A NEUTRON BRILLOUIN-SCATTERING STUDY OF MG70ZN30

Inelastic neutron scattering measurements have been made with momentum transfers within the first pseudo-Brillouin zone of a magnesium/zinc glass at 100 K and 297 K. We describe the application of a new spectro meter (PHAROS) for these studies. Because the longitudinal velocity of sound is about 4300 m s(-1), a high incident neutron energy (0.187 eV ) was required. Data have been obtained down to Q similar or equal to 8 nm(-1) and HBAR omega = 25 meV well inside the first pseudo-Brilloui n zone of the glass. Evidence for two predominantly longitudinal excit ations in the Mg70Zn30 glass is presented. Their positions agree appro ximately with theoretical predictions.