Collectivity of Exotic Heavy Fe Isotopes

The properties of exotic neutron-rich nuclei between the proton shell closures Z = 20 and Z = 28 are of particular interest for the understanding of the shell structure for large neutron excess. Effects related to the energy gap between the neutron fp and 1g9/2 shells lead to a strong variation of collectivity for nuclei around N = 40. Whereas (68)Ni was found to have doubly magic properties, this was not observed in neighbouring nuclei. Recent shell model calculations for the neutron rich iron isotopes clearly reveal the difficulty to describe nuclei in this mass region and resulted in large deviations of the predicted collectivity depending on the valence space. However, no experimental data on the transition strength existed for the very exotic nucleus (66)Fe at N = 40. Here we present the newest results on absolute transition strengths of the lowest excited states in (62,64,66)Fe measured model independently using the recoil distance Doppler-shift (RDDS) method. The experiments were performed at NSCL at Michigan State University with the Cologne/NSCL plunger device using Coulomb excitation in inverse kinematics at energies of 80 MeV/u. Our results yield a much higher collectivity for (64,66)Fe than expected and allow tests of new calculations.