Long - Term Behaviour of Cavity Phase Difference in NBS - 6 *

A critical review of all beam-reversal phase-shift measurements carried out on NBS-6 (NBS-5) since 1973 is presented. The analysis takes into account a recent study of frequency shifts in atomic-beam frequency standards resulting from overlapping tails of adjacent lines. A strongly correlated variation of the cavity phase difference with time is shown, which appears to fit nicely on an exponential decay curve with a time constant of 3.7 years. The suggestion is made that some mechanical relaxation in the microwave structure may be responsible for this phenomenon. Furthermore, the line-pulling model seems to suggest a better explanation for observations previously attributed to distributed cavity phase shift. Verification of these interpretations through more involved evaluations may allow substantial reductions in the error budget for the standard.