The dependence of electron damage on crystal orientation in the HVEM

Irradiation of Cu and Fe by fast electrons in a high-voltage electron microscope is known to produce visible damage preferentially at certain crystallographic orientations. Concepts from the semi-classical theory of electron diffraction are used, in addition to the usual Bloch wave approach, in describing this direction dependence. The damage rate is enhanced by a factor of up to 2·6 at the centre of Cu (220) bend contours at 600 kV, due to the large excitation of the most strongly-bound state. The width of the denuded zone at the top surface is found to be important in determining which systematic orientations produce well-defined bands of damage. The absence of damage patterns for many orientations of Fe is discussed. No marked increase in visible damage is expected for electrons incident close to principal crystal directions.