Barrier distribution of quasi-elastic backward scattering

In order to study the nucleus-nucleus interaction in Pb-based cold fusion, we have measured excitation functions for quasi-elastic scattering of Ti-48, Cr-54, Fe-16, Ni-64, Zn-70 and Kr-86 projectiles on Pb-208 target at backward angles. The barrier distributions were derived from the first derivative of measured quasi-elastic scattering cross sections relative to the Rutherford scattering cross section. The centroids of the barrier distributions show a deviation from several predicted barrier heights toward the low energy side. The shape of the barrier distributions is well reproduced by the results of a coupled-channel calculation taking account of the coupling effects of two phonon excitations of the quadrupole vibration for the projectiles and of the octupole vibration for the Pb-208 target.