Calculation of the ultracold neutron upscattering loss probability in fluid walled storage bottles using experimental measurements of the liquid thermomechanical properties of fomblin

Presently, the most accurate values of the free neutron beta-decay lifetime result from measurements using fluid-coated ultracold neutron (UCN) storage bottles. The purpose of this work is to investigate the temperature-dependent UCN loss rate from these storage systems. To verify that the surface properties of fomblin films are the same as the bulk properties, we present experimental measurements of the properties of a liquid 'fomblin' surface obtained by the quasielastic scattering of laser light. The properties include the surface tension and viscosity as functions of temperature. The results are compared to measurements of the bulk fluid properties. We then calculate the upscattering rate of UCNs from thermally excited surface capillary waves on the liquid surface and compare the results to experimental measurements of the UCN lifetime in fomblin-fluid-walled UCN storage bottles, and show that the excess storage loss rate for UCN energies near the fomblin potential can be explained. The rapid temperature dependence of the fomblin storage lifetime is explained by our analysis