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.

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

1. ↑ Rutherford and McClung, Phil. Trans. A. p. 25, 1901.
2. ↑ Meyer and Schweidler, Wien Ber. 113, July, 1904.