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

摘要

Ultracold neutrons (UCNs) were produced in a 4 liter volume of superfluidhelium using the PF1B cold neutron beam facility at the Institut Laue-Langevinand then extracted to a detector at room temperature. With a convertertemperature of 1.08 K the number of accumulated UCNs was counted to be$91,\!700 \pm 300$. From this, we derive a volumetric UCN production rate of$(6.9 \pm 1.7)\,\mathrm{cm^{-3}\,s^{-1}}$, which includes a correction forlosses in the converter during UCN extraction caused by a short storage time,but not accounting for UCN transport and detection efficiencies. Theup-scattering rate of UCNs due to excitations in the superfluid was studied byscanning the temperature between 1.2-2.4 K. Using the temperature-dependent UCNproduction rate calculated from inelastic neutron scattering data in theanalysis, the only UCN up-scattering process found to be present was fromtwo-phonon scattering. Our analysis rules out contributions from the otherscattering processes to $\lesssim 10\%$ of their predicted levels.