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 Ni284068 was directly probed via the two-neutron transfer reaction Ni66(t,p)Ni68 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 4-16 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 g9/2 orbital for the 02+, 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 21+ state [29(3)%] is incompatible with these calculations.