Analysis of two-nucleon transfer reactions in the Ne 20 + Cd 116 system at 306 MeV

Background: Heavy-ion induced two-nucleon transfer reactions are powerful tools to reveal peculiar aspects of the atomic nucleus, such as pairing correlations, single-particle and collective degrees of freedom, and more. Also, these processes are in competition with the direct meson exchange in the double charge exchange reactions, which have recently attracted great interest due to their possible connection to neutrinoless double-β decay. In this framework, the exploration of two-nucleon transfer reactions in the Ne20+Cd116 collision at energies above the Coulomb barrier is particularly relevant since the Cd116 nucleus is a candidate for the double-β decay. Purpose: We want to analyze selected transitions to low-lying 0+ and 2+ states of the residual nuclei in the Cd116(Ne20,Ne22)Cd114 two-neutron pickup and Cd116(Ne20,O18)Sn118 two-proton stripping reactions at 306 MeV incident energy and determine the role of the couplings with inelastic transitions. Methods: We measured the excitation energy spectra and absolute cross sections for the two reactions using the MAGNEX large acceptance magnetic spectrometer to detect the ejectiles. We performed direct coupled reaction channels and sequential distorted wave Born approximation calculations using the double folding São Paulo potential to model the initial and final state interactions. The spectroscopic amplitudes for two- and single-particle transitions were derived by different nuclear structure approaches: microscopic large-scale shell model, interacting boson model-2 and quasiparticle random phase approximation. Results: The calculations are able to reproduce the experimental cross sections for both two-neutron and two-proton transfer reactions. The role of couplings with the inelastic channels are found to be important in the two-proton transfer case. A competition between the direct and the sequential process is found in the reaction mechanism. For the two-proton transfer case, the inclusion of the 1g7/2 and 2d5/2 orbitals in the model space is crucial. Conclusions: The approach presented here, combination of nuclear structure and reaction mechanism ingredients, is a very promising tool for the analysis of two-neutron and two-proton transfer reactions between heavy nuclei.