Two-nucleon transfer reactions induced by $sup 16$O on $sup 42$Ca and $sup 48$Ca

The reactions $sup 42$,$sup 48$Ca($sup 16$O,$sup 18$O)$sup 40$,$sup 46$Ca and $sup 42$,$sup 48$Ca($sup 16$O,$sup 14$O)$sup 44$,$sup 50$Ti were studied at 56-MeV bombarding energy. Angular distributions for transitions to the ground and excited states of the residual nuclei and of the outgoing particles are analyzed in the framework of both ''cluster'' and microscopic distorted-wave Born-approximation calculations. The measured transfer cross sections are not described by calculations using optical-model parameters which fit elastic scattering. Agreement is substantially improved by a systematic change in the exit channel parameters. Cross sections for excited states relative to ground-state transitions are compared to relative strengths obtained from light-ion reactions and from simple shell-model calculations. (AIP)