Radio-activity/Appendix A
APPENDIX A.
PROPERTIES OF THE α RAYS.
A brief account is given here of some investigations made by the
writer on the properties of the α rays from radium—investigations
which were not completed in time for the results to be incorporated
in the text.
The experiments were undertaken primarily with a view of determining accurately the value of e/m of the α particle from radium, in order to settle definitely whether or not it is an atom of helium. In the previous experiments of the writer, Becquerel, and Des Coudres, on this subject (sections 89, 90, and 91), a thick layer of radium in radio-active equilibrium has been used as a source of α rays. Bragg (section 103) has shown that the rays emitted from radium under such conditions are complex, and consist of particles projected over a considerable range of velocity. In order to obtain a homogeneous pencil of rays it is necessary to use a very thin layer of a simple radio-active substance as a source of rays. In the experiments that follow, this condition was fulfilled by using a fine wire which was made active by exposure for several hours in the presence of a large quantity of radium emanation. By charging the wire negatively the active deposit was concentrated upon the wire, which was made intensely active. The active deposit initially contains radium A, B, and C. The activity of radium A practically disappears in about fifteen minutes, and the α radiation is then due entirely to the single product radium C, since radium B is a rayless product. The activity of radium C decreases to about 15 per cent. of its initial value after two hours.
Magnetic deflection of the α rays. The photographic method
was employed to determine the deviation of the pencil of rays in a
magnetic field. The experimental arrangement is shown in Fig. 106.
The rays from the active wire, which was placed in a slot, passed
through a narrow slit and fell normally on a photographic plate, placed
at a known distance above the slit. The apparatus was enclosed in a brass tube which could be exhausted rapidly to a low pressure by means of a Fleuss pump. The apparatus was placed in a strong uniform magnetic field parallel to the plane of the slit. The magnetic field was reversed every ten minutes, so that on developing the plate two narrow bands were observed, the distance between which represented twice the deviation from the normal of the pencil of rays by the magnetic field. The width of the band was found to be the same whether the magnetic field was
applied or not, showing that the pencil of
rays was homogeneous and consisted of α
particles projected with the same velocity.
Fig. 106.
By placing the photographic plate at different distances from the slit it was found that the rays, after entering the magnetic field, described the arc of a circle of radius ρ equal to 42·0 cms. The strength of field H was 9470 C.G.S. units, so that the value of Hρ for the α particles expelled from radium C is 398,000. This is in good agreement with the maximum values of Hρ, previously found for radium rays (see section 92).
The electric deviation of the rays from radium C has not yet been accurately measured, but an approximate determination of e/m for the α particles can be obtained by assuming that the heating effect of radium C is a measure of the kinetic energy of the α particles expelled from it. We have seen in section 246 that the heating effect of the radium C present in one gram of radium in radio-active equilibrium is 31 gram calories per hour, which corresponds to an emission of energy of 3·6 × 10^5 ergs per second. Now when radio-active equilibrium is reached, the number of α particles expelled from radium C per second is equal to the number of α particles expelled per second from radium at its minimum activity. This number, n, is 6·2 × 10^{10} (section 93).
Then 1/2 mnv^2 = 3·6 × 10^5,
or [m/e]v^2 = 1·03 × 10^{16},
substituting the value of n, and the value of the ionic charge e. The value of e in this case has not been assumed, since n = i/e, where i was the measured current due to the charge carried by the α rays.
From the magnetic deflection, it is known that
(m/e)v = 3·98 × 10^5.
From these two equations we obtain
v = 2·6 × 10^9 cms. per second.
e/m = 6·5 × 10^3 electro-magnetic units.
These values are in surprisingly good agreement with the previous values of the writer and Des Coudres (section 91). On account of the uncertainty attaching to the value of n, not much weight can be attached to the determination by this method of the constants of the α particles.
Decrease of velocity of the α particles in passing through matter. Some experiments were made to determine the velocity of
the α particles from radium C after passing through known thicknesses
of aluminium. The previous apparatus was employed, and the distance
between the photographic bands was observed for successive layers of
aluminium foil, each ·00031 cms. thick, placed over the active wire.
The photographic plate was placed 2 cms. above the slit, and the
magnetic field extended 1 cm. below the slit. The amount of deviation
of the rays is inversely proportional to their velocity after
traversing the aluminium screens. The impressions on the plate were
clear and distinct, and about the same in all cases, showing that the
rays were still homogeneous after passing through the aluminium.
A clear photographic impression was obtained for 12 layers of foil, but it was not found possible to obtain any effect through 13 layers. This result shows that the photographic action of the rays, like the ionizing action, ceases very abruptly.
The results obtained are shown in the following table. Assuming that the value of e/m is constant, the third column gives the velocity of the α particles after traversing the aluminium. This is expressed in terms of V_{0}, the velocity of the α particle when the screens are removed.
Number of layers Distance between bands Velocity of
of aluminum foil on the plate α particles
0 1·46 mms. 1·00 V_{0}
5 1·71 " ·85 "
8 1·91 " ·76 "
10 2·01 " ·73 "
12 2·29 " ·64 "
13 No photographic effect
cent. of its initial value when it fails to produce any action on the photographic plate.
Now Bragg has shown (section 104) that the α particle produces approximately the same number of ions per cm. of path in air over its whole range. Consequently, the simplest assumption to make is that the energy of the α particle is diminished by a constant amount in traversing each layer of foil. After passing through 12 layers the kinetic energy is reduced to 41 per cent. of the maximum. Each layer of foil thus absorbs 4·9 per cent. of the maximum energy. The observed kinetic energy of the α particle after passing through successive layers of foil, and the value calculated on the above assumptions, are shown in the following table.
Number of layers Observed Calculated
of aluminum foil energy energy
0 100 100
5 73 75
8 58 61
10 53 51
12 41 41
The experimental and theoretical values agree within the limits of experimental error. We may thus conclude, as a first approximation, that the same proportion of the total energy is abstracted from the α particles in passing through equal distances of the absorbing screen.
Range of ionization and photographic action in air.
The abrupt falling off of the photographic impression after the rays
had passed through 12 layers of foil suggested that it might be
directly connected with the corresponding abrupt falling off of the
ionization in air, so clearly brought out by Bragg. This was found to
be the case. It was found experimentally that the absorption in each
layer of aluminium foil was equivalent to that produced by a distance
of ·54 cms. of air. Twelve layers of foil thus corresponded to 6·5 cms. of
air. Now Bragg found that the α rays from radium C ionize the air
for a distance 6·7 cms., and that the ionization then falls off very
rapidly. We may thus conclude that the α rays cease to affect the
photographic plate at the same velocity as that at which they cease to
ionize the gas. This is a very important result, and, as we shall see
later, suggests that the action on the photographic plate is due to an
ionization of the photographic salts. The velocity of the α particles from the different radio-active products
can at once be calculated, knowing the maximum range in air
of the α rays from each product. The latter have been experimentally
determined by Bragg. The velocity is expressed in terms of V_{0}, the
initial velocity of the α particles from radium C. The rays from
radium C are projected with a greater velocity than the rays from the
other products of radium.
Product Maximum range of Velocity of
α particles in air α particles
Radium 3 cms. ·82 V_{0}
Emanation 3·8 or 4·4 cms. ·87 or ·90 V_{0}
Rad. A 4·4 or 3·8 " ·90 or ·87 V_{0}
Rad. C 6·7 " 1·00 V_{0}
It is difficult to determine from the experiments whether the range 3·8 cms. belongs to the rays from the emanation or from radium A. The mean velocity of the α particles is thus ·90 V_{0}, and the maximum variation for the individual products does not vary more than 10 per cent. from the mean value.
The results of Becquerel, discussed in section 92, at once receive an explanation on the above results. The α particles, expelled from radium in radio-active equilibrium, have all ranges lying between 0 and 6·7 cms. of air. The velocity of the α particles which are able to produce a photographic impression varies between ·64 V_{0} and V_{0}. The particles which have only a short range in air are projected with a smaller velocity than those which have a greater range. The former are in consequence more bent by a magnetic field. It is thus to be expected that the apparent curvature of the path of rays in a uniform magnetic field will be greater close to the radium than at some distance away.
Range of phosphorescent action in air. Some experiments
were also made to see whether the action of the α rays in producing
luminosity in substances like zinc sulphide, barium platino-cyanide,
and willemite, ceased at the same distance as the ionizing action.
A very active wire was placed on a moveable plate, the distance of which from a fixed screen of phosphorescent substance could be varied. The distance at which the phosphorescent action ceased could be determined fairly accurately. Different thicknesses of aluminium foil were then placed over the active wire, and the corresponding distance at which the luminosity disappeared was measured. The results are shown graphically in Fig. 107, where the ordinates represent the distance of the phosphorescent screen from the active wire, and the abscissae the number of layers of aluminium foil, each ·00031 cms. thick.
Fig. 107.
It is seen that the curve joining the points is a straight line. 12·5 thicknesses of foil absorbed the rays to the same extent as 6·8 cms. of air, so that each thickness of aluminium corresponded in absorbing power to ·54 cms. of air. For a screen of zinc sulphide, the phosphorescent action ceased at a distance of air of 6·8 cms., showing that the photographic and phosphorescent ranges of the α rays in air were practically identical.
The experiments with barium platino-cyanide and willemite were more difficult, as the β and γ rays from the active wire produced a luminosity comparable with that produced by the α rays. Fairly concordant results, however, were obtained by introducing a thin sheet of black paper between the active wire and the screen. If the luminosity was sensibly changed, it was concluded that the α rays still produced an effect, and in this way the point of cessation of phosphorescent action could be approximately determined. For example, with eight thicknesses of foil over the active wire the additional thickness of air required to cut off the phosphorescent effect of the a rays was 2·5 cms. for willemite, and 2·1 cms. for barium platino-cyanide. The corresponding distance for zinc sulphide was 2·40 cms., a value intermediate between the other two.
Since eight layers of foil are equivalent to 4·3 cms. of air, the ranges in air of phosphorescent action for zinc sulphide, barium platinocyanide, and willemite correspond to 6·7, 6·8, and 6·4 cms. respectively. The differences observed are quite likely to be due to experimental error.
Discussion of results. We have seen that the ionizing,
phosphorescent, and photographic actions of the α rays emitted from
radium C cease after traversing very nearly the same distance of air.
This is a surprising result when it is remembered that the α particle,
after passing through this depth of air, still possesses a velocity of at
least 60 per cent. of its initial value. Taking the probable value of
the initial velocity of the α particle from radium C as 2·5 × 10^9 cms.
per sec., the ionizing, phosphorescent, and photographic actions cease
when the velocity of the α particle falls below 1·5 × 10^9 cms. per second,
that is, a velocity of about 1/20 of that of light. The particle still
possesses nearly 40 per cent. of its initial energy of projection at this
stage.
These results show that the property of the α rays of producing ionization in gases, of producing luminosity in some substances, and of affecting a photographic plate, ceases when the velocity of the α particle falls below a certain fixed value which is the same in each case. It seems reasonable, therefore, to suppose that these three properties of the α rays must be ascribed to a common cause. Now the absorption of the α rays in gases is mainly a consequence of the energy absorbed in the production of ions in the gas. When the α particles are completely absorbed in the gas, the same total amount of ionization is produced, showing that the energy required to produce an ion is the same for all gases. On the other hand, for a constant source of radiation, the ionization per unit volume of the gas is approximately proportional to its density. Since the absorption of the α rays in solid matter is approximately proportional to the density of the absorbing medium compared with air, it is probable that this absorption is also a result of the energy used up in producing ions in the solid matter traversed, and that about the same amount of energy is required to produce an ion in matter whether solid, liquid, or gaseous.
It is probable, therefore, that the production of ions in the phosphorescent material and in the photographic film would cease at about the same velocity for which the α particle is unable to ionize the gas. On this view, then, the experimental results receive a simple explanation. The action of the α rays in producing photographic and phosphorescent actions is primarily a result of ionization. This ionization may possibly give rise to secondary actions which influence the effects observed.
This point of view is of interest in connection with the origin of the "scintillations" observed in zinc sulphide and other substances when exposed to the action of the α rays. This effect is ascribed by Becquerel to the cleavage of the crystals under the bombardment of the α particles. These results, however, show that we must look deeper for the explanation of this phenomenon. The effect is primarily due to the production of ions in the phosphorescent material and not to direct bombardment, for we have seen that the α particle produces no scintillations when it still possesses a large amount of kinetic energy. It seems not unlikely that the scintillations produced by the α rays must be ascribed to the recombination of the ions which are produced by the α particle in the crystalline mass. It is difficult to see how this ionization could result in a cleavage of the crystals.
This close connection of the photographic and phosphorescent actions of the α rays with their property of producing ions, raises the question whether photographic and phosphorescent actions in general may not, in the first place, be due to a production of ions in the substance.
Ionization curve for the α rays from radium C. Mr
McClung, working in the laboratory of the writer, has recently
determined the relative ionization per unit path of the α particles
projected from radium C, using the method first employed by Bragg
and discussed in section 104. An active wire, exposed for several
hours to the emanation from radium, was used as a source of rays.
The α particles were homogeneous, since the film of radio-active
matter was extremely thin.
The relation between the ionization observed over the cross section of the narrow cone of rays and the distance from the source of rays is shown in Fig. 108.
The curve exhibits the same peculiarities as those given by Bragg for a thin film of matter of one kind. The ionization of the α particle per unit path increases slowly for about 4 cms. There is then a more rapid increase just before the α particle ceases to ionize the gas, and then a rapid falling off. The ionization does not appear to end so abruptly as is really the case, since there is a correction to be applied for the angle subtended by the cone of rays. The maximum range of the α rays in air was 6·7 cms., a number in agreement with that obtained by Bragg by measurements on the range of the rays from radium.
Fig. 108.
These results show that the ionization per unit path of the α particle increases at first slowly and then rapidly with decrease of velocity until the rays cease to ionize the gas.
Energy required to produce an ion. From the above results the energy required to produce an ion by collision of the α particle with the gas molecules can readily be deduced. The α particles, emitted from radium itself, are initially projected with a velocity ·88V_{0} where V_{0} is the initial velocity of projection of the α particles from
radium C. The α particles cease to ionize the gas at a velocity ·64V_{0}.
From this it can at once be deduced that ·48 of the total energy of
the α particle, shot out by radium itself, is absorbed when it ceases to
ionize the gas. Assuming that the heating effect of radium at its
minimum activity—25 gram calories per hour per gram—is a measure
of the kinetic energy of the expelled α particles, it can be calculated
that the kinetic energy of each α particle is 4·7 × 10^{-6} ergs. The
amount of energy absorbed when the α particle just ceases to ionize
the gas is 2·3 × 10^{-6} ergs. Assuming that this energy is used up in
ionization, and remembering that the α particle from radium itself
produces 86000 ions in its path (section 252), the average energy
required to produce an ion is 2·7 × 10^{-11} ergs. This is equivalent to
the energy acquired by an ion moving freely between two points
differing in potential by 24 volts.
Townsend found that fresh ions were produced by an electron for a corresponding difference of potential of 10 volts. Stark, from other data, obtained a value 45 volts, while Langevin considers that 60 volts is an average value. The value obtained by Rutherford and McClung for ionization by X-rays was 175 volts, and is probably too high.
Rayless changes. We have seen that the α particles from the
radio-active substances are projected with an average velocity not more
than 30 per cent. greater than the minimum velocity, below which
the α particles are unable to produce any ionizing, photographic, or
phosphorescent action. Such a conclusion suggests that the property
of the radio-active substances of emitting α particles has been detected
because the α particles were projected slightly above this minimum
velocity. A similar disintegration of matter may be taking place in
other substances at a rate much greater than in uranium without
producing much electrical effect, provided the α particles are projected
below the critical velocity.
The α particle, on an average, produces about 100,000 ions in the gas before it is absorbed, so that the electrical effect observed is about 100,000 times as great as that due to the charge carried by the α particles alone.
It is not unlikely that the numerous rayless products which have been observed may undergo disintegration of a similar character to the products which obviously emit α rays. In the rayless product the α particle may be expelled with a velocity less than 1·5 × 10^9 cms. per second and so fail to produce much electrical effect.
These considerations have an important bearing on the question whether matter in general is radio-active. The property of emitting α particles above the critical velocity may well be a property only of a special class of substances, and need not be exhibited by matter in general. At the same time the results suggest that ordinary matter may be undergoing transformation accompanied by the expulsion of α particles at a rate much greater than that shown by uranium, without producing appreciable electrical or photographic action.