In this chapter we shall discuss some of the secondary effects accompanying the emission of αparticles, including the emission of delta rays, the recoil accompanjdng the ejection of an α particle, the heating effect produced by the absorption of α particles, and the production of helium due to the accumulated α particles.

§ 29. Emission of delta rays. When a stream of α particles passes through a thin sheet of matter in a vacuum, a number of electrons are observed to be emitted from both sides of the plate. The energy of the great majority of these electrons is only a few volts, but the total number from each surface of the plate is of the order of 10 times the number of incident α particles. J. J. Thomson, who first studied this emission of electrons from a polonium plate, gave them the name of 8 (delta) rays. A large amount of work has been done to determine the factors involved in the liberation of these electrons, including the effect of velocity of the α particle, the nature of the bombarded material, the state of its surface, and the distribution with velocity of the escaping electrons. Before discussing these data, it is desirable to consider the origin of these electrons. It is clear that the escape of electrons, whether from a radioactive surface or a bombarded plate, is in a sense a secondary effect connected with the absorption of energy of the α rays in their passage through matter.

In fact, on modern views, the emission of δ rays is a necessary and inevitable consequence of the passage of α rays through matter.

§ 56. In the previous chapter those experiments on the scattering of α particles in passing through matter were described which confirm the essential assumption of the present theory of atomic structure, that the atoms of matter contain a positively charged nucleus having practically the whole mass of the atom associated with it. The experiments showed that the magnitude of the positive charge of the nucleus was fixed by the atomic number of the atom, or its position in the ascending series of the chemical elements. In the collisions examined in these experiments, in which no elements of smaller atomic number than copper were investigated, the atomic nucleus and the α particle behaved as point charges repelling each other with a force varying inversely as the square of the distance between them.

It is to be anticipated that divergences from the Coulomb law of force would appear when the collisions between the atomic nucleus and the α particle are sufficiently close, for the nuclei are generally supposed to have a complex structure and to be built up in some way from two common units, the electron and the hydrogen nucleus or proton. Since the closest distance of approach to a nucleus of an α particle of given energy is proportional to the charge of the nucleus, it is in the collisions of fast α particles with the nuclei of the lighter elements that deviations from the normal law of force might be expected to be most readily discovered.

§ 119. In previous chapters we have discussed some of the properties of radioactive nuclei and the types of radiation which accompany their transformations. The instability of these nuclei has given us important information on their structure, but unfortunately such information is not available in the case of the ordinary non-radioactive elements. Apart from their instability, there is no reason to believe that the nuclei of the radioactive elements differ in any marked way in their general type of structure from ordinary elements of high atomic weight. It is thus important to examine the properties of atomic nuclei in general to see whether we can obtain evidence to throw light on their structure and their connection with one another. In particular, it is of great interest to see whether any definite evidence can be obtained of the reasons why the property of radioactivity only manifests itself in any marked degree in the two elements of highest atomic weight, thorium and uranium, and their products of transformation.

In chapter vn an account has been given of the genesis of the nuclear theory of the atom and the evidence in its support. On this theory, the ordinary physical and chemical properties of the atom, excluding its mass, depend on the magnitude of the nuclear charge, for on this depends the number and distribution of the outer electrons.

§ 102. The discovery of the penetrating radiations known as the γ rays was made by observations of the ionisation in an electroscope, and an idea of the penetrating power was obtained by placing a screen of absorbing material between the source and the electroscope. In this way it was found that these radiations were far more penetrating than the β rays, a centimetre of lead being required in some cases to reduce the ionising effect to one-half. It was observed that the γ rays in passing through matter gave rise to swift β rays, and the ionisation in the electroscope was ascribed to β rays liberated from the walls and the gas.

The chief source of γ radiation in the early experiments was radium (B + C), and a number of experiments were made to test the law of absorption by the simple method outlined above. While it was found that with considerable thicknesses the absorption curve approximated closely to a simple exponential law, yet with small thicknesses a greater absorbability was noticed. This led naturally to the deduction that a large part of the radiation was homogeneous in character and that superimposed on this were softer radiations which were more rapidly absorbed.

Systematic observations were made by Soddy and Russell of the absorption of different metals which showed that for the lighter elements the absorption depended only on the mass per square centimetre of the absorber and did not vary with the atomic weight. For the heavier elements such as lead the absorption was twice as great as would have been expected from the results with the light elements.

§124. The apparent radioactivity of ordinary matter. It was pointed out by Schuster in 1903 that every physical property discovered for one element had later been found to be shared by all the other elements in varying degrees. On such general grounds it might perhaps be expected that the instability shown by the elements uranium, thorium, actinium, and their products should be a property common to all matter. It is indeed true that every substance which has been examined shows a feeble radioactivity which can be detected by the ionisation method, but it seems probable that in all cases, with the two exceptions of potassium and rubidium, this activity is to be ascribed to the presence of traces of bodies belonging to the well-known radioactive families rather than to an instability of the element itself.

The early investigations of C. T. R. Wilson on the rate of loss of charge of an insulated conductor in a closed vessel indicated that the ionisation was produced by a radiation proceeding from the walls of the vessel. This view was confirmed by the experiments of Struttf, who found that the rate of discharge of an electroscope depended on the material of which it was composed. In some cases markedly different rates of discharge were found for different specimens of the same metal, indicating that in these cases at least a large part of the effect was due to radioactive impurities. On the other hand, Campbell concluded from an extensive series of measurements that all metals showed a specific radioactivity and emitted characteristic radiations of the α ray type.

§ 88. The greater portion of the β ray emission of a radioactive body is formed by the disintegration electrons. Useful qualitative information can be obtained by investigation of the total emission by methods such as measuring the absorption in aluminium, but while these suffice to show the great difference in penetrating power of the β radiation from different bodies they are not suitable for a detailed analysis.

The chief difficulty in investigating the disintegration electrons lies in distinguishing them from those forming the β ray spectra and any other electrons ejected from the outside electronic structure of the atom by subsidiary processes.

The two lines of research which have yielded the most conclusive evidence have been the determination of the total number of electrons emitted by a known quantity of radioactive material, and further the investigation of the distribution of energy among the emitted electrons. The importance of the first type of experiment lies in the fact that since we know there must be one electron emitted from the nucleus of each disintegrating atom, any excess of electrons above this number must be due to secondary processes such as conversion of the γ rays, collisions and so on. We obtain in this way direct and valuable evidence on the extent to which such processes occur.

The distribution of energy among the disintegration electrons is particularly interesting, since it has brought to light a behaviour quite unlike that of the α ray bodies. Instead of a β ray body emitting electrons all of one speed from the nucleus, they appear to be distributed continuously over a wide range of velocity.

§ 42. The scattering of α particles and the nuclear theory of the atom. When α particles pass through matter, some of them are deviated from their original direction of motion and undergo the process known as scattering. The presence of this scattering was first shown by Rutherford by a photographic method. He found that the image of a narrow slit produced by a beam of α particles had sharply defined edges when the experiment was performed in an evacuated vessel. If air was admitted into the apparatus, or if the slit was covered with a thin sheet of matter, the photographic trace of the pencil of α rays was broadened and the intensity of the photographic effect faded off slowly on either side of the centre.

A detailed examination of the amount and character of the scattering of the α particles in passing through matter was first made by Geiger, using the scintillation method of detecting the particles. These experiments will be described later. It will be sufficient to say here that they showed that the scattering suffered by α particles in penetrating the atoms of matter is relatively very small. The average angle of scattering even by comparatively thick sheets of matter was only a few degrees. About the same time Geiger and Marsden made the very striking observation that some of the α particles in a beam incident on a sheet of matter have their directions changed to such an extent that they emerge again on the side of incidence, that is, they are deflected through angles greater than 90°.

§ 75. The investigation of radioactivity during the last twenty-five years has led to the accumulation of a wealth of data concerning the emission of energy in the form of α, β, and γ rays from the radioactive nuclei and in nearly all cases the rate of disintegration of the element has been determined. This information must have an intimate bearing on the structure of the radioactive nuclei and it provides a variety of quantitative tests which can be applied to any hypothesis of this structure. It is, however, only within very recent times that a picture, even of the most general type, has been given which will explain satisfactorily the spontaneous disintegration of a nucleus with the emission of an α particle, and no application of this has yet been made to the emission of the β, and γ rays.

The difficulty in the way of an adequate theory of the structure of the radioactive nuclei has been twofold. In the first place, early in the study of radioactivity it became clear that the time of disintegration of an atom was independent of its previous history and depended only on chance. Since a nuclear particle, say an α particle, must be held in the nucleus by an attractive field, it seemed necessary, in order to explain its ejection, to invent some mechanism which would provide a spontaneous revulsion to a repulsive field.

§ 10a. Comparison of the radiations. All the radioactive substances possess in common the power of acting on a photographic plate and of ionising the gas in their immediate neighbourhood. The intensity of the radiations may be compared by means of their photographic or electrical action, and in the case of the strongly radioactive substances by the luminosity excited in a phosphorescent screen.

Two general methods have been used to distinguish the types of radiation given out by a radioactive matter depending upon

1. (1) a comparison of the relative absorption of the rays by solids and gases,

2. (2) observations on the direction and magnitude of the deflection of the rays when exposed to the action of strong magnetic and electric fields.

Examined in this way, it has been found that there are three distinct primary types of radiation emitted from radioactive bodies which for brevity and convenience have been termed the α (alpha), β (beta), γ (gamma) rays.

1. (1) The a rays, which are very readily absorbed by thin metal foil or by a few centimetres of air, consist of a stream of positively charged atoms of helium, initially projected from the radioactive matter with high velocity, which varies for different substances between 1·4 × 109 and 2·2 × 109 cm./sec. Normally the α particle at the moment of its expulsion carries two positive charges and is to be identified with the nucleus of the helium atom.

2. […]

§ 22. Retardation of the α particle. The great majority of α particles in passing through matter travel in nearly straight lines and lose energy in ionising the matter in their path. Occasionally an α particle suffers a nuclear collision with an atom and is deflected through a large angle. These occurrences, though of great interest, are so rare that they do not seriously influence the average loss of energy when a large number of α particles are under examination. The laws of retardation of the α particle are best studied by making use of the homogeneous α radiation emitted by the very thin deposits of radium C, thorium C, and polonium. It is found experimentally that the reduction of velocity in traversing normally a uniform screen is nearly the same for all the α particles, so that a homogeneous pencil of rays remains nearly homogeneous on emerging from the screen. This effect is most clearly shown with the swifter α particles, e.g. those that have a range in air between 8·6 and 3 cm. With reduction of the velocity, the “straggling” of the α particles, i.e. inequalities in the velocity and range of the emergent α particles, becomes more and more prominent and the issuing pencil of α particles becomes very heterogeneous. The reduction of velocity is best studied by an arrangement similar to that shown in Fig. 7, where the absorbing sheet of matter is placed over the source and the deflection in a uniform magnetic field of the issuing pencil of α rays, in an exhausted chamber, is observed either by the photographic or the scintillation method.

§113. Measurement of the total energy emitted in the form of γ rays. It was early shown by Rutherford and Barnes that the γ rays could only account for a small fraction of the heating effect of radium and its products. Later Rutherford and Robinson, in the course of their work on the heating effect of the α rays (see § 32), were able to estimate the energy emitted in the form of γ rays by radium B and radium C as about 7 per cent, of the total disintegration energy of radon in equilibrium with its products. The method employed was to measure first the heating effect due to the α and β rays, the walls of the calorimeter being sufficiently thin to allow practically all the γ rays to escape, and then to determine the increased heating effect when a certain fraction of the γ rays were absorbed by lead screens placed inside the calorimeter. No great accuracy was possible as the small heating effect of the γ rays was measured as the difference between two large effects.

Ellis and Wooster devised a method of automatically compensating the large α ray and β ray effect, enabling that due to the γ rays to be measured directly. The calorimeter consisted of a hollow cylinder of four equal sectors, the two opposing ones A and B being respectively of lead and aluminium, while the intermediate ones were of insulating material. The difference of temperature between A and B was measured by a system of thermocouples.