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

TL;DR

This study measures ultracold neutron production and up-scattering in superfluid helium between 1.1 K and 2.4 K, identifying two-phonon scattering as the dominant up-scattering process and quantifying UCN production rates.

Contribution

First direct measurement of UCN production rate and up-scattering processes in superfluid helium across a range of temperatures, confirming two-phonon scattering as the main process.

Findings

UCN production rate of (6.9 ± 1.7) cm^{-3} s^{-1} at 1.08 K

Up-scattering dominated by two-phonon process, with other processes contributing less than 10%

Number of UCNs accumulated was 91,700 ± 300 at 1.08 K

Abstract

Ultracold neutrons (UCNs) were produced in a 4 liter volume of superfluid helium using the PF1B cold neutron beam facility at the Institut Laue-Langevin and then extracted to a detector at room temperature. With a converter temperature 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 for losses in the converter during UCN extraction caused by a short storage time, but not accounting for UCN transport and detection efficiencies. The up-scattering rate of UCNs due to excitations in the superfluid was studied by scanning the temperature between 1.2-2.4 K. Using the temperature-dependent UCN production rate calculated from inelastic neutron scattering data in the analysis, the only UCN up-scattering process found to be present was from two-phonon scattering. Our analysis rules out contributions from the other scattering processes to $\lesssim 10\%$ of their predicted levels.