Page:Radio-activity.djvu/182

 The initial current with 1 layer of aluminium over the active material is taken as 100. It will be observed that the current due

Polonium.                       Radium. +-+-++ +-+-++ +-+-++  +-+-++ +-+-++  +-+-++
 * Layers of|Current| Ratio of  |  |Layers of|Current|  Ratio of  |
 * aluminium|      |decrease for|  |aluminium|       |decrease for|
 * |      | each layer |  |         |       | 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   |            |
 * |      |            |  |    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     i/i_{0} = e^{-[Greek: lambda]d},

where i is the current for a thickness d, and i_{0} 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 [Greek: alpha] 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.