Theory of Inelastic Neutron Scattering from SuperfluidHe4with a Free Surface

It is shown that one can take advantage of total neutron reflection at a free surface to eliminate volume-proportional terms in inelastic neutron scattering. When this is done, the neutron inelastic-scattering cross section for superfluid ${\mathrm{He}}^4$ with a free surface is found, within an ideal-fluid model, to contain only ripplon peaks distinctly separated from a smooth background due to nonresonant single-phonon processes occurring near the surface of the liquid. Some numerical results are given for the predicted cross section.