Measurement of the fusion probability, PCN, for hot fusion reactions

Background: The cross section for forming a heavy evaporation residue in fusion reactions depends on the capture cross section, the fusion probability, PCN, 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. PCN is the least known of these quantities. Purpose: We want to determine PCN for the reactions of 101.2 MeV 18 O, 147.3 MeV 26 Mg, 170.9 MeV 30 Si, and 195.3 MeV 36 Sw ith 197 Au. 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 PCN for each reaction. Results: The values of PCN for the reactions of 101.2 MeV 18 O, 147.3 MeV 26 Mg, 170.9 MeV 30 Si, and 195.3 MeV 36 S with 197 Au are 0.66, 1.00, 0.06, and 0.13, respectively. Conclusions: The new measured values of PCN agree roughly with the semiempirical systematic dependence of PCN upon fissility for excited nuclei.