ar X iv : n uc l - ex / 9 90 70 03 v 1 6 J ul 1 99 9 Coupled - channels analysis of the 16 O + 208 Pb fusion barrier distribution

Analyses using simplified coupled-channels models have been unable to describe the shape of the previously measured fusion barrier distribution for the doubly magic O+Pb system. This problem was investigated by remeasuring the fission excitation function for O+Pb with improved accuracy and performing more exact coupled-channels calculations, avoiding the constant-coupling and first-order coupling approximations often used in simplified analyses. Couplings to the singleand 2-phonon states of Pb, correctly taking into account the excitation energy and the phonon character of these states, particle transfers, and the effects of varying the diffuseness of the nuclear potential, were all explored. However, in contrast to other recent analyses of precise fusion data, no satisfactory simultaneous description of the