Mechanisms of dissipation of an oscillating quartz tuning fork immersed in He II at high pressures

The dissipative processes that occur with immersing a vibrating tuning fork in superfluid helium are investigated. The tuning forks resonance width Δf of frequencies from 32 to 97 kHz was measured in the temperature range from 0.2 to 2.5 K and He II pressure from SVP to 24.9 atm. Some of the tuning forks were in the original can (closed tuning fork), and for some tuning forks the can was either completely or partially removed (opened fork). We found that for the open tuning forks two dissipation mechanisms are clearly revealed in the temperature dependence of Δf, namely, acoustic radiation and scattering of ballistic thermal excitations at low temperatures, and viscous friction at high temperatures. At low temperatures (below ∼ 0.8 K) acoustic dissipation dominates, and the model of quadrupole oscillator for a tuning fork can be applied. We found that acoustic radiation for closed tuning forks is less effective and appears at lower temperatures. The first experimental data on dissipative processes in the quartz tuning fork–He II system at increased liquid pressures are obtained. It is shown that, for high frequency tuning forks the resonance bandwidth decreases with increasing pressure, i.e., with increasing wavelength of sound λ, according to the law λ⁻⁵. At low frequencies and low temperatures, with increasing mean free path of thermal excitations the resonance bandwidth is well described by the model of ballistic scattering.