EVIDENCE FOR LONG FORMATION TIMES OF NEAR-BARRIER FUSION REACTIONS

High energy gamma rays from the decay of the giant dipole resonance bu ilt on highly excited states in 164Yb at E(ex) = 49 MeV formed in two different reactions have been measured. While standard statistical mod el calculations can describe the gamma-ray spectrum from the O-16 + Sm -148 reaction they fail to reproduce the gamma-ray spectra from the mo re symmetric reaction 64Ni + Mo-100. Simple model calculations which i nclude particle evaporation and gamma-ray decay during the formation p rocess suggest that the observed differences may be related to a long fusion time in the more symmetric reaction.