Neutron diffraction by perfect crystals excited into mechanical resonance vibrations

Our contribution presents results of investigations concerning neutron diffraction by vibrating perfect crystals of Si and SiO₂. They were excited in longitudinal, flexural, thickness and thickness-shear modes of vibration with resonance frequencies ranging from 1 to 1500 kHz. In this case, when the acoustic wavelength is much larger than the extinction distance the following phenomena caused by the deformation and Doppler shift were observed: broadening of the perfect crystal rocking curve, enhancement of the integrated reflectivity by up to two orders of magnitude and time modulation of the diffracted beam. When neutron time-off-flight through the vibrating crystal is comparable to the vibration period, multiple reflections having influence on the reflectivity take place. While investigating the forbidden reflections in the crystal excited into vibrations, a strong multiple diffraction phenomena resulting in “umweganregung” effect was observed. Also the vibrating crystals as neutron choppers with 1 μs pulses and a repetition rate of up to 100 kHz were treated.