Thomson 1904/Chapter 6
- Chapter I
- Representation of the Electric Field by Lines of Force
- Chapter II
- Electrical and Bound Mass
- Chapter III
- Effects Due to the Acceleration of Faraday Tubes
- Chapter IV
- The Atomic Structure of Electricity
- Chapter V
- The Constitution of the Atom
- Chapter VI
- Radio-Activity and Radio-Active Substances
Chapter VI
Radio-Activity and Radio-Active Substances
In 1896 Becquerel discovered that uranium and its salts possess the power of giving out rays which, like Rontgen and cathode rays, affect a photographic plate, and make a gas through which they pass a conductor of electricity. In 1898 Schmidt discovered that thorium possesses similar properties. This power of emitting rays is called radio-activity, and substances which possess the power are said to be radio-active.
This property of uranium led to a careful examination of a large number of minerals containing this substance, and M. and Mme. Curie found that some of these, and notably some specimens of pitch-blende, were more radio-active than equal volumes of pure uranium, although only a fraction of these minerals consisted of uranium. This indicated that these minerals contained a substance or substances much more radio-active than uranium itself, and a systematic attempt was made to isolate these substances. After a long investigation, conducted with marvellous skill and perseverance, M. and Mme. Curie, with the collaboration of MM. Bemont and Debierne, succeeded in establishing the existence of three new radio-active substances in pitch-blende: radium associated with the barium in the mineral, and closely resembling it in its chemical properties; polonium associated with the bismuth, and actinium with the thorium. They succeeded in isolating the first of these and determined its combining weight, which was found to be 225. Its spectrum has been discovered and examined by Demarcay. Neither polonium nor actinium has yet been isolated, nor have their spectra been observed. The activity of polonium has been found to be fugitive, dying away in some months after its preparation.
These radio-active substances are not confined to rare minerals. I have lately found that many specimens of water from deep wells contain a radio-active gas, and Elster and Geitel have found that a similar gas is contained in the soil.
These radio-active substances may be expected to be of the greatest possible assistance in the task of investigating problems dealing with the nature of the atom, and with the changes that go on in the atom from time to time. For the properties possessed by these substances are so marked as to make the detection of exceedingly minute quantities of them a matter of comparative ease. The quantity of these substances which can be detected is to the corresponding amount of the other elements which have to be detected by the ordinary methods of chemical analysis, in the proportion of a second to thousands of years. Thus, changes which would have to go on for almost geological epochs with the non-radio-active substances, before they became large enough to be detected, could with radio-active substances prove appreciable effects in the course of a few hours.
Character of the Radiation
Rutherford found that the radiation from uranium, and it has subsequently been found that the same is true for thorium and radium, is made up of three distinct types which he calls the a, /3, and y radiations.
The a radiation is very easily absorbed, being unable to penetrate more than a few millimetres of air at atmospheric pressure, the /8 radiation is much more penetrating, while the y radiation is the most penetrating of all. Investigations of the effects of magnetic and electric forces on these three types of radiation have shown that they are of entirely different characters. Becquerel showed that the ft rays were deflected by electric and magnetic forces, the direction of the deflection showing that the rays carried a charge of negative electricity. He determined, using the method described
in Chapter IV, the value of —, the ratio of the
m
charge to the mass of the carriers of the negative electricity; he found that it was about 10T, and that the velocity for some of the rays was more than twothird s that of light. He thus proved that the ft rays consisted of corpuscles travelling at prodigious speeds.
The a rays are not nearly so easily deflected as the ft rays, but Rutherford has recently shown that they can be deflected, and the direction of deflection shows that they carry a, positive charge. He finds, and his measurements have been con
firmed by Des Coudres, that the ratio of — is 6 X
m
103, and the velocity of these particles is 2 X 10* centimetres per second. The value of — shows that the carriers of the positive electrification have masses comparable with those of ordinary atoms;
thus — for hydrogen is 104 and for helium 2.5 X m
103. The very high velocity with which these are shot out involves an enormous expenditure of energy, a point to which we shall return later. One of the most interesting things about this result is
that the value of — shows that the atoms shot off m
are not the atoms of radium, indicating either that radium is a compound containing lighter elements or else that the atom of radium is disintegrating
into such elements. The value of — for the a
m
rays obtained by Rutherford and Des Coudres suggests the existence of a gas heavier than hydrogen but lighter than helium. The y rays, as far as we know, are not deflected either by magnetic or electric forces.
There is considerable resemblance between a radio-active substance and a substance emitting secondary radiation under the influence of Rontgen rays : the secondary radiation is known to contain radiation of the ($ and y types; and as part of the radiation is exceedingly easily absorbed, being unable to penetrate more than a millimetre or so of air at atmospheric pressure, it is possible that closer investigation may show that a rays, i.e.j positively electrified particles, are present also. This analogy raises the question as to whether there may not, in the case of the body struck by the Rbntgen rays, be a liberation of energy such as we shall see occurs in the case of the radio-active substances, the energy emitted by the radiating substances being greater than the energy in the Rontgen rays falling upon it; this excess of energy being derived from changes taking place in the atoms of the body exposed to the Rontgen rays. This point seems worthy of investigation, for it might lead to a way of doing by external agency what radio-active bodies can do spontaneously, i.e., liberate the energy locked up in the atom.
Emanation from Radio-Active Substances
Rutherford proved that thorium emits something which is radio-active and which is wafted about by currents of air as if it were a gas; in order to avoid prejudging the question as to the physical state in which the substance given off by radium exists, Rutherford called it the "emanation." The emanation can pass through water or the strongest acid and can be raised to temperatures at which platinum is incandescent without suffering any loss of radio-activity. In this inertness it resembles the gases argon and helium, the latter of which is almost always found associated with thorium. The radio-activity of the thorium emanation is very transient, sinking to half its value in about one minute.
The Curies found that radium also gives off a radio-active emanation which is much more persistent than that given off by thorium, taking about four days to sink to half its activity.
There seems every reason for thinking that those emanations are radio-active matter in the gaseous form; they can be wafted from one place to another by currents of air; like a gas they diffuse through a porous plug at a rate which shows that their density is very high. They diffuse gradually through air and other gases. The coefficient of diffusion of the radium emanation through air has been measured by Rutherford and Miss Brooks and they concluded that the density of the emanation was about eighty. The emanation of radium has been liquefied by Rutherford and Soddy; and I have, by the kindness of Professor Dewar, been able to liquefy the radio-active gas found in water from deep wells, which very closely resembles the emanation and is quite possibly identical with it. In short the emanations seem to satisfy every test of the gaseous state that can be applied to them. It is true that they are not capable of detection by any chemical tests of the ordinary type, nor can they be detected by spectrum analysis, but this is only because they are present in very minute quantities — quantities far too small to be detected even by spectrum analysis, a method of detection which is exceedingly rough when compared with the electrical methods which we are able to employ for radio-active substances. It is not, I think, an exaggeration to say that it is possible to detect with certainty by the electrical method a quantity of a radio-active substance less than one-hundred-thousandth part of the least quantity which could be detected by spectrum analysis.
Each portion of a salt of radium or thorium is giving off the emanation, whether that portion be on the inside or the outside of the salt; the emanation coming from the interior of a salt, however, does not escape into the air, but gets entangled in the salt and accumulates. If such a radioactive salt is dissolved in water, there is at first a great evolution of the emanation which has been stored up in the solid salt. The emanation can be extracted from the water either by boiling the water or bubbling air through it. The stored up emanation can also be driven off from salts in the solid state by raising them to a very high temperature.
Induced Radio-Activity
Rutherford discovered that substances exposed to the emanation from thorium become radio-active, and the Curies discovered almost simultaneously that the same property is possessed by the emanation from radium. This phenomenon is called induced radio-activity. The amount of induced radio-activity does not depend upon the nature of the substance on which it is induced; thus, paper becomes as radio-active as metal when placed in contact with the emanations of thorium or radium.
The induced radio-activity is especially developed on substances which are negatively electrified. Thus, if the emanation is contained in a closed vessel, in which a negatively electrified wire is placed, the induced radio-activity is concentrated on the negatively electrified wire, and this induced activity can be detected on negatively electrified bodies when it is too weak to be detected on unelectrified surfaces. The fact that the nature of the induced radio-activity does not depend on the substance in which it is induced points to its being due to a radio-active substance which is deposited from the emanation on substances with which it comes in contact.
Further evidence of this is afforded by an experiment made by Miss Gates, in which the induced radio-activity on a fine wire was, by raising it to incandescence, driven off the wire and deposited on the surrounding surfaces. The induced radio-activity due to the thorium emanation is very different from that due to the radium emanation, for whereas the activity of the thorium emanation is so transient that it drops to half its value in one minute, the induced radio-activity due to it takes about eleven hours to fall in the same proportion. The emanation due to radium, which is much more lasting than the thorium emanation, taking about four days instead of one minute to fall to half its value, gives rise to a very much less durable induced radio-activity, one fall, ing to half its value in about forty minutes instead of, as in the case of thorium, eleven hours. The emanation due to actinium is said only to be active for a few seconds, but the induced radio-activity due to it seems to be nearly as permanent as that due to radium.
Separation of the Active Constituent from Thorium
Rutherford and Soddy, in a most interesting and important investigation, have shown that the radio-activity of thorium is due to the passage of the thorium into a form which they call Th X, which they showed could be separated from the rest of the thorium by chemical means. When this separation has been effected the thorium left behind is for a time deprived of most of its radio-activity, which is now to be found in the T h X. The radio-activity of the thorium X slowly decays while that of the rest of the thorium increases until it has recovered its original activity. While this has been going on, the radio-activity of the Th JThas vanished. The time taken for the radio-activity of the T h X to die away to half its original value has been shown by Rutherford and Soddy to be equal to the time taken by the thorium from which the T 7i X has been separated to recover half its original activity. All these results support the view that the radio-active part of the thorium, the thorium X, is continually being produced from the thorium itself; so that if the activity of thorium X were permanent, the radio-activity of the thorium would continually increase. The radio-activity of the thorium X, however, steadily dies away. This prevents the unlimited increase of the radioactivity of the mixture, which will reach a steady value when the increase in the radio-activity due to the production of fresh T h X is balanced by the decay in the activity of that already produced. The question arises as to what becomes of the Tli X and the emanation when they have lost their radio-activity. This dead ThX, as we may call it, is accumulating all the time in the thorium; but inasmuch as it has lost its radio-activity, we have only the ordinary methods of chemical analysis to rely upon, and as these are almost infinitely less delicate than the tests we can apply to radioactive substances, it might take almost geological epochs to accumulate enough of the dead TJiX to make detection possible by chemical analysis. It seems possible that a careful examination of the minerals in which thorium and radium occur might yield important information. It is remarkable that helium is almost invariably a constituent of these minerals.
You will have noticed how closely, as pointed out by Rutherford and Soddy, the production of radio-activity seems connected with changes taking place in the radio-active substance. Thus, to take the case of thorium, which is the one on which we have the fullest information,we have first the change of thorium into thorium X, then the change of the thorium X into the emanation and the substance forming the a rays. The radioactivity of the emanation is accompanied by a further transformation, one of the products being the substance which produces induced radio-activity.
On this view the substance while radio-active is continually being transformed from one state to another. These transformations may be accompanied by the liberation of sufficient energy to supply that carried off by the rays it emits while radio-active. The very large amount of energy emitted by radio-active substances is strikingly shown by some recent experiments of the Curies on the salts of radium. They find that those salts give out so much energy that the absorption of this by the salt itself is sufficent to keep the temperature of the salt permanently above that of the air by a very appreciable amount — in one of their experiments as much as 1.5° C. It appears from their measurements that a gram of radium gives out enough energy per hour to raise the temperature of its own weight of water from the freezing to the boiling point. This evolution of energy goes on uninterruptedly and apparently without diminution. If, however, the views we have just explained are true, this energy arises from the transformation of radium into other forms of matter, and its evolution must cease when the stock of radium is exhausted; unless, indeed, this stock is continually being replenished by the transformation of other chemical elements into radium.
We may make a rough guess as to the probable duration of a sample of radium by combining the result that a gram of radium gives out 100 calories per hour with Rutherford's result that the a rays are particles having masses comparable with the mass of an atom of hydrogen projected with a velocity of about 2 X 109 centimetres per second; for let us suppose that the heat measured by the Curies is due to the bombardment of the radium salt by these particles, and to get a superior limit to the time the radium will last, let us make the assumption that the whole of the mass of radium gets transformed into the a particles (as a matter of fact we know that the emanation is produced as well as the a particles). Let a? be the life in hours of a grain of radium; then since the gram emits per hour 100 calories, or 4.2 X 109 ergs, the amount of energy emitted by the radium during its life is x X 4.2 X 109 ergs. If jVis the number of a particles emitted in this time, m the mass of one of them in grams, v the velocity, then the energy in the a particles is J Nmv*, but this is to be equal to a? X 4.2 X 10* ergs, hence £ Nm v* = x X 4.2 X 109; but if the gram of radium is converted into the a particles, Nm = 1, and by Rutherford's experiments v = 2
4 V 1018 10' X 10', hence we have « = | Jg * ^ = g
hours, or about 50,000 years.
From this estimate we should expect the life of a piece of radium to be of the order of 50,000 years. This result shows that we could not expect to detect any measurable changes in the space of a few months. In the course of its life the gram of radium will have given out about 5 X 1010 calories, a result which shows that if this energy is derived from transformations in the state of the radium, the energy developed in these transformations must be on a very much greater scale than that developed in any known chemical reactions. On the view we have taken the difference between the case of radium and that of ordinary chemical reactions is that in the latter the changes are molecular, while in the case of radium the changes are atomic, being of the nature of a decomposition of the elements. The example given on page (111) shows how large an amount of energy may be stored up in the atom if we regard it as built up of a number of corpuscles.
We may, I think, get some light on the processes going on in radium by considering the behavior of a model atom of the kind described on page 124, and which may be typified by the case of the corpuscles which when rotating with a high velocity are stable when arranged in a certain way, which arrangement becomes unstable when the energy sinks below a certain value and is succeeded by another configuration. A top spinning about a vertical axis is another model of the same type. This is stable when in a vertical position if the kinetic energy due to its rotation exceeds a certain value. If this energy were gradually to decrease, then, when it reached the critical value, the top would become unstable and would fall down, and in so doing would give a considerable amount of kinetic energy.
Let us follow, then, the behavior of an atom of this type, i.e., one which is stable in one configuration of steady motion when the kinetic energy of the corpuscles exceeds a certain value, but becomes unstable and passes into a different configuration when the kinetic energy sinks below that value. Suppose now that the atom starts with an amount of kinetic energy well above the critical value, the kinetic energy will decrease in consequence of the radiation from the rapidly moving corpuscles; but as long as the motion remains steady the rate of decrease will be exceedingly slow, and it may be thousands of years before the energy approaches the critical value. When it gets close to this value, the motion will be very easily disturbed and there will probably be considerable departure from the configuration for steady motion accompanied by a great increase in the rate at which kinetic energy is loss by radiation. The atom now emits a much greater number of rays and the kinetic energy rapidly approaches the critical value; when it reaches this value the crash comes, the original configuration is broken up, there is a great decrease in the potential energy of the system accompanied by an equal increase in the kinetic energy of the corpuscles. The increase in the velocity of the corpuscles may cause the disruption of the atom into two or more systems, corresponding to the emission of the a rays and the emanation.
If the emanation is an atom of the same type as the original atom, i.e., one whose configuration for steady motion depends on its kinetic energy, the process is repeated for the emanation, but in a very much shorter time, and is repeated again for the various radio-active substances, such as the induced radio-active substance formed out of the emanation.
We have regarded the energy emitted by radium and other radio-active substances as derived from an internal source, i.e., changes in the constitution of the atom; as changes of this kind have not hitherto been recognized, it is desirable to discuss the question of other possible sources of this energy. One source which at once suggests itself is external to the radium. We might suppose that the radium obtained its energy by absorbing some form o£ radiation which is passing through all bodies on the surface of the earth, but which is not absorbed to any extent by any but those which are radio-active. This radiation must be of a very penetrating character, for radium retains its activity when surrounded by thick lead or when placed in a deep cellar. We are familiar with forms of Rontgen rays, and of rays given out by radium itself, which can produce appreciable effects after passing through several inches of lead, so that the idea of the existence of very penetrating radiation does not seem so improbable as it would have done a few years ago. It is interesting to remember that very penetrating radiation was introduced by Le Sage more than a century ago to explain gravitation. Le Sage supposed that the universe was thronged with exceedingly small particles moving with very high velocities. He called these ultra-mundane corpuscles and assumed that they were so penetrating that they could pass through masses as large as the sun or the planets without suffering more than a very slight absorption. They were, however, absorbed to a slight extent and gave up to the bodies through which they passed a small fraction of their momentum. If the direction of the ultramundane corpuscles passing through a body were uniformly distributed, the momentum communicated by them to the body would not tend to move it in one direction rather than another, so that a body A alone in the universe and exposed to bombardment by Le Sage's corpuscles would remain at rest; if, however, there is a second body B in the neighborhood of A, B will shield off from A some of the corpuscles moving in the direction B A; thus, A will not receive as much momentum in this direction as it did when it was alone in the field, but in the latter case it only received enough momentum in this direction to keep it in equilibrium; hence, when B is present, the momentum in the opposite direction will get the upper hand so that A will move in the direction, A B, i.e., will be attracted to B. Maxwell pointed out that this transference of momentum from Le Sage's corpuscles to the body through which they were passing involved the loss of kinetic energy by the corpuscles; and that if the loss of momentum were sufficient to account for gravitation, the kinetic energy lost by the ultra-mundane corpuscles would be sufficient, if converted into heat, to keep the gravitating body white hot. The fact that all bodies are not white hot was urged by Maxwell as an argument against Le Sage's theory. It is not necessary, however, to suppose that the energy of the corpuscles is transformed into heat; we might imagine it transformed into a very penetrating radiation which might escape from the gravitating body. A simple calculation will show that the amount of kinetic energy transformed per second in each gram of the gravitating body must be enormously greater than that given out in the same time by one gram of radium.
"We have seen in the first chapter that waves of electric and magnetic force possess momentum in their direction of propagation; we might therefore replace Le Sage's corpuscles by very penetrating Rontgen rays. Those, if absorbed, would give up momentum to the bodies through which they pass, and similar consideration to those given by Le Sage would show that two bodies would attract each other inversely as the square of the distance between them. If the absorption of these waves per unit volume depended only upon, and was proportional to, the density, the attraction between the bodies would be directly proportional to the product of their masses. It ought to be mentioned that on this view any changes in gravitation would be propagated with the velocity of light; whereas, astronomers believe they have established that it travels with a very much greater velocity.
As in the case of Le Sage's corpuscles, the loss of momentum by the Rontgen rays would be accompanied by a loss of energy; for each unit of momentum lost v units of energy would be lost, v being the velocity of light. If this energy were transformed into that of rays of the same type as the incident rays, a little reflection will show that he absorption of the rays would not produce gravitational attraction. To get such attraction the transformed rays must be of a more penetrating type than the original rays. Again, as in the case of Le Sage's corpuscles, the absorption of energy from these rays, if they are the cause of gravitation, must be enormous — so great that the energy emitted by radium would be but an exceedingly small fraction of the energy being transformed within it. From these considerations I think that the magnitude of the energy radiated from radium is not a valid argument against the energy being derived from radiation. The reason which induces me to think that the source of the energy is in the atom of radium itself and not external to it is that the radio-activity of substances is, in all cases in which we have been able to localize it, a transient property. No substance goes on being radio-active for very long. It may be asked how can this statement be reconciled with the fact that thorium and radium keep up their activity without any appreciable falling off with time. The answer to this is that, as Rutherford and Soddy have shown in the case of thorium, it is only an exceedingly small fraction of the mass which is at any one time radio-active, and that this radio-active portion loses its activity in a few hours, and has to be replaced by a fresh supply from the non-radio-active thorium. Take any of the radio-active substances we have described, the ThX, the emanations from thorium or radium, the substance which produces induced radioactivity, all these are active for at the most a few days and then lose this property. This is what we should expect on the view that the source of the radio-activity is a change in the atom; it is not what we should expect if the source were external radiation.
