§ 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.

§ 79. Introduction. The β rays from radium and other radioactive bodies were early shown to consist of a stream of electrons projected with a wide range of speed, the swifter ones with a velocity close to that of light. Apart from a study of the apparent absorption and reflection of the β rays from different radioactive substances in traversing matter, further progress was comparatively slow, and it was not until recent years that a clear idea was obtained of the origin of the β rays and of their connection with radioactive transformations.

Compared with the complex emission of β rays, a study of the homogeneous emission of a particles and their absorption by matter presented a relatively simple and direct problem, and in a surprisingly short time the main facts of the material nature of the α rays and their origin were established and the results interpreted in terms of the transformation theory.

Radioactivity was then at once recognised to be an actual disintegration of the nucleus, which in the a ray case consisted in the emission of a helium nucleus and in the β ray case of an electron from the nucleus. These disintegration particles had to be distinguished from any subsidiary electrons that might be detached from the outer electronic levels during the disintegration. It will be seen later that this separation has been rendered possible by the recognition of the important part played by γ rays in the disintegration.

§ 34 a. Emission of α particles and probability variations. The rate of disintegration of all radioactive substances is expressed by a simple law, namely, that the number of atoms n breaking up per second is proportional to the number n of atoms present. Consequently n = λN, where λ is a constant characteristic for a particular radioactive substance. The rate of transformation of an element has been found to be a constant under all conditions. It is unaltered by exposing the active matter to extremes of temperature or by change of its physical or chemical state. It is independent of the age of the active matter or its concentration. It is unaffected by exposure to strong magnetic fields. Hevesy has shown that the disintegration of the primary radioactive element uranium is unaltered by exposing it to the β and γ radiation from a strong source of radium, although these rays, of great individual energy, might be expected to penetrate the atomic, nucleus.

Since the expulsion of an α or β particle results from an instability of the atomic nucleus, the failure to alter the rate of transformation shows that the stability of the atomic nucleus is not influenced to an appreciable extent by the forces at our command. This is not unexpected when we consider the enormous intensity of the forces, probably both electric and magnetic, which hold the charged parts of the nucleus together in such a minute volume.

§ 1. In studying the history of the rapid progress in our knowledge of atomic physics during the past thirty years, one cannot fail to be impressed with the outstanding importance of three fundamental discoveries which followed one another in rapid succession at the close of the last century. We refer to the discovery of the X rays by Röntgen in 1895, the discovery of the radioactivity of uranium by Becquerel early in 1896, and the proof of the independent existence of the negative electron in 1897 by Sir J. J. Thomson, Wiechert and Kaufmann. In a sense these discoveries mark the beginning of a new epoch in physics, for they provided new and powerful methods for attacking the fundamental problems of physics, such as the nature of electricity and the constitution and relation of the atoms of the elements. While the rapid development of pur knowledge in each of these new fields of enquiry has provided us with new and very valuable information on the nature of radiation and the interaction between radiation and matter, a new orientation of our views on this subject was given by the remarkable theory of quanta first put forward by Planck in 1900, although its full significance was not generally recognised for another decade. The application in 1913 by Bohr of the quantum theory to explain the origin of spectra and the arrangement of the electrons in the outer structure of the atom has proved of great significance to modern science.

§ 26 a. Theory of absorption of α particles. We have already discussed the experimental results which have been obtained on the retardation of the α particle in passing through matter, and the variation in atomic stopping power for the different elements. It is of great interest and importance to consider in some detail how far we are able to give a theoretical explanation of the observed facts.

It is clear that the α particles have such great energy of motion that they pass freely through the electronic structure of the atoms in their path, and it is only rarely that they pass close enough to the nucleus to experience a sensible deflection. In the calculations we shall disregard the scattering of the α particles and assume that the α particle travels in nearly a straight line through the matter in its path.

In consequence of its charge, it is evident that the α particle, in passing close to an atom or in penetrating it, must disturb the motions of the electrons in the atom. The amount of energy which is communicated to the electrons by the α particle can be calculated on certain assumptions, but it is difficult to be certain of their validity. The difficulties involved may be illustrated by the distinction made to-day between an “elastic” and “non-elastic” collision of an electron with an atom. Suppose, for example, that an electron of definite energy collides with a helium atom. For an energy less than 20 volts, the electron is supposed to make an elastic collision with the atom, i.e. it is deflected with a very slight reduction of its energy.

§ 95. The study of the pa'ssage of β particles through matter encounters serious difficulties, both experimental and theoretical, which in most cases can be traced back to the ease with which these particles are scattered under the conditions in which experiments can be carried out. This scattering together with the rate of loss of energy constitute the two fundamental phenomena with which we are concerned, but unfortunately it is difficult to study the loss of energy without being seriously inconvenienced by the scattering.

In this respect β particles are far less amenable to investigation than α particles, which in the great majority of cases pursue straight paths until near the end of their range.

A convenient method of giving a general survey of the behaviour of β particles in traversing matter is to compare them with α particles. From this comparison we shall see how to differentiate between those experiments which can be easily analysed to give information about the mode of interaction of a β particle with an atom, and those experiments which, while valuable from a practical standpoint, yet prove on detailed consideration to be essentially complicated.

In the first place there is a considerable difference in penetrating power. While a few centimetres of air suffices to stop α particles, almost a hundred times as much is required to stop β particles. If in this preliminary comparison we neglect the slightly longer paths followed by the β particles due to their being scattered, we see that in a given distance the average number of collisions of a definite type with the atomic electrons will be the same for both sets of particles.

§ 15. Absorption of α rays. Historical. The absorption of α rays by matter was first investigated by Rutherford in 1899 by the electric method. A layer of radioactive material was spread on a plate and the variation of the saturation current between this plate and another placed parallel to it was examined by an electrometer when successive screens of absorbing matter were placed over the active matter. The current was found at first to decrease approximately according to an exponential law with the thickness of matter traversed, but ultimately fell off more slowly. Experiments of this kind were first made with uranium, and led to the divisions of the radiation into two types, called the α and β rays. Observations by similar methods showed also that the α rays were rapidly absorbed in air and other gases. Many experiments of this kind were made in the early days of radioactivity by Owens, Rutherford and Miss Brooks, Meyer and Schweidler and others.

When more active materials were available, it was possible to obtain easily measurable effects with a definite pencil of α rays. Mme Curie examined the absorption of the α rays from a thin film of polonium by a different method. The rays from polonium passed through a circular opening in a metal plate covered with a wire gauze or a thin metal foil, and the ionisation of the rays, after passing through the hole, was measured between this plate and a plate placed 3 cm. above it.

§ 64. The idea of the artificial disintegration or transmutation of an element is one which has persisted since the Middle Ages. In the times of the alchemists the search for the “philosopher's stone,” by the help of which one form of matter could be converted into another, was pursued with confidence and hope under the direct patronage of rulers and princes, who expected in this way to restore their finances and to repay the debts of the state. In spite of this encouragement the successful transmutation of some common matter into gold was but seldom reported. Even in these cases the transmutation could not be repeated; either the alchemist had vanished or his supply of the “philosopher's stone” had been exhausted. The failures were many and the natural disappointment of the patron usually vented itself on the person of the alchemist; the search sometimes ended on a gibbet gilt with tinsel. But when the confidence of the patrons departed the hope of the alchemists still remained, for the idea of transmutation not only accorded with the desires of the man but was founded on the conceptions of the philosopher. According to Aristotle all bodies are formed from a fundamental substance, “primordial matter,” and the four elements—water, earth, air, and fire—differed from each other only by possession of different combinations of the properties of cold, wet, warm, and dry. By changing the properties one element should be changed into another. On this view, it was almost self-evident that bodies so closely allied as the metals could interchange their qualities.

In 1904 I published through the Cambridge University Press a collected account of radioactive phenomena entitled Radioactivity, followed a year later by a revised and enlarged edition. In 1912 a new volume was issued, entitled Radioactive Substances and their Radiations (Cambridge University Press), which endeavoured to give a concise account of our knowledge of radioactivity within the compass of a single volume.

The issue of this book was sold out soon after the conclusion of the War, and I was unable in the press of other work to find time for the preparation of a revised edition.

Since the publication in 1912, there has been a very rapid growth of our knowledge of the transformations of radioactive substances and of the radiations which accompany these transformations. The literature has rapidly expanded and many thousands of new papers, dealing with various aspects of the subject, have been published. It was felt that any attempt to give a collected account of the researches on this subject along the lines of the 1912 edition would have necessitated a very bulky volume. In the meantime, the need for such a publication had been met by the appearance of several new books. Professors Stefan Meyer and Egon v. Schweidler published, in 1916, Radioaktivität (Teubner, Berlin), followed in 1927 by a second enlarged edition. This excellent volume, which gives references to all the literature on this subject, is of great value to the scientific student. In 1928, Professor K. W. F. Kohlrausch published a collected account of radioactive researches entitled Radioaktivität, which appeared as one of the volumes of the Wien-Harms Handbuch der Experimental Physik.