Neutron diffraction from single-crystal silicon: The dependence of the thermal diffuse scattering on the velocity of sound

It is well known from the theory of one-phonon scattering of thermal neutrons by a crystal that the nature of the thermal diffuse scattering (TDS) near the Bragg peak depends on whether the neutron velocity is greater than or is less than the sound velocity in the crystal. For faster-than-sound neutrons the TDS rises to a peak coinciding with the Bragg peak, whereas for slower-than-sound neutrons the TDS tends to give a flat background across the Bragg reflection. Theses theoretical predictions are supported by experiments using pulsed neutron diffraction from single crystals of perfect silicon. In particular, the integrated TDS across a reflection undergoes a pronounced fall when the neutron velocity drops below the velocity of sound. (orig./BHO)