Blackbody radiation shift in primary frequency standards

The principal coefficient k_omicron for the blackbody radiation shift in primary frequency standards has been subject to discussion. We present extensive measurements of this shift performed on the SYRTE´s atomic fountain clock, FO1. The DC Stark shift is measured at a range of electric fields reaching down to twice the rms ambient thermal field E _{300 K} - Furthermore, the blackbody radiation shift is measured directly by inserting a heated graphite tube around the atom trajectory. The Stark measurement reaches an accuracy of 2 x 10 ^-3 for the principal coefficient. The direct measurement gives an accuracy of 4%. Both results confirm our earlier measurement [Simon et alVhys. Rev. A 57_-- , 436 (1990)]. In the analysis a 3 x 10 ^-3 correction, due to a sign error in the theory must be made, so that the coefficient reads k_omicron = -2.282(4) x 10^-10 Hz/(V/m)².