Atomic trajectory characterization in a fountain clock based on the spectrum of a hyperfine transition

We describe a new method to determine the position of the atomic cloud during its interaction with the microwave field in the cavity of a fountain clock. The positional information is extracted from the spectrum of the |F = 3, mF = 0〉 to |F = 4, mF = −1〉 hyperfine transition, which shows a position-dependent asymmetry when the magnetic C-field is tilted by a few degrees with respect to the cavity axis. Analysis of this spectral asymmetry provides the horizontal centre-of-mass position for the ensemble of atoms contributing to frequency measurements. With an uncertainty of the order of 0.1 mm, the obtained information is useful for putting limits on the systematic uncertainty due to distributed cavity phase gradients. The validity of the new method is demonstrated through experimental evidence.