| 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 |