Competition between pairing correlations and deformation from the odd-even mass staggering of francium and radium isotopes

Abstract

The masses of $^{222,224,226\text{--}233}\mathrm{Fr}$ and $^{233,234}\mathrm{Ra}$ have been determined with the Penning-trap mass spectrometer ISOLTRAP at the ISOLDE facility at CERN, including the previously unknown mass and half-life of $^{233}\mathrm{Fr}$. We study the evolution of the odd-even staggering of binding energies along the francium and radium isotopic chains and of its lowest-order estimator, ${\ensuremath{\Delta}}^{3}(N)$. An enhancement of the staggering of ${\ensuremath{\Delta}}^{3}(N)$ is observed towards neutron number $N=146$, which points to contributions beyond pairing correlations. These contributions are investigated in the Hartree-Fock and Hartree-Fock-Bogoliubov approaches, emphasizing the connections to the single-particle level density and nuclear deformation.