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The paper presents a neutron thermalization unit, “a source of ultracold neutrons”, designed for basic research in a beam research hull reactor. For new-generation experiments in the fields of weak-interaction physics and astrophysics, statistical accuracy associated with high density of ultracold neutrons is necessary. To achieve high density for helium-4 in the source chamber, which is used as a converter of cold neutrons into the ultracold ones, it should be at the temperature of about 1 K. In case of applying vacuum pumping of helium-4 vapors in ultracold neutron sources, it has not yet succeeded to obtain a temperature below 1.4 K. To achieve lower temperatures, the required saturated vapor pressure should be less than 50 Pa, which is impossible due to hydraulic losses. It is proposed to use a heat exchanger where helium-4 will be cooled by helium-3. The reason is that the temperature of helium-3 is more efficiently maintained by vacuum pumping since its saturated vapor pressure is an order of magnitude higher than that of helium-4. However, between two heliums the temperature drop occurs due to Kapitsa jump and thermal bridge between the helium capsule and heat exchanger. To solve this problem, we proposed optimization using numerical simulation on the basis of a mathematical model of thermal processes in a chamber with superfluid helium. The model takes into account the contact thermal resistance of Khalatnikov acoustic mismatch model with a correction coefficient. An example of such optimization is presented for the ultracold neutron source located in Gatchina. The mathematical model was implemented in the general solver based on the finite element method. A heat exchanger design geometry was proposed with the temperature drop equal to 0.2 K; the temperature of helium-4 was achieved by vacuum pumping of helium-3 vapors at the pressure of 850 Pa. The temperature fall from 1.4 K to 1 K will increase the density of ultracold neutrons by almost an order of magnitude, and increase statistical accuracy of experiments with ultracold neutrons carried out in a non-beam research reactor.

International audience; Currently the most accurate measurements of neutron lifetime are being performed at the Petersburg Nuclear Physics Institute with ultracold neutrons (UCNs) stored in a gravitational trap. A modified setup with a large gravitational trap and cooling to 10–15 K is presented. The results of measurements of temperature dependence of UCN losses in collisions with walls, which were coated with a perfluorinated grease (Fomblin UT 18), at 300–77 K, are detailed. The probable heat inflow to the trap is estimated, and the feasibility of cooling to indicated temperatures in experiments with the modified setup is demonstrated.

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.