| Polonium. | Radium. | ||||
|---|---|---|---|---|---|
| Layers of aluminium | Current | Ratio of decrease for each layer | Layers of aluminium | Current | Ratio of decrease for each layer |
| 0 | 100 | 0 | 100 | ||
| ·41 | ·48 | ||||
| 1 | 41 | 1 | 48 | ||
| ·31 | ·48 | ||||
| 2 | 12·6 | 2 | 23 | ||
| ·17 | ·60 | ||||
| 3 | 2·1 | 3 | 13·6 | ||
| ·067 | ·47 | ||||
| 4 | ·14 | 4 | 6·4 | ||
| ·39 | |||||
| 5 | 0 | 5 | 2·5 | ||
| ·36 | |||||
| 6 | ·9 | ||||
| 7 | 0 |
to the radium rays decreases very nearly by half its value for each additional thickness until the current is reduced to about 6% of the maximum. It then decays more rapidly to zero. Thus, for radium, over a wide range, the current decreases approximately according to an exponential law with the thickness of the screen, or
where i is the current for a thickness d, and i₀ the initial current. In the case of polonium, the decrease is far more rapid than would be indicated by the exponential law. By the first layer, the current is reduced to the ratio ·41. The addition of the third layer cuts the current down to a ratio of ·17. For most of the active bodies, the current diminishes slightly faster than the exponential law would lead one to expect, especially when the radiation is nearly all absorbed.
98. The increase of absorption of the α rays of polonium with the thickness of matter traversed has been very clearly shown in some experiments made by Mme Curie. The apparatus employed is shown in [Fig. 34].
Fig. 34.
The saturation current was measured between two parallel plates PP´ 3 cms. apart. The polonium A was placed in the metal box CC, and the rays from it, after passing through an opening in the lower plate P´, covered with a layer of thin foil T, ionized the gas between the plates. For a certain distance AT, of 4 cms. or more, no appreciable current was observed between P and P´. As the distance AT was diminished, the current increased in a very sudden manner, so that for a small variation of the distance AT there was a large increase of current. With still further decrease of distance the current increases in a more regular manner. The results are shown in the following table, where the screen T consisted of one and two layers of aluminium foil respectively. The current due to the rays, without the aluminium screen, is in each case taken as 100.
| Distance AT in cms. | 3·5 | 2·5 | 1·9 | 1·45 | 0·5 |
|---|---|---|---|---|---|
| For 100 rays transmitted by one layer | 0 | 0 | 5 | 10 | 25 |
| For 100 rays transmitted by two layers | 0 | 0 | 0 | 0 | 0·7 |
The metallic screen thus cuts off a greater proportion of the rays the greater the distance of air which the radiations traverse. The effects are still more marked if the plates PP´ are close together. Results similar but not so marked are found if radium is substituted for the polonium.