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

K. K.H. Leung, S. Ivanov, F. M. Piegsa, M. Simson, O. Zimmer

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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±1.2)s and the number of UCNs counted after accumulated was 91700±300. From this, we derive a volumetric UCN production rate of (6.9±1.7)cm-3s-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 2.4K. 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<1.95K 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.

Original languageEnglish
Article number025501
JournalPhysical Review C
Issue number2
StatePublished - 4 Feb 2016


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