Spin Diffusion in Gases at Low Temperatures

It is shown that the first approximation to the spin-diffusion coefficient $D$ of a gas at low temperatures involves a scattering cross section for distinguishable particles only, so that $D$ is different from the self-diffusion coefficient $D_0$. Quantum symmetry effects show up in the second approximation to $D$ but the correction to the first approximation is small. The theoretical values of $D$ for gaseous hydrogen and gaseous ${\mathrm{He}}^3$ agree quite well with experimental results.