a small thickness of radio-active material. The following saturation currents were observed71 for different thicknesses of uranium oxide between parallel plates sufficiently far apart for all the α rays to be absorbed in the gas between them.
Surface of uranium oxide 38 sq. cms.
| Weight of uranium oxide in grammes per sq. cm. of surface | Saturation current in amperes per sq. cm. 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 α 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 Schweidler72 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 α and β rays, the ratio of the ionization currents produced by them depends on the thickness of the radio-active layer under examination. The following comparative values of the current due to the α and β rays were obtained for very thin layers of active matter73. A weight of ⅒ gramme of fine powder, consisting of uranium oxide, thorium oxide, or radium chloride of activity 2000, was spread as uniformly as possible over an area of 80 sq. cms. The saturation current was observed between parallel plates 5·7 cms. apart. This distance was sufficient to absorb most of the α rays from the active substances. A layer of aluminium ·009 cm. thick absorbed all the α rays.
| Current due to α rays | Current due to β rays | Ratio of currents β/α | |
|---|---|---|---|
| Uranium | 1 | 1 | ·0074 |
| Thorium | 1 | ·27 | ·0020 |
| Radium | 2000 | 1350 | ·0033 |
In the above table the saturation current due to the α and β rays of uranium is, in each case, taken as unity. The third column gives the ratio of the currents observed for equal weights of substance. The results are only approximate in character, for the ionization due to a given weight of substance depends on its fineness of division. In all cases, the current due to the β rays is small compared with that due to the α rays, being greatest for uranium and least for thorium. As the thickness of layer increases, the ratio of currents β/α steadily increases to a constant value.
114. Comparison of the energy radiated by the α and β rays. It has not yet been found possible