The effect of X-ray beam intensity oscillations as a function of the amplitude of ultrasound excited in a crystal

Oscillations of the X-ray diffraction intensity with varying amplitude of acoustic vibrations excited in the crystal are investigated. The effect offers the possibility to measure ultrasound amplitudes with high sensitivity and high local resolution by an absolute method. The oscillation contrast can be increased by selecting the central part of the diffraction profile with the help of a narrow slit placed in front of the detector. A drastic increase in the oscillation contrast for coincidence between the ultrasound wavelength λs and the extinction length τ is explained. For λs ≈ τ and at a very small amplitude of displacement there appears a narrow (some few μm wide) intensive peak at the centre of the base of the Borrmann delta. The behaviour of the initial phase of oscillations in dependence on the vibration frequency gives a direct experimental confirmation of the scheme of splitting of the dispersion branches.