VIII. EXCITED RADIO-ACTIVITY
=175. Excited radio-activity.= One of the most interesting and remarkable properties of thorium, radium, and actinium, is their power of “exciting” or “inducing” temporary activity on all bodies in their neighbourhood. A substance which has been exposed for some time in the presence of radium or thorium behaves as if its surface were covered with an invisible deposit of intensely radio-active material. The “excited” body emits radiations capable of affecting a photographic plate and of ionizing a gas. Unlike the radio-elements themselves, however, the activity of the body does not remain constant after it has been removed from the influence of the exciting active material, but decays with the time. The activity lasts for several hours when due to radium and several days when due to thorium.
This property was first observed by M. and Mme. Curie[269] for radium, and independently by the writer[270] for thorium[271].
If any solid body is placed inside a closed vessel containing an emanating compound of thorium or radium, its surface becomes radio-active. For thorium compounds the amount of excited activity on a body is in general greater the nearer it is to the active material. In the case of radium, however, provided the body has been exposed for several hours, the amount of excited activity is to a large extent independent of the position of the body in the vessel containing the active material. Bodies are made active whether exposed directly to the action of the radio-active substance or screened from the action of the direct rays. This has been clearly shown in some experiments of P. Curie. A small open vessel a (Fig. 62) containing a solution of radium is placed inside a larger closed vessel V.
Plates A, B, C, D, E are placed in various positions in the enclosure. After exposure for a day, the plates after removal are found to be radio-active even in positions completely shielded from the action of the direct rays. For example, the plate D shielded from the direct radiation by the lead plate P is as active as the plate E, exposed to the direct radiation. The amount of activity produced in a given time on a plate of given area in a definite position is independent of the material of the plate. Plates of mica, copper, cardboard, ebonite, all show equal amounts of activity. The amount of activity depends on the area of the plate and on the amount of free space in its neighbourhood. Excited radio-activity is also produced in water if exposed to the action of an emanating compound.
=176. Concentration of excited radio-activity on the negative electrode.= When thorium or radium is placed in a closed vessel, the whole interior surface becomes strongly active. In a strong electric field, on the other hand, the writer found that the activity was confined entirely to the negative electrode. By suitable arrangements, the whole of the excited activity, which was previously distributed over the surface of the vessel, can be concentrated on a small negative electrode placed inside the vessel. An experimental arrangement for this purpose is shown in Fig. 63.
The metal vessel V containing a large amount of thoria is connected with the positive pole of a battery of about 300 volts. The wire AB to be made active is fastened to a stouter rod BC, passing through an ebonite cork inside a short cylinder D, fixed in the side of the vessel. This rod is connected with the negative pole of the battery. In this way the wire AB is the only conductor exposed in the field with a negative charge, and it is found that the whole of the excited activity is concentrated upon it.
In this way it is possible to make a short thin metal wire over 10,000 times as active per unit surface as the thoria from which the excited activity is derived. In the same way, the excited activity due to radium can be concentrated mainly on the negative electrode. In the case of thorium, if the central wire be charged positively, it shows no appreciable activity. With radium, however, a positively charged body becomes slightly active. In most cases, the amount of activity produced on the positive electrode is not more than 5% of the corresponding amount when the body is negatively charged. For both thorium and radium, the amount of excited activity on electrodes of the same size is independent of their material.
All metals are made active to equal extents for equal times of exposure. When no electric field is acting, the same amount of activity is produced on insulators like mica and glass as on conductors of equal dimensions.
=177. Connection between the emanations and excited activity.= An examination of the conditions under which excited activity is produced shows that there is a very close connection between the emanation and the excited activity. If a thorium compound is covered with several sheets of paper, which cut off the α rays but allow the emanation to pass through, excited activity is still produced in the space above it. If a thin sheet of mica is waxed down over the active material, thus preventing the escape of the emanation, no excited activity is produced outside it. Uranium and polonium which do not give off an emanation are not able to produce excited activity on bodies. Not only is the presence of the emanation necessary to cause excited activity, but the amount of excited activity is always proportional to the amount of emanation present. For example, de-emanated thoria produces very little excited activity compared with ordinary thoria. In all cases the amount of excited activity produced is proportional to the emanating power. When passing through an electric field the emanation loses its property of exciting activity at the same rate as the radiating power diminishes. This was shown by the following experiment.
A slow constant current of air from a gasometer, freed from dust by its passage through cotton-wool, passed through a rectangular wooden tube 70 cms. long. Four equal insulated metal plates A, B, C, D, were placed at regular intervals along the tube. The positive pole of a battery of 300 volts was connected with a metal plate placed in the bottom of the tube, while the negative pole was connected with the four plates. A mass of thoria was placed in the bottom of the tube under the plate A, and the current due to the emanation determined at each of the four plates. After passing a current of air of 0·2 cm. per second for 7 hours along the tube, the plates were removed and the amount of excited activity produced on them was tested by the electric method. The following results were obtained.
Relative Relative current due excited to emanation activity
Plate A 1 1
„ B ·55 ·43
„ C ·18 ·16
„ D ·072 ·061
Within the errors of measurement, the amount of excited activity is thus proportional to the radiation from the emanation, i.e. to the amount of emanation present. The same considerations hold for the radium emanation. The emanation in this case, on account of the slow loss of its activity, can be stored mixed with air for long periods in a gasometer, and its effects tested quite independently of the active matter from which it is produced. The ionization current due to the excited activity produced by the emanation is always proportional to the current due to the emanation for the period of one month or more that its activity is large enough to be measured conveniently by an electrometer.
If, at any time during the interval, some of the emanation is removed and introduced into a new testing vessel, the ionization current will immediately commence to increase, rising in the course of four or five hours to about twice its original value. This increase of the current is due to the excited activity produced on the walls of the containing vessel. On blowing out the emanation, the excited activity is left behind, and at once begins to decay. Whatever its age, the emanation still possesses the property of causing excited activity, and in amount always proportional to its activity, i.e. to the amount of emanation present.
These results show that the power of exciting activity on inactive substances is a property of the radio-active emanations, and is proportional to the amount of emanation present.
The phenomenon of excited activity cannot be ascribed to a type of phosphorescence produced by the rays from the emanation on bodies; for it has been shown that the activity can be concentrated on the negative electrode in a strong electric field, even if the electrode is shielded from the direct radiation from the active substance which gives off the emanation. The amount of excited activity does not seem in any way connected with the ionization produced by the emanation in the gas with which it is mixed. For example, if a closed vessel is constructed with two large parallel insulated metal plates on the lower of which a layer of thoria is spread, the amount of the excited activity on the upper plate when charged negatively, is independent of the distance between the plates when that distance is varied from 1 millimetre to 2 centimetres. This experiment shows that the amount of excited activity depends only on the amount of emanation emitted from the thoria; for the ionization produced with a distance of 2 centimetres between the plates is about ten times as great as with a distance of 1 millimetre.
=178.= If a platinum wire be made active by exposure to the emanation of thoria, its activity can be removed by treating the wire with certain acids[272]. For example, the activity is not much altered by immersing the wire in hot or cold water or nitric acid, but more than 80% of it is removed by dilute or concentrated solutions of sulphuric or hydrochloric acid. The activity has not been destroyed by this treatment but is manifested in the solution. If the solution be evaporated, the activity remains behind on the dish.
These results show that the excited activity is due to a deposit on the surface of bodies of radio-active matter which has definite properties as regards solution in acids. This active matter is dissolved in some acids, but, when the solvent is evaporated, the active matter is left behind. This active matter is deposited on the surface of bodies, for it can be partly removed by rubbing the body with a cloth, and almost completely by scouring the plate with sand or emery paper. If a negatively charged wire is placed in the presence of a large quantity of radium emanation, it becomes intensely active. If the wire, after removal, is drawn across a screen of zinc sulphide, or willemite, a portion of the active matter is rubbed off, and a luminous trail is left behind on the screen. The amount of active matter deposited is extremely small, for no difference of weight has been detected in a platinum wire when made extremely active. On examining the wire under a microscope, no trace of foreign matter is observed. It follows from these results that the matter which causes excited activity is many thousand times more active, weight for weight, than radium itself.
It is convenient to have a definite name for this radio-active matter, for the term “excited activity” only refers to the radiation from the active matter and not to the matter itself. The term “active deposit” will be generally applied to this matter. The active deposit from the three substances thorium, radium, and actinium is, in each case, derived from its respective emanation, and possesses the same general property of concentration on the negative electrode in an electric field and of acting as a non-volatile type of matter which is deposited from the gas on to the surface of bodies. These active deposits, while all soluble in strong acids, are chemically distinct from each other.
The term “active deposit” can, however, only be used when the matter is spoken of as a whole; for it will be shown later that the matter, under ordinary conditions, is complex and contains several constituents which have distinctive physical and chemical properties and also a distinctive rate of change. According to the theory advanced in section 136, we may suppose that the emanation of thorium, radium, and actinium is unstable and breaks up with the expulsion of an α particle. The residue of the atom of the emanation diffuses to the sides of the vessel or is removed to the negative electrode in an electric field. This active deposit is in turn unstable and breaks up in several successive stages.
The “excited activity” proper is the radiation set up by the active deposit in consequence of the changes occurring in it. On this view, the emanation is the parent of the active deposit in the same way that Th X is the parent of the emanation. The proportionality which always exists between the activity of the emanation and the excited activity to which it gives rise, is at once explained, if one substance be the parent of the other.
=179. Decay of the excited activity produced by thorium.= The excited activity produced in a body after a long exposure to the emanations of thorium, decays in an exponential law with the time, falling to half value in about 11 hours. The following table shows the rate of decay of the excited activity produced on a brass rod.
Time in Current hours
0 100
7·9 64
11·8 47·4
23·4 19·6
29·2 13·8
32·6 10·3
49·2 3·7
62·1 1·86
71·4 0·86
The results are shown graphically in Fig. 64, Curve A.
The intensity of the radiation I after any time t is given by
$$ \frac {I} {I₀} = e^{–λt} $$,
where λ is the radio-active constant.
The rate of decay of excited activity, like that of the activity of other radio-active products, is not appreciably affected by change of conditions. The rate of decay is independent of the concentration of the excited activity, and of the material of the body on which it is produced. It is independent also of the nature and pressure of the gas in which it decays. The rate of decay is unchanged whether the excited activity is produced on the body with or without an electric field.
The amount of excited activity produced on a body increases at first with the time, but reaches a maximum after an exposure of several days. An example of the results is given in the following table. In this experiment a rod was made the cathode in a closed vessel containing thoria. It was removed at intervals for the short time necessary to test its activity and then replaced.
Time in Current hours
1·58 6·3
3·25 10·5
5·83 29
9·83 40
14·00 59
23·41 77
29·83 83
47·00 90
72·50 95
96·00 100
These results are shown graphically in Curve B, Fig. 64. It is seen that the decay and recovery curves may be represented approximately by the following equations.
For the decay curve A,
$$ \frac {I} {I₀} = e^{–λt} $$ .
For the recovery curve B,
$$ \frac {I} {I₀} = 1 − e^{–λt} $$ .
The two curves are thus complementary to one another; they are connected in the same way as the decay and recovery curves of Ur X, and are susceptible of a similar explanation.
The amount of excited radio-activity reaches a maximum value when the rate of supply of fresh radio-active particles balances the rate of change of those already deposited.
=180. Excited radio-activity produced by a short exposure.= The initial portion of the recovery curve B, Fig. 64, is not accurately represented by the above equation. The activity for the first few hours increases more slowly than would be expected from the equation. This result, however, is completely explained in the light of later results. The writer[273] found that, for a short exposure of a body to the thorium emanation, the excited activity upon it after removal, instead of at once decaying at the normal rate, increased for several hours. In some cases the activity of the body increased to three or four times its original value in the course of a few hours and then decayed with the time at the normal rate.
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.
=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.
It was found that the intensity of the radiation measured by the α rays decreased rapidly for the first 10 minutes after removal, but about 15 minutes after removal reached a value which remained nearly constant for an interval of about 20 minutes. It then decayed to zero, finally following an exponential law, the intensity falling to half value in about 28 minutes. With longer times of exposure, the irregularities in the curve are not so marked.
Miss Brooks has recently determined the decay curves of the excited activity of radium for different times of exposure, measured by the α rays. The results are shown in Fig. 67, where the initial ordinates represent the activity communicated to the body from different times of exposure to a constant supply of emanation. It will be observed that in all cases there is a sudden initial drop of activity, which becomes less marked with increasing time of exposure. The activity, several hours after removal, decreases exponentially in all cases, falling to half value in about 28 minutes.
Not only do the curves of variation of the excited activity after removal depend upon the time of exposure to the emanation, but they also depend upon whether the α or β and γ rays are used as a means of measurement. The curves obtained for the γ rays are identical with those from the β rays, showing that these two types of rays always occur together and in the same proportion. The curves measured by the β rays are very different, especially for the case of a short exposure to the emanation. This is clearly shown in Fig. 68, which gives the β and γ ray curves for exposures of 10 minutes, 40 minutes, and 1 hour, and also the limiting case of an exposure of 24 hours.
About 25 minutes after removal, the activity decays approximately at the same rate in each case. For convenience of representation, the ordinates of the curves were adjusted so that they all passed through a common point. We shall see later (chapter XI) that the rates of decay are not identically the same until several hours after removal; but, in the above figure, it is difficult to represent the slight variations. It will be observed that for the short exposure of 10 minutes the activity measured by the β rays is small at first but rises to a maximum in about 22 minutes, and then dies away with the time. The curve of decay of activity, measured by the β rays for a long exposure, does not show the rapid initial drop which occurs in all the α ray curves. Curie and Danne[276] made an investigation of the curves of decay of excited activity for different times of exposure to the radium emanation, but apparently did not take into account the fact that measurements made by the α and β rays give quite different curves of decay. Some of the family of curves, given in their paper, refer to the α rays and others to the β rays. They showed, however, the important fact that the curve of decay obtained by them for a long exposure (which is identical with the β ray curve) could be empirically expressed by an equation of the form
$$ \frac {I_t} {I₀} = ae^{–λ_1 t} − (a − 1) e^{–λ_2 t} $$,
where I₀ is the initial intensity and _I__{t} the intensity after any time t; λ₁ = ¹⁄₂₄₂₀, λ₂ = ¹⁄₁₈₆₀. The numerical constant a = 4·20. After an interval of 2·5 hours, the logarithmic decay curve is nearly a straight line, that is, the activity falls off according to an exponential law with the time, decreasing to half value in about 28 minutes.
The full explanation of this equation, and of the peculiarities of the various decay curves of the excited activity of radium, will be discussed in detail in chapter XI.
As in the case of the excited activity from thorium, the rate of decay of the excited activity from radium is for the most part independent of the nature of the body made active. Curie and Danne (loc. cit.) observed that the active bodies gave off an emanation itself capable of exciting activity in neighbouring bodies. This property rapidly disappeared, and was inappreciable 2 hours after removal. In certain substances like celluloid and caoutchouc, the decay of activity is very much slower than for the metals. This effect becomes more marked with increase of time of exposure to the emanation. A similar effect is exhibited by lead, but to a less marked degree. During the time the activity lasts, these substances continue to give off an emanation.
It is probable that these divergencies from the general law are not due to an actual change in the rate of decay of the true excited activity but to an occlusion of the emanation by these substances during the interval of exposure. After exposure the emanation gradually diffuses out, and thus the activity due to this occluded emanation and the excited activity produced by it decays very slowly with the time.
=183. Active deposit of very slow decay.= M. and Mme Curie[277] have observed that bodies which have been exposed for a long interval in the presence of the radium emanation do not lose all their activity. The excited activity at first decays rapidly at the normal rate, falling to half value in about 28 minutes, but a residual activity, which they state is of the order of ½0,000 of the initial activity, always remains. A similar effect was observed by Giesel. The writer has examined the variation of this residual activity, and has found that it increases for several years. The results are discussed in detail in chapter XI. It will there be shown that this active deposit of slow transformation contains the radio-active constituents present in polonium, radio-tellurium and radio-lead.
=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. Debierne[278] found that the excited activity decays approximately according to an exponential law, falling to half value in 41 minutes. Giesel[279] 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 Brooks[280] 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.
The explanation of these curves is discussed in detail in chapter X, section 212.
=185. Physical and chemical properties of the active deposit.= On account of the slow decay of the activity of the active deposit from the thorium emanation, its physical and chemical properties have been more closely examined than the corresponding deposit from radium. It has already been mentioned that the active deposit of thorium is soluble in some acids. The writer[281] found that the active matter was dissolved off the wire by strong or dilute solutions of sulphuric, hydrochloric and hydrofluoric acids, but was only slightly soluble in water or nitric acid. The active matter was left behind when the solvent was evaporated. The rate of decay of activity was unaltered by dissolving the active matter in sulphuric acid, and allowing it to decay in the solution. In the experiment, the active matter was dissolved off an active platinum wire; then equal portions of the solutions were taken at definite intervals, evaporated down in a platinum dish, and the activity of the residue tested by the electric method. The rate of decay was found to be exactly the same as if the active matter had been left on the wire. In another experiment, an active platinum wire was made the cathode in a copper sulphate solution, and a thin film of copper deposited on it. The rate of decay of the activity was unchanged by the process.
A detailed examination of the physical and chemical properties of the active deposit of thorium has been made by F. von Lerch[282] and some important and interesting results have been obtained. A solution of the active deposit was prepared by dissolving the metal which had been exposed for some time in the presence of the thorium emanation. In most cases the active matter was precipitated with the metal. For example, an active copper wire was dissolved in nitric acid and then precipitated by caustic potash. The precipitate was strongly active. An active magnesium wire, dissolved in hydrochloric acid and then precipitated as phosphate, also gave an active precipitate. The activity of the precipitates decayed at the normal rate, i.e. the activity fell to half value in about 11 hours.
Experiments were also made on the solubility of the active deposit in different substances. A platinum plate was made active and then placed in different solutions, and the decrease of the activity observed. In addition to the acids already mentioned, a large number of substances were found to dissolve the active deposit to some extent. The active matter was however not dissolved to an appreciable extent in ether or alcohol. Many substances became active if added to the active solution and then precipitated. For example, an active solution of hydrochloric acid was obtained by dissolving the deposit on an active platinum wire. Barium chloride was then added and precipitated as sulphate. The precipitate was strongly active, thus suggesting that the active matter was carried down by the barium.
=186. Electrolysis of solutions.= Dorn showed that, if solutions of radiferous barium chloride were electrolysed, both electrodes became temporarily active, but the anode to a greater degree than the cathode. F. von Lerch has made a detailed examination of the action of electrolysis on a solution of the active deposit of thorium. The matter was dissolved off an active platinum plate by hydrochloric acid, and then electrolysed between platinum electrodes. The cathode was very active, but there was no trace of activity on the anode. The cathode lost its activity at a rate much faster than the normal. With an amalgamated zinc cathode on the other hand, the rate of decay was normal. When an active solution of hydrochloric acid was electrolysed with an electromotive force smaller than that required to decompose water, the platinum became active. The activity decayed to half value in 4·75 hours while the normal fall is to half value in 11 hours. These results point to the conclusion that the active matter is complex and consists of two parts which have different rates of decay of activity, and can be separated by electrolysis.
Under special conditions it was found possible to make the anode active. This was the case if the anion attached itself to the anode. For example, if an active hydrochloric solution was electrolysed with a silver anode, the chloride of silver formed was strongly active and its activity decayed at a normal rate. The amount of activity obtained by placing different metals in active solutions for equal times varied greatly with the metal. For example, it was found that if a zinc plate and an amalgamated zinc plate, which show equal potential differences with regard to hydrochloric acid, were dipped for equal times in two solutions of equal activity, the zinc plate was seven times as active as the other. The activity was almost removed from the solution in a few minutes by dipping a zinc plate into it. Some metals became active when dipped into an active solution while others did not. Platinum, palladium, and silver remained inactive, while copper, tin, lead, nickel, iron, zinc, cadmium, magnesium, and aluminium became active. These results strongly confirm the view that excited activity is due to a deposit of active matter which has distinctive chemical behaviour.
G. B. Pegram[283] has made a detailed study of the active deposits obtained by electrolysis of pure and commercial thorium salts. The commercial thorium nitrate obtained from P. de Haen gave, when electrolysed, a deposit of lead peroxide on the anode. This deposit was radio-active, and its activity decayed at the normal rate of the excited activity due to thorium. From solutions of pure thorium nitrate, no visible deposit was obtained on the anode, but it was, however, found to be radio-active. The activity decayed rapidly, falling to half value in about one hour. Some experiments were also made on the effect of adding metallic salts to thorium solutions and then electrolysing them. Anode and cathode deposits of the oxides or metals obtained in this way were found to be radio-active, but the activity fell to half value in a few minutes. The gases produced by electrolysis were radio-active, but this was due to the presence of the thorium emanation. The explanation of the results obtained by Pegram and von Lerch will be considered later in section 207. It will be shown that the active deposit of thorium contains two distinct substances which have different rates of transformation.
=187. Effect of temperature.= The activity of a platinum wire which has been exposed in the presence of the thorium emanation is almost completely lost by heating the wire to a white heat. Miss F. Gates[284] found that the activity was not destroyed by the intense heat, but manifested itself on neighbouring bodies. When the active wire was heated electrically in a closed cylinder, the activity was transferred from the wire to the interior surface of the cylinder in unaltered amount. The rate of decay of the activity was not altered by the process. By blowing a current of air through the cylinder during the heating, a part of the active matter was removed from the cylinder. Similar results were found for the excited activity due to radium.
F. von Lerch (loc. cit.) determined the amount of activity removed at different temperatures. The results are shown in the following table for a platinum wire excited by the thorium emanation[285].
Temperature Percentage of activity removed
Heated 2 minutes 800° C. 0
then „ ½ minute 1020° C. 16 more
„ „ ½ „ 1260° C. 52 „
„ „ ½ „ 1460° C. 99 „
The effect of heat on the volatilization of the active deposit of radium has been examined in detail by Curie and Danne. The interesting and important results obtained by them will be discussed in chapter XI, section 226.
=188. Effect of variation of E.M.F. on amount of excited activity from thorium.= It has been shown that the excited activity is confined to the cathode in a strong electric field. In weaker fields the activity is divided between the cathode and the walls of the vessel. This was tested in an apparatus[286] shown in Fig. 70.
A is a cylindrical vessel of 5·5 cms. diameter, B the negative electrode passing through insulating ends C, D. For a potential difference of 50 volts, most of the excited activity was deposited on the electrode B. For about 3 volts, half of the total excited activity was produced on the rod B, and half on the walls of the vessel. Whatever the voltage applied, the sum of the activities on the central rod and the walls of the cylinder was found to be a constant when a steady state was reached.
When no voltage was applied, diffusion alone was operative, and in that case about 13 per cent. of the total activity was on the rod B. The application of an electric field has thus no influence on the sum total of excited activity, but merely controls the proportion concentrated on the negative electrode.
A more detailed examination of the variation with strength of field of the amount on the negative electrode was made in a similar manner by F. Henning[287]. He found that in a strong electric field the amount of excited activity was practically independent of the diameter of the rod B, although the diameter varied between ·59 mm. and 6·0 mms. With a small voltage, the amount on the negative electrode varied with its diameter. The curves showing the relation between the amount of excited activity and voltage are very similar in character to those obtained for the variation of the current through an ionized gas with the voltage applied.
The amount of excited activity reaches a maximum when all the active matter is removed from the gas as rapidly as it is formed. With weaker fields, a portion diffuses to the sides of the vessel, and produces excited activity on the positive electrode.
=189. Effect of pressure on distribution of excited activity.= In a strong electric field, the amount of excited activity produced on the cathode is independent of the pressure down to a pressure of about 10 mms. of mercury. In some experiments made by the writer[288], the emanating thorium compound was placed inside a closed cylinder about 4 cms. in diameter, through which passed an insulated central rod. The central rod was connected to the negative pole of a battery of 50 volts. When the pressure was reduced below 10 mms. of mercury, the amount of excited activity produced on the negative electrode diminished, and was a very small fraction of its original value at a pressure of ⅒ mm. Some excited activity was in this case found to be distributed over the interior surface of the cylinder. It may thus be concluded that at low pressures the excited activity appears on both anode and cathode, even in a strong electric field. The probable explanation of this effect is given in the next section.
Curie and Debierne[289] observed that when a vessel containing an emanating radium compound was kept pumped down to a low pressure, the amount of excited activity produced on the vessel was much reduced. In this case the emanation given off by the radium was removed by the pump with the other gases continuously evolved from the radium compound. On account of the very slow decay of activity of the emanation, the amount of excited activity produced on the walls of the vessel, in the passage of the emanation through it, was only a minute fraction of the amount produced when none of the emanation given off was allowed to escape.
=190. Transmission of excited activity.= The characteristic property of excited radio-activity is that it can be confined to the cathode in a strong electric field. Since the activity is due to a deposit of radio-active matter on the electrified surface, the matter must be transported by positively charged carriers. The experiments of Fehrle[290] showed that the carriers of excited activity travel along the lines of force in an electric field. For example, when a small negatively charged metal plate was placed in the centre of a metal vessel containing an emanating thorium compound, more excited activity was produced on the sides and corners of the plate than at the central part.
A difficulty however arises in connection with the positive charge of the carrier. According to the view developed in section 136 and later in chapters X and XI, the active matter which is deposited on bodies and gives rise to excited activity, is itself derived from the emanation. The emanations of thorium and radium emit only α rays, i.e. positively charged particles. After the expulsion of an α particle, the residue, which is supposed to constitute the primary matter of the active deposit, should retain a negative charge, and be carried to the anode in an electric field. The exact opposite however is observed to be the case. The experimental evidence does not support the view that the positively charged α particles, expelled from the emanation, are directly responsible for the phenomena of excited activity; for no excited activity is produced in a body exposed to the α rays of the emanation, provided the emanation itself does not come in contact with it.
There has been a tendency to attach undue importance to this apparent discrepancy between theory and experiment. The difficulty is not so much to offer a probable explanation of the results as to select from a number of possible causes. While there can be little doubt that the main factor in the disintegration of the atom consists in the expulsion of an α particle carrying a positive charge, a complicated series of processes probably occurs before the residue of the atom is carried to the negative electrode. The experimental evidence suggests that one or more negative electrons of slow velocity escape from the atom at the same time as the particle. This is borne out by the recent discovery that the particle expelled from radium, freed from the ordinary β rays, and also from polonium, is accompanied by a number of slowly moving and consequently easily absorbed electrons. If two negative electrons escaped at the same time as the α particle, the residue would be left with a positive charge and would be carried to the negative electrode. There is also another experimental point which is of importance in this connection. In the absence of an electric field, the carriers remain in the gas for a considerable time and undergo their transformation in situ. There is also some evidence (section 227) that, even in an electric field, the carriers of the active deposit are not swept to the electrode immediately after the break up of the emanation, but remain some time in the gas before they gain a positive charge. It must be remembered that the atoms of the active deposit do not exist as a gas and by the process of diffusion would tend to collect together to form aggregates. These aggregates would act as small metallic particles, and, if they were electro-positive in regard to the gas, would gain a positive charge from the gas.
There can be little doubt that the processes occurring between the break up of the emanation and the deposit of the residue in the cathode in an electric field are complicated, and further careful experiment is required to elucidate the sequence of the phenomena.
Whatever view is taken of the process by which these carriers obtain a positive charge, there can be little doubt that the expulsion of an α particle with great velocity from the atom of the emanation must set the residue in motion. On account of the comparatively large mass of this residue, the velocity acquired will be small compared with that of the expelled α particle, and the moving mass will rapidly be brought to rest at atmospheric pressure by collision with the gas molecules in its path. At low pressures, however, the collisions will be so few that it will not be brought to rest until it strikes the boundaries of the vessel. A strong electric field would have very little effect in controlling the motion of such a heavy mass, unless it has been initially brought to rest by collision with the gas molecules. This would explain why the active matter is not deposited on the cathode at low pressures in an electric field. Some direct evidence of a process of this character, obtained by Debierne on examination of the excited activity produced by actinium, is discussed in section 192.
=191.= The following method has been employed by the writer[291] to determine the velocity of the positive carriers of excited activity of radium and thorium in an electric field. Suppose A and B (Fig. 71) are two parallel plates exposed to the influence of the emanation, which is uniformly distributed between them. If an alternating E.M.F. E₀ is applied between the plates, the same amount of excited activity is produced on each electrode. If, in series with the source of the alternating E.M.F., a battery of E.M.F. E₁ less than E₀ is placed, the positive carrier moves in a stronger electric field in one half alternation than in the other. A carrier consequently moves over unequal distances during the two half alternations, since the velocity of the carrier is proportional to the strength of the electric field in which it moves. The excited activity will in consequence be unequally distributed over the two electrodes. If the frequency of alternation is sufficiently great, only the positive carriers within a certain small distance of one plate can be conveyed to it, and the rest, in the course of several succeeding alternations, are carried to the other plate.
When the plate B is negatively charged, the E.M.F. between the plates is E₀ − E₁, when B is positive the E.M.F. is E₀ + E₁.
Let d = distance between the plates, T = time of a half alternation, ρ = ratio of the excited radio-activity on the plate B to the sum of the radio-activities on the plates A and B, K = velocity of the positive carriers for a potential-gradient of 1 volt per centimetre.
On the assumption that the electric field between the plates is uniform, and that the velocity of the carrier is proportional to the electric field, the velocity of the positive carrier towards B is
E₀ − E₁ ---------- K d
and, in the course of the next half alternation,
E₀ + E₁ ---------- K d
towards the plate A.
If x₁ is less than d, the greatest distances x₁, x₂ passed over by the positive carrier during two succeeding half alternations is thus given by
E₀ − E₁ x₁ = ---------- KT d
and
E₀ + E₁ x₂ = ---------- KT d
Suppose that the positive carriers are produced at a uniform rate of q per second for unit distance between the plates. The number of positive carriers which reach B during a half alternation consists of two parts:
(1) One half of those carriers which are produced within the distance x₁ of the plate B. This number is equal to
1 --- x₁ qT 2
(2) All the carriers which are left within the distance x₁ from B at the end of the previous half alternation. The number of these can readily be shown to be
1 x₁ --- x₁ ---- qT 2 x₂
The remainder of the carriers, produced between A and B during a complete alternation, will reach the other plate A in the course of succeeding alternations, provided no appreciable recombination takes place. This must obviously be the case, since the positive carriers travel further in a half alternation towards A than they return towards B during the next half alternation. The carriers thus move backwards and forwards in the changing electric field, but on the whole move towards the plate A.
The total number of positive carriers produced between the plates during a complete alternation is 2_dqT_. The ratio ρ of the number which reach B to the total number produced is thus given by
$$ \rho = \frac {\frac {1}{2} x_1qT + \frac {1}{2} x_1 \frac {x_1}{x_2} qT} {2dqT} = \frac {1}{4} \frac {x_1}{d} \frac {x_1 + x_2} {x_2} $$ .
Substituting the values of x₁ and x₂, we find that
$$ K = \frac {2 (E₀ + E_1) d^2} {E₀ (E₀ − E_1) T} \rho $$ .
In the experiments, the values of E₀, E₁, d, and T were varied, and the results obtained were in general agreement with the above equation.
The following were the results for thorium:
Plates 1·30 cms. apart.
E₀ + E₀ − Alternations ρ K E₁ E₁ per second
152 101 57 ·27 1·25
225 150 57 ·38 1·17
300 200 57 ·44 1·24
Plates 2 cms. apart.
E₀ + E₀ − Alternations ρ K E₁ E₁ per second
273 207 44 ·37 1·47
300 200 53 ·286 1·45
The average mobility K deduced from a large number of experiments was 1·3 cms. per sec. per volt per cm. for atmospheric pressure and temperature. This velocity is about the same as the velocity of the positive ion produced by Röntgen rays in air, viz. 1·37 cms. per sec. The results obtained with the radium emanation were more uncertain than those for thorium on account of the distribution of some excited activity on the positive electrode. The values of the velocities of the carriers were however found to be roughly the same for radium as for thorium.
These results show that the carriers of the active deposit travel in the gas with about the same velocity as the positive or negative ions produced by the radiations in the gas. This indicates either that the active matter becomes attached to positive ions, or that the active matter itself, acquiring in some way a positive charge, collects a cluster of neutral molecules which travel with it.
=192. Carriers of the excited activity from actinium and “emanium.”= Giesel[292] observed that “emanium” gave off a large quantity of emanation, and that this emanation gave rise to a type of radiation which he termed the E rays. A narrow metal cylinder containing the active substance was placed with the open end downwards, about 5 cms. above the surface of a zinc sulphide screen. The screen was charged negatively to a high potential by an electric machine, and the cylinder connected with earth. A luminous spot of light was observed on the screen, which was brighter at the edge than at the centre. A conductor, connected with earth, brought near the luminous spot apparently repelled it. An insulator did not show such a marked effect. On removal of the active substance, the luminosity of the screen persisted for some time. This was probably due to the excited activity produced on the screen.
The results obtained by Giesel support the view that the carriers of excited activity of “emanium” have a positive charge. In a strong electric field the carriers travel along the lines of force to the cathode, and there cause excited activity on the screen. The movement of the luminous zone on the approach of a conductor is due to the disturbance of the electric field. Debierne[293] found that actinium also gave off a large amount of emanation, the activity of which decayed very rapidly with the time, falling to half value in 3·9 seconds.
This emanation produces excited activity on surrounding objects, and at diminished pressure the emanation produces a uniform distribution of excited activity in the enclosure containing the emanation. The excited activity falls to half value in 41 minutes.
Debierne observed that the distribution of excited activity was altered by a strong magnetic field. The experimental arrangement is shown in Fig. 71A. The active matter was placed at M, and two plates A and B were placed symmetrically with regard to the source. On the application of a strong magnetic field normal to the plane of the paper, the excited activity was unequally distributed between the plates A and B. The results showed that the carriers of excited activity were deviated by a magnetic field in the opposite sense to the cathode rays, i.e. the carriers were positively charged. In some cases, however, the opposite effect was obtained. Debierne considers that the excited activity of actinium is due to “ions activants,” the motion of which is altered by a magnetic field. Other experiments showed that the magnetic field acted on the “ions activants” and not on the emanation.
The results of Debierne thus lead to the conclusion that the carriers of excited activity are derived from the emanation and are projected with considerable velocity. This result supports the view, advanced in section 190, that the expulsion of α particles from the emanation must set the part of the system left behind in rapid motion. A close examination of the mode of transference of the excited activity by actinium and the emanation substance is likely to throw further light on the processes which give rise to the deposit of active matter on the electrodes.
Footnote 269:
M. and Mme. Curie, C. R. 129, p. 714, 1899.
Footnote 270:
Rutherford, Phil. Mag. Jan. and Feb. 1900.
Footnote 271:
As regards date of publication, the priority of the discovery of “excited activity” belongs to M. and Mme. Curie. A short paper on this subject, entitled “Sur la radioactivité provoquée par les rayons de Becquerel,” was communicated by them to the Comptes Rendus, Nov. 6, 1899. A short note was added to the paper by Becquerel in which the phenomena of excited activity were ascribed to a type of phosphorescence. On my part, I had simultaneously discovered the emission of an emanation from thorium compounds and the excited activity produced by it, in July, 1899. I, however, delayed publication in order to work out in some detail the properties of the emanation and of the excited activity and the connection between them. The results were published in two papers in the Philosophical Magazine (Jan. and Feb. 1900) entitled “A radio-active substance emitted from thorium compounds,” and “Radio-activity produced in substances by the action of thorium compounds.”
Footnote 272:
Rutherford, Phil. Mag. Feb. 1900.
Footnote 273:
Rutherford, Phys. Zeit. 3, No. 12, p. 254, 1902. Phil. Mag. Jan. 1903.
Footnote 274:
Miss Brooks, Phil. Mag. Sept. 1904.
Footnote 275:
Rutherford and Miss Brooks, Phil. Mag. July, 1902.
Footnote 276:
Curie and Danne, C. R. 136, p. 364, 1903.
Footnote 277:
Mme Curie, Thèse, Paris, 1903, p. 116.
Footnote 278:
Debierne, C. R. 138, p. 411, 1904.
Footnote 279:
Giesel, Ber. d. D. Chem. Ges. No. 3, p. 775, 1905.
Footnote 280:
Miss Brooks, Phil. Mag. Sept. 1904.
Footnote 281:
Rutherford, Phys. Zeit. 3, No. 12, p. 254, 1902.
Footnote 282:
F. von Lerch, Annal. d. Phys. 12, p. 745, 1903.
Footnote 283:
Pegram, Phys. Review, p. 424, Dec. 1903.
Footnote 284:
Miss Gates, Phys. Review, p. 300, 1903.
Footnote 285:
A more complete examination of the effect of temperature on the excited activity of thorium has been made by Miss Slater (section 207).
Footnote 286:
Rutherford, Phil. Mag. Feb. 1900.
Footnote 287:
Henning, Annal. d. Phys. 7, p. 562, 1902.
Footnote 288:
Rutherford, Phil. Mag. Feb. 1900.
Footnote 289:
Curie and Debierne, C. R. 132, p. 768, 1901.
Footnote 290:
Fehrle, Phys. Zeit. 3, No. 7, p. 130, 1902.
Footnote 291:
Rutherford, Phil. Mag. Jan. 1903.
Footnote 292:
Giesel, Ber. d. D. Chem. Ges. 36, p. 342, 1903.
Footnote 293:
Debierne, C. R. 136, pp. 446 and 671, 1903; 138, p. 411, 1904.
IX. THEORY OF SUCCESSIVE CHANGES
=193. Introduction.= We have seen in previous chapters that the radio-activity of the radio-elements is always accompanied by the production of a series of new substances with some distinctive physical and chemical properties. For example, thorium produces from itself an intensely radio-active substance, Th X, which can be separated from the thorium in consequence of its solubility in ammonia. In addition, thorium gives rise to a gaseous product, the thorium emanation, and also to another substance which is deposited on the surface of bodies in the neighbourhood of the thorium, where its presence is indicated by the phenomenon known as “excited activity.”
A close examination of the origin of these products shows that they are not produced simultaneously, but arise in consequence of a succession of changes originating in the radio-element. Thorium first of all gives rise to the product Th X. The Th X produces from itself the thorium emanation, and this in turn is transformed into a non-volatile substance. A similar series of changes is observed in radium, with the exception that there is no product in radium corresponding to the Th X in the case of thorium. Radium first of all produces an emanation, which, like thorium, is transformed into a non-volatile substance. In uranium only one product, Ur X, has been observed, for uranium does not give off an emanation and in consequence does not produce excited activity on bodies.
As a typical example of the evidence, from which it is deduced that one substance is the parent of another, we will consider the connection of the two products Th X and the thorium emanation. It has been shown (section 154) that after the separation of Th X from a thorium solution, by precipitation with ammonia, the precipitated thorium hydroxide has lost to a large extent its power of emanating. This cannot be ascribed to a prevention of escape of the emanation produced in it, for very little emanation is observed when a current of air is drawn through the hydroxide in a state of solution, when most of the emanation present would be carried off. On the other hand, the solution containing the Th X gives off a large quantity of emanation, showing that the power of giving off an emanation belongs to the product Th X. Now it is found that the quantity of emanation given off by the separated Th X decreases according to an exponential law with the time, falling to half value in four days. The rate of production of emanation thus falls off according to the same law and at the same rate as the activity of the Th X measured in the ordinary manner by the α rays. Now this is exactly the result to be expected if the Th X is the parent of the emanation, for the activity of Th X at any time is proportional to its rate of change, i.e., to the rate of production of the secondary type of matter by the emanation in consequence of a change in it. Since the rate of change of the emanation (half transformed in 1 minute) is very rapid compared with the rate of change of Th X, the amount of emanation present will be practically proportional to the activity of the Th X at any instant, i.e., to the amount of unchanged Th X present. The observed fact that the hydroxide regains its power of emanating in the course of time is due to the production of fresh Th X by the thorium, which in turn produces the emanation.
In a similar way, excited activity is produced on bodies over which the emanation is passed, and in amount proportional to the activity of the emanation, i.e., to the amount of the emanation present. This shows that the active deposit, which gives rise to the phenomenon of excited activity, is itself a product of the emanation. The evidence thus seems to be conclusive that Th X is the parent of the emanation and that the emanation is the parent of the deposited matter.
=194. Chemical and Physical properties of the active products.= Each of these radio-active products is marked by some distinctive chemical and physical properties which differentiate it from the preceding and succeeding products. For example, Th X behaves as a solid. It is soluble in ammonia, while thorium is not. The thorium emanation behaves as a chemically inert gas and condenses at a temperature of −120° C. The active deposit from the emanation behaves as a solid and is readily soluble in sulphuric and hydrochloric acids and is only slightly soluble in ammonia.
The striking dissimilarity which exists in many cases between the chemical and the physical properties of the parent matter and the product to which it gives rise is very well illustrated by the case of radium and the radium emanation. Radium is an element so closely allied in chemical properties to barium that, apart from a slight difference in the solubility of the chlorides and bromides, it is difficult to distinguish chemically between them. It has a definite spectrum of bright lines similar in many respects to the spectra of the alkaline earths. Like barium, it is non-volatile at ordinary temperature. On the other hand, the emanation which is continually produced from radium is a radio-active and chemically inert gas, which is condensed at a temperature of −150° C. Both in its spectrum and in the absence of definite chemical properties, it resembles the argon-helium group of inert gases, but differs from these gases in certain marked features.
The emanation must be considered to be an unstable gas which breaks down into a non-volatile type of matter, the disintegration being accompanied by the expulsion of heavy atoms of matter (α particles) projected with great velocity. This rate of breaking up is not affected by temperature over the considerable range which has been examined. After a month’s interval, the volume of the emanation has shrunk to a small portion of its initial value. But the most striking property of the emanation, which, as we shall see later (chapter XII), is a direct consequence of its radio-activity, is the enormous amount of energy emitted from it. The emanation in breaking up through its successive stages emits about 3 million times as much energy as is given out by the explosion of an equal volume of hydrogen and oxygen, mixed in the proper proportions to form water; and yet, in this latter chemical reaction more heat is emitted than in any other known chemical change.
We have seen that the two emanations and the products Ur X, Th X lose their activity with the time according to a simple exponential law, and at a rate that is independent—as far as observation has gone—of the chemical and physical agents at our disposal. The time taken for each of these products to fall to half its value is thus a definite physical constant which serves to distinguish it from all other products.
On the other hand, the variation of the excited activity produced by these emanations does not even approximately obey such a law. The rate of decay depends not only on the time of exposure to the respective emanations, but also, in the case of radium, on the type of radiation which is used as a means of comparative measurement. It will be shown, in succeeding chapters, that the complexity of the decay is due to the fact that the matter in the active deposits undergoes several successive transformations, and that the peculiarities of the curves of decay, obtained under different conditions, can be explained completely on the assumption that two changes occur in the active deposit from both thorium and actinium and six in the active deposit from radium.
=195. Nomenclature.= The nomenclature to be applied to the numerous radio-active products is a question of great importance and also one of considerable difficulty. Since there are at least seven distinct substances produced from radium, and probably five from thorium and actinium, it is neither advisable nor convenient to give each a special name such as is applied to the parent elements. At the same time, it is becoming more and more necessary that each product should be labelled in such a way as to indicate its place in the succession of changes. This difficulty is especially felt in discussing the numerous changes in the active deposits from the different emanations. Many of the names attached to the products were given at the time of their discovery, before their position in the scheme of changes was understood. In this way the names Ur X, Th X were applied to the active residues obtained by chemical treatment of uranium and thorium. Since, in all probability, these substances are the first products of the two elements, it may be advisable to retain these names, which certainly have the advantage of brevity. The name “emanation” was originally given to the radio-active gas from thorium, and has since been applied to the similar gaseous products of radium and actinium.
Finding the name “radium emanation” somewhat long and clumsy, Sir William Ramsay[294] has recently suggested “ex-radio” as an equivalent. This name is certainly brief and is also suggestive of its origin; but at least six other ex-radios, whose parentage is as certain as that of the emanation, remain unnamed. A difficulty arises in applying the corresponding names ex-thorio, ex-actinio to the other gaseous products, for, unlike radium, the emanations of thorium and actinium are probably the second, not the first, disintegration product of the radio-elements in question. Another name thus has to be applied to the first product in these cases. It may be advisable to give a special name to the emanation, since it has been the product most investigated and was the first to be isolated chemically; but, on the other hand, the name “radium emanation” is historically interesting, and suggests a type of volatile or gaseous matter. Since the term “excited” or “induced” activity refers only to the radiations from the active body, a name is required for the radiating matter itself. The writer in the first edition of this book suggested the name “emanation X.”[295] This title was given from analogy to the names Ur X and Th X, to indicate that the active matter was product of the emanation. The name, however, is not very suitable, and, in addition, can only be applied to the initial product deposited, and not to the further products of its decomposition. It is very convenient in discussing mathematically the theory of successive changes to suppose that the deposited matter called A is changed into B, B into C, C into D, and so on. I have therefore discarded the name emanation X, and have used the terms radium A, radium B, and so on, to signify the successive products of the decomposition of the emanation of radium. A similar nomenclature is applied to thorium and actinium. This system of notation is elastic and simple, and I have found it of great convenience in the discussion of successive products. In speaking generally of the active matter, which causes excited activity, without regard to its constituents, I have used the term “active deposit.” The scheme of nomenclature employed in this book is clearly shown below:—
Radium Thorium Uranium Actinium
Radium Th X Ur X Actinium X emanation
Radium A Thorium Final Actinium (Active) emanation product emanation
Radium B Thorium Actinium (Active) A A (Active) (Active)
Radium C Thorium Actinium (Active) B B (Active) (Active)
Radium D Thorium Actinium (Active) C C (final) (final)
&c.
Each product on this scheme is the parent of the product below it. Since only two products have been observed in the active deposit of thorium and actinium, thorium C and actinium C respectively refer to their final inactive products. It will be shown in the next chapter that, as in the case of thorium, an intermediate product exists between actinium and its emanation. From analogy to the products Th X and Ur X, this substance is termed “actinium X.”
=196. Theory of Successive Changes.= Before considering the evidence from which these changes are deduced, the general theory of successive changes of radio-active matter will be considered. It is supposed that the matter A changes into B, B into C, C into D, and so on.
Each of these changes is supposed to take place according to the same law as a monomolecular change in chemistry, i.e., the number N of particles unchanged after a time t is given by
$$ M = N₀ e^{–λt} $$,
where N₀ is the initial number and λ the constant of the change.
Since dN/dt = -λ_N_, the rate of change at any time is always proportional to the amount of matter unchanged. It has previously been pointed out that this law of decay of the activity of the radio-active products is an expression of the fact that the change is of the same type as a monomolecular chemical change.
Suppose that P, Q, R represent the number of particles of the matter A, B, and C respectively at any time t. Let λ₁, λ₂, λ₃ be the constants of change of the matter A, B, and C respectively.
Each atom of the matter A is supposed to give rise to one atom of the matter B, one atom of B to one of C, and so on.
The expelled “rays” or particles are non-radio-active, and so do not enter into the theory.
It is not difficult to deduce mathematically the number of atoms of P, Q, R, ... of the matter A, B, C, ... existing at any time t after this matter is set aside, if the initial values of P, Q, R, ...are given. In practice, however, it is generally only necessary to employ three special cases of the theory which correspond, for example, to the changes in the active deposit, produced on a wire exposed to a constant amount of radium emanation and then removed, (1) when the time of exposure is extremely short compared with the period of the changes, (2) when the time of exposure is so long that the amount of each of the products has reached a steady limiting value, and (3) for any time of exposure.
There is also another case of importance which is practically a converse of Case 3, viz. when the matter A is supplied at a constant rate from a primary source and the amounts of A, B, C are required at any subsequent time. The solution of this can, however, be deduced immediately from Case 3 without analysis.
=197.= CASE 1. Suppose that the matter initially considered is all of one kind A. It is required to find the number of particles P, Q, R of the matter A, B, C respectively present after any time t.
Then
$$ P = ne^{–λ_1 t} $$,
if n is the number of particles of A initially present. Now dQ, the increase of the number of particles of the matter B per unit time, is the number supplied by the change in the matter A, less the number due to the change of B into C, thus
dQ/dt = λ₁_P_ − λ₂_Q_ (1). Similarly dR/dt = λ₂_Q_ − λ₃_R_ (2).
Substituting in (1) the value of P in terms of n,
$$ \frac {dQ} {dt} = λ_1 ne^{–λ_1 t} − λ_2 Q $$ .
The solution of this equation is of the form
$$ Q = n (ae^{–λ_1 t} + be^{–λ_2 t}) $$ ......(3).
By substitution it is found that a = λ₁/(λ₂ − λ₁).
Since Q = 0 when t = 0, b = –λ₁(λ₂ − λ₁).
Thus
$$ Q = \frac {nλ_1} {λ_1 − λ_2} (e^{–λ_2 t} - e^{–λ_1 t}) $$ .... (4).
Substituting this value of Q in (2), it can readily be shown that
$$ R = n (ae^{–λ_1 t} + be^{–λ_2 t} + ce^{–λ_3 t}) $$ ..... (5).
where
$$ a = \frac {λ_1 λ_2} {(λ_1 − λ_2) (λ_1 - λ_3)} $$,
$$ b = \frac {- λ_1 λ_2} {(λ_1 − λ_2) (λ_2 − λ_3)} $$,
$$ c = \frac {λ_1 λ_2} {(λ_1 − λ_3) (λ_2 - λ_3)} $$,
The variation of the values of P, Q, R with the time t, after removal of the source, is shown graphically in Fig. 72, curves A, B, and C respectively. In order to draw the curves for the practical case which will be considered later corresponding to the first three changes in radium A, the values of λ₁, λ₂, λ₃ were taken as 3·85 × 10⁻³, 5·38 × 10⁻⁴, 4·13 × 10⁻⁴ respectively, i.e., the times required for each successive type of matter to be half transformed are about 3, 21, and 28 minutes respectively.
The ordinates of the curves represent the relative number of atoms of the matter A, B, and C existing at any time, and the value of n, the original number of atoms of the matter A deposited, is taken as 100. The amount of matter B is initially zero, and in this particular case, passes through a maximum about 10 minutes later, and then diminishes with the time. In a similar way, the amount of C passes through a maximum about 37 minutes after removal. After an interval of several hours the amount of both B and C diminishes very approximately according to an exponential law with the time, falling to half value after intervals of 21 and 28 minutes respectively.
=198.= CASE 2. A primary source supplies the matter A at a constant rate and the process has continued so long that the amount of the products A, B, C, ... has reached a steady limiting value. The primary source is then suddenly removed. It is required to find the amounts of A, B, C, ... remaining at any subsequent time t.
In this case, the number n₀ of particles of A, deposited per second from the source, is equal to the number of particles of A which change into B per second, and of B into C, and so on. This requires the relation
n₀ = λ₁_P₀_ = λ₂_Q₀_ = λ₃_R₀_ (6),
where P₀, Q₀, R₀ are the maximum numbers of particles of the matter A, B, and C when a steady state is reached.
The values of P, Q, R at any time t after removal of the source are given by equations of the same form as (3) and (5) for a short exposure. Remembering the condition that initially
P = P₀ = n₀/λ₁, Q = Q₀ = n₀/λ₂, R = R₀ = n₀/λ₃,
it can readily be shown that
$$ P = \frac {n₀} {λ_1} e^{–λ_1 t} $$ .... (7),
$$ Q = \frac {n₀} {λ_1 − λ_2} (\frac {λ_1} {λ_2} e^{–λ_2 t} − e^{–λ_1 t}) $$ .... (8),
$$ R = n₀ (ae^{–λ_1 t} + be^{–λ_2 t} + ce^{–λ_3 t}) $$ .... (9),
where
$$ a = \frac {λ_2} {(λ_1 − λ_2) (λ_1 - λ_3)} $$,
$$ b = \frac {–λ_1} {(λ_1 − λ_2) (λ_2 - λ_3)} $$,
$$ c = \frac {λ_1 λ_2} {λ_3 (λ_1 − λ_3) (λ_2 − λ_3)} $$ .
The relative numbers of atoms of P, Q, R existing at any time are shown graphically in Fig. 73, curves A, B, C respectively. The number of atoms R₀ is taken as 100 for comparison, and the values of λ₁, λ₂, λ₃ are taken corresponding to the 3, 21, and 28-minute changes in the active deposit of radium. A comparison with Fig. 72 for a short exposure brings out very clearly the variation in the relative amounts of P, Q, R in the two cases. Initially the amount of R decreases very slowly. This is a result of the fact that the supply of C due to the breaking up of B at first, nearly compensates for the breaking up of C. The values of Q and R after several hours decrease exponentially, falling to half value in 28 minutes.
=199.= CASE 3. Suppose that a primary source has supplied the matter A at a constant rate for any time T and is then suddenly removed. Required the amounts of A, B, C at any subsequent time.
Suppose that n₀ particles of the matter A are deposited each second. After a time of exposure T, the number of particles P{T}_ of the matter A present is given by
$$ P_T = n₀ \int₀^T e^{–λ_1 t} dt = \frac {n₀} {λ_1} (1 - e^{–λ_1 T}) $$ .
At any time t, after removal of the source, the number of particles P of the matter A is given by
$$ P = P_T e^{–λ_1 t} = \frac {n₀} {λ_1} (1 − e^{–λ_1 T}) e^{–λ_1 t} $$ .
Consider the number of particles n₀dt of the matter A produced during the interval dt. At any later time t, the number of particles dQ of the matter B, which result from the change in A, is given (see equation 4) by
$$ dQ = \frac {n₀ λ_1} {λ_1 − λ_2} (e^{–λ_2 t} − e^{–λ_1 t}) dt = n₀ f(t) dt $$ .... (10).
After a time of exposure T, the number of particles Q{T}_ of the matter B present is readily seen to be given by
$$ Q_T = n₀ (f(T) dt + f(T − dt) dt + ... + f(0) dt) = n₀ \int₀^T f(t) dt $$ .
If the body is removed from the emanation after an exposure T, at any later time t the number of particles of B is in the same way given by
$$ Q = n₀ \int_t^{T+t} f(t) dt $$ .
It will be noted that the method of deduction of Q{T}_ and Q is independent of the particular form of the function f(t).
Substituting the particular value of f(t) given in equation (10) and integrating, it can readily be deduced that
$$ \frac {Q} {Q_T} = \frac {ae^{–λ_2 t} − be^{–λ_1 t}} {a - b} $$ .... (11),
where
$$ a = \frac {(1 − e^{–λ_2 T} {λ_2} $$,
$$ b = \frac {(1 − e^{–λ_1 T} {λ_1} $$,
In a similar way, the number of particles R of the matter C present at any time can be deduced by substitution of the value of f(t) in equation (5). These equations are, however, too complicated in form for simple application to experiment, and will not be considered here.
=200.= CASE 4. The matter A is supplied at a constant rate from a primary source. Required to find the number of particles of A, B, C at any subsequent time t, when initially A, B, C are absent.
The solution can be simply obtained in the following way. Suppose that the conditions of Case 2 are fulfilled. The products A, B, C are in radio-active equilibrium and let P₀, Q₀, R₀ be the number of particles of each present. Suppose the source is removed. The values of P, Q, R at any subsequent time are given by equations (7), (8) and (9) respectively. Now suppose the source, which has been removed, still continues to supply A at the same constant rate and let P₁, Q₁, R₁ be the number of particles of A, B, C again present with the source at any subsequent time. Now we have seen, that the rate of change of any individual product, considered by itself, is independent of conditions and is the same whether the matter is mixed with the parent substance or removed from it. Since the values of P₀, Q₀, R₀ represent a steady state where the rate of supply of each kind of matter is equal to its rate of change, the sum of the number of particles A, B, C present at any time with the source, and in the matter from which it was removed, must at all times be equal to P₀, Q₀, R₀, ..., that is
P₁ + P = P₀, Q₁ + Q = Q₀, R₁ + R = R₀.
This must obviously be the case, for otherwise there would be a destruction or creation of matter by the mere process of separation of the source from its products; but, by hypothesis, neither the rate of supply from the source, nor the law of change of the products, has been in any way altered by removal.
Substituting the values of P, Q, R from equations (7), (8), and (9), we obtain
$$ \frac {P_1} {P₀} = 1 − e^{–λ_1 t} $$,
$$ \frac {Q_1} {Q₀} = 1 − \frac {(λ_1 e^{–λ_2 t} - λ_2e^{–λ_1 t})} {λ_1 − λ_2} $$,
$$ \frac {R_1} {R₀} = 1 − λ_3 (ae^{–λ_1 t} + be^{–λ_2 t} + ce^{–λ_3 t}) $$,
where a, b, and c have the values given after equation (9). The curves representing the increase of P, Q, R, are thus, in all cases, complementary to the curves shown in Fig. 73. The sum of the ordinates of the two curves of rise and decay at any time is equal to 100. We have already seen examples of this in the case of the decay and recovery curves of Ur X and Th X.
=201. Activity of a mixture of products.= In the previous calculations we have seen how the number of particles of each of the successive products varies with the time under different conditions. It is now necessary to consider how this number is connected with the activity of the mixture of products.
If N is the number of particles of a product, the number of particles breaking up per second is λ_N_, where λ is the constant of change. If each particle of each product, in breaking up, emits one α particle, we see that the number of α particles expelled per second from the mixture of products at any time is equal to λ₁_P_ + λ₂_Q_ + λ₃_R_ + ..., where P, Q, R, ... are the numbers of particles of the successive products A, B, C, .... Substituting the values of P, Q, R already found from any one of the four cases previously considered, the variation of the number of α particles expelled per second with the time can be determined.
The ideal method of measuring the activity of any mixture of radio-active products would be to determine the number of α or β particles expelled from it per second. In practice, however, this is inconvenient and also very difficult experimentally.
Certain practical difficulties arise in endeavouring to compare the activity of one product with another. We shall see later that, in many cases, all of the successive products do not emit α rays. Some give out β and γ rays alone, while there are several “rayless” products, that is, products which do not emit either α, β, or γ rays. In the case of radium, for example, radium A gives out only α rays, radium B no rays at all, while radium C gives out α, β, and γ rays.
In practice, the relative activity of any individual product at any time is usually determined by relative measurements of the saturation ionization current produced between the electrodes of a suitable testing vessel.
Let us consider, for example, the case of a product which gives out only α rays. The passage of the α particles through the gas produces a large number of ions in its path. Since the α particles from any individual product are projected with the same average velocity under all conditions, the relative amount of the ionization produced per second in the testing vessel serves as an accurate means of determining the variation of its activity. No two products, however, emit α particles with the same average velocity. We have seen that the rays from some products are more readily stopped in the gas than others. Thus the relative saturation current, due to two different products in a testing vessel, does not serve as an accurate method of comparing the relative number of α particles expelled per second. The ratio of the currents will in general depend upon the distance between the plates of the testing vessel, and, unless the relative ionization due to the average α particle from the two products is known from other data, the comparison of the currents can, at best, be only an approximate guide to the relative number of α particles escaping into the gas.
=202.= Some examples will now be considered to show how the factors, above considered, influence the character of the curves of activity obtained under different experimental conditions. For the purpose of illustration, we shall consider the variation after removal of the excited activity on a body exposed for different times to a constant supply of the radium emanation. The active deposit on removal consists in general of a mixture of the products radium A, B, and C. The nature of the rays from each product, the time for each product to be transformed, and the value of λ are tabulated below for convenience:—
Product Rays T. λ (sec⁻¹)
Radium A α rays 3 min. 3·85 × 10⁻³
Radium B no rays 21 min. 5·38 × 10⁻⁴
Radium C α, β, γ 28 min. 4·13 × rays 10⁻⁴
Since only the product C gives rise to β and γ rays, the activity measured by either of these types of rays will be proportional to the amount of C present at any time, i.e. to the value of R at any time. For a long exposure, the variation of activity with time measured by the β and γ rays will thus be represented by the upper curve CC of Fig. 73, where the ordinates represent activity. This curve will be seen to be very similar in shape to the experimental curve for a long exposure which is given in Fig. 68.
Since radium B does not give out rays, the number of α particles expelled from the active deposit per second is proportional to λ₁_P_ + λ₃_R_. The activity measured by the α rays, using the electrical method, is thus proportional at any time to λ₁_P_ + _K_λ₃_R_, where K is a constant which represents the ratio of the number of ions, produced in the testing vessel, by an α particle from C compared with that from an α particle emitted by A.
It will be seen later that, for this particular case, K is nearly unity. Taking K = 1, the activity at any time after removal is proportional to λ₁_P_ + λ₃_R_.
CASE 1. We shall first consider the activity curve for a short exposure to the radium emanation. The relative values of P, Q, and R at any time corresponding to this case are graphically shown in Fig. 74. The activity measured by the α rays at any time will be the sum of the activities due to A and C separately.
Let curve AA (Fig. 74) represent the activity due to A. This decreases exponentially, falling to half value in 3 minutes. In order to show the small activity due to C clearly in the Figure, the activity due to A is plotted after an interval of 6 minutes, when the activity has been reduced to 25 per cent. of its maximum value. The activity due to C is proportional to λ₃_R_, and in order to represent the activity due to C to the same scale as A, it is necessary to reduce the scale of the ordinates of curve CC in Fig. 72 in the ratio λ₃/λ₁.
The activity due to C is thus represented by the curve CCC, Fig. 74. The total activity is thus represented by a curve A + C whose ordinates are the sum of the ordinates of A and C.
This theoretical activity curve is seen to be very similar in its general features to the experimental curve shown in Fig. 66, where the activity from a very short exposure is measured by the α rays.
CASE 2. The activity curve for a long exposure to the emanation will now be considered. The activity after removal of A and C is proportional to λ₁_P_ + λ₃_R_, where the values of P and R are graphically shown in Fig. 75 by the curves AA, CC. Initially after removal, λ₁_P₀_ = λ₃_R₀_, since A and C are in radio-active equilibrium, and the same number of particles of each product break up per second. The activity due to A alone is shown in curve AA, Fig. 75. The activity decreases exponentially, falling to half value in 3 minutes. The activity due to C at any time is proportional to R, and is initially equal to that of A. The activity curve due to C is thus represented by the curve CC, which is the same curve as the upper curve CC of Fig. 73. The activity of A and C together is represented by the upper curve A + C (Fig. 75), where the ordinates are equal to the sum of the ordinates of the curves A and C. This theoretical curve is seen to be very similar in shape to the experimental curve (Fig. 67) showing the decay of activity of the active deposit from a long exposure measured by the α rays.
=203. Effect of a rayless change on the activity curves.= Certain important cases occur in the analysis of radio-active changes, when one of the products does not give rise to rays and so cannot be detected directly. The presence of this rayless change can, however, be readily observed by the variations which occur in the activity of the succeeding product.
Let us consider, for example, the case where the inactive matter A, initially all of one kind, changes into the matter B which gives out rays. The inactive matter A is supposed to be transformed according to the same law as the radio-active products. Let λ₁, λ₂ be the constants of the change of A and B respectively. If n is the number of particles of A, initially present, we see from the equation (4), section 197, that the number of particles of the matter B present at any time is given by
$$ Q = \frac {nλ_1} {λ_1 − λ_2} (e^{–λ_2 t} - e^{λ_1 t}) $$ .
Differentiating and equating to zero, it is seen that the value of Q passes through a maximum at a time T given by the equation
$$ λ_2 e^{–λ_2 T} = λ_1 e^{–λ_1 T} $$ .
For the sake of illustration, we shall consider the variation of the activity of the active deposit of thorium, due to a very short exposure to the emanation. Thorium A gives out no rays, and thorium B gives out α, β, and γ rays, while thorium C is inactive.
The matter A is half transformed in 11 hours, and B is half transformed in 55 minutes. The value of λ₁ = 1·75 x 10⁻⁵(sec.)⁻¹ and λ₂ = 2·08 x 10⁻⁴(sec.)⁻¹.
The activity of the mixture of products A + B is due to B alone, and will, in consequence, be always proportional to the amount of B present, that is, to the value of Q.
The variation of activity with time is shown graphically in Fig. 76. The activity rises from zero to a maximum in 220 minutes and then decays, finally decreasing, according to an exponential law, with the time, falling to half value in 11 hours.
This theoretical curve is seen to agree closely in shape with the experimental curve (Fig. 65), which shows the variation of the activity of the active deposit of thorium, produced by a short exposure in presence of the emanation.
There are several points of interest in connection with an activity curve of this character. The activity, some hours after removal, decays according to an exponential law, not at the rate of the product B, from which the activity rises, but at the same rate as the first rayless transformation. This will also be the case if the rayless product has a slower rate of change than the succeeding active product. Given an activity curve of the character of Fig. 76, we can deduce from it that the first change is not accompanied by rays and also the period of the two changes in question. We are, however, unable to determine from the curve which of the periods of change refers to the rayless product. It is seen that the activity curve is unaltered if the values of λ₁, λ₂, that is, if the periods of the products are interchanged, for the equation is symmetrical in λ₁, λ₂. For example, in the case of the active deposit of thorium, without further data it is impossible to decide whether the period of the first change has a value of 55 minutes or 11 hours. In such cases the question can only be settled by using some physical or chemical means in order to separate the product A from B, and then testing the rate of decay of their activity separately. In practice, this can often be effected by electrolysis or by utilizing the difference in volatility of the two products. If now a product is separated from the mixture of A and B which loses its activity according to an exponential law, falling to half value in 55 minutes (and such is experimentally observed), we can at once conclude that the active product B has the period of 55 minutes.
The characteristic features of the activity curve shown in Fig. 76 becomes less marked with increase of the time of exposure of a body to the emanation, that is, when more and more of B is mixed with A at the time of removal. For a long time of exposure, when the products A and B are in radio-active equilibrium, the activity after removal is proportional to Q, where
$$ Q = \frac {n₀} {λ_1 − λ_2} (\frac {λ_1} {λ_2} e^{–λ_2 t} − e^{λ_1 t}) $$,
(see equation 8, section 198). The value of Q, in this case, does not increase after removal, but at once commences to diminish. The activity, in consequence, decreases from the moment of removal, but more slowly than would be given by an exponential law. The activity finally decays exponentially, as in the previous case, falling to half value in 11 hours.
In the previous case we have discussed the activity curve obtained when both the active and inactive product have comparatively rapid rates of transformation. In certain cases which arise in the analysis of the changes in actinium and radium, the rayless product has a rate of change extremely slow compared with that of the active product. This corresponds to the case where the active matter B is supplied from A at a constant rate. The activity curve will thus be identical in form with the recovery curves of Th X and Ur X, that is, the activity I at any time t will be represented by the equation
$$ \frac {I_t} {I₀} = 1 − e^{–λ_2 t} $$,
where I₀ is the maximum value of the activity and λ₂ the constant of change of B.
=204.= In this chapter we have considered the variation with time, under different conditions, of the number of atoms of the successive products, when the period and number of the changes are given. It has been seen that the activity curves to be expected under various conditions can be readily deduced from the simple theory. In practice, however, the investigator has been faced with the much more difficult inverse problem of deducing the period, number, and character of the products, by analysis of the activity curves obtained under various conditions.
In the case of radium, where at least seven distinct changes occur, the problem has been one of considerable difficulty, and a solution has only been possible by devising special physical and chemical methods of isolation of some of the products.
We shall see later that two rayless changes occur in radium and actinium and one in thorium. It is at first sight a very striking fact that the presence of a substance which does not emit rays can be detected, and its properties investigated. This is only possible when the rayless product is transformed into another substance which emits rays; for the variation of the activity of the latter may be such as to determine not only the period but also the physical and chemical properties of the parent product. In the two following chapters the application of the theory of successive changes will be shown to account satisfactorily for the complicated processes occurring in the radio-elements.
Footnote 294:
Ramsay, Proc. Roy. Soc. p. 470, June, 1904; C. R. 138, June 6, 1904.
Footnote 295:
Phil. Mag. February, 1904.
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
$$ \frac {I} {I₀} = e^{–λt} $$,
and
$$ \frac {I_t} {I₀} = 1 − e^{–λt} $$,
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[296] and by Rutherford and Grier[297]. 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[298] 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.
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[299], 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[300] 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[301] 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
$$ Q = \frac {nλ_1} {λ_1 − λ_2} (e^{–λ_2 t} - e^{–λ_1 t}) $$ .... (4).
The value of Q{t}_ passes through a maximum Q{T}_ at the time T when
$$ \frac {λ_2} {λ_1} − e^{-(λ_1–λ_2) T} $$ .
The maximum activity I{T}_ is proportional to Q{T}_ and
$$ \frac {I_t} {I_T} = \frac {Q_t} {Q_T} = \frac {e^{–λ_2 t} - e^{–λ_1 t}} {e^{–λ_2 T} − e^{–λ_1 T}} $$ .
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 λ₁, λ₂. Since the above equation is symmetrical with regard to λ₁, λ₂, 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 λ₁ and λ₂ 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 λ₁.
Then λ₁ = 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 λ₁ and T in the equation, the value of λ₂ comes out to be
λ₂ = 2·08 × 10⁻⁴ (sec)⁻¹.
This value of λ₂ corresponds to a change in which half the matter is transformed in 55 minutes.
Substituting now the values of λ₁, λ₂, T, the equation reduces to
$$ \frac {I_t} {I_T} = 1\cdot37 (e^{–λ_2 t} − e^{–λ_2 t}) $$ .
The agreement between the results of the theoretical equation and the observed values is shown in the following table:
Time in Theoretical Observed value minutes value of 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[302] 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[303], 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[304] 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 λ₁ = 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 λ₂ = 2·08 × 10⁻⁴, i.e., in which half the matter is transformed in 55 minutes[305].
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,
$$ \frac {I_t} {I₀} = \frac {Q} {Q₀} = \frac {λ_2} {λ_2 - λ_1} e^{–λ_1 t} − \frac {λ_1} {λ_1 − λ_2} e^{–λ_2 t} $$
$$ = \frac {λ_2 e^{–λ_1 t}} {λ_2 - λ_1} (1 − \cdot083 e^{−1\cdot90 × 10^{−4} t} ) $$ .
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
$$ \frac {I_t} {I₀} = \frac {Q} {Q_T} = \frac {ae^{–λ_2 t} - be^{–λ_1 t}} {a − b} $$,
where I₀ is the initial value of the activity, immediately after removal, and
$$ a = \frac {1 − e^{–λ_2 T}} {λ_2} $$,
$$ b = \frac {1 − e^{–λ_1 T}} {λ_1} $$,
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 λ₁, λ₂ are substituted. Miss Brooks[306] 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.
=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[307] 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 39 intervals of 24 hours
After 3 more precipitations at 22 intervals of 24 hours and 3 at intervals of 8 hours
After 3 more each of 8 hours 24
After 6 more each of 4 hours 25
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 λ₁ and λ₂ 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 λ₁ = ·0072 and of λ₂ = ·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
$$ Q = \frac {λ_1 n₀} {λ_1 − λ_2} (e^{–λ_2 t} - e^{–λ_1 t}) $$,
and the activity I at any time of the two together is proportional to λ₁_P_ + _K_λ₂_Q_, where K is the ratio of the ionization of B compared with that of A.
Then
$$ \frac {I_t} {I₀} = \frac {λ_1 P + Kλ_2 Q} {λ_1 n₀} $$
$$ = e^{–λ_1 t} (1 + \frac {Kλ_2} {λ_2 − λ_1} (1 − e^{-(λ_2 − λ_1) t})) $$,
where I₀ is the initial activity due to n₀ 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}_/I₀ 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
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
$$ \frac {I_t} {I₀} = (1 + \frac {Kλ_2} {λ_2 − λ_1}) e^{–λ_1 t} $$,
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).
A table of the transformation products of thorium is shown below, with some of their physical and chemical properties.
Product Time to be λ (sec)⁻¹ Radiations Physical and half chemical transformed properties
Thorium α rays Insoluble in ammonia
Th. X 4 days 2·00 × α rays Soluble in 10⁻⁶ ammonia
Emanation 54 secs. 1·28 × α rays Inert gas, 10⁻² condenses −120° C.
Thorium A 11 hours 1·75 × no rays Soluble in 10⁻⁵ strong acids. Volatile at a white heat. B can be separated from A by electrolysis and by difference of volatility.
Thorium B 55 mins. 2·1 × α, β, γ rays Same 10⁻⁴
? — — — -
=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[308] 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[309]. Recent work has verified this supposition. Giesel[310] and Godlewski[311] 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.
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
$$ \frac {I_t} {I_T} = \frac {e^{–λ_2 t} − e^{–λ_1 t}} {e^{–λ_2 T} − e^{–λ_1 T}} $$,
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.
Products Time to be Rays Some Physical and half Chemical properties transformed
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 strong acids.
Actinium B 2·15 mins. α, β and γ Volatilized at 100°C. B can be separated from A by electrolysis
Footnote 296:
Soddy, Trans. Chem. Soc. 81, p. 460, 1902.
Footnote 297:
Rutherford and Grier, Phil. Mag. Sept. 1902.
Footnote 298:
Becquerel, C. R. 131, p. 137, 1900.
Footnote 299:
Meyer and Schweidler, Wien Ber. Dec. 1, 1904.
Footnote 300:
Meyer and Schweidler, Wien Ber. 113, July, 1904.
Footnote 301:
Rutherford, Phil. Trans. A. 204, pp. 169–219, 1904.
Footnote 302:
Pegram, Phys. Rev. p. 424, December, 1903.
Footnote 303:
Miss Slater, Phil. Mag. 1905.
Footnote 304:
von Lerch, Ann. de d.] Phys. November, 1903.
Footnote 305:
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.
Footnote 306:
Miss Brooks, Phil. Mag. Sept. 1904.
Footnote 307:
Rutherford and Soddy, Trans. Chem. Soc. 81, p. 837, 1902. Phil. Mag. Nov. 1902.
Footnote 308:
Miss Brooks, Phil. Mag. Sept. 1904.
Footnote 309:
Rutherford, Phil. Trans. A. p. 169, 1904.
Footnote 310:
Giesel, Ber. d. D. Chem. Ges. p. 775, 1905.
Footnote 311:
Godlewski, Nature, p. 294, Jan. 19, 1905.
Footnote 312:
Debierne, C. R. 138, p. 411, 1904.
Footnote 313:
Miss Brooks, Phil. Mag. Sept. 1904.