Spin-diffusion coefficient in gaseous helium between 1 and 0.5 K

We report on measurements of the nuclear-spin diffusion coefficient in gaseous helium below 1 K. Both D₃₃ (self-diffusion in ³He) and D₃₄ (diffusion of ³He in ⁴He) are obtained from T₂ measurements in applied magnetic gradients. We operate at low densities, where transport properties are governed by two-body collisions. Predictions from various helium-helium potentials have been computed and are compared with the experimental results. A rough agreement is obtained, although theoretical values for D₃₃ seem somewhat too small. Relative variations with temperature are well described by theory between 4.2 and 0.5 K (within 10%).