exposure, is expressed by an equation of the above form. It thus remains to fix the values of λ_{1}, λ_{2}. Since the above equation is symmetrical with regard to λ_{1}, λ_{2}, it is not possible to settle from the agreement of the theoretical and experimental curve which value of λ refers to the first change. The curve of variation of activity with time is unaltered if the values of λ_{1} and λ_{2} are interchanged.

It is found experimentally that the activity 5 or 6 hours after removal decays very approximately according to an exponential law with the time, falling to half value in 11 hours. This is the normal rate of decay of thorium for all times of exposure, provided measurements are not begun until several hours after the removal of the active body from the emanation. This fixes the value of the constants of one of the changes. Let us assume for the moment that this gives the value of λ_{1}.

Then λ_{1} = 1·75 × 10^{-5} (sec)^{-1}.

Since the maximum activity is reached after an interval T = 220 minutes (see Fig. 65), substituting the values of λ_{1} and T in the equation, the value of λ_{2} comes out to be

λ_{2} = 2·08 × 10^{-4} (sec)^{-1}.

This value of λ_{2} corresponds to a change in which half the matter is transformed in 55 minutes.

Substituting now the values of λ_{1}, λ_{2}, T, the equation reduces to

I_{t}/I_{T} = 1·37(e^{-λ_{2}t} - e^{-λ_{1}t}).

The agreement between the results of the theoretical equation and the observed values is shown in the following table:

+———————-+——————————+—————————-+
|Time in minutes|Theoretical value of| Observed value of |
| | I_{t}/I_{T} | I_{t}/I_{T} |
+———————-+——————————+—————————-+
| 15 | ·22 | ·23 |
| 30 | ·38 | ·37 |
| 60 | ·64 | ·63 |
| 120 | ·90 | ·91 |
| 220 | 1·00 | 1·00 |
| 305 | ·97 | ·96 |
+———————-+——————————+—————————-+

After 5 hours the activity decreased nearly exponentially with the time, falling to half value in 11 hours.

It is thus seen that the curve of rise of activity for a short exposure is explained very satisfactorily on the supposition that two changes occur in the deposited matter, of which the first is a rayless change.

Further data are required in order to fix which of the time constants of the changes refers to the first change. In order to settle this point, it is necessary to isolate one of the products of the changes and to examine the variation of its activity with time. If, for example, a product can be separated whose activity decays to half value in 55 minutes, it would show that the second change is the more rapid of the two. Now Pegram^[7] has examined the radio-active products obtained by electrolysis of thorium solutions. The rates of decay of the active products depended upon conditions, but he found that, in several cases, rapidly decaying products were obtained whose activity fell to half value in about 1 hour. Allowing for the probability that the product examined was not completely isolated by the electrolysis, but contained also a trace of the other product, this result would indicate that the last change which gives rise to rays is the more rapid of the two.

This point is very clearly brought out by some recent experiments of Miss Slater^[8], who has made a detailed examination of the effect of temperature on the active deposit of thorium.

A platinum wire was made active by exposure for a long interval to the thorium emanation, and then heated for a few minutes to any desired temperature by means of the electric current. The wire, while being heated, was surrounded by a lead cylinder in order that any matter driven off from it should be collected on its surface. The decay of activity both of the wire and of the lead cylinder was then tested separately. After heating to a dull red heat, no sensible diminution of the activity was observed at first, but the rate of decay of the activity on the wire was found to be more rapid than the normal. The activity of the lead cylinder was small at first but increased to a maximum after about 4 hours and then decayed at the normal rate with the time.

These results are to be expected if some thorium A is volatilized from the wire; for the rise of activity on the lead cylinder is very similar to that observed on a wire exposed for a short time in the presence of the thorium emanation, i.e., under the condition that only thorium A is initially present.

On heating the wire above 700° C. the activity was found to be reduced, showing that some thorium B had also been removed. By heating for a few minutes at about 1000° C. nearly all the thorium A was driven off. The activity on the wire then decayed exponentially with the time, falling to half value in about 1 hour. After heating for a minute at about 1200° C. all the activity was removed. These results show that thorium A is more volatile than B, and that the product which gives out rays, viz. thorium B, has a period of about 55 minutes.

Another series of experiments was made, in which an active aluminium disc was placed in an exhausted tube, and exposed to the cathode ray discharge. Under these conditions, a part of the activity of the disc was removed. When the disc was made the anode, the loss of activity was usually 20 to 60 per cent. for half-an-hour's exposure. If the disc was made the cathode, the loss was much greater, amounting to about 90 per cent. in 10 minutes. Part of the active matter removed from the disc was collected on a second disc placed near it. This second disc on removal lost its activity at a far more rapid rate than the normal. The rate of decay on the first disc was also altered, the activity sometimes even increasing after removal. These results indicate that, in this case, the apparent volatility of the products is reversed. Thorium B is driven off from the disc more readily than thorium A. The rates of decay obtained under different conditions were satisfactorily explained by supposing that the surfaces of the discs after exposure to the discharge were coated with different proportions of thorium A and B.

The escape of thorium B from the disc under the influence of the discharge seems rather to be the result of an action similar to the well-known "sputtering" of electrodes than to a direct influence of temperature.

The results obtained by von Lerch^[9] on the electrolysis of a solution of the active deposit also admit of a similar interpretation. Products were obtained on the electrodes of different rates of decay, losing half their activity in times varying from about 1 hour to 5 hours. This variation is due to the admixture of the two products in different proportions. The evidence, as a whole, thus strongly supports the conclusion that the active deposit from thorium undergoes two successive transformations as follows:

(1) A "rayless" change for which λ_{1} = 1·75 × 10^{-5}, i.e., in which half the matter is transformed in 11 hours;

(2) A second change giving rise to α, β and γ rays, for which λ_{2} = 2·08 × 10^{-4}, i.e., in which half the matter is transformed in 55 minutes^[10]. It is, at first sight, a somewhat unexpected result that the final rate of decay of the active deposit from thorium gives the rate of change not of the last product itself, but of the preceding product, which does not give rise to rays at all.

A similar peculiarity is observed in the decay of the excited activity of actinium, which is discussed in section 212.

For a long exposure in the presence of a constant supply of thorium emanation, the equation expressing the variation of activity with time is found from equation (8), section 198,

I_{t}/I_{0} = Q/Q_{0} = (λ_{2}/(λ_{2} - λ_{1}))e^{-λ_{1}t} - (λ_{1}/(λ_{1} - λ_{2}))e^{-λ_{2}t}
= ((λ_{2}e^{-λ_{1}t})/(λ_{2} - λ_{1}))(1 - ·083e^{-1·90 × 10^{-4}t}).

About 5 hours after removal the second term in the brackets becomes very small, and the activity after that time will decay nearly according to an exponential law with the time, falling to half value in 11 hours. For any time of exposure T, the activity at time t after the removal (see equation 11, section 199) is given by

I_{t}/I_{0} = Q/Q_{T} = (ae^{-λ_{2}t} - be^{-λ_{1}t})/(a - b),

where I_{0} is the initial value of the activity, immediately after removal, and

a = (1 - e^{-λ_{2}T})/λ_{2}, b = (1 - e^{-λ_{1}T})/λ_{1}.

By variation of T the curves of variation of activity for any time of exposure can be accurately deduced from the equation, when the values of the two constants λ_{1}, λ_{2} are substituted. Miss Brooks^[11] has examined the decay curves of excited activity for thorium for different times of exposure and has observed a substantial agreement between experiment and theory.

The results are shown graphically in Fig. 78. The maximum value of the activity is, for each time of exposure, taken as 100. The theoretical and observed values are shown in the Figure.

Fig. 78. 208. Analysis of the decay and recovery curves of Th X. The peculiarities of the initial portions of the decay and recovery curves of Th X and thorium respectively (Curves A and B, Fig. 47, p. 221), will now be considered. It was shown that when the Th X was removed from the thorium by precipitation with ammonia, the radiation increased about 15 per cent. during the first day, passed through a maximum, and then fell off according to an exponential law, decreasing to half value in four days. At the same time the activity of the separated hydroxide decreased for the first day, passed through a minimum, and then slowly increased again, rising to its original value after the lapse of about one month.

When a thorium compound is in a state of radio-active equilibrium, the series of changes in which Th X, the emanation, and thorium A and B are produced, go on simultaneously. Since a state of equilibrium has been reached for each of these products, the amount of each product changing in unit time is equal to the amount of that product supplied from the preceding change in unit time. Now the matter Th X is soluble in ammonia, while thorium A and B are not. The Th X is thus removed from the thorium by precipitation with ammonia, but A and B are left behind with the thorium. Since the active deposit is produced from the emanation, which in turn arises from Th X, on the removal of the parent matter Th X, the radiation due to this active deposit will decay, since the rate of production of fresh matter no longer balances its own rate of change. Disregarding the initial irregularity in the decay curve of the active deposit, its activity will have decayed to half value in about 11 hours, and to one quarter value at the end of 22 hours. As soon, however, as the Th X has been separated, new Th X is produced in the thorium compound. The activity of this new Th X is not, however, sufficient to compensate at first for the loss of activity due to the change in the active deposit, so that, as a whole, the activity will at first decrease, then pass through a minimum, then increase again.

The correctness of this point of view has been tested by Rutherford and Soddy^[12] as follows: If the precipitated thorium hydroxide after the removal of Th X is put through a series of precipitations with ammonia at short intervals, the Th X is removed almost as fast as it is formed, and, at the same time, the activity of thorium B in the thorium decays.

The following table indicates the results obtained. A portion of the precipitated hydroxide was removed after each series of precipitations and its activity tested in the usual way.

                                                              Activity of
                                                          hydroxide per cent.
After 1 precipitation 46
After 3 precipitations at intervals of 24 hours 39
After 3 more precipitations at intervals of 24 hours and
    3 at intervals of 8 hours 22
After 3 more each of 8 hours 24
After 6 more each of 4 hours 25

Fig. 79.

The differences in the last three numbers are not significant, for it is difficult to make accurate comparisons of the activity of thorium compounds which have been precipitated under slightly different conditions. It is thus seen that as a result of successive precipitations, the activity is reduced to a minimum of about 25 per cent. The recovery curve of the activity of this 23 times precipitated hydroxide is shown in Fig. 79. The initial drop in the curve is quite absent, and the curve, starting from the minimum, is practically identical with the curve shown in Fig. 48, which gives the recovery curve of thorium hydroxide after the first two days. This residual activity—about 25 per cent. of the maximum—is non-separable from the thorium by any chemical process that has been tried.

The initial rise of activity of Th X, after it has been separated, will now be considered. In all cases it was found that the activity of the separated Th X had increased about 15 per cent. at the end of 24 hours, and then steadily decayed, falling to half value in about four days.

This peculiarity of the Th X curve follows, of necessity, from the considerations already advanced to explain the drop in the recovery curve. As soon as the Th X is separated, it at once produces from itself the emanation, and this in turn produces thorium A and B. The activity due to B at first more than compensates for the decay of activity of the Th X itself. The total activity thus increases to a maximum, and then slowly decays to zero according to an exponential law with the time. The curve expressing the variation of the activity of the separated Th X with time can be deduced from the theory of successive changes already considered in chapter IX. In the present case there are four successive changes occurring at the same time, viz. the change of Th X into the emanation, of the emanation into thorium A, of A into B, and of B into an inactive product. Since, however, the change of the emanation into thorium A (about half changed in one minute) is far more rapid than the changes occurring in Th X or thorium A and B, for the purposes of calculation it may be assumed without serious error that the Th X changes at once into the active deposit. The 55 minute change will also be disregarded for the same reason.

Let λ_{1} and λ_{2} be the constants of decay of activity of Th X and of thorium A respectively. Since the activity of Th X and of thorium A falls to half value in 4 days and 11 hours respectively, the value of λ_{1} = ·0072 and of λ_{2} = ·063, where 1 hour is taken as the unit of time.

The problem reduces to the following: Given the matter A (thorium X) all of one kind, which changes into B (thorium B), find the activity of A and B together at any subsequent time. This corresponds to Case I. (section 197). The amount Q of B at any time T is given by

Q = (λ_{1}n_{0}/(λ_{1} - λ_{2}))(e^{-λ_{2}t} - e^{-λ_{1}t}),

and the activity I at any time of the two together is proportional to λ_{1}P + Kλ_{2}Q, where K is the ratio of the ionization of B compared with that of A.

Then I_{t}/I_{0} = (λ_{1}P + Kλ_{2}Q)/(λ_{1}n_{0}) = e^{-λ_{1}t}[1 + (Kλ_{2}/(λ_{2} - λ_{1}))(1 - e^{-(λ_{2} - λ_{1})t})],

where I_{0} is the initial activity due to n_{0} particles of Th X.

By comparison of this equation with the curve of variation of the activity of Th X with time, shown in Fig. 47, it is found that K is almost ·44. It must be remembered that the activity of the emanation and Th X are included together, so that the activity of thorium B is about half of the activity of the two preceding products.

The calculated values of I_{t}/I_{0} for different values of t are shown in the second column of the following table, and the observed values in the third column.

+—————+—————-+————+
| Time |Theoretical|Observed|
| | value | value |
+—————+—————-+————+
| 0 | 1·00 | 1·00 |
| ·25 days| 1·09 | — |
| ·5 " | 1·16 | — |
| 1 " | 1·15 | 1·17 |
| 1·5 " | 1·11 | — |
| 2 " | 1·04 | — |
| 3 " | ·875 | ·88 |
| 4 " | ·75 | ·72 |
| 6 " | ·53 | ·53 |
| 9 " | ·315 | ·295 |
|13 " | ·157 | ·152 |
+—————+—————-+————+

Fig. 80.

The theoretical and observed values thus agree within the limit of error in the measurements. The theoretical curve is shown in Curve A, Fig. 80 (with the observed points marked, for comparison). The curve B shows the theoretical curve of the decay of the activity of Th X and the emanation, supposing there is no further change into the active deposit. Curve C shows the difference curve between the curves A and B, i.e. the proportion of the activity at different times due to the active deposit. The activity due to the latter thus rises to a maximum about two days after removal of the Th X, and then decays with the time at the same rate as the Th X itself, i.e. the activity falls to half value every four days. When t exceeds four days, the term e^{-(λ_{2} - λ_{1})t} in the theoretical equation is very small. 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. A table of the transformation products of thorium is shown below, with some of their physical and chemical properties.

+————-+—————————-+—————————-+————————+—————————-+
| Product | Time to be half |λ (sec)^{-1}| Radiations | Physical and |
| | transformed | | |chemical properties|
+————-+—————————-+—————————-+————————+—————————-+
|Thorium | | | α rays |Insoluble in |
| [v] | | | | ammonia |
|Th. X | 4 days | 2·00 × 10^{-6} | α rays |Soluble in ammonia |
| [v] | | | | |
|Emanation| 54 secs. | 1·28 × 10^{-2} | α rays |Inert gas, |
| [v] | | | | condenses 120° C.|
|Thorium A| 11 hours} | 1·75 × 10^{-5} | no rays |} Soluble in |
| | | } | | |} strong acids. |
| | | } Active | | |} Volatile at a |
| | | } deposit | | |} white heat. |
| [v] | } | | |} B can be |
|Thorium B| 55 mins.} | 2·1 × 10^{-4} |α, β,|} separated from A |
| | | | | γ rays |} by electrolysis |
| | | | | |} and by difference|
| [v] | | | |} of volatility. |
|  ? | — | — | — | — |
+————-+—————————-+—————————-+————————+—————————-+

211. Transformation products of Actinium. It has previously been pointed out (sections 17 and 18) that the actinium of Debierne and the emanium of Giesel contain the same radio-active constituent. Both give out a short-lived emanation which imparts activity to the surface of bodies. Recently, thanks to Dr Giesel of Braunschweig, preparations of "emanium" have been placed on the market, and most of the investigations that are described later have been made with this substance.

Actinium X. Actinium and thorium are very closely allied in radio-active properties. Both emit an emanation which is rapidly transformed, but the rate of change of the actinium emanation is still more rapid than that of thorium, the activity decreasing to half value in 3·7 seconds. Miss Brooks^[13] has analysed the active deposit from the emanation of actinium, and has shown that two successive changes occur in it, very similar in character to those observed in the active deposit of thorium. It thus seemed probable, from analogy, that an intermediate product, corresponding to Th X in thorium, would be found in actinium^[14]. Recent work has verified this supposition. Giesel^[15] and Godlewski^[16] independently observed that a very active substance could be separated from "emanium," very similar in chemical and physical properties to Th X in thorium. This product will, from analogy, be called "actinium X." The same method, which was used by Rutherford and Soddy to separate Th X from thorium, is also effective in separating actinium X from actinium. After precipitation of the active solution with ammonia, actinium X is left behind in the filtrate. After evaporation and ignition, a very active residue remains. At the same time, the precipitated actinium loses a large proportion of its activity.

Giesel observed the separation of an active product, using a fluorescent screen to detect the radiations. A very complete examination of the product actinium X has been made by Godlewski in the laboratory of the writer.

After separation of actinium X, the activity, whether measured by the α or β rays, increases about 15 per cent. during the first day, and afterwards decays exponentially with the time, falling to half value in 10·2 days. The activity of the separated actinium was small at first but steadily increased with the time, reaching a practical maximum after an interval of sixty days. After the first day, the decay and recovery curves of activity are complementary to one another. The curves of rise and decay are shown graphically in Fig. 82, curves I and II respectively.

Godlewski observed that a solution of actinium, freed from actinium X, gave out very little emanation, while a solution of actinium X gave off the emanation in large quantity. The amount of emanation from the solution was measured by observing the activity produced in a testing vessel, similar to that shown in Fig. 51, when a constant current of air was passed through the solution. The emanating power of actinium X decreased exponentially with the time at the same rate as that at which the actinium X lost its activity. At the same time the actinium solution increased in emanating power, reaching its original value after about 60 days. The behaviour of actinium and thorium is thus quite analogous, and the explanation advanced to explain the decay and recovery curves of thorium applies equally well to the corresponding curves of actinium.

Fig. 82.

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^[17] observed that the excited activity produced by actinium decayed to half value in about 41 minutes. Miss Brooks^[18] 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.

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

I_{t}/I_{T} = (e^{-λ_{2}t} - e^{-λ_{1}t})/(e^{-λ_{2}T} - e^{-λ_{1}T}),

where λ_{1} and λ_{2} 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 λ_{1} = ·0194 (min.)^{-1}. By comparison with the curve, the value of λ_{2} 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 change 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.

Fig. 84.

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.

+—————+———————-+——————————————-+—————————————+
| Products |Time to be half| Rays |Some Physical and Chemical|
| | transformed | | properties |
+—————+———————-+——————————————-+—————————————+
|Actinium |  ? | No rays |Insoluble in ammonia |
|Actinium X| 10·2 days |α, (β and γ)|Soluble in ammonia |
|Emanation | 3·9 secs. | α rays |Behaves as a gas |
|Actinium A| 35·7 mins. | No rays |Soluble in ammonia and |
|Actinium B| 2·15 mins. | α, β and γ | strong acids. Volatilized|
| | | | at 100°C. B can |
| | | | be separated from A by |
| | | | electrolysis |
+—————+———————-+——————————————-+—————————————+

1. Soddy, Trans. Chem. Soc. 81, p. 460, 1902.
2. Rutherford and Grier, Phil. Mag. Sept. 1902.
3. Becquerel, C. R. 131, p. 137, 1900.
4. Meyer and Schweidler, Wien Ber. Dec. 1, 1904.
5. Meyer and Schweidler, Wien Ber. 113, July, 1904.
6. Rutherford, Phil. Trans. A. 204, pp. 169-219, 1904.
7. Pegram, Phys. Rev. p. 424, December, 1903.
8. Miss Slater, Phil. Mag. 1905.
9. von Lerch, Ann. de Phys. November, 1903.
10. The 'rayless change' certainly does not give out α rays, and special experiments showed that no appreciable amount of β rays were present. On the other hand, the second change gives out all three types of rays.
11. Miss Brooks, Phil. Mag. Sept. 1904.
12. Rutherford and Soddy, Trans. Chem. Soc. 81, p. 837, 1902. Phil. Mag. Nov. 1902.
13. Miss Brooks, Phil. Mag. Sept. 1904.
14. Rutherford, Phil. Trans. A. p. 169, 1904.
15. Giesel, Ber. d. D. Chem. Ges. p. 775, 1905.
16. Godlewski, Nature, p. 294, Jan. 19, 1905.
17. Debierne, C. R. 138, p. 411, 1904.
18. Miss Brooks, Phil. Mag. Sept. 1904.