Influence of the Entrance Channel in the Fusion Reaction 318 MeV 74Ge+74Ge

Entrance channel effects in the fusion of heavy ions have been studied by using the Ge-74+Ge-74 reaction at 318 MeV. The population of the yrast superdeformed band in Gd-144 shows an increase when compared with the results obtained in the more asymmetric Ti-48+Mo-100 reaction at 215 MeV. The relative yields of the different evaporation residues produced in the Ge-74+Ge-74 and in the Ti-48+Mo-100 reactions are very similar, with the exception of the Gd-145,Gd-144 residual nuclei (3n and 4n decay channels) which are populated with a larger yield in the symmetric reaction. Statistical model calculations reproduce qualitatively such effect if a fission delay is explicitly taken into account. Effects related to fusion barrier fluctuations seem to be important in determining the spin distributions of the compound nucleus. The spectra of the high energy gamma-rays emitted in the 74Ge+74Ge reaction have been measured as a function of the gamma-ray multiplicity as well as in coincidence with selected evaporation residues. They are reproduced by standard statistical model calculations with GDR parameters taken from systematics, demonstrating that, in agreement with dynamical model prediction, the emission of gamma-rays from the dinucleus formed in the earlier stage of the collision is unimportant.