Ultracold-neutron production and up-scattering in superfluid helium between 1.1 K and 2.4 K

Abstract

Ultracold neutrons (UCNs) were produced in superfluid helium using the PF1B cold-neutron beam facility at the Institut Laue-Langevin. A 4-liter beryllium-coated converter volume with a mechanical valve and windowless stainless-steel extraction system were used to accumulate and guide UCNs to a detector at room temperature. At a converter temperature of 1.08 K the total storage time constant in the vessel was $(20.3\ifmmode\pm\else\textpm\fi{}1.2)\phantom{\rule{0.28em}{0ex}}\mathrm{s}$ and the number of UCNs counted after accumulated was $91\phantom{\rule{0.16em}{0ex}}700\ifmmode\pm\else\textpm\fi{}300$. From this, we derive a volumetric UCN production rate of $(6.9\ifmmode\pm\else\textpm\fi{}1.7)\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, which includes a correction for losses in the converter during UCN extraction caused by the short storage time, but not accounting for UCN transport and detection efficiencies. The up-scattering rate of UCNs caused by excitations in the superfluid was studied by scanning the temperature between $1.2$ K and $2.4\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Using the temperature-dependent UCN production rate calculated from inelastic neutron scattering data, the only UCN up-scattering process found to occur was from two-phonon scattering. Our analysis for $T<1.95\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ rules out the contributions from roton-phonon scattering to $<29%$ (95% C.I.) and from one-phonon absorption to $<47%$ (95% C.I.) of their predicted levels.