For the production of high-density ultracold neutrons (UCNs), we placed 0.8 K superfluid helium in a cold neutron moderator. We resolved previous heat-load problems in the spallation neutron source that were particularly serious below 1 K. With a proton-beam power of $400\mathrm{MeV}\times 1\mu \mathrm{A}$, a UCN production rate of $4\mathrm{UCN}{\mathrm{cm}}^{-3}{\mathrm{s}}^{-1}$ at the maximum UCN energy of $E_c=210\mathrm{neV}$ and a storage lifetime of 81 s were obtained. A cryogenic test showed that the production rate can be increased by a factor of 10 with the same storage lifetime by increasing the proton-beam power as well as ${}^3\mathrm{He}$ pumping speed.

• Received 27 July 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.134801