Onset of collectivity in neutron-rich Fe isotopes: Toward a new island of inversion?

Abstract

The lifetimes of the first excited $2$${}^{+}$ states in $^{62}\mathrm{Fe}$ and $^{64}\mathrm{Fe}$ have been measured for the first time using the recoil-distance Doppler shift method after multinucleon transfer reactions in inverse kinematics. A sudden increase of collectivity from $^{62}\mathrm{Fe}$ to $^{64}\mathrm{Fe}$ is observed. The experimental results are compared with new large-scale shell-model calculations and Hartree-Fock-Bogolyubov--based configuration-mixing calculations using the Gogny D1S interaction. The results give a deeper understanding of the mechanism leading to an onset of collectivity near $^{68}\mathrm{Ni}$, which is compared with the situation in the so-called island of inversion around $^{32}\mathrm{Mg}$.