Page:Radio-activity.djvu/211

 of substances such as granite, brick, and cement, which give out secondary rays of nearly twice the penetrating power of other substances.

β and γ rays.

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 * |        | Secondary  |                    | Aluminium  |
 * Radiator   | Density | Radiation  |(Sec. Rad.)/Density | ·085 cm.   |
 * |        |            |                    |  λ  |
 * Mercury      |   13·6  |    147     |     10·8           |            |
 * Lead         |   11·4  |    141     |     12·4           |    18·5    |
 * Copper       |    8·8  |     79     |      9·0           |    20      |
 * Brass        |    8·4  |     81     |      9·6           |    21      |
 * Iron (wrought)|   7·8  |     75     |      9·6           |    20      |
 * Tin          |    7·4  |     73     |      9.9           |    20·3    |
 * Zinc         |    7·0  |     79     |     11·3           |            |
 * Granite      |    2·7  |     54     |     20·0           |    12·4    |
 * Slate        |    2·6  |     53     |     20·4           |    12·1    |
 * Aluminium    |    2·6  |     42     |     16·1           |    24      |
 * Glass        |    2·5  |     44     |     17·6           |    24      |
 * Cement       |    2·4  |     47     |     19·6           |    13·5    |
 * Brick        |    2·2  |     49     |     22·3           |    13·0    |
 * Ebonite      |    1·1  |     32     |     29·1           |    26      |
 * Water        |    1·0  |     24     |     24·0           |    21      |
 * Ice          |     ·92 |     26     |     28·2           |            |
 * Paraffin solid|    ·9  |     17     |     18·8           |    21      |
 * "   liquid|     ·85 |     16     |     18·8           |            |
 * Mahogany     |     ·56 |     21·4   |     38·2           |    23      |
 * Paper        |     ·4? |     21·0   |     52             |    22      |
 * Millboard    |     ·4? |     19·4   |     48             |    20·5    |
 * Papier-mâché |      |     21·9   |                    |            |
 * Basswood     |     ·36 |     20·7   |     57             |    22      |
 * Pine         |     ·35 |     21·8   |     62             |    21      |
 * X ray screen |         |     75·2   |                    |    23·6    |
 * X ray screen |         |     75·2   |                    |    23·6    |

The secondary radiation not only comes from the surface of the radiator but from a considerable depth. The amount of secondary rays increases with the thickness of the radiator, and, in the case of glass and aluminium, reaches a practical maximum for a plate about 3 mms. thick.

In the above table, the secondary radiation arises from both the β rays and γ rays together. When the β rays were cut off by a layer of lead 6·3 mms. thick, placed between the radium and the radiator, the effect on the electroscope was reduced to less than 20 per cent. of its former value, showing that the β rays supplied