Role of direct mechanism in two-nucleon T=0 transfer reactions in light nuclei using the (Li 6,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 (α,d) or (Li6,α) probes. In the past, most of the isoscalar np-transfer studies were performed with the (α,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 (Li6,α) angular distributions in light targets. Method: Differential cross sections of (Li6,α) reactions at a beam energy of 20 MeV were measured with C12 and F19 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 C12(Li6,α)N∗14 and F19(Li6,α)Ne∗21 reactions. Despite the relatively low cross section for the reactions with the asymmetric F19 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.