Quadrupole collectivity in neutron-rich Cd isotopes

The proximity to the closed shells at Z = 50 and N = 82 makes the neutron-rich Cd isotopes a perfect test case for nuclear theories. The energy of the first excited 2⁺-state in the even ¹²²⁻¹²⁸ shows an irregular behaviour as the Cd isotopes exhibit only a slight increase for ¹²²Cd to ¹²⁶Cd and even a decrease from ¹²⁶Cd to ¹²⁸Cd. This anomaly can so far not be reproduced by shell model calculations. Only beyond mean field calculations with a resultant prolate deformation are capable to describe this anomalous behaviour. In order to gain more information about the neutron-rich Cd isotopes a Coulomb excitation experiment was performed with MINIBALL at REX-ISOLDE, CERN. The extracted transition strengths B (E2,0⁺gs → 2⁺₁) for ¹²²,¹²⁴,¹²⁶,¹²⁸Cd agree with beyond mean field calculations. The spectroscopic quadrupole moments Qs (2⁺₁) are compared with measurements on odd neutron-rich Cd isotopes.