Precision hfs of 126Cs(T1/2=1.63m) by ABMR

The hfs separation Δν of 126Cs(T1/2=1.63m) in the 6sS1/22 ground state was obtained in a precision measurement near zero magnetic field by means of atomic beam magnetic resonance with laser optical pumping on-line with the CERN-PSB-ISOLDE mass separator. The result, Δν=3629.514 (0.001) MHz, corrects significantly a previous published value from a high-field experiment. With our result, the precision of the nuclear magnetic moment, μ(Cs126)≈0.776μN, is now limited by the influence of extended nuclear structure on the hfs (the Bohr-Weisskopf effect).