Scattering of ultracold neutrons on a surface of liquid helium
Pavel D. Grigoriev

Senior researcher of the L.D. Landau Institute for Theoretical Physics of Russian Academy of Sciences

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 analyze the stability of the lowest quantum states, 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. There are two main scattering mechanism of UCN scattering on helium surface: by He vapor atoms and by ripplons, the quanta of surface waves. The UCN scattering by He vapor dominates at high temperature T>0.65K but falls down exponentially fast as temperature decreases. The second scattering mechanism by ripplons falls down linearly with and dominates at low temperature. The total scattering time of these UCNs at 0.7 K is found to exceed 1 hour, and rapidly increases with decreasing temperature. Such low scattering rates should enable high-precision measurements of the sequence of discrete energy levels, thus providing improved tests of short-range gravity. The system might also be useful for neutron β-decay experiments.
Publications: P.D. Grigoriev, O. Zimmer, A.D. Grigoriev, T. Ziman, Neutrons on a surface of liquid helium, Phys. Rev. C 94, 025504 (2016); arXiv:1509.06343.