Evolution of the fundamental mode of acoustic oscillations in a cavity filled with gaseous helium has been studied during immersion of this resonator into a helium transport Dewar vessel. Critical behavior of the parameter of oscillation decay has been observed on approaching the level below which thermoacoustic instability arises.

Исследована эволюция основной моды акустических колебаний в резонаторе, заполненном газообразным гелием, при погружении его в транспортный дьюар. Обнаружено критическое поведение параметра затухания при приближении к уровню, ниже которого возникает термоакустическая неустойчивость.

The WWR-M reactor at PNPI is going to be equipped with an ultacold neutron source of high density. Method of UCN production is based on their accumulation in the super fluid helium due to particular qualities of that quantum liquid. The possibility of maintaining the temperature T = 1.371K with a thermal load of P = 60W was shown experimentally, while the theoretical load is expected to be P=30W. The project envisages the installation on UCN beams the experimental setups of various research projects such as searching for the nEDM, measure the neutron lifetime, and the observation of neutron to antineutron oscillation. In addition to UCN beams, three beams of cold and verycold neutrons are planned. Six experimental setups will be installed on these beams. At present, a vacuum container of the UCN source has been manufactured and the manufacture of low-temperature deuterium and helium parts of the source has been started.

A project of the source of ultracold neutrons for the WWR-M reactor based on superfluid helium for ultracold-neutron production has been developed. The full-scale source model, including all required cryogenic and vacuum equipment, the cryostat, and the ultracold-neutron source model has been created. The superfluid helium temperature T = 1.08 K without a heat load and T = 1.371 K with a heat load on the simulator of P = 60 W has been achieved in experiments at a technological complex of the ultracold-neutron source. The result proves the feasibility of implementing the ultracold-neutron source at the WWR-M reactor and the possibility of applying superfluid helium in nuclear engineering.

Запуск полномасштабной модели источника ультрахолодных нейтронов со сверхтекучим гелием© А

The evolution of the meniscus of a helium crystal near the (0001) face is traced during a change in the boundary conditions at the chamber wall in the temperature range 0.5–0.9 K. The critical behavior of the contact angle is studied. An anisotropy is detected in the crystal-glass interface energy. New data on the temperature dependence of the elementary-step energy are obtained.

The WWR-M reactor of the Petersburg Nuclear Physics Institute provides a unique opportunity for creating conditions of low radiative heat release (∼4 × 10 −3 W/g) at a sufficiently high neutron flux (∼3 × 10 12 neutrons/(cm 2 s)). This opportunity can be implemented in the reactor thermal column, which represents a 1-m-diameter channel adjacent to the reactor core. This diameter of the channel allows the arrangement of the core gamma shielding made of bismuth (15 cm thick), a graphite premoderator (300 dm 3 ) at a temperature of 20 K, and a converter with superfluid helium (35 dm 3 ) at a temperature of 1.2 K. Calculations show that the heat release in the source (20 W) can be removed by pumping helium vapor, and the density of ultracold neutrons in an experimental trap will be ∼10 4 neutrons/cm 3 , which is higher than that of existing sources of ultracold neutrons by two to three orders of magnitude.

The equilibrium shape of the interface between superfluid and crystalline 4 He near the (0001) orientation is analyzed. A feature is observed in the edge angle as a function of the wall inclination with respect to the gravitational field. The step energy on the basal plane of the crystal is measured.

Equilibrium shape of the interface between superfluid and crystalline 4 He near the (0001) orientation is analyzed. We observe a singular dependence of the contact angle on the wall inclination with respect to the gravity. The energy of a step on the basal plane is measured. From the analysis of the meniscus profile the step-step interaction constant is estimated.

: We have studied the nucleation of bubbles in pure superfluid helium-4 at temperatures down to 65 mK. We have found that the nucleation is a stochastic process, and that at temperatures below 600 mK the nucleation rate is independent of temperature. These results are consistent with the assumption that the nucleation takes place via quantum tunneling.

A strong anisotropy of the positive-charge mobility is observed in hcp 4He crystals. The mobility activation energies in the principal directions are determined.