Fig. 97: General experimental set-up for acoustic excitation of the Bragg-Fresnel lens. The double crystal monochromator was tuned for an energy of 9.89 keV. The circular Bragg-Fresnel lens with 0.62 m focal length provides 2D focusing under nearly backscattering conditions. The lithium niobite transducer with a fundamental frequency of about 30 MHz generated the quasi-longitudinal acoustic waves perpendicular to the crystal surface. The transducer was driven by computer through the high-frequency generator and amplifier.