Neutron-Rich [sup 62,64,64]Fe Show Enhanced Collectivity: The Washout of N = 40 in Terms of Experiment, Valence Proton Symmetry and Shell Model

Probing shell structure at a large neutron excess has been of particular interest in recent times. Neutron-rich nuclei between the proton shell closure Z=20 and Z=28 offer an exotic testing ground for shell evolution. The development of the N=40 gap between neutron fp and 1 g(9/2) shells gives rise to highly interesting variations of collectivity for nuclei in this region. While Ni-68 shows double magic properties in level energies and transition strength, this was not observed in neighbouring nuclei. Especially neutron-rich Fe isotopes proved particularly resistant to calculational approaches using the canonical valence space (fpg) resulting in important deviations of the predicted collectivity. Only an inclusion of the d(5/2)-orbital could solve the problem [1]. Hitherto no transition strengths for Fe-66 have been reported. We determined B(E2, 2(1)(+) -> 0(1)(+)) values from lifetimes measured with the recoil distance Doppler-shift method using the Cologne plunger for radioactive beams at National Superconducting Cyclotron Laboratory at Michigan State University. Excited states were populated by projectile Coulomb excitation for Fe-62,Fe-64,Fe-66. The data show a rise in collectivity for Fe isotopes towards N=40. Results [2] are interpreted by means of modified version of the Valence Proton symmetry [3] and compared to shell model calculation using a new effective interaction recently developed for the fpgd valence space [4].