Nous avons mesuré le temps de vie des phonons thermiques dans la silice vitreuse par spectroscopie Brillouin. Les résultats montrent que, en addition à celle due aux systèmes à deux niveaux, il existe une contribution substantielle due à la diffusion qui est indépendante de la température et probablement d'origine structurale. abstract.-The lifetimes of thermal phonons in fused silica have been measured using Brillouin spectroscopy. The results show that in addition to that due to two level

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... ems there is a substantial contribution to the scattering which is temperature independent and is probably structural in origin. Introduction.-A wave travelling through a structurally disordered material must be attenuated by the disorder. It is difficult in general to calculate the properties of phonons in disordered systems, but it is possible to do so in the long wavelength limit using a Debye approximation and treating the phonons as plane waves. Such a calculation is detailed in reference (1). We will begin by outlining the work reported in (1). It reveals that amorphous materials are unique in that, in addition to the usual scattering which increases in strength as the fourth power of the phenon frequency, the so-called "Rayleigh scattering", there is a contribution that increases linearly with frequency. This latter contribution is unique to strongly scattering systems.