One-step and two-step contributions to two-nucleon transfer reactions

Qualitative arguments are given to show that although first-order distorted-wave Born approximation calculations are adequate for the treatment of single-nucleon transfer reactions, there is little reason to trust them for two-nucleon or multinucleon transfer reactions. A first-plus-second Born approximation calculation is presented for the ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$(${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$)${}_{}{}^{206}{}_{}{}^{}\mathrm{Pb}$ ground state transition, at bombarding energies below and slightly above the Coulomb barrier. It is found that successive transfer (two-step) processes account for almost all the observed cross section.