Project of the ultracold and cold neutron source at the WWR-M reactor with superfluid helium as a moderator

The WWR-M reactor of the Petersburg Nuclear Physics Institute provides a unique opportunity for creating conditions of low radiative heat release ($\sim$4 $\times$ 10$^{-3}$ W/g) at a sufficiently high neutron flux ($\sim$3 $\times$ 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 $\sim$10$^4$ neutrons/cm$^3$, which is higher than that of existing sources of ultracold neutrons by two to three orders of magnitude.