Thermal diffuse scattering in time-of-flight neutron diffraction

The nature of the thermal diffuse scattering (TDS) from long-wavelength acoustic modes of vibration in a single crystal is examined for time-of-flight neutron diffraction. In the neighbourhood of the Bragg reflections there may be gaps or 'windows' in the energy change of the scattered neutrons, in the wavelengths of both incident and scattered radiation, and in the total time of flight. TDS is forbidden within these windows and rises to a steep maximum at the edges: one maximum is due to phonon emission (Stokes process) and the other to phonon absorption (anti-Stokes). The edges of the windows are determined by the sound velocity in the crystal. The sound velocity is readily derived, without employing energy analysis, by measuring the positions of the edges of the time-of-flight window.