Page:Radio-activity.djvu/212

 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.

+-+-++-+ +-+-++-+ +-+-++-+
 * Radiator | β and γ rays | γ rays | Röntgen |
 * 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   |

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.

Becquerel has shown that the secondary rays set up by the β rays are deflected by a magnet and consist of negatively