Transparency oscillations of a silicon single crystal in passing from axial to planar channelling

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Abstract
The transparency coefficient, T, of thin (2.5μ) Si single crystals relative to a well collimated 8.16 MeV He ion beam has been measured as a function of angle or between the beam direction and the [110] axis of crystals rotated in the (110) plane. Curve T(α) has been found to display minima at α = 0.15°, 0.34° and 0.5°. Computer simulation of experimental conditions has shown that the first minimum is a result of competition of two processes: increase of the radius of ring-shaped angular distribution with increasing a and ion capture in the (110) planar channel. The remaining two minima are due to particle dechannelling from channel (110) resulting from resonance enhancement of transverse particle oscillations in the channel. Similar calculations have been carried out for the transition from axial [110] to planar (001) channelling. It has been shown that in this case the difference in the conditions resonance result in spatial separation of the ion beams that have passed through channels (001) with and without displaced arrangement of rows [110].