Exchange collisions between ground-state hydrogen atoms are considered. The density matrix after collision is calculated for pairs of atoms which initially had the same density matrix. The result is applied to the hydrogen maser with the assumption that only exchange collisions and the escape of atoms from the storage bulb influence the linewidth for the field-independent hyperfine transition. Under normal operating conditions a frequency shift of roughly 5% of the exchange collision contribution to the linewidth is predicted.

• Received 23 July 1963

DOI:https://doi.org/10.1103/PhysRev.132.2154