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Recently, various two-neutron transfer studies using the (18 O, 16 O) reaction were performed with a large success. This was achieved because of a combined use of the microscopic quantum description of the reaction mechanism and of the nuclear structure. In the present work we use this methodology to study the two-neutron transfer reaction of the 18 O + 64 Ni system at 84 MeV incident energy, to the ground and first 2 + excited state of the residual 66 Ni nucleus. All the experimental data were measured by the large acceptance MAGNEX spectrometer at the Instituto Nazionale di Fisica Nucleare-Laboratori Nazionali del Sud (Italy). We have performed exact finite range cross section calculations using the coupled channel Born approximation (CCBA) and coupled reaction channel (CRC) method for the sequential and direct two-neutron transfers, respectively. Moreover, this is the first time that the formalism of the microscopic interaction boson model (IBM-2) was applied to a two-neutron transfer reaction. From our results we conclude that for two-neutron transfer to the ground state of 66 Ni, the direct transfer is the dominant reaction mechanism, whereas for the transfer to the first excited state of 66 Ni, the sequential process dominates. A competition between long-range and short-range correlations is discussed, in particular, how the use of two different models (Shell model and IBM's) help to disentangle long-and short-range correlations.

In this work we study the simultaneous and sequential two-neutron transfer mechanisms to the 28 Si nucleus induced by (t,p) and ( 18 O, 16 O) reactions. New experimental cross sections for the 28 Si( 18 O, 16 O) 30 Si 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 shell model calculation with psdmod interaction for the target overlaps where the 1p 3/2 , 1p 1/2 , 1d 3/2 , 1d 5/2 and 2s 1/2 orbitals are included as valence sub-space. We show that simultaneous and sequential two-neutron transfer are competing mechanisms for the population of the ground state in 30 Si. A systematic analysis of the two-neutron transfer induced by the ( 18 O, 16 O) indicates that static deformation of target nuclei impacts on the two-neutron transfer mechanism.

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