NeutronRadiographyofHeliumII

We have used a neutron radiography technique to investigate the spatial distribution of 3 He atoms in very dilute liquid 3 He- 4 He mixtures at temperatures below the superfluid transition temperature. By imposing heat currents and monitoring the subsequent redistribution of 3 He within the sample volume we obtain a direct measurement of the relevant mass diffusion coefficient. Data from these experiments provides a striking demonstration of the manner in which the distribution of impurity atoms within the liquid can be influenced by the presence of a free surface.