1974/06/01 by Roland Schotter, R. Schotter · 1 citation
Physics and Astronomy · #Atomic and Subatomic Physics Research #Quantum, superfluid, helium dynamics #Cold Atom Physics and Bose-Einstein Condensates
paper · doi:10.1063/1.1694859
The propagation of plane pressure waves in rarefied He, Ne, Ar, and Kr has been investigated for frequencies up to 80 times the collision rate (corresponding to pressures of roughly 0.003 Torr at the experimental frequency of 0.975 MHz) along distances extending four “mean wavelengths” 2πcm/ω into the interior of the gas. This large range was obtained by the use of high Q barium titanate sandwich transducers and narrow-band phase-sensitive signal detection. For small transmitter-receiver distances standing waves were observed at all pressures, i.e., even for very long mean free paths, as the result of incomplete accommodation of the atoms at the transducer surfaces. For large distances and small pressures the experiment resembles the free molecule propagation characteristics. While increasing the pressure, the influence of binary collisions is first recognized in the attenuation, which increases. Only for higher pressures does the phase velocity change toward lower values. Apart from the standing wave region the experiment fits the theory of J. K. Buckner and J. H. Ferziger [Phys. Fluids 9, 2315 (1966)] in an excellent way.