One aspect of the topic of "Compact, Highly Stable Ion Clock" (NPO-43075), NASA Tech Briefs, Vol. 32, No. 5 (May 2008), page 63, is examined in more detail. To recapitulate: A developmental miniature mercury-ion clock has stability comparable to that of a hydrogen-maser clock. The ion-handling components are housed in a sealed vacuum tube, wherein a getter pump is used to maintain the partial vacuum, and the evacuated tube is backfilled with mercury vapor in a buffer gas.
The development has included a study of gas-induced shifts of the clock frequency and of alternatives to the traditional use of helium as the buffer gas. The frequency-shifting effects of three inert gases (helium, neon, and argon) and three getterable gases (hydrogen, nitrogen, and methane) were measured. Neon was determined to be the best choice for the buffer gas: The pressure-induced frequency pulling by neon was found to be only about two- fifths of that of helium. Furthermore, because neon diffuses through solids much more slowly than does helium, the operational lifetime of a tube backfilled with neon could be considerably longer than that of a tube backfilled with helium.