Energy dependence of two-nucleon transfer reactions on light nuclei

The differential cross section has been measured at several incident proton energies between 20 and 45 MeV for the ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}(p,{}_{}{}^3{}_{}{}^{}\mathrm{He}){}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}\mathrm{}(p,t)\mathrm{}{}_{}{}^{13}{}_{}{}^{}\mathrm{N}$, ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}(p,{}_{}{}^3{}_{}{}^{}\mathrm{He}){}_{}{}^{13}{}_{}{}^{}\mathrm{C}$, and ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}\mathrm{}(p,t)\mathrm{}{}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ reactions. Data on the energy dependence of other transfer reactions on the same nuclei, i.e., ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}\mathrm{}(p,d)\mathrm{}{}_{}{}^{17}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}(p,\alpha ){}_{}{}^{11}{}_{}{}^{}\mathrm{C}$, and ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}(p,\alpha ){}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ reactions have also been obtained. These data are compared with distorted wave Born approximation (DWBA) predictions. The comparison indicates that the energy dependence of the cross sections is poorly explained by DWBA calculations based on single-step pickup which give a maximum in the cross section in the energy region where angular momentum matching conditions are achieved. The absence of corresponding maxima in the experimental excitation functions may indicate that other reaction mechanisms contribute substantially to the cross section. Theoretical approaches as successive pickup of the nucleons and preequilibrium emission are also discussed.