Competition between direct and sequential two-neutron transfers in the O 18 + Si 28 collision at 84 MeV

In this work we study the simultaneous and sequential two-neutron transfer mechanisms to the Si28 nucleus induced by (t,p) and (O18,O16) reactions. New experimental cross sections for the Si28(O18,O16)Si30 reaction at 84 MeV are also presented. Direct reaction calculations are carried out within the exact finite range coupled reaction channel, for the simultaneous transfer of the two-neutron cluster, and the second-order distorted wave Born approximation, for the sequential transfer. Two different models are considered to describe the two-neutron cluster. The spectroscopic information was obtained from a shell-model calculation with a psdmod interaction for the target overlaps where the 1p3/2, 1p1/2, 1d3/2, 1d5/2, and 2s1/2 orbitals are included as valence subspace. We show that simultaneous and sequential two-neutron transfers are competing mechanisms for the population of the ground state in Si30. A systematic analysis of the two-neutron transfer induced by the (O18,O16) reaction indicates that the static deformation of target nuclei impacts the two-neutron transfer mechanism.