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 710^15 and 510^16 cm^-3 display the expected magnitude and T^-1/2 divergence predicted for the spin-wave quality factor. 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.