Measurement of the $^3$He mass diffusion coefficient in superfluid $^4$He over the 0.45 – 0.95 K temperature range

We have directly measured the mass diffusion coefficient D of He-3 in super fluid He-4 at temperatures lower than were previously possible. The experimental technique utilizes scintillation light produced when neutrons react with He-3 nuclei, and allows measurement of the He-3 density integrated along the trajectory of a well-defined neutron beam. By measuring the change in He-3 density near a heater as a function of applied heat current, we are able to infer values for D with 20% accuracy. At temperatures below 0.7 K and for concentrations of order 10(-4), we find D = [2.0(-1.)(2.4)] T(-(6.5-/+1.2))cm(2)/s, in agreement with a theoretical approximation.