Scattering of Fast Electrons by Helium

Abstract

When a charged particle collides with another charged particle, the force between them varying inversely as the square of the distance, wave mechanics and classical mechanics give the same scattering formula when the particles are unlike, and different formulas when they are identical. In the particular case of scattering of electrons by electrons, wave mechanics predicts a smaller scattering than does classical mechanics, in a ratio which has a minimum of 1 to 2 at 45\ifmmode^\circ\else\textdegree\fi{}. The scattering of 2000 and 4000 volt electrons by helium atoms has been investigated. For such energy values, the scattering at considerable angles (>20\ifmmode^\circ\else\textdegree\fi{}) is due to the nuclei and to atomic electrons acting independently of each other. Thus it is possible to measure the ratio of the scattering by the atomic electrons to that by the nucleus and compare it with the values given by the two theories. The elastically scattered electrons are to be identified with those scattered by the nucleus while the inelastically scattered electrons are to be identified with those scattered by atomic electrons. The results are in quantitative agreement with the wave mechanical theory of collisions between electrons. A subsidiary result of the investigation is that the distribution of energies among the inelastically scattered electrons may be used to show that the velocity of the atomic electrons has a value close to that given by the Bohr theory.