Sound Velocity, Phase Separation, and Lambda Transitions of He3–He4 Mixtures

Abstract

The velocity of first sound has been measured in saturated liquid He3–He4 mixtures ranging in composition from 20 to 90% He3 between 0.5° and 2.3°K. The velocities decrease with increasing He3 concentration due to the large increase in compressibility. The temperature Ts at which a liquid mixture first begins to separate into two layers of different composition is identified by a break in the temperature derivative of the sound velocity. The values of Ts are compared with and found to agree well with all other recent direct measurements. Breaks also occur in the temperature derivatives of first sound velocity and signal amplitude at temperatures which are in good agreement with most recent determinations of lambda transition temperatures for concentrations up to 30%. For higher concentrations, the values obtained are in good agreement with λ points determined from second sound measurements but differ markedly from results based on the disappearance of boiling. Possible reasons for the differences are discussed. A smoothed table of λ temperature as a function of concentration is given up to the intersection of the λ line and the phase separation region at 68% He3 and 0.86°K. The vapor pressure of pure He4 at its lambda point is 37.80 ± 0.01 mm Hg (at 0°C and standard gravity) based on the measured vapor pressure at the minimum sound signal and a calculated displacement of the sound minimum from the lambda point.