Accurate rubidium atomic fountain frequency standard

The design, operating parameters and the accuracy evaluation of the NPL Rb atomic fountain are described. The atomic fountain employs a double magneto-optical arrangement that allows a large number of Rb-87 atoms to be trapped, a water-cooled temperature-stabilized interrogation region and a high quality factor interrogation cavity. From the uncertainties of measured and calculated systematic frequency shifts, the fractional frequency accuracy is estimated to be 3.7 x 10(-16). The fractional frequency stability, limited predominantly by noise in the local oscillator, is measured to be 7 x 10(-16) after one day of averaging. Based on the proposed quasi-continuous regime of operation of the fountain, the accuracy of the Rb standard of 5 x 10(-17) reachable in two days of averaging is predicted.