High energy diffraction (E > 100 keV) is well suited for structural analysis of bulk materials. In most cases both absorption and scattering are small enabling the study of bulk samples with a thickness of several cm. As a benefit of modern synchrotron sources the high brightness can be exploited to perform measurements in a time resolved mode with resolutions in the nanosecond range. Exploiting diffraction patterns implies that structural modifications are observed on an atomic level. Although the detector system was far from being optimized, we were able to follow the structural response to an external perturbation by high energy X-ray diffraction with a 20 ns time resolution. In a collaboration with the ILL and the Universities of Bochum and Würzburg we have followed the time evolution of the strain field of an ultra sound wave with a frequency of 8.20 MHz corresponding to an acoustic wavelength of 1.14 mm in a Si 111 crystal. High energy radiation at 300 keV probing the bulk is of crucial importance for this case because the strain field vanishes at the surface of the crystal. The figure shows an intensity map as a function of rocking angle and time. A standing, longitudinal ultrasonic wave is characterized by regions of compression and dilatation with amplitudes varying in time. At a certain moment the sound amplitude is zero and the crystal is free of strain which corresponds to the points of narrowest rocking widths in the figure. Under this condition the crystal behaves like a dynamical scatterer and the integrated diffracted intensity is low. After this zero crossing point the regions of anti-nodes develop reaching their maximum value a quarter of a period later. Now, the width of the curve corresponds to the maximum strain and the crystal behaves as a kinematical scatterer. The scattering geometry averaged over a large crystal volume including zones of compression and dilatation, and thus the whole distribution of lattice planes is seen in a snapshot of time, and the frequency for one sound period of 122 ns seems to be doubled in the figure.
|Title of host publication||ESRF Highlights|
|Number of pages||1|
|State||Published - 1 Jan 1996|