Dynamical adjustment of crystal reflectivity by large-amplitude ultrasonic excitation for synchrotron X-ray monochromatization

A new technique of double-crystal X-ray monochromatization is described that provides a higher flux at a given bandwidth than conventional perfect-double-crystal-monochromator systems and, furthermore, permits dynamical adjustment of the energy resolution. The new technique is based on the excitation of perfect crystals by ultrasonic waves (USWs), which induces a continuous increase in reflectivity with increasing USW amplitude. Theoretical expressions in analytical form have been derived for both the differential and integrated reflection coefficients of USW-excited crystals for large amplitudes of vibration. For Si(440), the perfect-crystal reflection width of 3 × 10^-6 rad may be broadened to a width of more than 10^-4 rad by USW excitation. First experimental studies of the dependence of the overall reflectivity of an X-ray double-crystal monochromator system on the amplitude of USW excitation prove the feasibility of the new technique. Potential applications include optimization of flux and bandwidth in resonant absorption, scattering or diffraction studies using synchrotron radiation.