Role of direct mechanism in two-nucleon T=0 transfer reactions in light nuclei using the (Li6,α) probe

Abstract

Background: Two-nucleon transfer reactions provide a unique tool to understand the correlation between nucleon pairs. Two-nucleon $(pp,$ $nn$, and $np)$ transfer reactions can occur via isoscalar $(T=0,$ $S=1)$ or isovector $(T=1,$ $S=0)$ processes. In particular, the isoscalar pair transfer can be induced by the $(\ensuremath{\alpha},d)$ or $(^{6}\mathrm{Li},\ensuremath{\alpha})$ probes. In the past, most of the isoscalar $np$-transfer studies were performed with the $(\ensuremath{\alpha},d)$ reaction, but this probe is strongly momentum mismatched with respect to other two-nucleon transfer reactions.Purpose: We aim to investigate the interplay between direct and sequential reaction mechanisms from the analysis of experimental $(^{6}\mathrm{Li},\ensuremath{\alpha})$ angular distributions in light targets.Method: Differential cross sections of $(^{6}\mathrm{Li},\ensuremath{\alpha})$ reactions at a beam energy of 20 MeV were measured with $^{12}\mathrm{C}$ and $^{19}\mathrm{F}$ targets. The interplay between direct and sequential transfer mechanisms in the experimental angular distributions was investigated with coupled-reaction-channels calculations.Results: The experimental angular distributions of isoscalar $np$ transfer were compared with theoretical calculations assuming a direct or a sequential reaction mechanism. Direct $np$-transfer calculations describe successfully most of the angular distributions. The sequential transfer mechanism is about two orders of magnitude smaller than the direct process.Conclusions: The present results suggest a significant $np$ correlation in the $^{12}\mathrm{C}(^{6}\mathrm{Li},\ensuremath{\alpha})^{14}\mathrm{N}^{*}$ and $^{19}\mathrm{F}(^{6}\mathrm{Li},\ensuremath{\alpha})^{21}\mathrm{Ne}^{*}$ reactions. Despite the relatively low cross section for the reactions with the asymmetric $^{19}\mathrm{F}$ target, the direct transfer mechanism remains dominant over the sequential process. Further studies including measurements with other asymmetric $sd$-shell nuclei will be required to fully understand the isoscalar and isovector $np$-transfer mechanism in this nuclear region.