Neutron-rich isotopes represent a steady source of new information on the behaviour of the nucleus. Sometimes, unexpected phenomena occur such as halo-nuclei or the disappearance of the well-established magic numbers. Spin-isospin parts in the nucleon-nucleon interaction, e.g., the proton-neutron tensor force (in particular, the strongly attractive monopole parts) are expected to modify shell structure in exotic nuclei. These potential changes in the intrinsic shell structure are of fundamental interest.
At the National Superconducting Cyclotron Laboratory we studied the distribution in single-particle strength in the neutron-rich 67,69,71Ni isotopes via one-neutron knockout reactions, a well-established technique to address this specific issue. The secondary 68,70,72Ni beams were produced by fragmentation reactions of a primary 82Se beam impinging on a Be target, and purification and separation of the beam was achieved with the A1900 fragment separator. The secondary beams were transported to the S800 large-acceptance spectrometer, tuned to accept the one-neutron knock-out fragments. The de-excitation gamma-rays were measured by means of the GRETINA tracking array.
In the same measurement 68Ni was populated by 2-neutrons knock-out and the energy of the first excited state in 68Ni was determined to be Ex(0+)=1603 keV. The comparison of the relative B(E2) transition probabilities between low-spin states and shell-model calculation gives support to the shape-coexistence in low-lying states of 68Ni.