Production and accumulation of isotopically pure helium-4 for ultracold neutron source

A high-intensity ultracold neutron source (UCN) is being created at the NRC “Kurchatov Institute” – PNPI. It is going to be placed at the PIK reactor, for scientific research in the field of physics of fundamental interactions. The UCN source will use superfluid helium, which will make it possible to achieve a UCN density of 2.2 $\cdot$ 10$^3$ cm$^{-3}$. To achieve these parameters, the commercial helium used as a converter has been purified from the helium-3 isotope by 3 orders of magnitude. Due to this it became possible to increase the neutron storage time in helium from 3.1 to 38.6 s. The helium was purified using a superleak filter made of pressed Al$_2$O$_3$ powder. As a result of this work, 43 m$^3$ of isotopically pure $^4$He was produced and stored.