A simple method of estimating the screening radius of an atom
In the discussions of the electron scattering theory as applied to electron microscopy it is usually assumed that the scattering at small angles is so greatly affected by the shielding of the nucleus by the atomic electrons, that the scattering cross section becomes small and approximately constant after a limiting angle θ min and can be neglected entirely. The magnitude of the limiting angle is found to be of the order of 10−2 radian or more. Practical observations show, however, that the introduction of a diaphragm in the objective lens to reduce the angular aperture below 10−2 radian improves the image contrast considerably. It appears, therefore, that although the scattering at angles in the neighbourhood of θ min is theoretically complicated, it is not strictly justifiable to ignore it completely.