Abstract
The frequency shift in cesium beam atomic frequency standards resulting from the second order doppler shift is described in terms of a truncated Maxwellian velocity distribution in the atomic beam. A means of establishing the upper and lower limits of the atomic velocities accepted by the standard, based on fitting of experimental and calculated Ramsey resonance patterns, is outlined. A detailed analysis of the influence of both the velocity distribution and the exciting power level is given, and it is shown that an asymmetric resonance results, with asymmetry greatest for a full Maxwellian distribution. Results are given for the NRC 2.1 m cesium standard, Cs III. An increase of the second order doppler shift correction for Cs III from 0.003 Hz to 0.0072 Hz is required, and the effect of this on the NRC UTA time scale is discussed.
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