Accuracy evaluation of ITCsF2: a nitrogen cooled caesium fountain

For almost two decades, caesium fountain primary frequency standards (PFS) have represented the best realization of the definition of the second in the International System of units. Their accuracy has progressively improved with time, reaching a few parts in 1016. In this paper, we present the accuracy evaluation of ITCsF2, the new Cs fountain PFS developed at the Italian National Metrological Institute, designed to be operated at cryogenic temperature to reduce the blackbody radiation shift. The short-term stability of the ITCsF2 fountain is 2 × 10−13τ−1/2 when operated at high atomic density, and the relative inaccuracy reaches 2.3 × 10−16. We also report four calibrations of International Atomic Time with a relative frequency agreement of (−1.7 ± 3.2)× 10−16, between ITCsF2 and the average of the other fountains operated in the world during the reference periods.