Microscopic structure of coexisting 0+ states in Ni68 probed via two-neutron transfer

The structure of low-spin states originating from shape-coexisting configurations in Ni-68(40)28 was directly probed via the two-neutron transfer reaction Ni-66(t, p)Ni-68 in inverse kinematics using a radioactive ion beam on a radioactive target. The direct feeding to the first excited 0(+) state was measured for center-of-mass angles 4 degrees - 16 degrees and amounts to an integral of 4.2(16)% relative to the ground state. The observed difference in feeding of the 0(+) states is explained by the transfer of neutrons, mainly in the pf shell below N = 40 for the ground state, and across N = 40 in the g(9/2) orbital for the 0(2)(+), based on second-order distorted-wave Born approximation calculations combined with state-of-the-art shell-model two-nucleon amplitudes. However, the direct feeding to the 2(1)(+) state [29(3)%] is incompatible with these calculations.