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 × 109 cms. per sec., the ionizing, phosphorescent, and photographic actions cease when the velocity of the α particle falls below 1·5 × 109 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].
Fig. 108.