Enhanced Quadrupole Collectivity at $N=40$: The Case of Neutron-Rich Fe Isotopes

The transition rates for the $2_1^{+}$ states in ${}^{62,64,66}\mathrm{Fe}$ were studied using the recoil distance Doppler-shift technique applied to projectile Coulomb excitation reactions. The deduced $E2$ strengths illustrate the enhanced collectivity of the neutron-rich Fe isotopes up to $N=40$. The results are interpreted using the generalized concept of valence proton symmetry which describes the evolution of nuclear structure around $N=40$ as governed by the number of valence protons with respect to $Z\approx 30$. The trend of collectivity suggested by the experimental data is described by state-of-the-art shell-model calculations with a new effective interaction developed for the $fpgd$ valence space.

Figure 1

Typical results from the $\gamma$-ray line-shape fits to the data for the $2^{+}\to 0^{+}$ transitions in ${}^{62,64,66}\mathrm{Fe}$.

Figure 2

(a) Energies $E(2^{+})$ and (b) $B(E2;2^{+}\to 0^{+})$ values for even-even nuclei with $Z=24–36$ and $N=22–58$. The stars indicate the present data of ${}^{62,64,66}\mathrm{Fe}$. The $B(E2)$ data of Kr in (b) are divided by 2. In (c), the $B(E2)$ data of the Fe and Cr isotopes are compared to the scaled $B(E2)$ values for the Se and Kr isotopes, respectively, as well as the shell-model predictions (SM) for ${}^{62,64,66,68}\mathrm{Fe}$ and ${}^{60}\mathrm{Cr}$ isotopes.