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[1] 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.
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| Weight of uranium oxide | Saturation current |
| in grammes per sq. cm. | in amperes per sq. cm. |
| of surface | of surface |
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| ·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} |
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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[2] 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
