INVESTIGATION OF THE FUSION OF HEAVY NEARLY SYMMETRICAL SYSTEMS

Excitation functions in the vicinity of the Coulomb barrier have been measured for the formation of evaporation residues in Mo-100-induced f usion reactions with Zr-90,92,96, Mo-92,96,98,100, Ru-104 and Pd-110 a s well as for the system Zr-96 + Zr-96. From these data the fusion pro bability in central collisions was extracted covering a range of 4 ord ers of magnitude. At the fusion barriers expected from systematics we find that the fusion probability is suppressed by one to three orders of magnitude. It is rising very gradually at higher energies and reach es for the heaviest systems saturation only at energies as high as 30 MeV above the barrier. The observed hindrance of the fusion process in creases roughly with the growing Coulomb repulsion between the collisi on partners, but there is also a distinct influence of their individua l nuclear structure. The data are compared to the extra-push model, th e surface-friction model and the diabatic fusion model. A parameterisa tion of the extra-push energy and its fluctuation in terms of a macros copic quantity like the Coulomb repulsion combined with a microscopic quantity characterizing the nuclear structure is proposed. As a byprod uct of this work a new alpha emitter, Po-191, could be identified. Its half-life is (15.5(-2.5)+6) ms, the alpha energy is (7314 +/- 20) keV .