more than 80 per cent. of the secondary radiation. The following table shows the relative amount of secondary rays from different substances when exposed to β and γ rays together and to γ rays alone. The amount from lead in each case is taken as a standard and equal to 100. The amount of secondary radiation found by Townsend from soft X rays is added for comparison.
Secondary Radiations.
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| Radiator | β and γ rays | γ rays | Röntgen |
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| Lead | 100 | 100 | 100 |
| Copper | 57 | 61 | 291 |
| Brass | 58 | 59 | 263 |
| Zinc | 57 | . . . | 282 |
| Aluminium | 30 | 30 | 25 |
| Glass | 31 | 35 | 31 |
| Paraffin | 12 | 20 | 125 |
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It will be observed that the relative amounts are about the same for the γ rays alone as for the β and γ rays together. On the other hand, the amount of secondary radiation set up by X rays is very different, lead for example giving much less than brass or copper. The secondary rays from the γ rays alone are slightly less penetrating than for the β and γ rays together, but are far more penetrating than the secondary radiation from the X rays examined by Townsend.
The amount of secondary radiation set up by the β and γ rays is mainly independent of the state of the surface of the radiator. About the same amount is obtained from iron as from iron filings; from liquid as from solid paraffin; and from ice as from water[1].
Becquerel has shown that the secondary rays set up by the β rays are deflected by a magnet and consist of negatively
- ↑ In a recent paper (Phil. Mag. Feb. 1905), McClelland has, in the main, confirmed the experimental results obtained by Eve. An electrometer was used instead of an electroscope. He finds, in addition, that the amount of secondary radiation depends on the angle of incidence of the primary rays, and is greatest for an angle of 45°. In a letter to Nature (Feb. 23, p. 390, 1905), he states that more recent experiments have shown that the amount of secondary radiation from different substances is a function of their atomic weights rather than of their densities. In every case examined, the amount of secondary radiation increases with the atomic weight, but is not proportional to it.
