Page:Radio-activity.djvu/215



113. Comparison of the ionization produced by the [Greek: alpha] and β rays. With unscreened active material the ionization produced between two parallel plates, placed as in Fig. 17, is mainly due to the [Greek: alpha] rays. On account of the slight penetrating power of the [Greek: alpha] rays, the current due to them practically reaches a maximum with a small thickness of radio-active material. The following saturation currents were observed for different thicknesses of uranium oxide between parallel plates sufficiently far apart for all the [Greek: alpha] rays to be absorbed in the gas between them. Surface of uranium oxide 38 sq. cms.

+++ +++ +++
 * Weight of uranium oxide |   Saturation current   |
 * in grammes per sq. cm.  | in amperes per sq. cm. |
 * of surface        |       of surface       |
 * ·0036             |    1·7 × 10^{-13}      |
 * ·0096             |    3·2 × 10^{-13}      |
 * ·0189             |    4·0 × 10^{-13}      |
 * ·0350             |    4·4 × 10^{-13}      |
 * ·0955             |    4·7 × 10^{-13}      |

The current reached about half its maximum value for a weight of oxide ·0055 gr. per sq. cm. If the [Greek: alpha] rays are cut off by a metallic screen, the ionization is then mainly due to the β rays, since the ionization produced by the γ rays is small in comparison. For the β rays from uranium oxide it has been shown (section 86) that the current reaches half its maximum value for a thickness of 0·11 gr. per sq. cm.

Meyer and Schweidler have found that the radiation from a water solution of uranium nitrate is very nearly proportional to the amount of uranium present in the solution.

On account of the difference in the penetrating power of the [Greek: alpha] and β rays, the ratio of the ionization currents produced by them