The angular distribution of photoelectrons from the K shell

Abstract

In an investigation with the cloud chamber on the properties of X-rays, Wilson (1923) observed that although photoelectrons due to a narrow pencil of X-rays were ejected from the gas atoms in all directions, there was, especially in the case of high-energy electrons, a preponderance with forward components in their velocities of ejection. Wilson’s pictures threw light on results obtained earlier by workers in X-ray fields, who found an excess of electrons ejected forwards from thin metal foils by X-rays, and have since inspired a number of experiments designed to determine with precision the angular distribution of photoelectrons. A survey of the different methods developed for this purpose has been given by Compton and Allison (1935). Of these the cloud expansion chamber has proved the most satisfactory, in spite of the inevitable disadvantages associated with statistical methods. If a suitable gas is chosen, the shells from which the electrons are ejected can usually be identified, and the directions of ejection can be determined from measurements relating to the initial portions of the cloud tracks. No trouble is ever encountered in distinguishing photoelectron tracks from those due to recoil electrons. The method has been criticized by Watson (1927), who suggested the possibility of nuclear scattering within the radius of the first drop. This objection has been answered by Kirchner (1927), who was able to show from an application of Rutherford’s theory of nuclear scattering to the problem, that under normal conditions of observation the effect of nuclear scattering on the initial directions of ejection as measured from track photographs could be neglected. The most likely source of error appears to lie in the systematic exclusion of tracks in certain angular intervals determined by the experimental arrangement (e. g. positions of cameras, lighting, etc.), and much thought has been given by different workers (Kirchner 1927; Williams, Nuttall and Barlow 1928) as to the best means for ensuring representative samples of tracks for measurement. In this respect our experience leads us to believe that a requirement of fundamental importance is sufficient and uniform illumination throughout the whole chamber.