Precision hfs of $^{126}$Cs(T$_{1/2}$=1.63m) by ABMR

A precision measurement of the $^{126}$Cs(T$_{1/2}$=1.63m) hfs separation $\Delta\nu$ in the ground $6s\,^{2}\!S\!_{1/2}$ state was obtained by means of atomic beam magnetic resonance on-line with the CERN-PSB-ISOLDE mass separator. The result is $\Delta\nu$ = 3629.514(0.001)MHz. The ultimate goal is a systematic study of the influence of extended nuclear structure on hfs(Bohr-Weisskopf effect). These hfs data can serve to gain knowledge of neutron distributions, of recent interest in nuclear astrophysics and parity non-conservation effects in atomic interactions.