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Journal of Experimental and Theoretical Physics Letters

Quasielastic neutron scattering by diffusive adsorbed hydrogen as a possible cause of the energy spreading of ultracold neutrons during long storage in traps

Yu. N. Pokotilovskiǐ

Published Jan 1, 1999• 3 citations
Publisher

Abstract

Small cooling and heating of ultracold neutrons during long storage in traps have been observed in recent experiments. It is shown that quasielastic neutron scattering due to the diffusive motion of hydrogen at the surface of adsorbed hydrogenous contaminants on the surface may be a possible cause of the spreading of the energy of ultracold neutrons during long storage in traps.

DOI
10.1134/1.567989

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arXiv:0903.0458•Mar 3, 2009•0 citations
Small cooling and heating of ultracold neutrons during long storage in traps have been observed in recent experiments. It is shown that quasielastic neutron scattering due to the diffusive motion of hydrogen at the surface of adsorbed hydrogenous contaminants on the surface may be a possible cause of the spreading of the energy of ultracold neutrons during long storage in traps.
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 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 of these UCNs at 0.70.70.7 K is found to exceed 1 h, 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 $\ensuremath{\beta}$-decay experiments. We also sketch new experimental propositions for level population and trapping of ultracold neutrons above a flat horizontal mirror.
The capture of trapped ultracold neutrons (UCNs) by closed hydrogenous impurities within a solid coating of the trap is discussed as a possible cause of observed anomalously large losses of UCNs in solid material UCN traps. Then significant losses of UCNs arise only if resonances occur in the UCN-impurity scattering amplitude. For a large size impurity, higher partial waves in the UCN-impurity interaction are important, and they are taken into account in the present paper. The method of the calculation is applicable to irregular shape impurities as well. A small distortion of an impurity shape, if it splits the resonance, can increase the UCN losses by a few times. UCN losses in the beryllium trap are calculated assuming they are due to the UCN capture by ice spherical impurities within the coating of the trap walls. Both s- and p-wave resonances contribute significantly to the UCN losses considered. As an example, observed anomalous large UCN losses are achieved if the average radius of the impurity is about 600 Angstroms and the impurity density is about 3*10^{14}/cm^3. A distortion of the spherical shape of the impurity could increase the UCN losses and therefore decrease the impurity density.