Neutrons on a surface of liquid helium

TL;DR

This paper explores the potential of using ultracold neutrons stored in quantum states above liquid helium for high-precision gravity measurements and neutron experiments, analyzing stability and scattering effects.

Contribution

It provides a detailed analysis of UCN stability and scattering mechanisms near liquid helium, proposing new experimental methods for neutron trapping and level population.

Findings

Total scattering time exceeds one hour at 0.7 K

Scattering rates decrease rapidly with temperature

Potential for high-precision gravity and neutron decay experiments

Abstract

We investigate the possibility of ultracold neutron (UCN) storage in quantum states defined by the combined potentials of the Earth's gravity and the neutron optical repulsion by a horizontal surface of liquid helium. We analyse the stability of the lowest quantum state, which is most susceptible to perturbations due to surface excitations, against scattering by helium atoms in the vapor and by excitations of the liquid, comprised of ripplons, phonons and surfons. This is an unusual scattering problem since the kinetic energy of the neutron parallel to the surface may be much greater than the binding energies perpendicular. The total scattering time constant of these UCNs at 0.7 K is found to exceed one hour, and rapidly increasing with decreasing temperature. Such low scattering rates should enable high-precision measurements of the scheme of discrete energy levels, thus providing improved access to short-range gravity. The system might also be useful for neutron beta-decay experiments. We also sketch new experimental concepts for level population and trapping of UCNs above a flat horizontal mirror.