On the origin of the anomalous behaviour of 2+ excitation energies in the neutron-rich Cd isotopes

Recent experimental results obtained using p decay and isomer spectroscopy indicate ail Unusual behaviour of the energies of the first excited 2(+) states in neutron-rich Cd isotopes approaching the N = 82 shell Closure. To explain the unexpected trend, changes of the nuclear structure far off stability have been suggested, namely a quenching of the N = 82 shell gap already in Cd-130, only two proton holes away from doubly magic Sn-132. We study the behaviour of the 2(+) energies in the Cd isotopes from N = 50 to N = 82, i.e. across the entire span of a major neutron shell using modern beyond mean field techniques and the Gogny force. We demonstrate that the observed low 2(+) excitation energy in Cd-128 close to the N = 82 shell closure is a consequence of the doubly magic character of this nucleus for oblate deformation favoring thereby prolate configurations rather than spherical ones. (C) 2008 Elsevier B.V. All rights reserved.