X-ray-induced thinning of 3He and 3He/4He mixture films

Films of isotopic mixtures of helium have been studied using x-ray specular reflectivity techniques. In contrast with superfluid He-4 films, x-ray exposure causes a reduction in the thickness of He-4 films above the superfluid transition as well as films of pure He-3 and He-3/He-4 mixtures. One proposed model that could account for this effect is a charging model, in which thinning is caused by electrostatic pressure of free charges that accumulate on the helium surface. Unfortunately, this model is not fully consistent with all of the experimental observations. A localized heating model, in which indirect heating of the film causes it to thin would explain the data if there were dissipative film flow in the He-3/He-4 mixtures at temperatures where the bulk is superfluid. We argue that various published experimental results suggest such an effect. In this model, film thinning data for dilute He-3/He-4 films indicates dissipation that is linear in He-3 content of the film over two orders of magnitude.