### Abstract:

Abstract: Background: The cross section for forming a heavy evaporation residue in fusion reactions depends on the capture cross section, the fusion probability, P[subscript CN], i.e., the probability that the projectile-target system will evolve inside the fission saddle point to form a completely fused system rather than reseparating (quasifission), and the survival of the completely fused system against fission. P[subscript CN] is the least known of these quantities.
Purpose: We want to determine P[subscript CN] for the reactions of 101.2 MeV ¹⁸O, 147.3 MeV ²⁶Mg, 170.9 MeV ³⁰Si, and 195.3 MeV ³⁶S with ¹⁹⁷Au.
Methods: We measured the fission fragment angular distributions for these reactions and used the formalism of Back to deduce the fusion-fission and quasifission cross sections. From these quantities we deduced P[subscript CN] for each reaction.
Results: The values of P[subscript CN] for the reactions of 101.2 MeV ¹⁸O, 147.3 MeV ²⁶Mg, 170.9 MeV ³⁰Si, and 195.3 MeV ³⁶S with ¹⁹⁷Au are 0.66, 1.00, 0.06, and 0.13 respectively. Conclusions: The new measured values of P[subscript CN] agree roughly with the semiempirical systematic dependence of P[subscript CN] upon fissility for excited nuclei.