Fig. 46.
A small electroscope was mounted on one side of a lead platform 1·2 cms. thick, which rested on a lead cylinder 10 cms. high and 10 cms. in diameter. The radium was placed at the bottom of a hole reaching to the centre of the cylinder.
On applying a strong magnetic field, at right angles to the plane of the paper, so as to bend the secondary rays from the platform towards the electroscope, the rate of discharge was much increased. On reversing the field, the effect was much diminished. Since the γ rays are not themselves deflected by a magnetic field, this result shows that the secondary radiation is quite different in character from the primary rays, and consists of electrons projected with a velocity (deduced from the penetrating power) of about half the velocity of light. We have already pointed out that the emission of electrons from a substance traversed by the rays will account sufficiently well for the charge observed by Paschen, without the necessity of assuming that the γ rays carry a negative charge of electricity.
The secondary radiation set up by Röntgen rays, like that due to the β and γ rays, consists in part of electrons projected with considerable velocity. These three types of rays seem about equally efficient in causing the expulsion of electrons from the substance through which they pass. We have seen that the X and γ rays are, in all probability, electromagnetic pulses set up by the sudden starting or stopping of electrons, and, since these rays in turn cause the removal of electrons, the process appears to be reversible. Since the β rays pass through some thickness of matter before their energy of motion is arrested, theory would lead us to expect that a type of soft X rays should be generated in the absorbing matter.
PART VI.
113. Comparison of the ionization produced by the α and β rays. With unscreened active material the ionization produced between two parallel plates, placed as in [Fig. 17], is mainly due to the α rays. On account of the slight penetrating power of the α rays, the current due to them practically reaches a maximum with a small thickness of radio-active material. The following saturation currents were observed[[181]] 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.