Atomic hydrogen spin-exchange collisions in a cryogenic maser

The results of an experiment in which the oscillation frequency of a cryogenic hydrogen (H) maser is studied as a function of the H density and the 1420-MHz resonator tuning are presented. The results are interpreted in terms of a theory which predicts that hyperfine interactions between colliding H atoms should include a frequency shift not predicted by classical spin-exchange theories. This effect has the potential to limit the ultimate frequency stability of H masers operating at cryogenic temperatures. The authors found qualitative agreement between the data and the magnitude of the predicted frequency shift. The sign of the effect which is observed however, is unmistakably opposite to that predicted by theory