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 ⁷⁴Ge+⁷⁴Ge reaction at 318 MeV. The population of the yrast superdeformed band in ¹⁴⁴Gd shows an increase when compared with the results obtained in the more asymmetric ⁴⁸Ti+¹⁰⁰Mo reaction at 215 MeV. The relative yields of the different evaporation residues produced in the ⁷⁴Ge+⁷⁴Ge and in the ⁴⁸Ti+¹⁰⁰Mo reactions are very similar, with the exception of the ^145,144Gd 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 γ-rays emitted in the ⁷⁴Ge+⁷⁴Ge reaction have been measured as a function of the γ-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 γ-rays from the dinucleus formed in the earlier stage of the collision is unimportant.