Abstract
We report X-ray and neutron scattering results in vitreous silica. As in earlier data, elear evidence is found for sound wave modes at the energies of the boson peak. We have observed by X-ray experiments a elear dispersion of Brillouin peaks and we have detected experimental evidence of inelastic scattering in the second Brillouin zone. By neutron scattering experiments we have found coherence effects in S(Q, E). These effects are consistent with average acoustic phonon dispersion curves, which are similar in character to the dispersion curves found in crystals but, obviously, broadened by the topological disorder.