Fusion and capture of heavy ions above the barrier: Analysis of experimental data with the surface friction model

Abstract

A systematic analysis of experimental data on fusion and capture of heavy ions above the barrier over a wide mass and energy range is performed within the surface friction model. The essential feature of this model originally proposed by Gross, Satpathy and Nayak is that the friction acts between the juxtaposed surfaces of the colliding nuclei-therefore its name; the surface friction leads to oblate deformations during the approach phase and to strong prolate deformations in the exit channel. The model used in the present paper is an extended version of the surface friction model by allowing for independent and different (quadrupole) deformations for projectile and target, the corresponding dissipative forces and statistical fluctuations in all variables. Thus one is not only able to reproduce a large variety of fusion data but can simultaneously describe at least the rough features of deep-inelastic collisions. Only three universal parameters enter the model: the strengths of the radial friction, of the tangential friction and of the intrinsic damping of the deformation modes. As the friction acts between the juxtaposed surfaces of the colliding nuclei the dissipative processes take place at distances 1–2fm larger than within a proximity description. For a large variety of systems the present calculations are compared with experimental fusion and capture data, with spherical proximity model calculations and with extra push model predictions. Our analysis leads to the conclusion that heavy-ion fusion and capture above the barrier at least up to energies where incomplete fusion processes start to occur are essentially determined by properties of the entrance channel. This means that fusion (or capture) occurs when trajectories pass over the saddle of the energy surface as a function of the radial distance and the deformation degrees of freedom. The success of the calculations suggests that fusion and capture data do not give information on the dynamics of the exit channel (in particular on neck formation) with the exception of fast fission systems.