Large Shape Staggering in Neutron-Deficient Bi Isotopes

The changes in the mean-square charge radius (relative to $^{209}$Bi), magnetic dipole, and electric quadrupole moments of $^{187;188;189;191}$Bi were measured using the in-source resonance-ionization spectroscopy technique at ISOLDE (CERN). A large staggering in radii was found in $^{187;188;189}$Bi$^g$ , manifested by a sharp radius increase for the ground state of $^{188}$Bi relative to the neighboring $^{187;189}$Bi$^g$. A large isomer shift was also observed for $^{188}$Bi$^m$. Both effects happen at the same neutron number, $N =$ 105, where the shape staggering and a similar isomer shift were observed in the mercury isotopes. Experimental results are reproduced by mean-field calculations where the ground or isomeric states were identified by the blocked quasiparticle configuration compatible with the observed spin, parity, and magnetic moment.