Fusion-fission dynamics at high excitation energies studied by neutron emission.

Abstract

Neutron emission in coincidence with fusion-fission events and evaporation residues was measured in the heavy-ion reactions $^{141}+(316$ MeV) $^{40}$Ar and $^{175}$Lu+(192 MeV) ${\mathrm{}}^{12}$C. Both reactions are leading to similar composite systems and excitation energies as the previously investigated reaction $^{165}$Ho${+\mathrm{}}^{20}$Ne. In order to determine the lifetimes of the composite systems prior to scission and to study entrance channel and angular-momentum effects, the results for all three systems are compared.From measured cross sections of fission and evaporation residues, the angular momentum intervals leading to fission are deduced to be 50--109 \ensuremath{\Elzxh} and 49--62 \ensuremath{\Elzxh} for Pr+Ar and Lu+C, respectively. The corresponding prescission neutron multiplicities are deduced to be ${M}_{\mathrm{n}{}^{\mathrm{presc}=3.6\ifmmode\pm\else\textpm\fi{}0.6}}$ and 6.3\ifmmode\pm\else\textpm\fi{}0.8, whereas the respective postscission multiplicities are ${M}_{\mathrm{n}{}^{\mathrm{post}=4.4\ifmmode\pm\else\textpm\fi{}0.4}}$ and 3.6\ifmmode\pm\else\textpm\fi{}0.6. For the system $^{175}$Lu${+\mathrm{}}^{12}$C it is found that 0.5\ifmmode\pm\else\textpm\fi{}0.2 preequilibrium neutrons are emitted. In contrast to the evaporative neutrons, a strong anisotropy ${a}_{\mathrm{n}{}^{\mathrm{PE}=2.2\ifmmode\pm\else\textpm\fi{}0.6}}$ relative to the reaction plane defined by one fission fragment and the beam direction is observed.From the prescission neutron multiplicities, the evaporation time of the system prior to scission is deduced using the statistical model to \ensuremath{\approxeq}(3--12)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ s. Nucleus deformation effects and neutron emission from not fully accelerated fission fragments are taken into account. The unexpected long prescission lifetimes can be explained as long transition times to the scission point caused by a large two-body viscosity. Under this assumption the viscosity parameter of the highly excited nuclei has been determined in a first approximation to \ensuremath{\mu}\ensuremath{\approxeq}0.1 TP. The results might be understood also assuming a mixture of a two-body and one-body friction.