Formation cross sections of neutron deficient polonium isotopes in asymmetric and symmetrical heavy ion fusion

Excitation functions are measured for the fusion reactions 27 Al+ 175 Lu and 31 P+ 169 Tm leading to the isotopes 192–198 Po. A comparison to the data for the reactions 100 Mo + 92–100 Mo shows that characteristics of the compound nucleus evaporation do not depend on the bombarding ion mass in the whole range of the entrance channel asymmetry up to the fully symmetrical systems. It is also shown that a satisfactory description of the experimental data is possible in the frame of the statistical deexcitation model only at an assumption involving, in comparison to theory predictions, a considerable reduction of macroscopic fission barriers for neutron-deficient isotopes of Bi.