Influence of the Atmosphere on a Rubidium Clock’s Frequency Aging

A number of mechanisms have been proposed to explain the phenomenon of frequency aging in the rubidium atomic clock. Helium permeation is one such mechanism. Briefly, the four millitorr of helium in the Earth’s atmosphere can permeate into the resonance cell, changing the clock frequency via the pressure shift of the 0-0 hyperfine transition. On orbit, any He in the resonance cell would permeate out, again changing the buffer gas pressure in the resonance cell. Unfortunately, studies examining this hypothesis have not been particularly clear cut. Here, we report on a multi-year study comparing the frequency aging rates of three Rb clocks in vacuum and in air. Our findings indicate that the atmosphere does play a role in frequency aging, at least for one family of Rb clocks, and adds evidence to the helium permeation hypothesis. However, when combined with on-orbit data, it appears that frequency aging in vapor-cell clocks is likely driven by more than one mechanism.