Dissipation mechanisms of a tuning fork oscillations in superfluid solutions of <sup>3</sup>Не-<sup>4</sup>Не
In the temperature range, 0.1-2.5 K, the amplitude-frequency characteristics of tuning forks immersed in superfluid <sup>3</sup>Не-<sup>4</sup>Не solutions were measured. The resonant frequency of the tuning fork and the width of the resonance were measured as a function of temperature at a solution concentration of 5% and 15% <sup>3</sup>Не, and also for comparison in pure <sup>4</sup>Не. The obtained experimental data for the main mechanisms of dissipation - viscous friction, radiation tuning fork of the first and second sounds were analyzed. For the separation and evaluation of the contributions of viscous attenuation and radiation of the first and second sound, “closed” (in the factory capsule) and “open” (without the capsule) tuning forks were used. It was established that the resonance width, which is a measure of dissipation of oscillating tuning forks, is larger in solutions than in pure <sup>4</sup>Не, and increases with increasing concentration of <sup>3</sup>Не. It is shown that the available analytical expression for the contribution of viscous friction well describes the experimental data for <sup>4</sup>Не only in the hydrodynamic region. For solutions, the viscosity contribution agrees with the experiment only at high temperatures (above 1.4 K). For an “open” tuning fork, the contribution of the radiation of the first sound is consistent with the calculation only for <sup>4</sup>Не, and in the case of solutions, the calculation gives underestimated values compared with the experiment. Experiments with a “closed” tuning fork allowed us to estimate the possible contribution of second-sound radiation to the dissipation of the tuning fork in the solution. It turned out that this contribution has a non-monotonic dependence with a maximum at temperatures of 0.6-0.8 K. The calculation of the ratio of energy loss by a tuning fork due to a thermal diffusion wave to loss due to the radiation flux of a second sound wave in super fluid solutions conducted on the basis of literature data, gave an estimate with an accuracy of 10<sup>-3</sup>-10<sup>-4</sup>.