A Novel Technique for Blackbody Shift Evaluation in Cs With a Fountain Standard

The uncertainty in the evaluation of the frequency shift caused in the Cs hyperfine transition by the alternating current Stark effect due to blackbody radiation is presently the dominant term in the accuracy budget of Cs fountain frequency standards. The bias is calculated with a formula wherein it is proportional to a coefficient beta. The latter must be known to the nearest 1% if the accuracy goal is in the low 10 ^-16. This, in turn, has required 10^-16 uncertainty in frequency measurements used to evaluate beta. The novel technique discussed here relaxes this latter requirement by shifting the burden to the short-term stability of the experiment, hence allowing that kind of accuracy on beta even if the accuracy of the standard is lower. The method is based on fast temperature cycling of the drift tube while keeping everything else at a constant temperature. In this way, all frequency biases, other than the one under study, are rejected as common mode in a differential measurement, and the uncertainty on the relative shift can be better than the accuracy of the frequency standard