PART IV.. The γ or Very Penetrating Rays.

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, 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 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 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.

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. 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

$$ \frac {I} {I₀} = e^{–λ d} $$,

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