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

We have directly measured the mass diffusion coefficient D of 3He in superfluid 4He at temperatures lower than were previously possible. The experimental technique utilizes scintillation light produced when neutrons react with 3He nuclei, and allows measurement of the 3He density integrated along the trajectory of a well-defined neutron beam. By measuring the change in 3He 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+2.4−1.2]T−(6.51.2) cm2/s, in agreement with a theoretical approximation.