THERMAL-CHARACTERISTICS OF COMPOSITE SYSTEMS FORMED IN FUSION OF SI-28 WITH SN-118 AND SN-124 NUCLEI

Neutrons from the fusion reactions of 120-150 MeV Si-28 with Sn-118 an d 124Sn target nuclei have been measured in coincidence with evaporati on residues, in two series of complementary experiments using either a 4pi neutron multiplicity meter or a neutron time-of-flight spectromet er. Both the energy spectra and multiplicity distributions reveal sign ificant quantitative differences in the decay patterns of the compound nuclei 146Gd and Gd-152 formed in the two reactions studied. It is sh own that these differences cannot be understood in terms of decay casc ades 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 de cay model, if different effective level density parameters are allowed for the evaporation chains of the two composite systems.