Critical positions in elastic electron scattering by argon

The electron energy where a minimum in the differential cross section attains its smallest value is known as the critical energy, and the corresponding scattering angle is the critical angle. A comparison is made between critical positions in argon derived from six calculations and six experiments. These include a direct measurement, and several independent determinations of absolute cross sections, from which the phaseshifts have been derived. The theoretical models yield significant differences in the predicted critical energies. The experimental values have a large scatter, but the critical positions derived from the experiment of Williams and Willis are in closest agreement with the direct measurement of Kessler et al. Measured critical energies tend to lie significantly higher than calculated values, and so the superiority of any theoretical model cannot be established.