Role of direct mechanism in two-nucleon T=0 transfer reactions in light nuclei using the (6Li, α) probe

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 (alpha, d) or (Li-6, alpha) probes. In the past, most of the isoscalar np-transfer studies were performed with the (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 (Li-6, alpha) angular distributions in light targets. Method: Differential cross sections of (Li-6, alpha) reactions at a beam energy of 20 MeV were measured with C-12 and F-19 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 C-12(Li-6, alpha)N-14(*) and F-19(Li-6, alpha)Ne-21(*) reactions. Despite the relatively low cross section for the reactions with the asymmetric F-19 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.