Charge radii and electromagnetic moments of Bi214−218 : Exploring the “southern” border of the Z>82 octupole-deformation region

The changes in the mean-squared charge radii relative to ²⁰⁹Bi₁₂₆ (δr²N,126) and the magnetic dipole and electric quadrupole moments in ^214-218Bi have been measured using the in-source resonance-ionization spectroscopy technique at ISOLDE (CERN). Magnetic moments of odd-odd bismuth nuclei have been analyzed by the additivity relation. Previous tentative spin-parity and configuration assignments based on the β-decay feeding patterns have been supported. A normal odd-even staggering in charge radii of bismuth isotopes with $N>126$ has been observed. The new data for the δr² of bismuth isotopes allow a study of the isotonic dependencies in the charge radii, revealing jumps in δr²132,126 and δr²134,126 at $Z=84$. This pattern could be explained by a sudden onset of octupole deformation at $N=132$ and 134 when going from polonium ($Z=84$) to astatine ($Z=85$).