Chapter IX: Part III: The α Rays (1)

=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.

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 0·93
mica to absorb all α rays

(4) Radium covered with mica and 0·92
magnetic field applied

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_
_H_ρ = ---- _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,

2ρ_d₁_ = _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_ 2_d₁_ _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 _H_ρ 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. _H_ρ
from the slit

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 _H_ρ for complete deviation of the α rays was 390,000. The results are thus in good agreement. Since

_m_
_H_ρ = ----- _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 _H_ρ 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.

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 × 10⁹ 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 Current in arbitrary units
of lower
plate

Without magnetic With magnetic field
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 4_N_ 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 Current Ratio of Layers of Current Ratio of
aluminium decrease aluminium decrease
for each for each
layer 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

$$ \frac {i} {i₀} = e^{λ d} $$,

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.

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 0 0 5 10 25
transmitted by one
layer

For 100 rays 0 0 0 0 0·7
transmitted by two
layers

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.

=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).

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

$$ \frac {I} {I₀} = e^{–λ x} $$,

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

$$ I₀ e^{–λ x} $$,

and at the upper plate

$$ I₀ e^{–λ (l + x)} $$ .

The total number of ions produced between the parallel plates _A_ and _B_ is therefore proportional to

$$ e^{–λ x} − e^{–λ (l + x)} = e^{–λ x} (1 - e^{–λ l}) $$ .

Since the factor

$$ 1 = e^{–λ l} $$

is a constant, the saturation current between _A_ and _B_ varies as

$$ e^{–λ x} $$,

_i.e._ it decreases according to an exponential law with the distance traversed.

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