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

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 ${}^{32}\mathrm{Mg}$.

Figure 1

Typical identification plot showing the energy loss of the ions in the ionization chamber of VAMOS as a function of the mass-over-charge ratio, which is determined from the time of flight through the spectrometer and the magnetic rigidity. The lines indicate the loci of ions with $A=64$ and $Z=26$, respectively. The most intense peak corresponds to elastically scattered ${}^{64}\mathrm{Ni}$ target nuclei. The data is shown for only one of 21 silicon detectors mounted in VAMOS and for one single charge state ($Q=23$). The identification of the ions was performed in the same way for all detectors and all charge states reaching the focal plane.

Figure 2

Spectra showing the fully Doppler shifted and degraded components of the $2_1^{+}\to 0_1^{+}$ transitions for three target-degrader distances in ${}^{64}\mathrm{Fe}$ and for one distance in ${}^{62}\mathrm{Fe}$. The $\gamma$ rays were observed at angles ranging from 160${}^{°}$ to 180${}^{°}$ with respect to the recoil direction.

Figure 3

Systematic of $B(E2;2_1^{+}\to 0_1^{+})$ and $E(2_1^{+})$ values for even-even nuclei around ${}^{68}\mathrm{Ni}$. The data are taken from the compilation of Raman et al. [29] complemented by newer results for Cr [12, 21], Fe [11], Ni [23], and Zn [8] isotopes. The panels on the right compare the experimental results for the chain of Fe isotopes with theoretical calculations (see text).