Electromagnetic moments of 215,217Bi: Probing shell evolution beyond N=126

The nuclear properties of bismuth isotopes (Z=83), with just one valence proton above the closed spherical shell at Z=82, are expected to be governed by a single unpaired proton. However, already in semimagic 209Bi (Z=83, N=126), the magnetic moment (μ) strongly deviates from the single-particle Schmidt value. A near linear decrease in μ with the increase of N after the N=126 magic number was observed up to N=130. In order to test whether this trend is kept at N > 130 and to reveal the underlying mechanisms, an investigation of 215,217Bi (N=132,134) has been undertaken. The magnetic dipole and electric quadrupole moments of the Iπ=9/2− nuclear ground states in these isotopes have been measured for the first time using the in-source resonance-ionization spectroscopy technique at ISOLDE (CERN). It has been shown that the linearly decreasing trend of μ(209,211,213Big) is broken in 215,217Bi with a nearly constant value of μ observed. Experimental data have been compared to calculations in the framework of the configuration-interaction shell model with the monopole-based universal VMU+LS interaction. The peculiarities in the behavior of μ(Bi, 9/2−) with increasing neutron number are explained as being due to the shell evolution, change of the neutron orbitals occupancies and strong configuration mixing beyond N=130. Also, the difference in the μ trends for bismuth (Z=83) and astatine (Z=85) isotopes with N > 126 are reproduced by the shell-model calculations. It is shown that monopole interaction plays noticeable role in the description of the peculiarities of the μ behaviour. Additionally, the extension of the application of the VMU interaction to the μ isotopic trends for heavy nuclei is important for further study of the capabilities of this promising version of the shell-model calculations.