Résumé
New neutron spectroscopic results on glassy selenium are presented. For the first time both Brillouin scattering under kinematic conditions and the boson peak were observed. Both thermal neutron three-axis and cold neutron time-of-flight spectroscopies were necessary. Their absolute calibration reveals the strength of the various spectral contributions in this frequency range. The most plausible interpretation of the overall data is that longitudinal acoustic waves enter a regime of strong scattering between 3 and 4 meV. Umklapp scattering of the neutrons via the static structure factor is also seen. It provides unique information on transverse acoustic modes. Its fast disappearance beyond 5 meV is another strong indication that acoustic excitations cease to propagate.