Ion channeling in natural diamond. II. Critical angles

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Abstract
In this paper the critical angles for ions channeling in diamond single crystals are presented and considered. Careful attention was paid to experimental precautions to ensure reliable results. In diamond the thermal vibration amplitude is much less than the Thomas-Fermi screening distance even at ordinary temperatures, and this crystal can be expected to furnish a stringent test of theoretical critical-angle expressions which depend on the ratio of these two quantities. Data were taken for different axes and planes, for different ions and ion energies, and for different crystal temperatures. It is shown that most of the diamond data are in fact well represented by the established semiempirical expressions due to Barrett and can also be predicted accurately by the procedure of Varelas and Sizmann. This provides fresh confirmation of the continuum model for channeling and of some of the physical considerations underlying these theoretical approaches. The areas of disagreement serve to highlight important second-order effects of crystal geometry and can be understood within the terms of current theory.