Nuclear-spin waves in polarized atomic hydrogen gas: Temperature and density dependence in the hydrodynamic and Knudsen regimes

Accurate measurements on the nuclear-spin waves observed in the NMR spectrum of spin-polarized atomic hydrogen provide a quantitative test of theory. Results for temperatures between 160 and 537 mK and densities between 7×${10}^{15}$ and 5×${10}^{16}$ ${\mathrm{cm}}^{\mathrm{-}3}$ display the expected magnitude and $T^{\mathrm{-}1/2}$ divergence predicted for the spin-wave quality factor μ. It is shown that spin waves persist at low enough densities (Knudsen regime) that elastic wall collisions are more probable than atom-atom exchange collisions. An anomalous increase in spin-wave widths at low temperatures is explained in terms of the enhanced effects of sticking collisions of the atoms with the walls.