Neutrons on a surface of liquid helium

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 β-decay experiments. We also sketch new experimental concepts for level population and trapping of UCNs above a flat horizontal mirror.

Helium film may greatly increase the storage time of ultracold neutrons in material traps

“…However, the amplitude of neutron-ripplon scattering is small even from the lowest neutron energy level, so that the corresponding scattering time 1/w R exceeds many hours at T < 0.5K. [44] Our preliminary results show that the amplitude of neutron-ripplon scattering from higher levels is even smaller. Moreover, the linear temperature dependence of neutron-ripplon scattering rate w R (T ) allows its effective extrapolation to zero temperature.…”

“…Liquid He introduces new scattering mechanism of UCN: the He vapor atoms and the soft thermal excitations -the quanta of surface waves, called ripplons. The corresponding scattering rates were recently calculated for a neutron on the lowest vertical level, [44] and these estimates are qualitatively valid for higher levels. The energy of 4 He evaporation is 7.17K per atom.…”

“…The ripplons, the quanta of surface waves, are gapless excitations with soft spectrum: ω 2 q = (σ He /ρ He ) q 2 + a −2 H q. The neutron scattering rate w R on ripplons depends linearly on temperature [44] and, formally, cannot be discarded even below 0.5K. Luckily, the amplitude of neutron-ripplon interaction is small, and the corresponding neutron scattering time 1/w R exceeds many hours at T < 0.5K.…”

“…The corresponding scattering rates for a neutron on the lowest vertical level were studied recently. [44] The concentration of 4 He vapor exponentially decreases with temperature, n V ∝ exp (−7.17/T [K]), and can be disregarded below 0.5K. However, the neutron scattering rate w R on ripplons depends linearly on temperature [44] and, formally, cannot be discarded even below 0.5K.…”

“…[44] The concentration of 4 He vapor exponentially decreases with temperature, n V ∝ exp (−7.17/T [K]), and can be disregarded below 0.5K. However, the neutron scattering rate w R on ripplons depends linearly on temperature [44] and, formally, cannot be discarded even below 0.5K. However, the amplitude of neutron-ripplon scattering is small even from the lowest neutron energy level, so that the corresponding scattering time 1/w R exceeds many hours at T < 0.5K.…”