  We have 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 neutron 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 of 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^-(6.5 -/+ 1.2) cm^2/s, in agreement with a
theoretical approximation.
