PART IV.
The γ or very penetrating Rays.
105. In addition to the α and β rays, the three active substances, uranium, thorium, and radium, all give out a radiation of an extraordinarily penetrating character. These γ rays are considerably more penetrating than the X rays produced in a “hard” vacuum tube. Their presence can readily be 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[[167]], 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[[168]].
Using a few milligrams of radium bromide, the γ rays can be detected in a dark room by the luminosity they excite in the mineral willemite or a screen of platinocyanide of barium. The α and β rays are completely absorbed by placing a thickness of 1 centimetre of lead over the radium, and the rays which then pass through the lead consist entirely of γ 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 centimetres thick are placed between the radium and the screen. These rays also produce ionization in gases and are best investigated by the electrical method. The presence of the γ rays from 30 mgrs. of radium bromide can be observed in an electroscope after passing through 30 cms. of iron.
106. Absorption of the γ rays. In an examination of the active substances by the electrical method, the writer[[169]] found that both uranium and thorium gave out γ rays in amount roughly proportional to their activity. An electroscope of the type shown in [Fig. 12] was employed. This was placed on a large lead plate ·65 cm. thick, the active substance being placed in a closed vessel beneath.
The discharge due to the natural ionization of the air in the electroscope was first observed. The additional ionization due to the active substance must be that produced by rays which have passed through the lead plate and the walls of the electroscope. The following table shows that the discharge due to these rays decreases approximately according to an exponential law with the thickness of lead traversed.
| Thickness of lead | Rate of discharge |
|---|---|
| ·62 cms. | 100 |
| „ + ·64 cms. | 67 |
| „ + 2·86 „ | 23 |
| „ + 5·08 „ | 8 |
Using 100 grs. of uranium and thorium, the discharge due to the rays through 1 cm. of lead was quite appreciable, and readily measured. The results showed that the amount of γ rays was about the same for equal weights of thorium and uranium oxides. The penetrating power was also about the same as for the radium rays.
Fig. 44.
The writer showed that the absorption of the γ rays from radium was approximately proportional to the density of the substance traversed. A more detailed examination of the absorption of these rays in various substances has been recently made by McClelland[[170]]. The curve ([Fig. 44]) shows the decrease of the ionization current in a testing vessel due to the β and γ rays with successive layers of lead. It is seen that the β rays are almost completely stopped by 4 mms. of lead; the ionization is then due entirely to the γ rays.
In order to leave no doubt that all the β rays were absorbed, the radium was covered with a thickness of 8 mms. of lead, and measurements of the coefficient of absorption λ were made for additional thicknesses. The average value of λ was calculated from the usual equation
where d is the thickness of matter traversed. The following table shows the value of λ, (I) for the first 2·5 mms. of matter traversed (after initially passing through 8 mms. of lead), (II) for the thickness 2·5 to 5 mms., (III) for 5 to 10 mms., (IV) 10 to 15 mms.
TABLE A.
| Substance | I | II | III | IV |
|---|---|---|---|---|
| Platinum | 1·167 | |||
| Mercury | ·726 | ·661 | ·538 | ·493 |
| Lead | ·641 | ·563 | ·480 | ·440 |
| Zinc | ·282 | ·266 | ·248 | ·266 |
| Aluminium | ·104 | ·104 | ·104 | ·104 |
| Glass | ·087 | ·087 | ·087 | ·087 |
| Water | ·034 | ·034 | ·034 | ·034 |
In the above table, the absorption in aluminium, glass and water was too small to determine with accuracy the variation of λ with distance traversed. It will be observed that, for the denser substances, the coefficient of absorption decreases with the distance through which the rays have passed. This indicates that the rays are heterogeneous. The variation of λ is more marked in heavy substances.
Table B gives the values of λ divided by density for the above numbers. If the absorption were directly proportional to the density, the quotient would be the same in all cases.
TABLE B.
λ divided by density.
| Substance | I | II | III | IV |
|---|---|---|---|---|
| Platinum | ·054 | |||
| Mercury | ·053 | ·048 | ·039 | ·036 |
| Lead | ·056 | ·049 | ·042 | ·037 |
| Zinc | ·039 | ·037 | ·034 | ·033 |
| Aluminium | ·038 | ·038 | ·038 | ·038 |
| Glass | ·034 | ·034 | ·034 | ·034 |
| Water | ·034 | ·034 | ·034 | ·034 |
The numbers in column I vary considerably, but the agreement becomes closer in the succeeding columns, until in column IV the absorption is very nearly proportional to the density.
It is seen that the absorption of all three types of rays from radio-active substances is approximately proportional to the density of the substance traversed—a relation first observed by Lenard for the cathode rays. This law of absorption thus holds for both positively and negatively electrified particles projected from the radio-active substances, and also for the electromagnetic pulses which are believed to constitute the γ rays; although the absorption of the α rays, for example, is 10,000 times greater than for the γ rays. We have seen in section 84 that the value of the absorption constant λ for lead is 122 for the β rays from uranium. The value for the γ rays from radium varies between ·64 and ·44, showing that the γ rays are more than 200 times as penetrating as the β rays.
107. Nature of the rays. In addition to their great penetrating power, the γ rays differ from the α and β rays in not being deflected to an appreciable degree by a magnetic or electric field. In a strong magnetic field, it can be shown, using the photographic method, that there is an abrupt discontinuity between the β and γ rays, for the former are bent completely away from the latter. This indicates that, as regards the action of a magnetic field, there is no gradual transition of magnetic properties between the β and γ rays. Paschen[[171]] has examined the γ rays in a very intense magnetic field, and, from the absence of deflection of these rays, has calculated that, if they consist of electrified particles carrying an ionic charge, and projected with a velocity approaching that of light, their apparent mass must be at least 45 times greater than that of the hydrogen atom.
It now remains for us to consider whether the γ rays are corpuscular in character, or whether they are a type of electromagnetic pulse in the ether similar to Röntgen rays. They resemble Röntgen rays in their great penetrating power and in their absence of deflection in a magnetic field. Earlier experiments seemed to indicate an important difference between the action of γ and X rays. It is well known that ordinary X rays produce much greater ionization in gases such as sulphuretted hydrogen and hydrochloric acid gas, than in air, although the differences in density are not large. For example, exposed to X rays, sulphuretted hydrogen has six times the conductivity of air, while with γ rays the conductivity only slightly exceeds that of air. The results obtained by Strutt, in this connection, have already been given in section 45. It is there shown that the relative conductivity of gases exposed to γ rays (and also to α and β rays) is, in most cases, nearly proportional to their relative densities; but, under X rays, the relative conductivity for some gases and vapours is very much greater than for the γ rays. It must be remembered, however, that the results obtained by Strutt were for “soft X rays,” whose penetrating power was very much less than that of the γ rays. In order to see if the relative conductivity of gases produced by X rays depended upon their penetrating power, A. S. Eve[[172]] made some experiments with a very “hard” X ray bulb, which gave an unusually penetrating type of rays.
The results of the measurements are shown in the table below, where the conductivity for each type of rays is expressed relative to air as unity. The results obtained for “soft” X rays by Strutt and by Eve for γ rays are added for comparison.
It is seen that the hard rays show a much closer agreement than the soft rays with the density law found for the γ rays. The high values previously obtained for the vapours of chloroform and carbon tetrachloride are greatly reduced, and are very nearly the same as for the γ rays. On the other hand, the vapour of methyl iodide is an exception, and still shows a high conductivity. The γ rays were, however, forty times as penetrating as the hard X rays, and it is probable that the value of methyl iodide would be reduced with still more penetrating X rays.
Relative conductivities of gases.
| Gas | Relative Density | “Soft” X rays | “Hard” X rays | γ rays |
|---|---|---|---|---|
| Hydrogen | ·07 | ·11 | ·42 | ·19 |
| Air | 1·0 | 1·0 | 1·0 | 1·0 |
| Sulphuretted Hydrogen | 1·2 | 6 | ·9 | 1·23 |
| Chloroform | 4·3 | 32 | 4·6 | 4·8 |
| Methyl Iodide | 5·0 | 72 | 13·5 | 5·6 |
| Carbon Tetrachloride | 5·3 | 45 | 4·9 | 5·2 |
The hard X rays were found to give far more secondary radiation than the γ rays, but this effect is probably also a function of the penetrating power of the primary rays. It will be seen later ([section 112]) that γ rays give rise to a secondary radiation of the β ray type. This has also been observed for the X rays.
Considering the experimental evidence as a whole, there is undoubtedly a very marked similarity between the properties of γ and X rays. The view that the γ rays are a type of very penetrating X rays, also receives support from theoretical considerations. We have seen ([section 52]) that the X rays are believed to be electromagnetic pulses, akin in some respects to short light waves, which are set up by the sudden stoppage of the cathode ray particles. Conversely, it is also to be expected that X rays will be produced at the sudden starting, as well as at the sudden stopping, of electrons. Since most of the β particles from radium are ejected from the radium atom with velocities much greater than the cathode particles in a vacuum tube, X rays of a very penetrating character will arise. But the strongest argument in support of this view is derived from an examination of the origin and connection of the β and γ rays from radio-active substances. It will be shown later that the α ray activity observed in radium arises from several disintegration products, stored up in the radium, while the β and γ rays arise only from one of these products named radium C. It is found, too, that the activity measured by the γ rays is always proportional to the activity measured by the β rays, although by separation of the products the activity of the latter may be made to undergo great variations in value.
Thus the intensity of the γ rays is always proportional to the rate of expulsion of β particles, and this result indicates that there is a close connection between the β and γ rays. Such a result is to be expected if the β particle is the parent of the γ ray, for the expulsion of each electron from radium will give rise to a narrow spherical pulse travelling from the point of disturbance with the velocity of light.
108. There is another possible hypothesis in regard to the nature of these rays. It has been shown (sections 48 and 82) that the apparent mass of an electron increases as the speed of light is approached; theoretically it should be very great when the velocity of the electron is exceedingly close to the velocity of light. In such a case, a moving electron would be difficult to deflect by a magnetic or electric field.
The view that the γ rays are electrons carrying a negative charge and moving with a velocity nearly equal to that of light has recently been advocated by Paschen[[173]]. He concluded from experiment that the γ rays like the β rays carried a negative charge. We have seen ([section 85]) that Seitz also observed that a small negative charge was communicated to bodies on which the γ rays impinged, but the magnitude of this charge was much smaller than that observed by Paschen. I do not think that much weight can be attached to observations that a small positive or negative charge is communicated to bodies on which the γ rays fall, for it will be shown later that a strong secondary radiation, consisting in part of electrons, is set up during the passage of the γ rays through matter. It is not improbable that the small charge observed is not a direct result of the charge carried by the γ rays, but is an indirect effect due to the secondary radiations emitted from the surface of bodies. There is no doubt that a thick lead vessel, completely enclosing a quantity of radium, acquires a small positive charge, but this result would follow whether the γ rays carry a charge or not, since the secondary radiations from the lead surface consist of projected particles which carry with them a negative charge.
On this corpuscular theory of the nature of the γ rays, each electron must have a large apparent mass, or otherwise it would be appreciably deflected by an intense magnetic field. The energy of motion of the electron must, in consequence, be very great, and, if the number of the electrons constituting the γ rays is of the same order of magnitude as the number of the β particles, a large heating effect is to be expected when the γ rays are stopped in matter. Paschen[[174]] made some experiments on the heat emission of radium due to the γ rays; he concluded that the γ rays were responsible for more than half of the total heat emission of radium and carried away energy at the rate of over 100 gram calories per hour per gram of radium. This result was not confirmed by later experiments of Rutherford and Barnes[[175]], who found that the heating effect of the γ rays could not be more than a few per cent. of the total heat emission of radium. These results will be considered later in [chapter XII].
The weight of evidence, both experimental and theoretical, at present supports the view that the γ rays are of the same nature as the X rays but of a more penetrating type. The theory that the X rays consist of non-periodic pulses in the ether, set up when the motion of electrons is arrested, has found most favour, although it is difficult to provide experimental tests to decide definitely the question. The strongest evidence in support of the wave nature of the X rays is derived from the experiments of Barkla[[176]], who found that the amount of secondary radiation set up by the X rays on striking a metallic surface depended on the orientation of the X ray bulb. The rays thus showed evidence of a one-sidedness or polarization which is only to be expected if the rays consist of a wave motion in the ether.