Analyzing powers for Pb207(t,p)209Pb at 17 MeV as evidence for multistep contributions

Analyzing powers and differential cross sections for Pb207(t,p)209Pb have been studied at 17 MeV in order to identify the effects of second-order (t,p) transfer mechanisms. Angular distributions for transitions to seven single-particle states in Pb209 and to the first pairing vibration state in Pb209 were measured for the angular range 6?(detheta70°. The observed L dependence of the differential cross sections is adequately reproduced by one-step finite-range distorted-wave Born approximation calculations, but the absolute magnitude of these transitions is underpredicted by factors ranging from 1.25 to 2.5. The analyzing powers for the transitions to the seven well-known single-particle states are significantly different from each other and do not follow one-step distorted-wave Born approximation predictions or any other simple dependence on the transferred angular momentum L. The observed analyzing powers for the L=5 transfers to the (9/2)+ ground state and the (11/2)+ state at 0.778 MeV differ even in sign. It is found that the inclusion of two-step second-order distorted-wave Born approximation channels, which treat the dominant single-neutron stripping channels as intermediate states, can yield qualitatively correct analyzing powers. In agreement with experiment, two-step calculations predict analyzing powers which depend strongly on the spins (j1,j2) of the transferred neutrons. However, quantitatively satisfactory results have only been obtained if an adiabatic scattering potential for the intermediate projectile is used. The adiabatic potential, obtained as the sum of a proton and a neutron potential, gives a considerably deeper absorptive part than the empirical deuteron optical model potential. This representation of the intermediate scattering state is supported by a recent two-nucleon transfer model by Austern and Kawai. The inclusion of sequential stripping channels also gives a needed enhancement of the absolute cross sections, although zero-range calculations do not always give quantitatively correct enhancements.