Macroscopic Dynamical Description of Rotating

(Au + Au)-System

Events with more than two heavy fragments have been abundantly observed in heavy-ion semi-peripheral (fission-like) reaction 197Au + 197Au at 15 MeV/nucleon [1]. This raised interesting questions about their origin and about the time-scale at which they occur. As a possible explanation of this process, the surface instability of the cylindrical neck that is formed along the path from contact to reseparation of the rotationg (Au+Au)-system is investigated in the present paper. For this purpose the Los Alamos finite-range macroscopic dynamical model [2] was used. The calculations were performed at relatively high angular momenta, L=100 to 300 ħ, for two types of dissipation mechanisms: two-body viscosity and one-body dissipation.Various initial nuclear deformations and initial kinetic energies in the fission direction were considered. The resulting dynamical evolution in the multidimensional deformation space always led to multifragment scission configurations suggesting that ternary and quaternary break-up can occur during the heavy-ion reaction studied.

[1] I. Skwira-Chalot, K. Siwek-Wilczyńska, J. Wilczyński, Int. J. Mod. Phys. E16 (2007) 511[2] N. Cârjan, A.J. Sierk and J.R. Nix, Nucl. Phys. A452 (1986) 381

N. Cârjan, K. Siwek-Wilczyńska, I. Skwira-Chalot, J. Wilczynski

a) The fission of the Au-like fragment is mainly asymmetric

b) The sum of the 2 fission fragments is heavier than Au (210>197); since there are also 16 evaporated nucleons the fissioning system is much heavier than Au.

It is improbable thatIt is improbable that the origin of

these ternary events is the sequential fission of Au-like nuclei excited after the target has transferred about 20 nucleons to the projectile.It must be something else.

Dynamical evolution strongly depends on the type of dissipation

Higher angular momentum preserves this feature but increases the middle- fragment mass m2

Initial kinetic energy in the fission direction produces larger middle mass. Note that the initial configuration is deformed in this example.

Strongly deformed initial configuration is more realistic: resembles better to a target + projectile in contact

However the initial deformation is not crucial for 2-body dissipation

The same as before but for higher initial kinetic energy in the fission direction

Effect of the initial kinetic energy in the case of 1-body dissipation and moderate initial deformation: it also leads to a larger middle mass.

The same as before but for a more elongated initial configuration: the effect of the kinetic energy in the fission direction is much smaller.

Comparison between the results for a compact and an elongated initial configuration: contrary to 2-body dissipation the outcome is very different

The same as before but for a higher kinetic energy in the fission direction