Evidence for long formation times of near-barrier fusion reactions

High energy γ rays from the decay of the giant dipole resonance built on highly excited states in Yb164 at Eex=49 MeV formed in two different reactions have been measured. While standard statistical model calculations can describe the γ-ray spectrum from the O16 + Sm148 reaction they fail to reproduce the γ-ray spectra from the more symmetric reaction Ni64 + Mo100. Simple model calculations which include particle evaporation and γ-ray decay during the formation process suggest that the observed differences may be related to a long fusion time in the more symmetric reaction.