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For twenty years of research, the processes of radiation and dissipation occurring during oscillations of quartz tuning forks in superfluid helium and its mixtures have turned from an object of research into a tool for studying the properties of helium. Quartz tuning forks are used to study various properties of helium - viscosity, thermal conductivity, radiation of the first and second sounds, and also as a precision temperature sensor. Experimental observations of these phenomena were carried out in a wide range of temperatures and pressures, but the results of observations have not yet been exhaustively described theoretically. The aim of this work is to study density and pressure oscillations to determine the conditions under which oscillations of a solid wall excite the first sound in superfluid helium and sound in supercritical helium, and to calculate the contributions of these processes to the formation of resonances during oscillations of closed tuning forks. In particular, the experimentally observed excitation of standing waves of pressure oscillations by an oscillating closed tuning fork, the appearance and properties of resonances depending on the temperature and pressure of helium are considered. As a result of the work, a model was built that described the physical features of the experimentally observed resonance phenomena.

A convective model describing the nature and structure of the roton is proposed. According to the model, the roton is a cylindrical convective cell with free horizontal boundaries. On the basis of the model, the characteristic geometric dimensions of the roton are estimated, and the spatial distribution of the velocity of the helium atoms and the perturbed temperature inside are described. It is assumed that the spatial distribution of rotons has a horizontally multilayer periodic structure, from which follows the quantization of the energy spectrum of rotons. The noted quantization allows us to adequately describe the energy spectrum of rotons. The convective model is quantitatively confirmed by experimental data on the measurement of the density of the normal component of helium II, the scattering of neutrons and light by helium II. The use of a convective model for describing the scattering of light by helium II made it possible to estimate the dipole moment of the roton, as well as the number of helium atoms participating in the formation of the roton.

Thermal and gas flow calculations of low-temperature part of the UCN source at WWR-M reactor are presented. The calculations were made on the basis of previously developed design documentation. Calculations show that the standard refrigerator Linde TCF-50 is able to maintain the operating temperature of the thermal screens and the condensation of 50 liters of the deuterium premoderapor in the chamber. Calculations of the temperature field of the thermal shield of the low-temperature part of the UCN source were used to select the location of the support structures for a superfluid helium vessel. Based on the above calculations, an estimate of heat inflows at the UCN source chamber was made, which amounted to P = 35 W. The capacity of the vacuum system for pumping helium vapor and helium liquefier is sufficient to maintain 35 liters of the superfluid UCN converter. At present, the UCN source is at the manufacturing stage.