Ultrasonic Velocity and Dispersion in Liquid Helium II from 0.15 to 1.8°K

From the measurement of changes in the delay experienced by an ultrasonic pulse traversing a fixed path, the temperature dependence of the velocity of sound in liquid helium under its saturated vapor pressure has been determined from 0.15 to 1.8\ifmmode^\circ\else\textdegree\fi{}K at the frequencies 1.00, 3.91, and 11.9 Mc/sec. A phase-comparison technique made it possible to resolve velocity changes amounting to a few mm/sec. With increasing temperature, the velocity ${u}_{1}$ increases above its asymptotic limit ${u}_{10}$ at 0\ifmmode^\circ\else\textdegree\fi{}K, passes through a maximum near 0.7\ifmmode^\circ\else\textdegree\fi{}K, and decreases rapidly at higher temperatures. The velocity difference over any temperature interval that extends from below 0.2 to above 1.1\ifmmode^\circ\else\textdegree\fi{}K is independent of frequency, but within the temperature range indicated ${u}_{1}$ increases with frequency and the position of its maximum shifts to higher temperatures. At (1.00, 3.91, 11.9) Mc/sec the maximum velocity lies (5, 7, 14) cm/sec above ${u}_{10}$ and occurs at (0.65, 0.70, 0.72)\ifmmode^\circ\else\textdegree\fi{}K. The dispersion is greatest in the neighborhood of 0.9\ifmmode^\circ\else\textdegree\fi{}K, where the attenuation coefficient goes through a maximum. Within experimental error, the theory of Khalatnikov and Chernikova accounts for the separation between the velocity-versus-temperature curves at the three frequencies. Combination of the velocity changes measured in these experiments with values of the absolute velocity obtained by other workers above 1\ifmmode^\circ\else\textdegree\fi{}K yields ${u}_{10}={238.3}_{0}\ifmmode\pm\else\textpm\fi{}{0.1}_{3}$ m/sec.

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