Rutherford, Ernest. Radioactive Substances and their Radiations. London: Cambridge University Press, 1913. 

Chapter I-Radio-Active Substances

[4] 2. Radio-Active Substances.  The term “radio-active” is now generally applied to a class of substances, such as uranium, thorium, radium, and their compounds, which possess the property of spontaneously emitting radiations capable of passing through plates of metal and other substances opaque to ordinary light. The characteristic property of these radiations, besides their penetrating power, is their action on a photographic plate and their power of discharging electrified bodies. In addition, a strongly radio-active body like radium is able to cause marked phosphorescence and fluorescence on some substances placed near it. In the above respects the radiations possess properties analogous to Roentgen rays, but it will be shown that, for the major part of the radiations emitted, the resemblance is only superficial.

            The most remarkable property of the radio-active bodies like uranium and thorium is their power of radiating energy spontaneously and continuously at a constant rate, without, as far as is known, the action upon them of any external exciting cause. The phenomena at first sight appear to be in direct contradiction to the laws of conservation of energy, since no obvious change with time occurs in the radiating material. The phenomena appear still more remarkable when it is considered that the radio-active bodies must have been steadily radiating energy since the time of their formation in the earth’s crust.

             [6] 3. Discovery of radio-activity.   The discovery by Roentgen in 1895 of the X rays and their remarkable properties created the most intense interest throughout the scientific world. It occurred to several physicists to try whether ordinary bodies emitted a similar radiation, which was able to pass through matter opaque to ordinary light. Since the production of X rays by a discharge through a vacuum tube appeared to be connected with the strong phosphorescence and florescence excited on the glass by the cathode rays, it was natural at first to examine those substances which phosphoresced brightly under the action of light. Several observers at first affirmed the presence of such a penetrating radiation from phosphorescent calcium sulphide; but these results have not stood the test of more rigorous experiment. Following out his same idea, Professor Henri Becquerel exposed a number of phosphorescent substances enveloped in black paper under a photographic plate. The results [7] were entirely negative. It then occurred to him to try experiments with the salts of uranium, the phosphorescence of which had been previously investigated by him. Among these salts were some fine crystals of the double sulphate of uranium and potassium, which he had prepared about fifteen years before. The crystals were first exposed to light and then enveloped with two layers of black paper, and below the photographic plate with a small piece of silver between. After an exposure of several hours and development, a distinct photographic effect was observed. The experiment was at once repeated with a plate of glass .01 mm thick between the uranium salt and the photographic plate in order to cut off effects due to possible vapours. A distinct but slightly feebler photographic impression was again obtained.

            The results of these experiments, which were communicated to the Academy of Science at Paris on February 24^th, 1896, indicated that the salt of uranium emitted a type of radiation capable of penetrating through a considerable thickness of matter opaque to ordinary light. These simple experiments mark the discovery of a new property of matter-radio-activity-the further investigation of which was lead to such remarkable consequences.

            [9] From his earlier experiments, Becquerel concluded that the rays from uranium were intermediate in properties between ordinary light and X rays, since he observed some evidence of reflection, refraction, and polarisation. The results were not confirmed by the later experiments of LeBon, Rutherford and Becquerel himself. While there is no evidence of direct reflection, there is always a diffuse reflection of the penetrating radiation striking an obstacle, so that the shadow of an absorbing screen is always surrounded by a diffuse border. The absence of the properties of light waves necessarily follows from our present knowledge of nature of the uranium rays; for the radiation, mainly responsible for the photographic effects, consists of a flight of charged particles projected with great velocity.

            The radiations from uranium are complex in character, and will be shown later to consist of three distinct types known as the alpha, beta and gamma rays. The alpha rays, which are very easily absorbed by thin sheets of metal foil or by gases, consist of a stream of positively charged particles projected with great velocity. The beta rays are far more penetrating and are identical with the negatively charged particles constituting the cathode rays. The gamma rays, which are very penetrating, appear to be analogous in properties to the X rays. With a thin layer of any uranium compound, the electric discharge is due mainly to [10] the alpha rays. The electric effect due to the gamma rays is very feeble in comparison and is difficult to detect unless several hundred grams of material are employed as a source of radiation.

            The radiations from uranium are thus analogous, as regards their photographic and electrical actions, to X rays, but compared with the rays from an ordinary X ray tube, these actions are extremely feeble. While with X rays a strong impression is produced on a photographic plate in a few minutes or even by a single discharge, a day’s exposure to the uranium rays is required to produce a well-marked action, even though the uranium compound, enveloped in black paper, is placed close to the plate. The discharging action, while very easily measurable by suitable methods, is also compared with that produced by X rays from an ordinary focus tube.

Chapter II-Ionisations of Gases

            [26] 12. Ionisation of gases by radiation.  The most important property possessed by the radiations from radio-active matter is their power of discharging bodies whether positively or negatively electrified. As this property has been made the basis of a method for an accurate quantitative analysis and comparison of the radiation, the variation of the rate of discharge under different conditions and the processes underlying it will be considered in some detail.

            In order to explain the similar discharging power of Roentgen rays, the theory has been put forward that the rays produce positively and negatively charged carriers throughout the volume of the gas surrounding the charged body, and that the rate of production is proportional to the intensity of the radiation. These carriers, or ions as they have been termed, move with a uniform velocity through a gas under a constant electric field, and their velocity varies directly to the strength of the field.

            [27] -This theory accounts also for all the characteristics properties of gases made concluding by the rays from active substances, though there are certain differences observed between the conductivity phenomena produced by the active substances and by X rays. These differences are in part the result of unequal absorption of the two types of rays. Unlike Roentgen rays, a large proportion of the radiation from active consists of rays which are absorbed in their passage through a few centimeters of air. The ionisation of the gas is thus not uniform, but falls off rapidly with increase of distance from the active substance. In addition, there are marked differences in the distribution of ions produced by X rays and by alpha rays.

 Chapter III-Methods of Measurement

            [87] 34. Methods of Measurement.  Three general methods have been employed for examination of the radiations from radio-active bodies, depending on 

(1)   The action of the rays on a photographic plate,

(2)   The ionisation action of the rays on the surrounding gas,

(3) The luminosity produced by the rays on a screen of platinocyanide of barium, zinc sulphide, or similar substance.

The photographic method has been used very widely, especially in the earlier development of the subject, but has gradually been displaced by the electrical method, as a quantitative determination of the radiations became more and more necessary. In certain directions, however, it possesses distinct advantages over the electrical method. For example, it has proved a very valuable means of investigating the curvature of the path of the rays, when deflected by a magnetic or electric field, and has allowed us to determine the constants of these rays with considerable accuracy.

On the other hand, as a general method of study of the radiations, it is open to many objections. A day’s exposure is generally required to produce an appreciable darkening of the sensitive film when exposed to a weak source of radiation like uranium or thorium. It cannot, in consequence, be employed to investigate the radiations of those active products which rapidly lose their activity. Moreover, it is shown that the darkening of a photographic plate can be produced by many agents which do not give out rays like those of the radio-active bodies. This darkening of the plate is produced under the most [88] varied conditions, and very special precautions are necessary when long exposures to a weak source of radiation are required.

The chief drawback of the photographic methods lies in the difficulty of deducing the intensity of the intensity of the radiation from measurements of the density of the photographic plate impression. In addition, the relative photographic effect of the different types of radiation is dependent upon the thickness and rapidity of the films and plates employed. In some cases, for example using thick layers of uranium oxide, the photographic effect is largely due to beta rays. In most cases, however, where thin films of radio-active matter are used, the photographic effect in a vacuum or close to the active film is mainly due to the alpha rays. A detailed study of the photographic effects due to the alpha rays has been made by Kinoshita, and some of his conclusions will be discussed later.  The electrical method, on the other hand, offers a rapid and accurate method of quantitatively examining the radiations. It can be used as a means of measurement of all the types of radiation emitted, excluding light waves, and is capable of accurate measurement over a wide range. With proper precautions it can be used to measure effects produced by radiations of extremely small intensity.

The luminosity produced by the alpha, beta and gamma rays in barium platinocyanide and willemite is only of service in qualitative work. On the other hand, the luminosity in the form of scintillations produced in zinc sulphide by the alpha rays has proved in valuable in quantitative work. It has afforded a direct method of counting the number of alpha particles and has been widely employed in researches upon the alpha rays.

Chapter IV-The Alpha rays-Comparison of the Radiations

[114] 51. The Three Types of Radiation.  All the radio-active 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 radio-active substances, by the power they possess of lighting up a phosphorescent screen. Such comparisons, however, do not throw much light on the question whether the radiations are of the same or of different kinds, for it is well known that such different types of radiations as the short waves of ultra-violet light, Roentgen and cathode rays, all possess the property of producing ions throughout the volume of a gas, lighting up a fluorescent screen, and acting on a photographic plate. Neither can the ordinary optical methods be employed to examine the radiations under consideration, as they show no race of regular reflection, refraction, or polarisation.

Two general methods can be used to distinguish the types of the radiations given out by the same body, and also to compare the radiations from the different active substances. These methods are as follows:

(1). By observing whether the rays are appreciably deflected in a magnetic and an electric field.

(2). By comparing the relative absorption of the rays by solids and gases.

            [115] Examined in these ways, it has been found that there are three distinct types of radiation emitted from radio-active bodies, which for brevity and convenience have been termed by the writer alpha, beta, and gamma rays.

            (i). The alpha rays are very readily absorbed by thin metal foil and by a few centimeters of air. They have been shown to consist of positively charged atoms of helium projected with a velocity of about 1/15 the velocity of light. They are deflected by intense magnetic and electric fields, but the amount of deviation is minute in comparison with the deviation, under the same conditions, of the cathode rays produced in a vacuum tube.

            (ii). The beta rays are on the average far more penetrating in character than the alpha rays, and consist of negatively charged bodies projected with velocities of the same order as the velocity of light. They are far more readily deflected than the alpha rays, and are in type identical with the cathode rays produced in a vacuum tube.

            (iii). The gamma rays are extremely penetrating, and non-deviable by a magnetic or electric field. The true nature is not definitely settled, but they are analogous in most respects to very penetrating Roentgen rays.

            The four best known radio-active substances, uranium, thorium, radium and actinium, when in equilibrium with their products, all give out theses three types of rays.

            52. Comparison of the rays. The rays emitted from the active bodies thus present a very close analogy with the rays which are produced in a highly exhausted vacuum tube when an [116] electric discharge passes through it. The alpha rays are analogous to the canal rays, discovered by Goldstein, which have been shown by Wein to consist of positively charged bodies projected with great velocity. The beta rays are the same as the cathode rays, while the gamma rays resemble the Roentgen rays. In a vacuum tube, a large amount of electric energy is expanded in producing the rays, but, in the radio-active bodies, the rays are emitted spontaneously, and at a rate uninfluenced by any chemical or physical agency. The alpha and beta rays from the active bodies are projected with much greater velocity than the corresponding rays in a vacuum tube, while the gamma rays are much greater penetrating power than Roentgen rays.

            [117] 53. Ionising and penetrating power of the rays. Of the three types of rays, the alpha rays produce most of the ionisation in the gas and the gamma rays the least. For example, using a thin layer of a radium compound spread on the lower of two parallel plates 5 centimetres apart, the amount of ionisation due to the alpha, beta, and gamma rays is of the relative order 10,000, 100 and 1. These numbers are only rough approximations and the differences become less marked as the thickness of the radio-active layer is increased. Since each type of rays from radio-active substances is usually complex and consists of radiations which are absorbed to an unequal extent, it is difficult to give an accurate comparison of the relative penetrating power of the three types of rays. As a rough working rule, it may be taken that the beta rays are about 100 times as penetrating as the alpha rays and the gamma rays from 10 to 100 times as penetrating as the beta rays.

            It is often convenient to know what thickness of matter is sufficient to absorb a specific type of radiation. A thickness of 0.006 centimetres of aluminum or mica or a sheet of ordinary writing paper is sufficient to absorb completely all the alpha rays. With such a screen over the active material, the external effects are due only to the beta and gamma rays, which pass through with a very slight absorption. Most of the beta rays are absorbed in 5 millimetres of aluminum or 1 millimetre of lead. The radiation passing through such screens consists very largely of the gamma rays. As a rough working rule, it may be taken that a thickness of matter required to absorb any type of rays is inversely proportional to the density of the substance, i.e. the absorption is proportional to the density. This rule holds approximately for light substances, but, in heavy substances like lead and mercury, the radiations are more readily absorbed than the density rule would lead us to expect.

The Alpha Rays

            [118] 55. The Alpha rays. The magnetic deviation of the beta rays was discovered towards the end of 1899, at a comparative early stage in the history of radio-activity, but three years elapsed before the true character of the alpha rays was disclosed. It was natural that great prominence should have been given in the early stages of the subject to the beta rays, on account of their great penetrating power and marked action in causing phosphorescence in many substances. The alpha rays were, in comparison, very little studied, and their importance was not generally recognized. It will, however, be shown that the alpha rays play a far more important part in radio-active processes than the beta rays, and that the greater [119] portion of the energy emitted in the form of heat and of ionising radiations is due to them.

            The nature of the alpha rays was difficult to determine, for a magnetic field sufficient to cause considerable deviation of the beta rays produced no appreciable effect on the alpha rays. It was suggested by several observers that they were in reality, secondary rays set up by the beta 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 alpha rays only. Later work also showed that the matter, which gave rise to the beta rays from uranium, could be chemically separated from uranium, while the intensity of the alpha rays was unaffected. These and other results show that the alpha and beta rays are produced in many cases quite independent of one another. The view that they were an easily absorbed type of Roentgen rays failed to explain a characteristic property of the alpha 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. This peculiarity of the absorption of the alpha rays was first clearly shown by Mme Curie in 1900 using polonium as a source of radiation. In explanation, she suggested that the alpha rays consisted of projected particles, which lost their energy by passing through matter. From observations of the ionisation produced in gases by the alpha and beta rays Strutt in 1901 suggested that the alpha rays might be analogous to the canal rays produced in a vacuum tube, and thus might consist of positively charged particles projected with great velocity. A similar suggestion was made in 1902 by Sir William Crookes.

            [128] 59. Counting the alpha particles.  The total number of alpha particles expelled per second from one gram of radium was initially estimated by Rutherford by measuring the positive charge carried by the alpha particles emitted from a known quantity of radium in the form of a thin film. On assumption that each alpha particle carried an ionic charge 3.4x10^-10 electrostatic [129] units, the results showed that 6.2x10^10 alpha particles are expelled per second from one gram of radium itself, and four times this number when the radium is in equilibrium with its three alpha ray products. The uncertainty, however, of the actual magnitude of the charge carried by the alpha particle rendered it desirable to determine this number by an independent method. We shall later that the alpha particle from radium C of range about 7 centimetres, produces about 2.37x10^5 ions in its path in air before absorption. If each ion carries a charge 4.65x10^-10 units, this corresponds to a transfer of 1.10x10^-4 electrostatic units in a strong electric field. This quantity of electricity is small, but should be detectable using a very sensitive electroscope or electrometer. The detection of a single alpha particle, however, by its direct electrical effect, while not impossible, is beset by many experimental difficulties. 

            [132] This expression holds provided that each portion of the active source can fire particles through the aperture. As a result of a number of experiments, it was found that the number of alpha particles emitted per second from the product radium C in equilibrium with one gram of radium was 3.4x10^10. From other evidence, it is known that the same number of alpha particles is emitted per second from one gram radium itself and from each of its three alpha ray products in equilibrium with it. Consequently the number of alpha particles expelled from one gram of radium itself is 3.4x10^10, and from radium in equilibrium 13.6x10^10.

            [135] 61. The charge carried by the alpha particle.  Since the number of alpha particles can be directly counted, the charge carried by each alpha particle can be deduced by measuring the total charge carried by a counted number of alpha particles. The determination of the positive charge carried by the alpha particles was at first difficult on account of the secondary electrical effects produced when alpha rays fall upon matter. J.J. Thomson showed that a film of polonium emitted, in addition to alpha rays, a great number of negatively charged particles, which he termed the delta rays. These particles, which are identical with electrons, always appear when the alpha particles impinge on matter, whether in a liquid, solid, or gaseous state. To determine the charge carried by the alpha rays it is necessary to eliminate the effects due to the delta rays. This can be done by placing the active matter in a strong magnetic field. Under such conditions, the delta particles, which emitted at a slow speed from a surface, describe a small orbit, and return to the surface from which they set out. In order to measure accurately the charge carried by the alpha rays, Rutherford and Geiger employed radium C as a source of radiation.

            [141] 64. Retardation of the alpha particle.  We have seen in section 57 that the alpha particles diminish in velocity in passing through matter. Using a pencil of homogenous rays, the reduction of velocity in traversing normally a uniform screen is nearly the same for all the alpha particles, so that a homogeneous pencil of rays remain nearly homogeneous after traversing the screen. In the ordinary arrangement, the effects of scattering of the alpha particles in passing through matter does not come in appreciably, but we shall see later that there is some evidence that a small fraction of the alpha particles owing to scattered suffer a greater reduction of velocity than the average. In discussing the laws of retardation of the alpha particles, it is convenient  to connect the velocity of an alpha particle with its range of ionisation in air. It will be seen later that Bragg has shown that the ionisation due to homogeneous pencil of alpha rays ends comparatively abruptly after they have traversed a certain distance of air. This distance is called the range of the alpha particle in air.   The magnitude of the range is inversely proportional to the density of the air, and thus depends on the pressure and temperature. The range in air is usually given for air at standard pressure and at laboratory temperature. For example, the range of the alpha particles from a thin film of radium C is 7.06 centimetres at 20 degrees C. on placing a uniform screen, for example of aluminum, over the source, the range of ionisation is reduced say to 5 centimetres. In this case, the screen of aluminum is said to have the “stopping power” for the alpha particle equivalent to 2.06 centimetres of air.

Chapter V-The Beta Rays

[193] 76. Discovery of the Beta rays.  A discovery which gave a great impetus to the study of radiation from active bodies was made in 1899 by Geitel, and confirmed shortly afterwards by Meyer and Schweidler and Becquerel and P. Curie. It was observed that the preparations of radium gave out some rays which were deviable by a magnetic field and very similar to the cathode produced rays in a vacuum tube. The initial observation of Elster and Geitel that a magnetic field altered the conductivity produced in air by radium rays, led Geitel to examine the effect of a magnetic field on the radiations. In his experiments, the radio-active preparation was placed in a small vessel between the poles of an electromagnet. The vessel was arranged to give a pencil of rays which was approximately perpendicular to the field. The rays caused a small fluorescent patch on the screen. On exciting the electromagnet, the fluorescent zone was observed to broaden out on one side. On reversing the field, the deflection of the zone was in the opposite direction. The deviation of the rays thus indicated was in the same direction and of the same order of magnitude as that for cathode produced in a vacuum tube.

            Meyer and Schweidler showed the deflection of the rays by the alteration of the conductivity of the air when a magnetic field was applied, while Becquerel employed a photographic method. Curie found that the rays from radium consisted of two kinds, [194] one apparently non-deviable and easily absorbed (now known as the alpha rays), the other more penetrating and deviable by a magnetic field (now known as the beta rays). Under ordinary conditions, the ionisation effect due to the beta rays was small compared with that due to the alpha rays. Later investigations have shown that ordinary preparations of uranium, radium, thorium and actinium emit beta as well as alpha rays, while ionium and polonium emit only alpha rays.

            [198] 80. Charge carried by the beta rays.  The experiments of Perrin and J.J. Thomson have shown that the cathode rays carry with them a negative charge, while Lenard found that they still retained this charge after passing through a thin screen of matter. When beta rays, or cathode rays, are stopped in matter, they give up their negative charge, and the quantity of negative electricity communicated to the plate is a measure of the quantity of electricity carried by the absorbed particles.

            The total charge carried by the beta rays emitted per second from a gram of uranium or thorium is very small, and can only be detected by very delicate measurements. Using a milligram of radium, the charge carried by the beta rays from it can be measured easily by an electroscope or ordinary electrometer.

            Suppose that an active preparation of radium is spread on a metal plate connected with earth, and that the beta rays are absorbed by a parallel plate connected with an electrometer. If the rays are negatively charged, the top plate should receive a negative charge increasing with the time. On account, however, of the great ionisation produced by the rays between the plates, any charge given to one of them is almost instantly dissipated. In many cases, the plate does become charged to a definite positive or negative potential depending on the metal, but this is due to the [199] contact difference of potential between the plates, and would be produced whether the rays were charged or not. The ionisation of the gas between the plates is greatly diminished by placing over the active material a metal screen which absorbs the alpha rays, but allows the beta rays to pass through with little absorption.

            [240] 92. The retardation of the beta particles.  The laws of absorption of beta particles by matter is very similar in solids, liquids and gases. Since it is known that the passage of the beta particle through gases is accompanied by ionisation and consequently by absorption of energy, it seems probable that ionisation is also produced by the beta particle in passing through liquid and solid matter. It is thus to be expected that the velocity of the beta particles should decrease in passing through matter. This has been simply shown in the case of the alpha rays, where homogeneous radiations are available. In the case of the beta rays, the diminution of velocity has been more difficult to determine, partly on account of the difficulty of obtaining homogeneous beta radiation, and partly on account of the marked scattering of the beta rays in passing through matter. The initial experiments on the absorption of beta rays first led to the conclusion that the reduction of velocity in passing through matter be small.

[117] 54. Difficulties of comparative measurements.  It is difficult to make quantitative or even qualitative measurements [118] of the relative intensity of the three types of rays from active substances. The three general methods employed depend upon the action of the rays in ionising the gas, in acting on a photographic plate, and in causing phosphorescent or fluorescent effects in certain substances. In each of these methods the fraction of the rays which is absorbed and transformed into another form of energy is different for each type of ray. Even when one specific kind of ray is under observation, comparative measurements are rendered difficult by the complexity of that type of rays. For example, the beta rays from the radium consist of negatively charged particles projected with a wide range of velocity, and, in consequence, they are absorbed in different amounts in passing through a definite thickness of matter. In each case only a fraction of the energy absorbed is transformed into the particular type of energy, whether ionic, chemical, or luminous, which serves as a means of measurement.

    The necessity of taking into account the relative ionising and photographic effects produced by the alpha and beta rays is well illustrated by the earlier experiments with uranium. With unscreened material, the ionisation observed is due mainly to the alpha rays while the photographic effect is largely due to the beta rays. The failure to appreciate this difference led at first to considerable confusion and apparent contradiction in the effects observed.

Chapter VI-The Gamma or very Penetrating Rays

            [257] 96. In addition to the alpha and beta rays, a number of the radio-active substances gives out a radiation of an extraordinarily penetrating character called the gamma rays. These rays are considerably more penetrating than X rays produced in a “hard” vacuum tube. Their presence can be readily observed for an active substance like radium, but is difficult to detect for uranium and thorium unless a large quantity of active material is used.

            Villard, using the photographic method, first drew attention to the fact that radium gave out these very penetrating rays, and found that they were non-deviable by a magnetic field. This result was confirmed by Becquerel.  

            Using a few milligrams of radium, the gamma rays can be detected in a dark room by the luminosity they excite in the mineral willemite or in a screen of platinocyanide of barium. The alpha and beta rays are completely absorbed by placing a thickness of 2 millimetres of lead over the radium, and the rays which then pass through the lead consist entirely of gamma rays. The very great penetrating power of these rays is easily observed by noting the slight diminution of the luminosity of the screen when plates of metal several centimeters thick are placed between the radium the screen. These rays also produce ionisation in gases and are best investigated by the electrical method. The presence of the gamma rays from 30 milligrams of radium can be observed in an electroscope after passing through 30 centimetres of iron.

            [286] 104. Nature of the gamma rays.  In addition to their great penetrating power, the gamma rays differ from the alpha and beta rays in not being deflected by the magnetic or dielectric field. In a strong magnetic field, it can be shown by the photographic method that there is an abrupt discontinuity between the beta and gamma rays, for the former are bent completely away from the latter. Paschen passed gamma rays through a very intense magnetic field and observed no deviation. He calculated that if the gamma rays consisted of charged particles with nearly the velocity of light, they have an apparent mass certainly greater than 45 times that of the atom of hydrogen. The heating effects of the gamma rays from radium are of the same magnitude as that due to the gamma rays, so that the last possibly is excluded.

            It has generally been supposed that X rays and gamma rays are analogous types of radiation, the main difference being that the gamma rays are on the whole far more penetrating that the X rays. There are at first sight certain apparent differences in properties between the X rays and the gamma rays which might indicate a difference of nature. For example, ordinary X rays produce relatively far more ionisation in complex gases and vapours compared with air than penetrating gamma rays. A further study has shown, however, that this relative ionisation is not a constant but depends on the penetrating power of the X rays employed. Eve showed that the high relative values of the conductivity for some complex gases were greatly reduced when very penetrating X rays were used. Barkla has shown that “characteristic” radiations are set [287] up in some elements when X rays traverse them. Such an effect has not so far been generally observed for gamma rays, but we have seen that Gray has obtained some evidence that a characteristic radiation is set up in heavy elements by the soft gamma rays from radium E (Section 103). The X rays in traversing matter liberate a type of beta radiation of moderate velocity and penetrating power; the gamma rays also liberate beta particles but in still greater velocity. There is at present no definite evidence to indicate that the X rays and gamma rays are fundamentally different kinds of radiation. There is every reason to believe that the gamma rays arising from radio-active matter would show identical properties with X rays of the same penetrating power.

            We have seen (Section 33) that the X rays were initially believed to be narrow spherical electromagnetic pulses set up by the sudden stopping of electrons by matter. On this view, the cathode rays are to be regarded as the parents of the X rays. In a similar way, there appears to be little doubt that the appearance of gamma rays from radio-active matter is connected with the expulsion of beta rays, for gamma rays have only been observed from products which emit beta rays. At present, there is no definite evidence of the mode of origin of the gamma ray in the atom. It may, for example, be due to the rapid acceleration or retardation of an escaping electron in moving through the strong electric field inside the atom.

Chapter VII-Properties of the Radiations

            [305] 110. Photographic action of the rays. Active preparations of all radio-active substances produce a marked action on the photographic plate. In the case of the alpha rays, the photographic action disappears when the alpha particles lose their power of ionisation and of producing scillations in zinc sulphide. Fairly clear radiographs of objects can be obtained with the beta of gamma rays. On account of the marked scattering of the beta rays, the edges of objects are not so clearly defined as with X rays. Radiographs obtained by the gamma rays show a better definition than beta rays; but to obtain the best effects it is desirable to deflect away the beta rays which accompany the gamma rays with a magnetic field. There is not the same contrast for substances of different density which are shown by radiographs with X rays. For example, the bones of the hand do not show out so clearly with gamma rays as with X rays.

Chapter III-Methods of Measurement