PART III.
The α Rays.
87. The α rays. The magnetic deviation of the β rays was discovered towards the end of 1899, at a comparatively early stage in the history of radio-activity, but three years elapsed before the true character of the α rays was disclosed. It was natural that great prominence should have been given in the early stages of the subject to the β rays, on account of their great penetrating power and marked action in causing phosphorescence in many substances. The α rays were, in comparison, very little studied, and their importance was not generally recognized. It will, however, be shown that the α rays play a far more important part in radio-active processes than the β rays, and that the greater portion of the energy emitted in the form of ionizing radiations is due to them.
88. The nature of the α rays. The nature of the α rays was difficult to determine, for a magnetic field sufficient to cause considerable deviation of the β rays produced no appreciable effect on the α rays. It was suggested by several observers that they were, in reality, secondary rays set up by the β or cathode rays in the active matter from which they were produced. Such a view, however, failed to explain the radio-activity of polonium, which gave out α rays only. Later work also showed that the matter, which gave rise to the β rays from uranium, could be chemically separated from the uranium, while the intensity of the α rays was unaffected. These and other results show that the α and β rays are produced quite independently of one another. The view that they are an easily absorbed type of Röntgen rays fails to explain a characteristic property of the α rays, viz. that the absorption of the rays in a given thickness of matter, determined by the electrical method, increases with the thickness of matter previously traversed. It does not seem probable that such an effect could be produced by a radiation like X rays, but the result is to be expected if the rays consist of projected bodies, which fail to ionize the gas when their velocity is reduced below a certain value. From observations of the relative ionization produced in gases by the α and β rays, Strutt[[138]] suggested in 1901 that the α rays might consist of positively charged bodies projected with great velocity. Sir William Crookes[[139]], in 1902, advanced the same hypothesis. From a study of the α rays of polonium Mme. Curie[[140]] in 1900 suggested the probability that these rays consisted of bodies, projected with great velocity, which lost their energy by passing through matter.
The writer was led independently to the same view by a mass of indirect evidence which received an explanation only on the hypothesis that the rays consisted of matter projected with great velocity. Preliminary experiments with radium of activity 1000 showed that it was very difficult to determine the magnetic deviation of the α rays. When the rays were passed through slits sufficiently narrow to enable a minute deviation of the rays to be detected, the ionizing effect of the issuing rays was too small to be measured with certainty. It was not until radium of activity 19,000 was obtained that it was possible to detect the deviation of these rays in an intense magnetic field. How small the magnetic deviation is may be judged from the fact that the α rays, projected at right angles to a magnetic field of 10,000 C.G.S. units, describe the arc of a circle of about 39 cms. radius, while under the same conditions the cathode rays produced in a vacuum tube would describe a circle of about ·01 cm. radius. It is therefore not surprising that the α rays were for some time thought to be non-deviable in a magnetic field.
89. Magnetic deviation of the α rays. The general method employed[[141]] to detect the magnetic deviation of the α rays was to allow the rays to pass through narrow slits and to observe whether the rate of discharge of an electroscope, due to the issuing rays, was altered by the application of a strong magnetic field. [Fig. 32] shows the general arrangement of the experiment. The rays from a thin layer of radium of activity 19,000 passed upwards through a number of narrow slits G, in parallel, and then through a thin layer of aluminium foil, ·00034 cm. thick, into the testing vessel V. The ionization produced by the rays in the testing vessel was measured by the rate of movement of the leaves of a gold-leaf electroscope B. The gold-leaf system was insulated inside the vessel by a sulphur bead C, and could be charged by means of a movable wire D, which was afterwards earthed. The rate of movement of the gold-leaf was observed through small mica windows in the testing vessel by means of a microscope provided with a micrometer eye-piece.
Fig. 32.
In order to increase the ionization in the testing vessel, the rays passed through 20 to 25 slits of equal width, placed side by side. This was arranged by cutting grooves at regular intervals in side-plates into which brass plates were slipped. The width of the slit varied in different experiments between ·042 cm. and ·1 cm. The magnetic field was applied perpendicular to the plane of the paper, and parallel to the plane of the slits. The rays are thus deflected in a direction perpendicular to the plane of the slits and a very small amount of deviation is sufficient to cause the rays to impinge on the sides of the plate where they are absorbed.
The testing vessel and system of plates were waxed to a lead plate P so that the rays entered the vessel V only through the aluminium foil. It is necessary in these experiments to have a steady stream of gas passing downwards between the plates in order to prevent the diffusion of the emanation from the radium upwards into the testing vessel. The presence in the testing vessel of a small amount of this emanation, which is always given out by radium, would produce great ionization and completely mask the effect to be observed. For this purpose, a steady current of dry electrolytic hydrogen of about 2 c.c. per second was passed into the testing vessel; it then streamed through the porous aluminium foil, and passed between the plates carrying the emanation with it away from the apparatus. The use of a stream of hydrogen instead of air greatly simplifies the experiment, for it increases the ionization current due to the α rays in the testing vessel, and at the same time greatly diminishes that due to the β and γ rays. This is caused by the fact that the α rays are much more readily absorbed in air than in hydrogen, while the rate of production of ions due to the β and γ rays is much less in hydrogen than in air. The intensity of the α rays after passing between the plates is consequently greater when hydrogen is used; and since the rays pass through a sufficient distance of hydrogen in the testing vessel to be largely absorbed, the total amount of ionization produced by them is greater with hydrogen than with air.
The following is an example of an observation on the magnetic deviation:—
Pole-pieces 1·90 × 2·50 cms.
Strength of field between pole-pieces 8370 units.
Apparatus of 25 parallel plates of length 3·70 cms., width ·70 cm., with an average air-space between plates of ·042 cm.
Distance of radium below plates 1·4 cm.
| Rate of discharge of electroscope in volts per minute | |
|---|---|
| (1) Without magnetic field | 8·33 |
| (2) With magnetic field | 1·72 |
| (3) Radium covered with thin layer of mica to absorb all α rays | 0·93 |
| (4) Radium covered with mica and magnetic field applied | 0·92 |
The mica plate, ·01 cm. thick, was of sufficient thickness to absorb completely all the α rays, while it allowed the β rays and γ rays to pass through without appreciable absorption. The difference between (1) and (3), 7·40 volts per minute, gives the rate of discharge due to the α rays alone; the difference between (2) and (3), 0·79 volts per minute, that due to the α rays not deviated by the magnetic field employed.
The amount of α rays not deviated by the field is thus about 11% of the total. The small difference between (3) and (4) measures the small ionization due to the β rays, for they would be completely deviated by the magnetic field; (4) comprises the effect of the γ rays together with the natural leak of the electroscope in hydrogen.
In this experiment there was a good deal of stray magnetic field acting on the rays before they reached the pole-pieces. The diminution of the rate of discharge due to the α rays was found to be proportional to the strength of field between the pole-pieces. With a more powerful magnetic field, the whole of the α rays were deviated, showing that they consisted entirely of projected charged particles.
In order to determine the direction of deviation of the rays, the rays were passed through slits one mm. in width, each of which was half covered with a brass strip. The diminution of the rate of discharge in the testing vessel for a given magnetic field in such a case depends upon the direction of the field. In this way it was found that the rays were deviated in the opposite sense to the cathode rays. Since the latter consist of negatively charged particles, the α rays must consist of positively charged particles.
These results were soon after confirmed by Becquerel[[142]], by the photographic method, which is very well adapted to determine the character of the path of the rays acted on by a magnetic field. The radium was placed in a linear groove cut in a small block of lead. Above this source, at a distance of about 1 centimetre, was placed a metallic screen, formed of two plates, leaving between them a narrow opening parallel to the groove. Above this was placed the photographic plate. The whole apparatus was placed in a strong magnetic field parallel to the groove. The strength of the magnetic field was sufficient to deflect the β rays completely away from the plate. When the plate was parallel to the opening, there was produced on it an impression, due to the α rays alone, which became more and more diffuse as the distance from the opening increased. This distance should not exceed 1 or 2 centimetres on account of the absorption of the rays in air. If, during the exposure, the magnetic field is reversed for equal lengths of time, on developing the plate two images of the α rays are observed which are deflected in opposite directions. This deviation, even in a strong field, is small though quite appreciable and is opposite in sense to the deviation observed for the β or cathodic rays from the same material.
M. Becquerel[[143]], by the same method, found that the α rays from polonium were deviated in the same direction as the α rays from radium; and thus that they also consist of projected positive bodies. In both cases, the photographic impressions were sharply marked and did not show the same diffusion which always appears in photographs of the β rays.
90. Electrostatic deviation of the α rays. If the rays are charged bodies, they should be deflected in passing through a strong electric field. This was found by the writer to be the case, but the electric deviation is still more difficult to detect than the magnetic deviation, as the intensity of the electric field must of necessity be less than that required to produce a spark in the presence of radium. The apparatus was similar to that employed for the magnetic deviation ([Fig. 32]) with this exception, that the brass sides which held the plates in position, were replaced by ebonite. Alternate plates were connected together and charged to a high potential by means of a battery of small accumulators. The discharge in the electroscope, due to the α rays, was found to be diminished by application of the electric field. With plates ·055 cm. apart and 4·5 cms. high, the diminution was only 7% with a P.D. of 600 volts between the slits. With a special arrangement of plates, with slits only ·01 cm. apart, the discharge was diminished about 45% with an electric field corresponding to 10,000 volts per cm.
91. Determination of the constants of the rays. If the deviation of the rays in both an electric and magnetic field is known, the values of the velocity of the rays, and the ratio e/m of the charge of the particle to its mass can be determined by the method, first used by J. J. Thomson for the cathode rays, which is described in section 50. From the equations of a moving charged body, the radius of curvature ρ of the path of the rays in a magnetic field of strength H perpendicular to the path of the rays is given by
m
Hρ = ---- V .
e
If the particle, after passing through a uniform magnetic field for a distance l1, is deviated through a small distance d1 from its original direction,
2ρd1 = l12
or
l12 e H
d1 = ----- --- --- (1).
2 m V
If the rays pass through a uniform electric field of strength X and length l2 with a deviation d2,
1 Xel22
d2 = --- ----- (2),
2 mV2
since Xe/m is the acceleration of the particle, at right angles to its direction, and l2/V is the time required to travel through the electric field.
From equations (1) and (2)
d1 l22 X
V = ----- ----- --- ,
d2 l12 H
and
e 2d1 V
---- = ------ --- .
m l12 H
The values of V and e/m are thus completely determined from the combined results of the electric and magnetic deviation. It was found that
V = 2·5 × 109 cms. per sec.
e/m = 6 × 103.
On account of the difficulty of obtaining a large electrostatic deviation, these values are only approximate in character.
The results on the magnetic and electric deviation of the α rays of radium have been confirmed by Des Coudres[[144]], by the photographic method. Some pure radium bromide was used as a source of radiation. The whole apparatus was enclosed in a vessel which was exhausted to a low vacuum. In this way, not only was he able to determine the photographic action of the rays at a much greater distance from the source, but he was also able to apply a stronger electric field without the passage of a spark. He found values of the constants given by
V = 1·65 × 109 cms. per sec.
e/m = 6·4 × 103.
These values are in very good agreement with the numbers found by the electric method. The α rays from radium are complex, and probably consist of a stream of positively charged bodies projected at velocities lying between certain limits. The amount of deviation of the particles in a magnetic field will thus differ according to the velocity of the particle. The photographic results of Becquerel seem to indicate that the velocity of the rays of radium can vary only within fairly narrow limits, since the trajectory of the rays in a magnetic field is sharply marked and not nearly as diffuse as in similar experiments with the β rays. The evidence, however, discussed in the following section, shows that the velocities of the α particles from a thick layer of radium vary over a considerable range.
92. Becquerel[[145]] has examined the amount of magnetic deviation of the α rays at different distances from the source of the rays in a very simple way. A narrow vertical pencil of the rays, after its passage through a narrow slit, fell on a photographic plate, which was inclined at a small angle to the vertical and had its lower edge perpendicular to the slit. The trajectory of the rays is shown by a fine line traced on the plate. If a strong magnetic field is applied parallel to the slit, the trajectory of the rays is displaced to the right or left according to the direction of the field. If equal times of exposure are given for the magnetic field in the two directions, on developing the plate two fine diverging lines are found traced on the plate. The distance between these lines at any point is a measure of twice the average deviation at that point, corresponding to the value of the magnetic field. By measuring the distance between the trajectories at various points, Becquerel found that the radius of curvature of the path of the rays increased with the distance from the slit. The product Hρ of the strength of the field and the radius of curvature of the path of the rays is shown in the following table.
| Distance in mms. from the slit | Hρ |
|---|---|
| 1 | 2·91 × 105 |
| 3 | 2·99 „ |
| 5 | 3·06 „ |
| 7 | 3·15 „ |
| 8 | 3·27 „ |
| 9 | 3·41 „ |
The writer (loc. cit.) showed that the maximum value of Hρ for complete deviation of the α rays was 390,000. The results are thus in good agreement. Since
m
Hρ = ----- V
e
these results show that the values either of V or of e/m for the projected particles vary at different distances from the source. Becquerel considered that the rays were homogeneous, and, in order to explain the results, has suggested that the charge on the projected particles may gradually decrease with the distance traversed, so that the radius of curvature of the path steadily increases with the distance from the source. It, however, seems more probable that the rays consist of particles projected with different velocities, and that the slower particles are more quickly absorbed in the gas. In consequence of this, only the swifter particles are present some distance from the source.
This conclusion is borne out by some recent experiments of Bragg and Kleeman[[146]] on the nature of the absorption of α particles by matter, which are discussed in more detail in sections 103 and 104. They found that the α particles from a thick layer of radium are complex, and have a wide range of penetrating power and presumably of velocity. This is due to the fact that the α particles emitted from the radium come from different depths. Since their velocity is reduced in their transit through matter, a pencil of α rays will consist of particles which differ considerably in speed. Those which are just able to emerge from the radium will be absorbed in a very short depth of air, while those that come from the surface will be able to pass through several centimetres of air before they lose their power of ionizing the gas. Since the α particles have different velocities, they will be unequally deflected by the magnetic field, the slower moving particles describing a more curved path than the swifter ones. Consequently, the outer edge of the trace of the pencil of rays on the photographic plate, as obtained by Becquerel, will be the locus of the points where the photographic action of the α particles end. It was found that the α particles are most efficient as ionizers of the gas just before their power of ionizing ends. The loss of ionizing power of the α particles seems to be fairly abrupt, and, for particles of the same velocity, to occur always after traversing a definite distance in air. On the assumption that the photographic as well as the ionizing action is most intense just before the particles are stopped, and ceases fairly abruptly, Bragg has been able to account numerically for the measurements (see above table) recorded by Becquerel. Quite apart from the special assumptions required for such a quantitative comparison of theory with experiment, there can be little doubt that the increase of value of Hρ with distance can be satisfactorily explained as a consequence of the complex character of the pencil of rays[[147]].
Becquerel states that the amount of deviation, in a given magnetic field, was the same for the α rays of polonium and of radium. This shows that the value of
m
--- V
e
is the same for the α rays from the two substances. Since the α rays from polonium are far more readily absorbed than the α rays from radium, this result would indicate that the value of m/e is greater for the α particles of polonium than of radium. Further experimental evidence is required on this important point.
93. Charge carried by the α rays. We have seen that the negative charge carried by the β particles has been readily measured. Since there is reason to believe ([section 229]) that four α particles are expelled from radium for each β particle, it is to be expected that the positive charge carried by the α particles should be determined still more readily. All the initial experiments, however, made to detect this charge, gave negative results; and, before successful results were obtained, it was found necessary to eliminate some secondary actions, which at first completely masked the effects to be looked for.
In consequence of the importance of this question, a brief account will be given of the methods of measurement adopted and the special experimental difficulties which have arisen.
In the first place, it must be remembered that only a small fraction of the α rays, emitted from a layer of powdered radium bromide, escape into the surrounding gas. On account of the ease with which the α rays are stopped in their passage through matter, only those escape which are expelled from a superficial layer, and the rest are absorbed by the radium itself. On the other hand, a much larger proportion of the β rays escape, on account of their greater power of penetration. In the second place, the α particle is a far more efficient ionizer of the gas than the β particle, and, in consequence, if the charge carried by the α rays is to be determined by methods similar to those employed for the β rays (see section 80), the pressure of the gas surrounding the conductor to be charged must be very small in order to eliminate, as far as possible, the loss of charge resulting from the ionization of the residual gas by the α rays[[148]].
The experimental arrangement used by the writer is shown in [Fig. 33].
A thin film of radium was obtained on a plate A by evaporation of a radium solution containing a known weight of radium bromide. Some hours after evaporation, the activity of the radium, measured by the α rays, is about 25 per cent. of its maximum value, and the β rays are almost completely absent. The activity measured by the α and β rays is then slowly regained, and recovers its original value after about a month’s interval (see [chapter XI.]). The experiments were made on the active plate when its activity was a minimum, in order to avoid complications due to the presence of β rays. The film of radium was so thin that only a very small fraction of the α rays was absorbed.
Fig. 33.
The active plate A was insulated in a metal vessel D, and was connected to one pole of the battery, the other pole being earthed. The upper electrode, which was insulated and connected with a Dolezalek electrometer, consisted of a rectangular copper vessel BC, the lower part of which was covered with a thin sheet of aluminium foil. The α rays passed through the foil, but were stopped by the copper sides of the vessel. This arrangement was found to reduce the secondary ionization produced at the surface of the upper plate. The outside vessel D could be connected with either A or B or with earth. By means of a mercury pump, the vessel was exhausted to a very low pressure. If the rays carry a positive charge, the current between the two plates measured by the electrometer should be greater when A is charged positively. No certain difference, however, between the currents in the two directions was observed, even when a very good vacuum was obtained. In some arrangements, it was found that the current was even greater when the lower plate was negative than when it was positive. An unexpected experimental result was also noticed. The current between the parallel plates at first diminished with the pressure, but soon reached a limiting value which was not altered however good a vacuum was produced. For example, in one experiment, the current between the two parallel plates, placed about 3 mms. apart, was initially 6·5 × 10-9 amperes and fell off directly as the pressure. The current reached a limiting value of about 6 × 10-12 amperes, or about ¹⁄₁₀₀₀ of the value at atmospheric pressure. The magnitude of this limiting current was not much altered if the air was replaced by hydrogen.
Experiments of a similar character have been made by Strutt[[149]] and J. J. Thomson[[150]]; using an active bismuth plate coated with radio-tellurium (polonium) after Marckwald’s method. This substance emits only α rays, and is thus especially suitable for experiments of this kind. Strutt employed the method used by him to show the charge carried by the β rays ([Fig. 27]). He found, however, that, even in the lowest possible vacuum, the electroscope rapidly lost its charge and at the same rate whether it was charged positively or negatively. This is in agreement with the results found by the writer with radium.
In the experiments of J. J. Thomson, the electroscope was attached to a metal disc placed 3 cms. from the plate of radio-tellurium. A very low vacuum was produced by Dewar’s method by absorbing the residual gas in cocoanut charcoal immersed in liquid air. When the electroscope was charged negatively, an extremely slow rate of leak was observed, but when charged positively the leak was about 100 times greater. This showed that the polonium gave out large quantities of negative electricity, but not enough positive to be detected. By placing the apparatus in a strong magnetic field, the negative particles were prevented from reaching the electroscope and the positive leak was stopped.
These results indicate that these negative particles are not projected with sufficient velocity to move against the repulsion exerted by the electrified body, and are bent by a magnetic field. There thus seems little doubt that a stream of negative particles (electrons) is projected from the active surface at a very slow speed. Such low velocity electrons are also projected from uranium and radium. It is probable that these electrons are a type of secondary radiation, set up at the surfaces on which the α rays fall. The particles would be extremely readily absorbed in the gas, and their presence would be difficult to detect except in low vacua. J. J. Thomson at first obtained no evidence that the α particles of polonium were charged; but in later experiments, where the plates were closer together, the electroscope indicated that the α rays did carry a positive charge.
In order to see whether the positive charge due to the α rays from radium could be detected when the slow moving ions were prevented from escaping by a magnetic field, I placed the apparatus of [Fig. 33] between the pole-pieces of a large electromagnet, so that the magnetic field was parallel to the plane of the plates[[151]]. A very marked alteration was observed both on the magnitude of the positive and negative currents. In a good vacuum, the upper plate received a positive charge, independently of whether the lower plate was charged positively or negatively or was connected with earth. After the magnetic field had reached a certain value, a great increase in its strength had no appreciable effect on the magnitude of the current.
The following table illustrates the results obtained when the two plates were 3 mms. apart, and were both coated with thin aluminium foil.
| Potential of lower plate | Current in | arbitrary units | |
|---|---|---|---|
| Without magnetic field | With magnetic field | ||
| 0 | — | +·36 | |
| +2 volts | 2·0 | +·46} | |
| } | ·39 | ||
| -2 „ | 2·5 | +·33} | |
| +4 „ | 2·8 | +·47} | |
| } | ·41 | ||
| -4 „ | 3·5 | +·35} | |
| +8 „ | 3·1 | +·56} | |
| } | ·43 | ||
| -8 „ | 4·0 | +·31} | |
| +84 „ | 3·5 | +·77} | |
| } | ·50 | ||
| -84 „ | 5·2 | +·24} |
Let n be the number of α particles, carrying a charge e, which are absorbed in the upper plate. Let ι₀ be the current due to the slight ionization of the residual gas.
If only a small potential is applied to the lower plate, this current should be equal in magnitude but opposite in sign when the potential is reversed. Let ι1 be the charge per sec. communicated to the upper electrode when the lower plate is charged positively and ι2 the value when charged negatively. Then
ι1 = ι₀ + ne,
ι2 = ι₀ + ne;
adding we get
ι1 + ι2
ne = ------ .
2
Now in the third column of the above table it is seen that (ι1 + ι2)/2 has the values ·39, ·41, ·43 for 2, 4, and 8 volts respectively. The numbers are thus in fairly good agreement. Similar results were obtained when a brass plate was substituted for the upper electrode shown in the figure. Taking into consideration that the magnitude of ne is independent of the strength of the magnetic field above a certain small value, and the good agreement of the numbers obtained with variation of voltage, I think that there can be no doubt that the positive charge communicated to the upper electrode was carried by the α particles. This positive charge was not small, for using a weight of ·48 mgrs. radium bromide spread in a thin foil over an area of about 20 sq. cms., the charge communicated by the particles corresponded to a current 8·8 × 10-13 amperes, and, with the Dolezalek electrometer employed, it was necessary to add a capacity of ·0024 microfarads to the electrometer system.
In these experiments, the film of radium bromide was so thin, that only a very small percentage of the α particles was stopped by the radium itself. Assuming that each α particle carries the same charge as an ion, viz. 1·1 × 10-19 coulombs, and remembering that half of the α particles are absorbed in the lower plate, the total number N of α particles expelled per second from one gram of radium bromide (at its minimum activity) can be deduced. In two separate experiments where the amount of radium used was ·194 and ·484 mgrs. respectively, the values of N were in close agreement and equal to 3·6 × 1010. Now it will be shown later that in radium there are three other products in radio-active equilibrium, each of which probably gives out the same number of α particles as radium itself. If this is the case, the total number of α particles expelled per second from 1 gram of radium bromide in radio-active equilibrium is 4N or 1·44 × 1011. Assuming the composition of radium bromide as RaBr2, the number per second per gram of radium is 2·5 × 1010. This number will be found to be in very good agreement with that deduced from indirect data ([chapter XIII].). The value of N is of great importance in determining the magnitude of various quantities in radio-active calculations.
94. Mass and energy of the α particle. It has been pointed out that the α rays from radium and polonium are analogous to the Canal rays of Goldstein, for both carry a positive charge and are difficult to deflect by a magnetic field. The experiments of Wien have shown that the velocity of projection of the canal rays varies with the gas in the tube and the intensity of the electric field applied, but it is generally about ⅒ of the velocity of the α particle from radium. The value of e/m is also variable, depending upon the gas in the tube.
It has been shown that for the α rays of radium
e
V = 2·5 × 109 and ------- = 6 × 103.
m
Now the value of e/m for the hydrogen atom, liberated in the electrolysis of water, is 104. Assuming the charge carried by the α particle to be the same as that carried by the hydrogen atom, the mass of the α particle is about twice that of the hydrogen atom. Taking into consideration the uncertainty attaching to the experimental value of e/m for the α particle, if the α particle consists of any known kind of matter, this result indicates that it consists either of projected helium or hydrogen. Further evidence on this important question is given in section 260.
The α rays from all the radio-active substances and their products, such as the radio-active emanations and the matter causing excited activity, possess the same general properties and do not vary very much in penetrating power. It is thus probable that in all cases the α rays from the different radio-active substances consist of positively charged bodies projected with great velocity. Since the rays from radium are made up in part of α rays from the emanation stored in the radium, and from the excited activity which it produces, the α rays from each of these products must consist of positively charged bodies; for it has been shown that all the α rays from radium are deviated in a strong magnetic field.
The kinetic energy of each projected particle is enormous, compared with its mass. The kinetic energy of each α particle is
1 1 m
--- mV2 = --- --- V2e = 5·9 × 10-6 ergs.
2 2 e
Taking the velocity of a rifle bullet as 105 cms. per second, it is seen that, mass for mass, the energy of motion of the α rays is 6 × 108 times as great as that of the rifle bullet. In this projection of bodies atomic in size with great velocity probably lies the principal cause of the heating effects produced by radium ([chapter XII]).
95. Atomic disintegration. The radio-activity of the radio-elements is an atomic and not a molecular property. The rate of emission of the radiations depends only on the amount of the element present and is independent of its combination with inactive substances. In addition, it will be shown later that the rate of emission is not affected by wide variations of temperature, or by the application of any known chemical or physical forces. Since the power of radiating is a property of the radio-atoms, and the radiations consist for the most part of positively and negatively charged masses projected with great velocity, it is necessary to suppose that the atoms of the radio-elements are undergoing disintegration, in the course of which parts of the atom escape from the atomic system. It seems very improbable that the α and β particles can suddenly acquire their enormous velocity of projection by the action of forces existing inside or outside the atom. For example, the α particle would have to travel from rest between two points differing in potential by 5·2 million volts in order to acquire the kinetic energy with which it escapes. Thus it seems probable that these particles are not set suddenly in motion, but that they escape from an atomic system in which they were already in rapid oscillatory or orbital motion. On this view, the energy is not communicated to the projected particles, but exists beforehand in the atoms from which they escape. The idea that the atom is a complicated structure consisting of charged parts in rapid oscillatory or orbital motion has been developed by J. J. Thomson, Larmor and Lorentz. Since the α particle is atomic in size, it is natural to suppose that the atoms of the radio-active elements consist not only of the electrons in motion, but also of positively charged particles whose mass is about the same as that of the hydrogen or helium atom.
It will be shown later that only a minute fraction of the atoms of the radio-element need break up per second in order to account for the radiations even of an enormously active element like radium. The question of the possible causes which lead to this atomic disintegration and the consequences which follow from it will be discussed later in [chapter XIII].
96. Experiments with a zinc sulphide screen. A screen of Sidot’s hexagonal blend (phosphorescent crystalline zinc sulphide) lights up brightly under the action of the α rays of radium and polonium. If the surface of the screen is examined with a magnifying glass, the light from the screen is found not to be uniformly distributed but to consist of a number of scintillating points of light. No two flashes succeed one another at the same point, but they are scattered over the surface, coming and going rapidly without any movement of translation. This remarkable action of the radium and polonium rays on a zinc sulphide screen was discovered by Sir William Crookes[[152]], and independently by Elster and Geitel[[153]], who observed it with the rays given out from a wire which has been charged negatively either in the open air or in a vessel containing the emanation of thorium.
In order to show the scintillations of radium on the screen, Sir William Crookes has devised a simple apparatus which he has called the “Spinthariscope.” A small piece of metal, which has been dipped in a radium solution, is placed several millimetres away from a small zinc sulphide screen. This screen is fixed at one end of a short brass tube and is looked at through a lens fixed at the other end of the tube. Viewed in this way, the surface of the screen is seen as a dark background, dotted with brilliant points of light which come and go with great rapidity. The number of points of light per unit area to be seen at one time falls off rapidly as the distance from the radium increases, and, at several centimetres distance, only an occasional one is seen. The experiment is extremely beautiful, and brings vividly before the observer the idea that the radium is shooting out a stream of projectiles, the impact of each of which on the screen is marked by a flash of light.
The scintillating points of light on the screen are the result of the impact of the α particles on its surface. If the radium is covered with a layer of foil of sufficient thickness to absorb all the α rays the scintillations cease. There is still a phosphorescence to be observed on the screen due to the β and γ rays, but this luminosity is not marked by scintillations to any appreciable extent. Sir William Crookes showed that the number of scintillations was about the same in vacuo as in air at atmospheric pressure. If the screen was kept at a constant temperature, but the radium cooled down to the temperature of liquid air, no appreciable difference in the number of scintillations was observed. If, however, the screen was gradually cooled to the temperature of liquid air, the scintillations diminished in number and finally ceased altogether. This is due to the fact that the screen loses to a large extent its power of phosphorescence at such a low temperature.
Not only are scintillations produced by radium, actinium, and polonium, but also by the emanations and other radio-active products which emit α rays. In addition, F. H. Glew[[154]] has found that they can be observed from the metal uranium, thorium compounds and various varieties of pitchblende. In order to show the scintillations produced by pitchblende, a flat surface was ground, and a transparent screen, whose lower surface was coated with zinc sulphide, placed upon it. Glew has designed a modified and very simple form of spinthariscope. A transparent screen, coated on one side with a thin layer of zinc sulphide, is placed in contact with the active material, and the scintillations observed by a lens in the usual way.
Since there is no absorption in the air, the luminosity is a maximum. The relative transparency of different substances placed between the active material and the screen may, in this way, be directly studied.
The production of scintillations appears to be a general property of the α rays from all radio-active substances. The scintillations are best shown with a zinc sulphide screen; but are also observed with willemite (zinc silicate), powdered diamond, and potassium platinocyanide (Glew, loc. cit.). If a screen of barium platinocyanide is exposed to the α rays from radium, the scintillations are difficult to observe, and the luminosity is far more persistent than for a zinc sulphide screen exposed under the same conditions. The duration of the phosphorescence in this case probably accounts for the absence of visible scintillations.
There can be no doubt that the scintillations result from the continuous bombardment of the sensitive screen by the α particles. Each of these particles moves with enormous velocity, and has a considerable energy of motion. On account of the ease with which these particles are stopped, most of this energy is given up at the surface of the screen, and a portion of the energy is in some way transformed into light. Zinc sulphide is very sensitive to mechanical shocks. Luminosity is observed if a penknife is drawn across the screen, or if a current of air is directed on to the screen. The disturbance effected by the impact of the α particle extends over a distance very large compared with the size of the impinging particle, so that the spots of light produced have an appreciable area. Recently Becquerel[[155]] has made an examination of the scintillations produced by different substances, and has concluded that the scintillations are due to irregular cleavages in the crystals composing the screen, produced by the action of the α rays. Scintillations can be mechanically produced by crushing a crystal. Tommasina[[156]] found that a zinc sulphide screen removed from the action of the radium rays for several days, showed the scintillations again when an electrified rod was brought near it.
The number of scintillations produced in zinc sulphide depends upon the presence of a slight amount of impurity and on its crystalline state. It can be shown that even with the most sensitive zinc sulphide screens, the number of scintillations is probably only a small fraction of the total number of α particles which fall upon it. It would appear that the crystals are in some way altered by the bombardment of the α particles, and that some of the crystals occasionally break up with emission of light[[157]].
Although the scintillations from a particle of pure radium bromide are very numerous, they are not too numerous to be counted. Close to the radium, the luminosity is very bright, but by using a high power microscope the luminosity can still be shown to consist of scintillations. Since the number of scintillations probably bears no close relation to the number of α particles emitted, a determination of the number of scintillations would have no special physical significance. The relation between the number of α particles and the number of scintillations would probably be variable, depending greatly on the exact chemical composition of the sensitive substance and also upon its crystalline state.
97. Absorption of the α rays by matter. The α rays from the different radio-active substances can be distinguished from one another by the relative amounts of their absorption by gases or by thin screens of solid substances. When examined under the same conditions, the α rays from the active substances can be arranged in a definite order with reference to the amount of absorption in a given thickness of matter.
In order to test the amount of absorption of the α rays for different thicknesses of matter, an apparatus similar to that shown in [Fig. 17], p. 98, was employed[[158]]. A thin layer of the active material was spread uniformly over an area of about 30 sq. cms., and the saturation current observed between two plates 3·5 cms. apart. With a thin layer[[159]] of active material, the ionization between the plates is due almost entirely to the α rays. The ionization due to the β and γ rays is generally less than 1% of the total.
The following table shows the variation of the saturation current between the plates due to the α rays from radium and polonium, with successive layers of aluminium foil interposed, each ·00034 cm. in thickness. In order to get rid of the ionization due to the β rays from radium, the radium chloride employed was dissolved in water and evaporated. This renders the active compound, for the time, nearly free from β rays.
The initial current with 1 layer of aluminium over the active material is taken as 100. It will be observed that the current due
| Polonium. | Radium. | ||||
|---|---|---|---|---|---|
| Layers of aluminium | Current | Ratio of decrease for each layer | Layers of aluminium | Current | Ratio of decrease for each layer |
| 0 | 100 | 0 | 100 | ||
| ·41 | ·48 | ||||
| 1 | 41 | 1 | 48 | ||
| ·31 | ·48 | ||||
| 2 | 12·6 | 2 | 23 | ||
| ·17 | ·60 | ||||
| 3 | 2·1 | 3 | 13·6 | ||
| ·067 | ·47 | ||||
| 4 | ·14 | 4 | 6·4 | ||
| ·39 | |||||
| 5 | 0 | 5 | 2·5 | ||
| ·36 | |||||
| 6 | ·9 | ||||
| 7 | 0 |
to the radium rays decreases very nearly by half its value for each additional thickness until the current is reduced to about 6% of the maximum. It then decays more rapidly to zero. Thus, for radium, over a wide range, the current decreases approximately according to an exponential law with the thickness of the screen, or
where i is the current for a thickness d, and i₀ the initial current. In the case of polonium, the decrease is far more rapid than would be indicated by the exponential law. By the first layer, the current is reduced to the ratio ·41. The addition of the third layer cuts the current down to a ratio of ·17. For most of the active bodies, the current diminishes slightly faster than the exponential law would lead one to expect, especially when the radiation is nearly all absorbed.
98. The increase of absorption of the α rays of polonium with the thickness of matter traversed has been very clearly shown in some experiments made by Mme Curie. The apparatus employed is shown in [Fig. 34].
Fig. 34.
The saturation current was measured between two parallel plates PP´ 3 cms. apart. The polonium A was placed in the metal box CC, and the rays from it, after passing through an opening in the lower plate P´, covered with a layer of thin foil T, ionized the gas between the plates. For a certain distance AT, of 4 cms. or more, no appreciable current was observed between P and P´. As the distance AT was diminished, the current increased in a very sudden manner, so that for a small variation of the distance AT there was a large increase of current. With still further decrease of distance the current increases in a more regular manner. The results are shown in the following table, where the screen T consisted of one and two layers of aluminium foil respectively. The current due to the rays, without the aluminium screen, is in each case taken as 100.
| Distance AT in cms. | 3·5 | 2·5 | 1·9 | 1·45 | 0·5 |
|---|---|---|---|---|---|
| For 100 rays transmitted by one layer | 0 | 0 | 5 | 10 | 25 |
| For 100 rays transmitted by two layers | 0 | 0 | 0 | 0 | 0·7 |
The metallic screen thus cuts off a greater proportion of the rays the greater the distance of air which the radiations traverse. The effects are still more marked if the plates PP´ are close together. Results similar but not so marked are found if radium is substituted for the polonium.
It follows from these experiments that the ionization per unit volume, due to a large plate uniformly covered with the radio-active matter, falls off rapidly with the distance from the plate. At a distance of 10 cms. the α rays from uranium, thorium, or radium have been completely absorbed in the gas, and the small ionization then observed in the gas is due to the more penetrating β and γ rays. The relative amount of the ionization observed at a distance from the source will increase with the thickness of the layer of active matter, but will reach a maximum for a layer of a certain thickness. The greater proportion of the ionization, due to unscreened active matter, is thus entirely confined to a shell of air surrounding it not more than 10 cms. in depth.
Fig. 35.
99. The α rays from different compounds of the same active element, although differing in amount, have about the same average penetrating power. Experiments on this point have been made by the writer[[160]] and by Owens[[161]]. Thus in comparing the relative power of penetration of the α rays from the different radio-elements, it is only necessary to determine the penetrating power for one compound of each of the radio-elements. Rutherford and Miss Brooks[[162]] have determined the amount of absorption of the α rays from the different active substances in their passage through successive layers of aluminium foil ·00034 cm. thick. The curves of absorption are given in [Fig. 35]. For the purpose of comparison in each case, the initial current with the bare active compound was taken as 100. A very thin layer of the active substance was used, and, in the case of thorium and radium, the emanations given off were removed by a slow current of air through the testing vessel. A potential difference of 300 volts was applied between the plates, which was sufficient to give the maximum current in each case.
Curves for the minerals organite and thorite were very nearly the same as for thoria.
For comparison, the absorption curves of the excited radiations of thorium and radium are given, as well as the curve for the radio-elements uranium, thorium, radium, and polonium. The α radiations may be arranged in the following order, as regards their power of penetration, beginning with the most penetrating.
Thorium}
Radium } excited radiation.
Thorium.
Radium.
Polonium.
Uranium.
The same order is observed for all the absorbing substances examined, viz., aluminium, Dutch metal, tinfoil, paper, and air and other gases. The differences in the absorption of the α rays from the active bodies are thus considerable, and must be ascribed either to a difference of mass or of velocity of the α particles or to a variation in both these quantities.
Since the α rays differ either in mass or velocity, it follows that they cannot be ascribed to any single radio-active impurity common to all radio-active bodies.
100. Absorption of the α rays by gases. The α rays from the different radio-active substances are quickly absorbed in their passage through a few centimetres of air at atmospheric pressure and temperature. In consequence of this, the ionization of the air, due to the α rays, is greatest near the surface of the radiating body and falls off very rapidly with the distance (see [section 98]).
Fig. 36.
A simple method of determining the absorption in gases is shown in [Fig. 36]. The maximum current is measured between two parallel plates A and B kept at a fixed distance of 2 cms. apart, and then moved by means of a screw to different distances from the radio-active surface. The radiation from this active surface passed through a circular opening in the plate A, covered with thin aluminium foil, and was stopped by the upper plate. For observations on other gases besides air, and for examining the effect at different pressures, the apparatus is enclosed in an air-tight cylinder.
If the radius of the active surface is large compared with the distance of the plate A from it, the intensity of the radiation is approximately uniform over the opening in the plate A, and falls off with the distance x traversed according to an exponential law. Thus
where λ is the “absorption constant” of the radiation for the gas under consideration[[163]]. Let
x = distance of lower plate from active material,
l = distance between the two fixed plates.
The energy of the radiation at the lower plate is then
and at the upper plate
The total number of ions produced between the parallel plates A and B is therefore proportional to
Since the factor
is a constant, the saturation current between A and B varies as
i.e. it decreases according to an exponential law with the distance traversed.
Fig. 37.
The variation of the current between A and B with the distance from a thin layer of uranium oxide is shown in [Fig. 37] for different gases. The initial measurements were taken at a distance of about 3·5 mms. from the active surface. The actual values of this initial current were different for the different gases, but, for the purposes of comparison, the value is in each case taken as unity.
It will be seen that the current falls off with the distance approximately in a geometrical progression, a result which is in agreement with the simple theory given above. The distance through which the rays pass before they are absorbed is given below for different gases.
| Gas | Distance in mms. to absorb half of radiation |
|---|---|
| Carbonic acid | 3 |
| Air | 4·3 |
| Coal-gas | 7·5 |
| Hydrogen | 16 |
The results for hydrogen are only approximate, as the absorption is small over the distance examined.
The absorption is least in hydrogen and greatest in carbonic acid, and follows the same order as the densities of the gases. In the case of air and carbonic acid, the absorption is proportional to the density, but this rule is widely departed from in the case of hydrogen. Results for the relative absorption by air of the α rays from the different active bodies are shown in [Fig. 38].
Fig. 38.
The initial observation was made about 2 mms. from the active surface, and the initial current is in each case taken as 100. The current, as in the case of uranium, falls off at first approximately in geometrical progression with the distance. The thickness of air, through which the radiation passes before the intensity is reduced to half value, is given below.
| Distance in mms. | |
|---|---|
| Uranium | 4·3 |
| Radium | 7·5 |
| Thorium | 10 |
| Excited radiation from Thorium and Radium | 16·5 |
The order of absorption by air of the radiations from the active substances is the same as the order of absorption by the metals and solid substances examined.
101. Connection between absorption and density. Since in all cases the radiations first diminish approximately according to an exponential law with the distance traversed, the intensity I after passing through a thickness x is given by
where λ is the absorption constant and I₀ the initial intensity.
The following table shows the value of λ with different radiations for air and aluminium.
| Radiation | λ for aluminium | λ for air |
|---|---|---|
| Excited radiation | 830 | ·42 |
| Thorium | 1250 | ·69 |
| Radium | 1600 | ·90 |
| Uranium | 2750 | 1·6 |
Taking the density of air at 20° C. and 760 mms. as 0·00120 compared with water as unity, the following table shows the value of λ divided by density for the different radiations.
| Radiation | Aluminium | Air |
|---|---|---|
| Excited radiation | 320 | 350 |
| Thorium | 480 | 550 |
| Radium | 620 | 740 |
| Uranium | 1060 | 1300 |
Comparing aluminium and air, the absorption is thus roughly proportional to the density for all the radiations. The divergence, however, between the absorption-density numbers is large when two metals like tin and aluminium are compared. The value of λ for tin is not much greater than for aluminium, although the density is nearly three times as great.
If the absorption is proportional to the density, the absorption in a gas should vary directly as the pressure, and this is found to be the case. Some results on this subject have been given by the writer (loc. cit.) for uranium rays between pressures of ¼ and 1 atmosphere. Owens (loc. cit.) examined the absorption of the α radiation in air from thoria between the pressures of 0·5 to 3 atmospheres and found that the absorption varied directly as the pressure.
The variation of absorption with density for the projected positive particles is thus very similar to the law for the projected negative particles and for cathode rays. The absorption, in both cases, depends mainly on the density, but is not in all cases directly proportional to it. Since the absorption of the α rays in gases is probably mainly due to the exhaustion of the energy of the rays by the production of ions in the gas, it seems probable that the absorption in metals is due to a similar cause.
102. Relation between ionization and absorption in gases. It has been shown ([section 45]) that if the α rays are completely absorbed in a gas, the total ionization produced is about the same for all the gases examined. Since the rays are unequally absorbed in different gases, there should be a direct connection between the relative ionization and the relative absorption. This is seen to be the case if the results of Strutt ([section 45]) are compared with the relative absorption constants ([section 100]).
| Gas | Relative absorption | Relative ionization |
|---|---|---|
| Air | 1 | 1 |
| Hydrogen | ·27 | ·226 |
| Carbon dioxide | 1·43 | 1·53 |
Considering the difficulty of obtaining accurate determinations of the absorption, the relative ionization in a gas is seen to be directly proportional to the relative absorption within the limits of experimental error. This result shows that the energy absorbed in producing an ion is about the same in air, hydrogen, and carbon dioxide.
103. Mechanism of the absorption of α rays by matter. The experiments, already described, show that the ionization of the gas, due to the α rays from a large plane surface of radio-active matter, falls off in most cases approximately according to an exponential law, until most of the rays are absorbed, whereupon the ionization decreases at a much faster rate. In the case of polonium, the ionization falls off more rapidly than is to be expected on the simple exponential law.
The ionization produced in the gas is due to the collision of the rapidly moving α particles with the molecules of the gas in their path. On account of its large mass, the α particle is a far more efficient ionizer than the β particle moving at the same speed. It can be deduced from the results of experiment that each projected α particle is able to produce about 100,000 ions in passing through a few centimetres of the gas before its velocity is reduced to the limiting value, below which it no longer ionizes the gas in its path.
Energy is required to ionize the gas, and this energy can only be obtained at the expense of the kinetic energy of the projected α particle. Thus it is to be expected that the α particle should gradually lose its velocity and energy of motion in its passage through the gas.
Since the rate of absorption of the α rays in gases is deduced from measurements of the ionization of the gas at different distances from the source of radiation, a knowledge of the law of variation of the ionizing power of the projected α particle with its speed is required in order to interpret the results. The experimental data on this question are, however, too incomplete to be applied directly to a solution of this question. Townsend[[164]] has shown that a moving electron produces ions in the gas after a certain limiting velocity is reached. The number of ions produced per centimetre of its path through the gas then rises to a maximum, and for still higher speeds continuously decreases. For example, Townsend found that the number of ions produced by an electron moving in an electric field was small at first for weak fields, but increased with the strength of the electric field to a maximum corresponding to the production of 20 ions per cm. of path in air at a pressure of 1 mm. of mercury. Durack[[165]] found that the electrons, generated in a vacuum tube, moving with a velocity of about 5 × 109 cms. per second produced a pair of ions every 5 cms. of path at 1 mm. pressure. In a later paper, Durack showed that for the electrons from radium, which are projected with a velocity greater than half the velocity of light, a pair of ions was produced every 10 cms. of path. The high speed electron from radium is thus a very inefficient ionizer and produces only about ¹⁄₁₀₀ of the ionization per unit path observed by Townsend for the slow moving electron.
104. In the case of the α particle, no direct measurements have been made upon the variation of the ionization with the velocity of the particle, so that the law of absorption of the rays cannot be deduced directly. An indirect attack upon the question has, however, been made recently by Bragg and Kleeman[[166]] who have formulated a simple theory to account for the experimental results which they have obtained upon the absorption of the α rays. The α particles from each simple type of radio-active matter are supposed to be projected with the same velocity, and to pass through a definite distance a in air at atmospheric pressure and temperature before they are all absorbed. As a first approximation the ionization per unit path is supposed to be the same over the whole length traversed before absorption, and to cease fairly suddenly at a definite distance from the source of radiation. This is in agreement with the observed fact that the ionization between parallel plates increases very rapidly when it approaches nearer than a certain distance to the radiant source. The range a depends upon the initial energy of motion of the α particle and will thus be different for different kinds of radio-active matter. If a thick layer of radio-active matter is employed, only the α particles from the surface have a range a. Those which reach the surface from a depth d have their range diminished by an amount ρd, where ρ is the density of the radio-active matter compared with air. This is merely an expression of the fact that the absorption of the α rays is proportional to the thickness and density of matter traversed. The rays from a thick layer of active matter will thus be complex, and will consist of particles of different velocity whose ranges have all values between 0 and a.
Suppose that a narrow pencil of α rays is emitted from a thick layer of radio-active material, and confined by metal stops as in [Fig. 39].
Fig. 39.
The pencil of rays passes into an ionization vessel AB through a fine wire gauze A. The amount of ionization is to be determined between A and B for different distances h from the source of the rays R to the plate A.
All the particles coming from a depth x of the material given by h = a – ρx will enter the ionization vessel. The number of ions produced in a depth dh of the ionization vessel is equal to nxdh, i.e. to
a – h
n ------ dh ,
ρ
where n is a constant.
If the depth of the ionization vessel be b, the total number of ions produced in the vessel is
This supposes that the stream of particles passes completely across the vessel. If not, the expression becomes
If the ionization in the vessel AB is measured, and a curve plotted showing its relation to h, the curve in the former case should be a straight line whose slope is nb/ρ and in the latter a parabola.
Thus if a thin layer of radio-active material is employed and a shallow ionization vessel, the ionization would be represented by a curve such as APM ([Fig. 40]), where the ordinates represent distances from the source of radiation, and the abscissae the ionization current between the plates AB.
Fig. 40.
In this case, PM is the range of the α particles from the lowest layer of the radio-active matter. The current should be constant for all distances less than PM.
For a thick layer of radio-active matter, the curve should be a straight line such as APB.
Curves of the above character should only be obtained when definite cones of rays are employed, and where the ionization vessel is shallow and includes the whole cone of rays. In such a case the inverse square law need not be taken into account.
In the experiments previously recorded (sections [99] and [100]), the ionization was measured between parallel plates several centimetres apart for a large area of radio-active material. Such an arrangement was necessary at the time at which the experiments were made, as only weak radio-active material was available. Measurable electrical effects could not then be obtained with narrow cones of rays and shallow ionization vessels, but this disadvantage is removed by the advent of pure radium bromide as a source of radiation.
The interesting experiments described by Bragg and Kleeman show that the theoretical curves are approximately realized in practice. The chief difficulty experienced in the analysis of the experimental results was due to the fact that radium is a complex radio-active substance and contains four radio-active products each of which gives rise to α rays which have different ranges. The general character of the results obtained from radium are shown graphically in [Fig. 41], curves A, B, C, D.
Fig. 41.
The ordinates represent the distance between the radium and the gauze of the testing vessel; the abscissae the current in the ionization vessel in arbitrary units. Five milligrams of radium bromide were used, and the depth of the ionization vessel was about 5 mms. Curve A is for a cone of rays of angle 20°. The initial current at a distance of 7 cms. is due to the β and γ rays and natural leak. This curve is initially parabolic, and then is made up of two straight lines. Curve B is for a smaller cone, and shows the straight line character of the curve to within a short distance of the radium. Curve C was obtained under the same condition as curve A, but with a layer of gold beater’s skin placed over the radium. The effect of this is to reduce all the ordinates of curve A by the same quantity. This is to be expected on the simple theory already considered. Curve D was obtained when the radium was heated so as to get rid of the emanation and its products. The α particles of greatest range are quite absent and the curve is simpler in character.
Fig. 42.
The complex character of the radium curves are more clearly brought out by a careful examination of a portion of the curve at distances between 2 and 5 cms. from the radium, using an ionization vessel of depth only 2 mms. The results are shown in [Fig. 42], where the curve is seen to consist approximately of four straight lines of different slopes represented by PQ, QR, RS, ST.
Such a result is to be expected, for it will be shown later that four distinct α ray products exist in radium when in radio-active equilibrium. Each of these products of radium emits an equal number of α particles per second, but the range of each is different. If a1 is the range of one stream, a2 of another, the ionization in the vessel AB, when two streams enter the vessel, should be
nb nb
---- (a1-h-b/2) + ----- (a2 – h – b/2),
ρ ρ
i.e.
nb
---- (a1 + a2 – 2h – b) .
ρ
Thus the slope of the curve should in this case be 2nb/ρ, while if only one stream enters, it should be nb/ρ. When three reach it, the slope should be 3nb/ρ and for four 4nb/ρ. These results are realized fairly closely in practice. The curve ([Fig. 42]) consists of four parts, whose slopes are in the proportion 16, 34, 45, 65, i.e. very nearly in the ratio 1, 2, 3, 4.
Experiments were also made with very thin layers of radium bromide, when, as we have seen ([Fig. 40]) a very different shape of curve is to be expected. An example of the results is shown in [Fig. 43], curves I., II. and III. Curve I. is obtained from radium bromide which has been heated to drive off the emanation, and curves II. and III. from the same substance several days later, when the emanation was again accumulating. The portion PQ, which is absent in the first curve, is probably due to the “excited” activity produced by the emanation. By careful examination of the successive changes in the curves after the radium has been heated to drive off the emanation, it is possible to tell the range of the α rays from each of the different products, and this has been done to some extent by Bragg and Kleeman.
It will be seen later that the results here obtained support in a novel way the theory of radio-active changes which has been advanced from data of quite a different character.
The inward slope of the curve in [Fig. 43] due to the radium indicates that the α particles become more efficient ionizers as their velocity decreases. This is in agreement with observations on the β rays. In some cases Bragg also observed that the α particles are the most efficient ionizers just before they lose their power of ionizing the gas.
Fig. 43.
Thus we may conclude from these experiments that the α particles from a simple radio-active substance traverse a definite distance in air, at a definite pressure and temperature, and that the ionization ends fairly abruptly. If the rays traverse a sheet of metal, the effective range of ionization is diminished by a distance corresponding to ρd, where ρ is the density of the material compared with air and d its thickness. The α rays from a thick layer of a simple radio-active substance consist of α particles of different velocities, which have ranges in air lying between 0 and the maximum range. The ionization of the particles per unit path is greatest near the end of its range, and decreases somewhat as we approach the radiant source. A complex source of rays like radium gives out four types of rays, each of which has a different but distinct range.
From this theory it is possible to calculate approximately the decrease of current to be observed when sheets of metal foil are placed over a large area of radio-active substance. This is the method that has been employed to obtain the curves of Figs. [35] and [38].
Suppose a very thin layer of simple radio-active matter is employed (for example a bismuth plate covered with radio-tellurium or a metal plate made active by exposure to the presence of the thorium or radium emanations) and that the ionization vessel is of sufficient depth to absorb the α rays completely.
Let d be the thickness of the metal plate, ρ its density compared with air. Consider a point P close to the upper side of the plate. The range of the particles moving from a point, when the path makes an angle θ with the normal at P, is a – ρd sec θ, where a is the range in air. The rays coming from points such that the paths make an angle with the normal greater than
will thus be absorbed in the plate. By integrating over the circular area under the point P, it is easy to show that the total ionization in the vessel is proportional to
The curves showing the relation between current and distance of metal traversed should thus be parabolic with respect to d. This is approximately the case for a simple substance like radio-tellurium. The curve for a thick layer of radium would be difficult to calculate on account of the complexity of the rays, but we know from experiment that it is approximately exponential. An account of some recent investigations made to determine the range of velocity over which the α particle is able to ionize the gas is given in [Appendix A]. The results there given strongly support the theory of absorption of the α rays discussed above.