Multichannel experimental and theoretical approach to the 12C(18O, 18 F) 12B single-charge-exchange reaction at 275 MeV. II. Competition between the meson exchange and the sequential transfer in the reaction mechanism

Abstract

Background: Single-charge-exchange reactions are among the most appropriate nuclear tools to study the response of nuclear systems to isovector interaction. Nowadays, the availability of powerful experimental setups and advanced nuclear models bring the possibility of the complete study of the reaction mechanisms involved in the nuclear reactions, also in the case of heavy projectiles. This new possibility allows one to access valuable information on key nuclear structure aspects, including those embedded in the widely searched neutrinoless double-beta decay. Purpose: We intend to elucidate the main nuclear structure and reaction features involved in the 18O + 12C collision at 275 MeV beam incident energy. In this paper, the main focus is to quantify the competition between the sequential two-step transfer and the direct meson-exchange reaction mechanisms. Methods: The energy spectra and cross-section angular distributions for the 12C(18O, 18 F) 12B single-charge- exchange reaction are measured by the MAGNEX magnetic spectrometer in the same experimental setup of the elastic and inelastic scattering and the one-nucleon transfer reaction channels. The cross sections for the sequential two-step transfer and the direct meson-exchange single-charge-exchange reaction mechanisms are evaluated in a single coherent theoretical calculation, using state-of-the-art nuclear structure and reaction theories. Results: The energy resolution achieved in the study of the 12C(18O, 18 F) 12B single-charge-exchange reaction allows one to separate the ground-to-ground-state transition and to identify other structures in the measured energy spectra. The coherent sum of the distorted wave Born approximation cross sections of the direct and sequential reaction mechanisms well describes the experimental cross-section angular distributions. The crucial role of the optical model distortion in the scattering of the incoming and outgoing waves was taken into account via the introduction of the coupled-channel local equivalent effective potential. Conclusions: Advanced nuclear structure and reaction models turned out to be appropriate tools for the detailed analysis of single-charge-exchange reactions originating in heavy-ion collisions. This is of particular relevance in several fields of nuclear physics. Moreover, it is inherent to the challenging project to provide valuable information on neutrinoless double-beta decay nuclear matrix elements from single- and double-charge-exchange cross-section measurements.