We have performed new Brillouin scattering measurements of the velocity and attenuation of hypersonic waves as a function of pressure P in vitreous silica. The hypersound-attenuation coefficient exhibits a sharp maximum located at 2 GPa which appears to coincide with the well-known minimum in sound velocity. The results are analyzed together with previous literature data. Internal friction measurements are described satisfactorily using a model including dissipative effects of anharmonicity and thermally activated relaxation of structural defects. From this model, the associated velocity changes were calculated, and subtracted from the data to obtain the bare velocities (the so-called unrelaxed velocities) as a function of T at different P. is almost constant at low T, then increases linearly above a temperature onset . We find evidence for an increase of with pressure P [1]. This is in agreement with numerical simulations in which the observed hardening is related to changes of the ring conformations with T [2,3]. [1] S. Ayrinhac, B. Rufflé, M. Foret, H. Tran, S. Clément, R. Vialla, J.-C. Chervin, P. Munsch, A. Polian, Phys. Rev. B. (2011) submitted. [2] L. Huang and J. Kieffer, Phys. Rev. B {\bf 69}, 224203 (2004). [3] Y. Liang, C.~R. Miranda, and S. Scandolo, Phys. Rev. B {\bf 75}, 024205 (2007).

We present a new optical arrangement which allows to avoid the broadening by finite aperture in Brillouin spectroscopy. In this system, all the rays scattered at the same angle by the whole scattering volume are collected on a single pixel of the area detector. This allows to use large collection angles, increasing the luminosity without lowering the accuracy of the frequency-shift and linewidth measurements. Several results of experimental checks are provided, showing the efficiency of the device.

Si la démarche qualité est un concept qui s'est formalisé depuis le début du XXe siècle dans le milieu industriel, son déploiement dans la recherche publique est plus récent. Sa mise en œuvre dans le milieu de la recherche est très particulière comme le démontrent certaines actions menées ces dernières années dans les différents organismes. Elle répond, généralement et à la fois, à des enjeux scientifiques, économiques et financiers... Les spécificités liées à la recherche peuvent constituer un frein à son déploiement mais aussi en être le moteur.

High-resolution Brillouin scattering experiments are performed under hydrostatic pressures ranging from 0 to 10 GPa in vitreous silica. The hypersound-attenuation coefficient exhibits a sharp maximum located at 2 GPa, which appears to coincide with the well-known minimum in sound velocity. Two main processes contribute to sound damping: the thermal relaxation of local defects and the anharmonic interactions with thermal vibrations. By analyzing the temperature dependence of the velocity and subtracting the changes produced by the above mechanisms, the unrelaxed velocity is found. It is constant at low temperatures and increases anomalously above a temperature onset.We show that the onset strongly depends on pressure, which is in agreement with a dynamical origin of the structural changes that produce the hardening.