Investigation of the ²⁰⁸Pb(¹⁸O, f) fission reaction: Mass-energy distributions of fission fragments and their correlation with the gamma-ray multiplicity

The mass-energy distributions of fragments originating from the fission of the compound nucleus Th-226 and their correlations with the multiplicity of gamma rays emitted from these fragments are measured and analyzed in O-18 + Pb-208 interaction induced by projectile oxygen ions of energy in the range E-lab = 78-198.5 MeV. Manifestations of an asymmetric fission mode, which is damped exponentially with increasing Elab, are demonstrated. Theoretical calculations of fission valleys reveal that only two independent valleys, symmetric and asymmetric, exist in the vicinity of the scission point. The dependence of the multiplicity of gamma rays emitted from both fission fragments on their mass, M-gamma(M), has a complicated structure and is highly sensitive to shell effects in both primary and final fragments. A two-component analysis of the dependence M-gamma(M) shows that the asymmetric mode survives in fission only at low partial-wave orbital angular momenta of compound nuclei. It is found that, for all Elab, the gamma-ray multiplicity M-gamma as a function of the total kinetic energy (TKE) of fragments, M-gamma(TKE), decreases linearly with increasing TKE. An analysis of the energy balance in the fission process at the laboratory energy of E-lab = 78 MeV revealed the region of cold fission of fragments whose total kinetic energy is TKE similar to Q(max).