FORMATION CROSS-SECTIONS OF NEUTRON-DEFICIENT POLONIUM ISOTOPES IN ASYMMETRIC AND SYMMETRICAL HEAVY-ION FUSION

Excitation functions are measured for the fusion reactions Al-27+Lu-17 5 and P-31+Tm-169 leading to the isotopes Po-192-198. A comparison to the data for the reactions Mo-100 + Mo-92-100 shows that characteristi cs bf the compound nucleus evaporation do not depend on the bombarding ion mass in the whole range of the entrance channel asymmetry up to t he fully symmetrical systems. It is also shown that a satisfactory des cription of the experimental data is possible in the frame of the stat istical deexcitation model only at an assumption involving, in compari son to theory predictions, a considerable reduction of macroscopic fis sion barriers for neutron-deficient isotopes of Bi.