Thermal characteristics of composite systems formed in fusion of ²⁸Si with ¹¹⁸Sn and ¹²⁴Sn nuclei

Neutrons from the fusion reactions of 120–150 MeV ²⁸Si with ¹¹⁸Sn and ¹²⁴Sn target nuclei have been measured in coincidence with evaporation residues, in two series of complementary experiments using either a 4π neutron multiplicity meter or a neutron time-of-flight spectrometer. Both the energy spectra and multiplicity distributions reveal significant quantitative differences in the decay patterns of the compound nuclei ¹⁴⁶Gd and ¹⁵²Gd formed in the two reactions studied. It is shown that these differences cannot be understood in terms of decay cascades proceeding through states of enhanced collective energy, such as the superdeformed states, suggested in earlier studies. Instead, they can be explained consistently within the framework of a statistical decay model, if different effective level density parameters are allowed for the evaporation chains of the two composite systems.