Brillouin scattering measurements of the velocity and attenuation of high frequency sound waves in superfluid helium

We have measured, from the Brillouin spectrum of scattered laser light, the velocity and attenuation of thermally driven high frequency first sound in liquid helium under its saturated vapor pressure between 1.25°K and the lambda point. The sound wavevector was fixed at 2.0339 × 10 ⁵ cm ^-1; the frequency varied from 768 MHz at our lowest temperature to 709 MHz near Tλ. We find no dispersion in the sound velocity except quite near Tλ. The attenuation shows a sharp rise near Tλ and, for lower temperatures, a definite peak whose maximum lies between 1.4 and 1.5°K. We compare our results with the theory of Landau and Khalatnikov near Tλ and with the theory of Khalatnikov and Chernikova at lower temperatures.