When a lead screen 3 mms. thick was placed over the radium—a thickness sufficient to absorb all the readily deflectable β rays—a small negative charge was still given to the plate, corresponding to ·29 per cent. of the maximum. This is a very much smaller value than was observed by Paschen (see [Fig. 30]).
| Thickness of Tin in mms. | I/I₀ | λ |
|---|---|---|
| 0·00834 | ·869 | 175 |
| 0·0166 | ·802 | 132·5 |
| 0·0421 | ·653 | 101·5 |
| 0·0818 | ·466 | 93·5 |
| 0·124 | ·359 | 82·5 |
| 0·166 | ·289 | 74·9 |
| 0·205 | ·230 | 71·5 |
| 0·270 | ·170 | 65·4 |
| 0·518 | ·065 } | 53} |
| 0·789 | ·031 } | 44} |
| 1·585 | ·0059} | 32} |
| 2·16 | ·0043} | 25} |
This difference may, in part, be due to the fact that, in Paschen’s experiments, a large proportion of the slow velocity electrons were absorbed in the glass tube of ·5 mm. thickness containing the radium.
Seitz also determined the relative thickness, compared with tin, of different substances which reduced the negative charge communicated to P by a definite amount. A few of the numbers are given below, and expressed in terms of tin as unity.
| Substance | Thickness Tin = 1 |
|---|---|
| Lead | ·745 |
| Gold | ·83 |
| Platinum | ·84 |
| Silver | 1 |
| Steel | 1·29 |
| Aluminium | 1·56 |
| Water | 1·66 |
| Paraffin | 1·69 |
The thickness required to stop a given proportion of the β rays thus decreases with the density, but not nearly so fast as the density increases. These results are difficult to reconcile with the density-law of absorption found by Lenard from the cathode rays, or with the results of the ionization method already considered. A further experimental examination of the whole question is very much to be desired.
86. Variation of the amount of radiation with the thickness of the layer of radiating material. The radiations are sent out equally from all portions of the active mass, but the ionization of the gas which is measured is due only to the radiations which escape into the air. The depth from which the radiations can reach the surface depends on the absorption of the radiation by the active matter itself.
Let λ be the absorption constant of the homogeneous radiation by the active material. It can readily be shown that the intensity I of the rays issuing from a layer of active matter, of thickness d, is given by
where I₀ is the intensity at the surface due to a very thick layer.