Experimental study of ultracold neutron production in pressurized superfluid helium

TL;DR

This study experimentally examined how pressure affects ultracold neutron production in superfluid helium, confirming pressure dependence for single-phonon processes but not for multiphonon processes, and found no overall increase in UCN production with pressure.

Contribution

It provides the first experimental verification of pressure effects on UCN production, distinguishing between single-phonon and multiphonon contributions in superfluid helium.

Findings

Pressure dependence confirmed for single-phonon UCN production.

No significant pressure effect observed for multiphonon UCN production.

Applying pressure does not increase overall UCN production rate.

Abstract

We have investigated experimentally the pressure dependence of the production of ultracold neutrons (UCN) in superfluid helium in the range from saturated vapor pressure to 20bar. A neutron velocity selector allowed the separation of underlying single-phonon and multiphonon pro- cesses by varying the incident cold neutron (CN) wavelength in the range from 3.5 to 10{\AA}. The predicted pressure dependence of UCN production derived from inelastic neutron scattering data was confirmed for the single-phonon excitation. For multiphonon based UCN production we found no significant dependence on pressure whereas calculations from inelastic neutron scattering data predict an increase of 43(6)% at 20bar relative to saturated vapor pressure. From our data we conclude that applying pressure to superfluid helium does not increase the overall UCN production rate at a typical CN guide.