The passage of an electron beam through a field-free enclosure

Abstract

A method which is frequently employed in investigating collision problems consists essentially of firing a directed beam of electrons into a collision chamber containing the gas to be investigated at a pressure of a few hundredths or thousandths of a millimetre. The collision chamber generally contains a system of “grids” between which various potentials can be put, so as to draw out of the path of the beam the products of collision which are then received on an electrode or Faraday cylinder placed behind the last grid. When these grids are all at the same potential the collision chamber is generally supposed to have energies determined by the temperature of the gas in the chamber. In particular, an apparatus employing an electron beam in a “field-free” enclosure has been used by a number of investigators to examine the angular scattering of electrons in gases. These experiments have considerable interest, and in view of the large amount of work that is being done in this comparatively new field of investigation it is of importance to determine to what extent an enclosed space around an electron beam is truly field-free; for it will be clearly seen that the existence of a potential gradient inside the collision chamber will, if sufficiently great, seriously alter the shape of the angular scattering curve.