The peak ring intensity (${I_R}^{\prime }$) and the background intensity (${I_B}^{\prime }$) for the most intense rings in the electron diffraction patterns of aluminium and thallium chloride have been measured, as a function of film thickness and accelerating voltage.

It is shown that part of the discrepancy between the results and the theories of Bethe and Morse is due to plural electron scattering.

A semi-empirical theory that includes the effect of plural scattering has been developed to explain the variation of ${I_R}^{\prime }$ and ${I_B}^{\prime }$ with specimen thickness. The contrast in the patterns ($\frac{{I_R}^{\prime }}{{I_B}^{\prime }}$) increases rapidly with accelerating voltage and falls rapidly with increase in film thickness. The maximum film thickness ($T_{max}$) that will yield an observable pattern increases less rapidly than the accelerating voltage, between 50 and 150 kv, in agreement with the results of Möllenstedt.

• Received 12 May 1952

DOI:https://doi.org/10.1103/PhysRev.87.970