Large Shape Staggering in Neutron-Deficient Bi Isotopes

The changes in the mean-square charge radius (relative to ²⁰⁹Bi), magnetic dipole, and electric quadrupole moments of 187,188,189,191Bi were measured using the in-source resonance-ionization spectroscopy technique at ISOLDE (CERN). A large staggering in radii was found in 187,188,189Biᵍ, manifested by a sharp radius increase for the ground state of ¹⁸⁸Bi relative to the neighboring 187,189Biᵍ. A large isomer shift was also observed for ¹⁸⁸Biᵐ. 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.