Probing Shell Closures in Neutron-Rich Nuclei

Abstract

The modification of shell structure in neutron-rich nuclei is at the center of a number of recent theoretical and experimental investigations. In this contribution results of two experimental campaigns will be presented that investigate shell structure in neutron-rich nuclei near 54Ca and 132Sn, respectively. In the first experiment we used a 450 MeV/u 86Kr beam from the GSI accelerator complex to produce neutron-rich nuclei via in-flight fragmentation. The primary fragments were separated and identified in the first half of the Fragment Separator FRS before they hit a secondary reaction target at the intermediate focal plane of the FRS, where secondary fragments were produced in a knock-out reaction. The second half of the FRS was used for the identification of the secondary fragments as well as for the measurement of their longitudinal momentum distribution, which gives insight into the angular momentum of the nucleon that was removed. Gamma-rays from excited states of the secondary fragments were detected by the MINIBALL segmented gamma-ray detector array, which allows for the determination of momentum distribution and population cross-section for individual excited states. Results for 55Ti will be presented, which are essential to probe shell model predictions in the vicinity of 54Ca. The MINIBALL array is also routinely used for Coulomb excitation experiments at the REX-ISOLDE accelerator at CERN. Results on the collectivity of the first excited state in even-even neutron-rich Cd and Xe isotopes will be presented, which are produced at ISOLDE as fission fragments using a UCx target. The results provide insights on the evolution of collectivity around 132Sn.