An eikonal treatment including two-step mechanisms for the (p,n) reaction on light nuclei

The reactions ⁷Li(p, n)⁷Be(g.s.), ⁷Li(p, n)⁷Be(0.43 MeV), and ¹³C(p, n)¹³N(g.s.) are treated in an eikonalized distorted-wave impulse approximation (DWIA), including the contribution of a two-step mechanism involving isovector correlations for the last reaction. Our calculation uses the free on-shell nucleon-nucleon t-matrix, with spin and isospin dependence, and incorporates antisymmetrization of it. We find agreement at roughly the 20% level with the forward experimental charge-exchange results in the energy range 120 to 200 MeV. The two-step contribution for the ¹³C case is rather small, being of the order of 10% at 0°; it interferes constructively, thus improving somewhat the agreement with experiment.