CHAPTER III.Radiation of the New Radio-active Substances.
In order to investigate the radiation emitted by radio-active bodies, any one of the properties of this radiation can be utilised. Thus the action of the rays on photographic plates may serve, or their property of ionisation of the air, which renders it a conductor, or their capacity for causing fluorescence of certain bodies. Henceforth, in speaking of these different methods of working, I shall use the expressions radiographic method, electrical method, fluoroscopic method.
The first two have been used from the beginning in the study of uranium rays; the fluoroscopic method can only be applied in the case of the new bodies which are strongly radio-active, for the feebly active bodies such as uranium and thorium produce no appreciable fluorescence. The electrical method is the only one which serves for exact determinations of intensity; the other two are specially adapted for giving qualitative results, and only furnish rough approximations. The results obtained with the three methods just considered are not strictly comparable the one with the other. The sensitive plate, the gas which is ionised, the fluorescent screen, are in reality receivers, which absorb the energy of the radiation, and transform it into another kind of energy, chemical energy, ionic energy, or luminous energy. Each receiver absorbs a fraction of the radiation, which depends essentially upon its nature. Later on, we shall see that the radiation is complex, that the fractions of the radiation absorbed by the different receivers may differ among themselves both quantitatively and qualitatively. Finally, it is neither evident, nor even probable, that the energy absorbed is entirely transformed by the receiver into the form that we wish for observation; part of this energy may be transformed into heat, into the evolution of secondary radiations which may or may not assist in the production of the observed phenomenon, into chemical action which differs from that under observation, &c., and here also the effective action of the receiver, with reference to the end we have in view, depends essentially upon the nature of that receiver.
Let us compare two radio-active substances, one containingradium and the other polonium, and which show an equal degree of activity in the condenser of Fig. 1. If each is covered with a thin leaf of aluminium, the second appears considerably less active than the first, and the same is the case when they are placed under the same fluorescent screen, if the latter is of sufficient thickness, or is placed at a certain distance from the two radio-active bodies.
Whatever be the method of research employed, the energy of radiation of the new radio-active substances is always found to be considerably greater than that of uranium and thorium. Thus it is that, at a short distance, they act instantaneously upon a photographic plate, whereas an exposure of twenty-four hours is necessary when operating with uranium and thorium. A fluorescent screen is vividly illuminated by contact with the new radio-active bodies, whilst no trace of luminosity is visible with uranium and thorium. Finally, the ionising action upon air is considerably stronger in the ratio of 106approximately. But it is, strictly speaking, not possible to estimate thetotal intensity of the radiation, as in the case of uranium, by the electrical method described at the beginning (Fig. 1). With uranium, for example, the radiation is almost completely absorbed by the layer of air between the plates, and the limiting current is reached at a tension of 100 volts. But the case is different for strongly radio-active bodies. One portion of the radiation of radium consists of very penetrating rays, which penetrate the condenser and the metallic plates, and are not utilised in ionising the air between the plates. Further, the limiting current cannot always be obtained for the tensions supplied; for example, with very active polonium the current remains proportional to the tension between 100 and 500 volts. Therefore the experimental conditions which give a simple interpretation are not realised, and, consequently, the numbers obtained cannot be taken as representing the measurement of the total radiation; they merely point to a rough approximation.
The researches of various physicists (MM. Becquerel, Meyer and von Schweidler, Giesel, Villard, Rutherford, M. and Mdme. Curie) have proved the complex nature of the radiation of radio-active bodies. It will be convenient to specify three kinds of rays, which I shall denote,according to the notation adopted by Mr. Rutherford, by the letters α, β, γ.
I. The α-rays are very slightly penetrating, and appear to constitute the principal part of the radiation. These rays are characterised by the laws by which they are absorbed by matter. The magnetic field acts very slightly upon them, and they were formerly thought to be quite unaffected by the action of this field. However, in a strong magnetic field, the α-rays are slightly deflected; the deflection is caused in the same manner as with cathode rays, but the direction of the deflection is reversed; it is the same as for the canal rays of the Crookes tubes.
II. The β-rays are less absorbable as a whole than the preceding ones. They are deflected by a magnetic field in the same manner and direction as cathode rays.
III. The γ-rays are penetrating rays, unaffected by the magnetic field, and comparable to Röntgen rays.
Fig. 4.
Fig. 4.
Fig. 4.
Consider the following imaginary experiment:—Some radium,R, is placed at the bottom of a small deep cavity, hollowed in a block of lead,P(Fig. 4). A sheaf of rays, rectilinear and slightly expanded, streams from the receptacle. Let us suppose that a strong uniform magnetic field is established in the neighbourhood of the receptacle, normal to the plane of the figure and directed towards the back. The three groups of rays, α, β, γ, will now be separated. Then rather faint γ-rays continue in their straight path without a trace of deviation. The β-rays are deflected in the manner of cathode rays, and describe circular paths in the plane of the figure. If the receptacle is placed on a photographic plate,A C, the portion,B C, of the plate which receives the β-rays is acted upon. Lastly,the α-rays form a very intense shaft which is slightly deflected, and which is soon absorbed by the air. These rays describe in the plane of the figure a path of great curvature, the direction of the deflection being the reverse of that with the β-rays.
If the receptacle is covered with a thin sheet of aluminium (0·1 m.m. thick), the α-rays are suppressed almost entirely, the β-rays are lessened, and the γ-rays do not appear to be absorbed to any great extent.
We have seen that the rays emitted by radio-active bodies have many properties common to cathode rays and to Röntgen rays. Cathode rays, as well as Röntgen rays, ionise the air, act on photographic plates, cause fluorescence, undergo no regular deflection. But the cathode rays differ from Röntgen rays in being deflected from their rectilinear path by the action of the magnetic field, and in the transportation of charges of negative electricity.
The fact that the magnetic field acts upon the rays emitted by radio-active substances was discovered almost simultaneously by MM. Giesel, Meyer and von Schweidler, and Becquerel. These physicists observed that the rays of radio-active substances are deflected by the magnetic field in the same manner and direction as the cathode rays; their observations were in relation to the β-rays.
M. Curie demonstrated that the radiation of radium comprises two groups of quite distinct rays, of which one is readily deflected by the magnetic field (β-rays), whilst the other seems to be unaffected by the action of this field (α- and γ-rays).
M. Becquerel did not find that the specimens of polonium prepared by us emitted rays of the cathode kind. On the contrary, he first noticed the effect of the magnetic field on a specimen of polonium prepared by himself. None of the polonium prepared by us ever gave rise to rays of the cathode order.
The polonium of M. Giesel only gives rise to these rays when recently prepared, and it is probable that the emission is due to the phenomenon of induced radio-activity of which we shall speak later.
The following are experiments which prove that one portion of the radiation of radium, and one portion only, consists of easily deflected rays (β-rays). These experiments were done according to the electrical method.
The radio-active bodyA(Fig. 5) sends forth radiations in the directionA Dbetween the platesPandP′. The platePis now at a potential of 500 volts, plateP′is connected to an electrometer and to a quartz electric piezometer. The intensity of the current passing through the air under the influence of the radiations is measured. The magnetic field can be established at will perpendicular to the plane of the figure over the whole regionE E E E. If the rays are deflected, even slightly, they no longer pass between the plates, and the current is suppressed. The region of the passage of the rays is surrounded with masses of lead,B,B′,B″, and by the armatures of the electro-magnet; when the rays are deflected, they are absorbed by the masses of leadBandB′.
Fig. 5.
Fig. 5.
Fig. 5.
The results obtained depend essentially on the distance,A D, of the radiating substance,A, from the condenser atD. If the distanceA Dis great enough (greater than 7 c.m.), most of the radium rays (90 to 100 per cent) arriving at the condenser are deflected and suppressed for a field of 2500 units. These are the β-rays. If the distanceA Dis less than 65 m.m., a smaller part of the rays are deflected by the action of the field; this portion is also entirely deflected by a field of 2500 units, and the proportion of the rays suppressed is not increased by increasing the field from 2500 to 7000 units.
The proportion of the rays not suppressed by the field increases with decrease of the distance,A D, between the radiating body and the condenser. For small distances, the rays which can be easily deflected form a very small fraction of the total radiation. The penetrating rays are therefore, for the most part, deviable rays of the cathode order (β-rays).
Under the experimental conditions just described, the action of the magnetic field on the α-rays could not be well observed for the fields employed. The chief radiation, apparently undergoing no deflection, observed at a short distance from the radiating source, consisted of α-rays; the undeflected radiation observed at a greater distance consisted of γ-rays.
If an absorbing lamina (aluminium or black paper) is placed in the path of the bundle of rays, those which passthrough are nearly all deflected by the field in such a way that, with the aid of the screen and the magnetic field, almost all the radiation is suppressed in the condenser, the remainder being due to the γ-rays, the proportion of which is small. The α-rays are absorbed by the screen.
An aluminium plate of 1/100 m.m. thickness is sufficient for the suppression of almost all the rays not readily deflected when the substance is far enough from the condenser; for smaller distances (34 m.m. and 51 m.m.) two pieces of this aluminium foil are necessary to give the same result.
Similar determinations were made with four substances containing radium (chlorides or carbonates) of very different activity; analogous results were obtained.
It may be remarked that, in all cases, the penetrating rays deflected by the magnet (β-rays) form only a small fraction of the total radiation; they influence but slightly the determinations in which the whole radiation is made use of to produce conductivity of the air.
The radiation emitted by polonium may be studied by the electrical method. When the distance,A D, of the polonium from the condenser is varied, no current is observed at first while the distance is fairly great; on nearing the polonium, the radiation suddenly becomes manifest with great intensity; the current then increases uniformly whilst approaching the polonium, but the magnetic field produces no appreciable effect under these conditions. The radiation of polonium is apparently limited in space, and does not pass into the air beyond a kind of sheath surrounding the substance to a thickness of several centimetres.
The interpretation of the experiments I have just described must be accompanied by some important general reservations. In speaking of the proportion of the rays deflected by the magnet, I refer only to that portion of the radiation capable of causing a current in the condenser. In employing the fluorescent action of the Becquerel rays, or their action on photographic plates, the proportion would probably be different—a measure of intensity having, as a rule, no meaning except for the method of measurement adopted.
The rays of polonium are α-rays. In the experiments just described, I observed no action of the magnetic field upon them, but the experimental conditions were such that a slight deflection would pass unnoticed.
The experiments made by the radiographic method confirmed the preceding results. Taking radium as the source of radiation, and receiving the impression on a plateparallel to the primitive shaft and normal to the field, a very clear print is obtained of two shafts separated by the action of the field, the one deflected, the other not deflected. The β-rays constitute the deflected beam; the α-rays, being very slightly deflected, are not to be distinguished from the undeflected bundle of the γ-rays.
The experiments of M. Giesel and MM. Meyer and von Schweidler showed that the radiation of the radio-active bodies is, in part at least, deflected by a magnetic field, and that this deflection resembles that of the cathode rays. M. Becquerel investigated the action of the field on the rays by the radiographic method. The experimental arrangement was that of Fig. 4. The radium was placed in the lead receptacle,P, and this receptacle was placed on the sensitive face of a photographic plate,A C, covered with black paper. The whole was placed between the poles of an electro-magnet, the magnetic field being normal to the plane of the figure.
If the field is directed to the back of this plane, the partB Cof the plate is acted upon by rays which, after having described circular paths, return to the plate and strike it at a right angle. These rays are β-rays.
M. Becquerel has demonstrated that the impression consists of a wide diffused band, a continuous spectrum indeed, showing that the sheaf of deviable rays emitted by the source is formed of an infinite number of radiations unequally deflected. If the gelatin of the plate be covered with different absorbent screens (paper, glass, metals), one portion of the spectrum is suppressed, and it is found that the rays most deflected by the magnetic field—otherwise those which have the smallest radius of curvature—are the most completely absorbed. With each screen, the impression on the plate begins at a certain distance from the source of radiation, this distance being proportional to the absorptive power of the screen.
The cathode rays are, as shown by M. Perrin, charged with negative electricity. Further, according to the experiments of M. Perrin and M. Lenard, they are capable of carrying their charge through the metallic envelopes connected to earth and through isolating screens. At every point where the cathode rays are absorbed, there is a continuous evolution of negative electricity. We have provedthat the same is the case for the deflected β-rays of radium.The deviable β-rays of radium are charged with negative electricity.
(Note.—Let the radio-active substance be placed on one of the plates of a condenser, this plate being connected to earth; the second plate is connected to an electrometer, it receives and absorbs the rays emitted by the substance. If the rays are charged, a continuous flow of electricity into the electrometer should be observed. In this experiment, carried out in air, we were not able to detect a charge accompanying the rays, but such an experiment is not delicate. The air between the plates being caused by the rays to conduct, the electrometer is no longer isolated, and can only respond to charges if these be sufficiently strong. In order that the α-rays may not interfere with the experiment, they may be suppressed by covering the source of radiation with a thin metallic screen. We repeated this experiment, without more success, by causing the rays to pass through the interior of a Faraday cylinder in connection with the electrometer).
According to the preceding experiments, it was evident that the charge of the rays of the radiating body employed was a weak one.
In order to fix a feeble evolution of electricity upon the conductor which absorbs the rays, this conductor should be completely insulated; this is effected by screening it from the air, either by placing it in a tube with a very perfect vacuum, or by surrounding it with a good solid dielectric. We employed the latter arrangement.
Fig. 6.
Fig. 6.
Fig. 6.
A conducting disc,M M(Fig. 6), is connected by the wire,t, to the electrometer; the disc and wire are completely enveloped by the insulating substancei i i i; the whole is again surrounded with the metallic covering,E E E E, which is in electric connection with the earth. The insulator,p p, and the metallic envelope are very thin upon one of the faces of the disc. This face is exposed to the radiation of the barium and radium salt,R, placed outside in a lead receptacle. The rays emitted by the radium penetrate the metallic envelope and the insulating lamina,p p, and are absorbed by the metallic disc,M M. The latterthen becomes the source of a continuous evolution of negative electricity, as determined by the electrometer, and is measured by means of a quartz piezometer.
The current thus created is very weak. With very active barium-radium chloride, forming a layer of 2·5 sq. c.m. in area, and of 0·2 c.m. in thickness, a current of magnitude 10–11ampères is obtained, the rays utilised having traversed, before being absorbed by the discM M, a thickness of aluminium of 0·01 m.m., and a thickness of ebonite of 0·3 m.m.
We used successively lead, copper, and zinc for the discM M, ebonite and paraffin for the insulator; the results obtained were the same.
The current diminishes with increasing distance from the source of radiation,R, also when a less active product is used.
We obtained the same results again when the discM Mis replaced by a Faraday cylinder filled with air, and covered outside with insulating material. The opening of the cylinder, closed by the thin insulating plate,p p, was opposite the radiating source.
Fig. 7.
Fig. 7.
Fig. 7.
Finally, we made the inverse experiment, which was to place the lead receptacle with the radium in the centre of the insulating material and in connection with the electrometer (Fig. 7), the whole being surrounded with the metallic covering connected to earth.
Under these conditions, it is evident from the electrometer that the radium has a positive charge equal in magnitude to the negative charge of the former experiment. The radium rays penetrate the thin dielectric plate,p p, and leave the conductor inside carrying with them negative electricity.
The α-rays of radium do not interfere in these experiments, being almost completely absorbed by a very thin layer of matter. The method just described is not suitable for the study of the charge of the rays of polonium, these rays very slightly penetrating. We observed no indication of any charge in the case of polonium, which gives rise to α-rays only; but, for the reason just given, no conclusion can be drawn from this.
Thus, in the case of the deflected β-rays of radium, as in the case of cathode rays, the rays carry a charge of electricity. But, hitherto, the existence of electric charges uncombined with matter has been unknown. In the study of the emission of the β-rays of radium, we are therefore led to make use of the theory which is in vogue for the study of cathode rays. In this ballistic theory, formulated by Sir William Crookes, since developed and completed by Prof. J. J. Thomson, the cathode rays consist of extremely minute particles, which are hurled from the cathode with great velocity, and which are charged with negative electricity. We might similarly conceive that radium sends into space negatively electrified particles.
A specimen of radium, enclosed in a solid thin perfectly insulated envelope, should become spontaneously charged to a very high potential. By the ballistic hypothesis the potential would increase until the potential difference of the surrounding conductors became sufficient to hinder the ejection of the electrified particles and to cause their return to the source of radiation.
We have performed an experiment on these lines. A specimen of very active radium was enclosed for some time in a glass vessel. In order to open the vessel, we made a trace on the glass with a glass cutter. Whilst so doing, we clearly heard the report of a spark, and upon examining the vessel with a magnifying glass, we observed that the glass had been pierced by a spark at the spot where it had been weakened by the scratch. The phenomenon produced is comparable to the rupture of the glass of an overcharged Leyden jar.
The same phenomenon occurred with another glass. Further, at the moment of the passing of the spark, M. Curie, who was holding the glass, felt the electric shock of discharge in his fingers.
Certain kinds of glass have good insulating properties. If the radium is enclosed in a sealed glass vessel, well insulated, it is to be expected that, at a given moment, the vessel will be spontaneously perforated.
Radium is the first example of a body which is spontaneously charged with electricity.
The β-rays of radium, being analogous to the cathode rays, should be deflected by an electric field in a manner similar to the latter;i.e., as would a particle of matternegatively charged and hurled into space with a great velocity. The existence of such a deflection has been demonstrated both by M. Dorn and M. Becquerel.
Let us consider the case of a ray which traverses the space situated between the two plates of a condenser. Suppose the direction of the ray parallel to the plates: when an electric field is established between the latter, the ray is subjected to the action of this uniform field along its whole path in the condenser l. By reason of this action the ray is deflected towards the positive plate and describes the arc of a parabola; on leaving the field, it continues its path in a straight line, following the tangent to the arc of the parabola at the point of exit. The ray can be received on a photographic plate perpendicular to its original direction. Observations are taken of the impression produced on the plate when the field is zero, and when it has a known value, and from that is deduced the value of the deflection, δ, which is the distance of the points in which the new direction of the ray and its original direction meet a common plane perpendicular to the original direction. If h is the distance of this plane from the condenser,i.e., at the edge of the field, we have, by a simple calculation,—
mbeing the mass of the moving particles,eits charge,vits velocity, and F the strength of the field.
The experiments of M. Becquerel enable him to assign a value approaching to δ.
When a material particle having a massmand a negative chargee, is projected with a velocityvinto a uniform magnetic field perpendicular to its initial velocity, this particle describes, in a plane normal to the field and passing through its initial velocity, an arc of a circle of radius ρ, so that—H being the strength of the field—we have the relation—
If, for the same ray, the deflection, δ, and the radius of curvature, ρ, be measured in a magnetic field, values could be found from these two experiments for the ratioemand for the velocity,v.
The experiments of M. Becquerel threw the first light upon this subject. They gave for the ratioema value approximately equal to 107absolute electro-magnetic units, and for v a magnitude of 1·6 × 1010. These values are of the same order of magnitude as those of the cathode rays.
Accurate experiments have been made on the same subject by M. Kaufmann. This physicist subjected a narrow beam of radium rays to the simultaneous action of an electric field and a magnetic field, the two fields being uniform and having a similar direction, normal to the original direction of the beam. The impression produced on a plate normal to the primitive beam and placed beyond the limits of the field with reference to the source, has the form of a curve, each point of which corresponds to one of the original beam. The most penetrating and least deflected rays are at the same time those with the greatest velocity.
It follows from the experiments of M. Kaufmann, that for the radium rays, of which the velocity is considerably greater than that of the cathode rays, the ratioemdecreases, while the velocity increases.
According to the researches of J. J. Thomson and Townsend, we may assume that the moving particle, which constitutes the ray, possesses a charge,e, equal to that carried by an atom of hydrogen during electrolysis, this charge being the same for all the rays. We are therefore led to the conclusion that the mass of the particle,m, increases with increase of velocity.
These theoretical considerations lead to the idea that the inertia of the particle is due to its state of charge during motion, the velocity of an electric charge in motion being incapable of modification without expenditure of energy. To state it otherwise, the inertia of the particle is of electro-magnetic origin, and the mass of the particle is—in part at least—a virtual mass or an electro-magnetic mass. M. Abraham goes further, and assumes that the mass of the particle is entirely an electro-magnetic mass. If, according to this hypothesis, the value of this mass,m, be calculated for a known velocity,v, we find thatmapproaches infinity whenvapproaches the velocity of light, and thatmapproaches a constant value when the velocity,v, is much less than that of light. The experiments of M. Kaufmann are in agreement with the results of this theory, the importance of which is great because it foreshadows the possibility of establishingmechanical bases upon the dynamical of little particles of matter charged in a state of motion.
These are the figures obtained by M. Kaufmann foremandv.
M. Kaufmann concludes, from comparison of his experiments with the theory, that the limiting value of the ratioemfor radium rays of relatively small velocity would be the same as the valueemfor cathode rays.
The most complete experiments of M. Kaufmann were made with a minute quantity of pure radium chloride, with which we provided him.
According to M. Kaufmann’s experiments, certain β-rays of radium possess a velocity very near to that of light. These rapid rays seem to possess great penetrating capacity towards matter.
In a recent work, Mr. Rutherford announced that, in a powerful electric or magnetic field, the α-rays of radium are slightly deflected, in the manner of particles positively electrified and possessing great velocity. Mr. Rutherford concludes from his experiments that the velocity of the α-rays is of the order of magnitude 2·5 × 109c.m.sec.and that the ratioemfor these rays is of the order of magnitude 6 × 103, which is 104times as great as for the deflected β-rays. We shall see later that these conclusions of Mr. Rutherford are in agreement with the properties already known of the α-radiation, and that they account, in part at least, for the law of absorption of this radiation.
The experiments of Mr. Rutherford have been confirmed by M. Becquerel. M. Becquerel has further demonstrated that polonium rays behave in a magnetic field like the α-raysof radium, and that, for the same field, they seem to have the same curvature as the latter.
It also appears from M. Becquerel’s experiments that the α-rays do not form a magnetic spectrum, but act rather like a homogeneous radiation, all the rays being equally deflected.
We have just seen that radium gives off α-rays comparable to the tube rays, β-rays comparable to cathode rays, and γ-rays which are penetrating and not deflected. Polonium gives off α-rays only. Amongst the other radio-active substances, actinium seems to behave like radium, but the study of its radiation has not yet advanced so far as in the case of radium. As regards the faintly radio-active bodies, we know to-day that uranium and thorium give rise to α-rays as well as β-rays (Becquerel, Rutherford).
As I have already mentioned, the proportion of β-rays increases with increase of distance from the source of radiation. These rays never occur alone, and for great distances the presence of γ-rays is always discernible. The presence of very penetrating, undeflected rays in the radiation of radium was first observed by M. Villard. These rays constitute only a small portion of the radiation measured by the electrical method, and their presence escaped our notice in our first experiments, so that we believed falsely that the radiation at great distances contained only rays capable of deflection.
The following are the numerical results obtained with experiments made by the electrical method with an apparatus similar to that of Fig. 5. The radium was only separated from the condenser by the surrounding air. I shall indicate by the letterdthe distance from the source of radiation to the condenser. The numbers of the second line represent the current subsisting when the magnetic field is acting, supposing the current obtained with no field equal to 100 for each distance. These numbers may be considered as giving the percentage of the total α- and γ-rays, the deflection of the α-rays having been scarcely observable with the conditions employed.
At great distances there are no α-rays, and the undeflected radiation is therefore of the γ kind only.
Experiments made at short distances:—
Experiments made at long distances with a product considerably more active than that which was used for the preceding series:—
It is thus evident that after a certain distance the proportion of undeflected rays in the radiation is approximately constant. These rays probably all belong to the γ species.
The following is another series of experiments in which the radium was enclosed in a very narrow glass tube, placed below the condenser and parallel to the plates. The rays emitted traversed a certain thickness of glass and air before entering the condenser:—
As in the preceding experiments, the number of the second line approximate to a constant value, when the distancedincreases, but the limit is reached for smaller distances than in the preceding series, because the α-rays have been more completely absorbed by the glass than the β- and γ-rays.
The following experiment shows that a thin sheet of aluminium (0·01 m.m. thick) absorbs principally α-rays. The product being placed 5 c.m. from the condenser, the proportion of rays other than β, when the magnetic field is acting, is about 71 per cent. When the same substance is covered with the sheet of aluminium, the distance remaining the same, the radiation transmitted is found to be almost totally deflected by the magnetic field, the α-rays having been absorbed by the aluminium. The same result is obtained when paper is used as the absorbing screen.
The greatest part of the radiation of radium consists of α-rays, which are probably emitted principally by the superficial layer of the radiating matter. When the thickness of the layer of radiating matter is varied, the intensity of the current increases with this thickness; the increase is not proportional to the thickness for the whole of the radiation; it is, moreover, more considerable for the β-rays than for the α-rays, so that the proportion of β-rays increases with the thickness of the active layer. The source ofradiation being placed at a distance of 5 c.m. from the condenser, it is found that for a thickness equal to 0·4 m.m. of the active layer, the total radiation is given by the number 28, and the proportion of the β-rays is 29 per cent. By making the layer 2 m.m. thick,i.e., five times as thick, a total radiation equal to 102, and a proportion of β-rays equal to 45 per cent are obtained. The total radiation which exists at this distance has therefore been increased in the ratio of 3·6, and the β-radiation has become five times as strong.
The preceding experiments were made by the electrical method. When the radiographic method is used, certain results seem to be in contradiction with what precedes. In the experiments of M. Villard, a beam of radium rays, subjected to the action of the magnetic field, was received on to a pile of photographic plates. The undeflected and penetrating γ-beam passed through all the plates, leaving its trace on each. The deflected β-beam produced an impression on the first plate only. This beam appeared therefore to contain no rays of great penetration.
On the contrary, in our experiments a beam which is propagated in the air contains at the greatest distances accessible to observation about 9/10 of β-rays, and the same is the case when the source of radiation is enclosed in a little sealed glass vessel. In M. Villard’s experiments, these deflected and penetrating β-rays did not affect the photographic plates beyond the first, because they are to a great extent diffused in all directions by the first solid obstacle encountered, and no longer form a beam. In our experiments the rays given off by radium and transmitted through the glass of the vessel were also probably scattered by the glass, but the vessel being very small would itself act as a source of β-rays at its surface, and we were able to follow the course of the latter to a great distance from the vessel.
The cathode rays of Crookes tubes can only traverse very thin screens (aluminium screens of 0·01 m.m. thickness). A beam of rays striking the screen normally is scattered in all directions; but the diffusion becomes less with diminishing thickness of the screen, and for very thin screens the emerging beam is practically the prolongation of the incident beam.
The deflected β-rays of radium behave in a similar manner, but the transmitted beam experiences, for the same thickness of screen, a much slighter modification. According to the experiments of M. Becquerel, the very readily deflected β-rays of radium (those with a relativelysmall velocity) are powerfully scattered by an aluminium screen of thickness 0·1 m.m.; but the penetrating and less deflected rays (rays of the cathode kind of great velocity) pass through this screen without being sensibly diffused, whatever be the inclination of the screen to the direction of the beam. The β-rays of great velocity penetrate without diffusion a much greater thickness of paraffin (several centimetres), and in this the curvature of the beam produced by the magnetic field can be traced. The thicker the screen, and the more absorbent the material of which it is composed, the greater is the modification of the deflected primitive beam, because, with increasing thickness of screen, diffusion occurs progressively among fresh groups of rays of increasing penetration.
The β-rays of radium experience a diffusion in passing through the air, which is very marked for readily deflected rays, but which is much slighter than that produced by equal thicknesses of solid substances. For this reason, the β-rays traverse long distances in the air.
Since the beginning of the researches on radio-active bodies, investigations of the absorption produced by different screens upon the rays given off by these bodies have been carried on. In a previous paper on this subject I gave figures (quoted at the beginning of this work) representing the penetrating power of uranium and thorium rays. Mr. Rutherford has made a special study of the radiation of uranium, and proved it to be heterogeneous. Mr. Owens has arrived at the same results for thorium rays. When the discovery of strongly radio-active bodies immediately followed upon this, the penetrating power of their rays was also studied by various physicists (Becquerel, Meyer and von Schweidler, Curie, Rutherford). The first observations brought to light the complexity of the radiation, which seems to be a general phenomenon, and common to the radio-active bodies. In them we have sources which give rise to a variety of radiations, each of which has a power of penetration proper to itself.
Radio-active bodies emit rays which are propagated both in the air andin vacuo. The propagation is rectilinear; this fact is proved by the distinctness and shape of the shadows formed by interposing bodies opaque to the radiation between the source and the sensitive plate or fluorescent screen which serves as receiver, the source being of small magnitude in comparison with its distance from the receiver.Various experiments demonstrating the rectilinear propagation of uranium, radium, and polonium rays have been made by M. Becquerel.
It is interesting to know the distance that rays can travel in air. We have found that radium emits rays which can be detected in the air at a distance of several metres from the source. In certain of our electrical determinations, the action of the source upon the air of the condenser made itself felt at a distance of between 2 and 3 metres. We have also obtained fluorescent effects and radiographic impressions at similar distances. The experiments are not easily carried out, except with very intense radio-active sources, because, independently of the absorption by the air, the action upon a given receiver varies inversely as the square of the distance from a source of small dimensions. This radiation, which travels a long distance in the case of radium, comprises rays of the cathode kind and rays which are undeflected; however, the deflected rays predominate, according to the results of the experiments already mentioned. The greater part of the radiation (α-rays) is, on the contrary, limited in air to a distance of about 7 c.m. from the source.
I made several experiments with radium enclosed in a little glass vessel. The rays emerging from the vessel, after traversing a certain space of air, were received in a condenser, which served to measure their ionising capacity by the usual electrical method. The distance,d, from the source to the condenser was varied, and the current of saturation,i, obtained in the condenser was measured. The following are the results of one of the series of determinations:—
After a certain distance, the intensity of radiation varies inversely as the square of the distance from the condenser.
The radiation of polonium is only propagated in air to a distance of a few centimetres (4 to 6 c.m.) from the source of radiation.
In the case of the absorption of radiations by solid screens, we find another fundamental difference between radium and polonium. Radium emits rays capable of penetrating great thicknesses of solid matter,e.g., several centimetres of lead or of glass. The rays which have passed through a great thickness of a solid body are extremely penetrating, and it is practically impossible to absorb them entirely by any material whatever. But these rays form only a small fraction of the total radiation, the greater part of which is absorbed by a slight thickness of solid matter.
Polonium emits rays which are readily absorbed, and which can only pass through extremely thin screens.
The following are figures showing the absorption produced by an aluminium lamina of thickness 0·01 m.m. This lamina was placed above and almost in contact with the substance. The direct radiation and that transmitted by the aluminium were measured by the electrical method (apparatus of Fig. 1); the current of saturation was practically obtained in every case. I have represented the activity of the radiating body bya, that of uranium being unity.
We see that radium compounds of different nature and activity give very similar results, as I have already pointed out in the case of uranium and thorium compounds at the beginning of this work. We see also that, taking into account the whole of the radiation, and with a given absorbent screen, the different radio-active bodies can be arranged in the following decreasing order of penetrating power:—Thorium, radium, polonium, uranium.
These results are similar to those which have been published by Mr. Rutherford.
Mr. Rutherford also finds that the order is the same when air is the absorbent substance. But it is probable that this order has no absolute value, and would not be maintainedindependently of the nature and thickness of the screen. Experiment shows, indeed, that the law of absorption is very different for polonium and radium, and that, for the latter, the absorption of the rays of each of the three groups must be considered separately.
Polonium is particularly well adapted to the study of α-rays, because the specimens which we possess emit no other kind of rays. I made a preliminary series of experiments with extremely active recently prepared specimens of polonium. I found the absorbability of the rays to increase with increase of thickness of the matter traversed. This singular law of absorption is contrary to that known for other kinds of radiation.
I employed for this research our apparatus for the determination of electrical conductivity arranged in the following manner:—
The two plates of a condenser,P PandP′ P′(Fig 8), are horizontally disposed in a metallic box,B B B B, connected to earth. The active body,A, placed in a thick metallic box,C C C C, connected with the plateP′ P′, acts upon the air of the condenser across a metallic sheet,T; the rays which pass through the sheet are alone utilised for producing the current, the electric field being limited by the sheet. The distance,A T, of the active body from the sheet may be varied. The field between the plates is established by means of a battery. By placing inAupon the active body different screens, and by adjusting the distanceA T, the absorption of rays which travel long or short distances in the air may be determined.