Evidence for long formation times of near-barrier fusion reactions

High energy \ensuremath{\gamma} rays from the decay of the giant dipole resonance built on highly excited states in $^{164}\mathrm{Yb}$ at ${\mathit{E}}_{\mathrm{ex}}$=49 MeV formed in two different reactions have been measured. While standard statistical model calculations can describe the \ensuremath{\gamma}-ray spectrum from the $^{16}\mathrm{O}$ + $^{148}\mathrm{Sm}$ reaction they fail to reproduce the \ensuremath{\gamma}-ray spectra from the more symmetric reaction $^{64}\mathrm{Ni}$ + $^{100}\mathrm{Mo}$. Simple model calculations which include particle evaporation and \ensuremath{\gamma}-ray decay during the formation process suggest that the observed differences may be related to a long fusion time in the more symmetric reaction.

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