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_-1.2^+2.4] T-(6.5∓1.2) cm²/s, in agreement with a theoretical approximation.