Determination of the velocity of sound in crystals by time-of-flight neutron diffraction

In a neutron Laue (time-of-flight) experiment, the nature of the thermal diffuse scattering by elastic waves depends on the ratio, β, of the sound velocity in the crystal to the neutron velocity. For slower-than-sound neutrons, there is a certain range of β for which a 'wavelength window' appears in the incident beam; TDS is forbidden for all wavelengths lying within this window. The window is best observed in back scattering, and for a scattering angle close to 1800C the centre of the window coincides with the Bragg wavelength. At the edges of the window, the TDS intensity rises abruptly to two sharp peaks, one due to phonon emission and the other to phonon absorption. The sound velocity is derived by measuring the time of flight of either peak. The method is illustrated by applying it to pyrolytic graphite, which was examined using the neutron spallation source ISIS. (orig.)