Onset of collectivity in Neutron-Rich iron isotopes: Toward a new island of inversion?

The lifetimes of the first excited 2(+) states in Fe-62 and Fe-64 have been measured for the first time using the recoil-distance Doppler shift technique, at GANIL. A U-238 beam of 6.5 AMeV impinged on Ni-64 target, and the target like products were slowed down by a degrader foil positioned at micrometer distance downstream of the target and identified in the VAMOS spectrometer on an event-by-event basis. The lifetimes were then determined from the intensities of the degraded and fully Doppler shifted components of the 2(+) -> 0(+) transition detected in EXOGAM detectors positioned at backward angles. The resulting lifetimes show a steep increase of the B(E2) values of the first excited 2(+) state from Fe-62 to Fe-64. A comparison with shell model calculations shows that the onset of collectivity is related to the occupation of neutron intruder orbitals. The large B(E2) value for Fe-64 is only reproduced if the valence space includes both the neutron g(9/2) and d(5/2) orbitals. The transition from spherical Ni-68 to more proton-deficient N 40 isotones has some similarity with the island of inversion around Mg-32. The developing quadrupole collectivity can in both cases be related to the occupation of neutron intruder orbitals which are at the same time quasi-SU(3) partners: (f(7/2), p(3/2)) for Mg-32 and (g(9/2), d(5/2)) for the neutron-rich Fe.