Recent progress in primary Cs beam frequency standards at the PTB

The deflection of cesium (Cs) atomic beams using quadrupole and hexapole magnets, or combinations of both, has been investigated. The beam efficiency and the atomic velocity distribution has been calculated for magnets with a cylindrical, conic or parabolic form, respectively. A combination of quadrupole and hexapole magnets allows the selection of atoms in the beam with a relative velocity range of a few percent. A new rectangular frequency modulation method has been developed using modulation widths of multiples of the line width enabling the detection of very small asymmetries of the Cs resonance. By improving the beam optics and lowering the average atomic velocity in the beam to less than 100 m/s, the relative uncertainty of the primary Cs standard CSI of the PTB could be reduced from 1.5 × 10⁻¹³ to 1 × 10⁻¹³. Measurements of the frequency of the International Atomic Time (TAI) since 1973 showed that on an average the relative frequency deviation from the nominal value is +(10.3 ± 1) × 10⁻¹³. TAI frequency measurements with CSI carried out between 1969 and 1973 had revealed a negative TAI-frequency drift.