The Scattering of 50-Kilovolt Electrons by Aluminum

Apparatus is described for scattering electrons with thin foils and measuring the intensity of scattering in a narrow pencil making any angle from 30° to 120° with the direction of incidence. Electrons with energies below 95 percent of the energy on incidence are excluded by a retarding voltage. The theory of nearly single scattering is discussed, and a general expression is found for the error introduced by slight plural scattering. In the case of Rutherford scattering this expression gives for the fractional error the value ${\epsilon }^2\left\{{csc}^2\right(\frac{\theta }{2})-\frac{1}{2}\}$ where $\theta$ is the angle at which scattering is measured and ${\epsilon }^2$ is the mean square deflection of particles traversing in the foil the same length of path as those scattered at the angle $\theta$. The relation of this result to Wentzel's criterion is shown. Electrons accelerated by 50 kv were scattered by thin composite foils of Al on collodion. A difference was found, though not explained, between the intensities of scattering at 90° on the two sides of the foil, which was inclined at 45° to the direction of incidence. With allowance made for this asymmetry in comparing the scattering at angles above and below 90°, relative intensities at angles from 30° to 120° were within 5 percent of those predicted by Mott. Comparison of the observed and predicted absolute intensities of the scattering from Al was subject to an uncertainty of about 20 percent, principally in the determination of the thickness of the Al film. Within this uncertainty, the observation agreed with the prediction. If the actual scattering differs from the predicted, it appears from these observations to be more probably less than greater.