UCN production by multiphonon processes in superfluid Helium under pressure

TL;DR

This paper investigates how pressure affects ultra-cold neutron production in superfluid helium via multiphonon processes, providing a model to predict production rates under different pressures, and concludes that pressure is not advantageous for UCN production.

Contribution

It introduces a pressure-dependent model for multiphonon scattering in superfluid helium, enabling accurate UCN production rate calculations under various pressures.

Findings

UCN production decreases with increased pressure.

Pressure dependence of the scattering function significantly impacts UCN yield.

Pressure is not beneficial for optimizing UCN production.

Abstract

Cold neutrons are converted to ultra-cold neutrons (UCN) by the excitation of a single phonon or multiphonons in superfluid helium. The dynamic scattering function S(q, omega) of the superfluid helium strongly depends on pressure, leading to a pressure- dependent differential UCN production rate. A phenomenological expression for the multiphonon part of the scattering function s(lambda) describing UCN production has been derived from inelastic neutron scattering data. When combined with the production rate from single phonon processes this allows us to calculate the UCN production for any incident neutron flux. For calculations of the UCN production from single phonon processes we propose to use the values for S*(SVP) = 0.118(8) and S*(20 bar) = 0.066(6). As an example we will calculate the expected UCN production rate at the cold neutron beam for fundamental physics PF1b at the Institut Laue Langevin. We conclude that UCN production in superfluid helium under pressure is not attractive.