J. Wilczyński

DYNAMICAL EVOLUTION OF THE 197Au + 197Au SYSTEM AT 15 MeV/NUCLEON

Collisions of a very heavy 197Au + 197Au system have been studied at an energy of 15 MeV/nucleon with the aim to investigate a possible change of the energy dissipation mechanism from one-body dissipation (presumably dominating at low excitation energies) to two-body dissipation. The experiment was carried out in 4π geometry using the CHIMERA multidetector array. A class of ternary events satisfying nearly complete balance of mass numbers was selected. Preliminary results suggest that the selected ternary events represent mostly prompt ternary partitions of the colliding system, in which the lightest fragment is quite massive. In the light of theoretical predictions of Cârjan, Sierk and Nix this result may imply that the observed ternary reactions are driven by the two-body dissipation mechanism already at 15 MeV/nucleon.

CALCULATIONS OF THE FORMATION CROSS SECTIONS FOR SYNTHESIS OF Z = 114–118 NUCLEI IN 48Ca INDUCED REACTIONS

Calculations of the evaporation-residue cross sections for production of super-heavy elements of Z = 114–118 in hot fusion (3n and 4n) reactions induced by 48Ca projectiles on Pu, Am, Cm and Cf targets are presented. Results of calculations are compared with the cross sections measured by Oganessian et al. in the experiments in which the Z = 114–118 elements had been discovered. Calculations were done for the ground-state masses and fission saddle-point energies of Myers and Swiatecki and Sobiczewski et al. Considerable differences in the theoretical masses result in large differences (of several orders of magnitude) in the predicted cross sections. The ground-state masses and fission barriers of Sobiczewski et al. seem to well reproduce the experimental cross sections.

TERNARY REACTIONS IN 197AU + 197AU COLLISIONS REVISITED

Ternary events originating from the 197Au + 197Au reaction at 15 MeV/nucleon, studied by using the multidetector array CHIMERA at LNS Catania, have been collected in almost complete 2π range of the forward hemisphere of CHIMERA thus extending the angular range of our previously reported preliminary data. Energy- and time calibrations for heavy fragments at large angles were supplemented with an analysis of coincidence events of binary strongly damped reactions. Contrary to the previous preliminary results, the new complete data clearly show presence of sequential fission processes in which one of Au-like nuclei undergoes fission after re-separation of the primary binary system. Mechanism of these two types of reactions observed in our data, sequential fission reactions and prompt ternary processes, are presently studied using comparisons with predictions of the stochastic BNV model of Baran, Colonna and Di Toro and the QMD model of Łukasik.

NEUTRON ENRICHMENT OF THE NECK-ORIGINATED INTERMEDIATE MASS FRAGMENTS IN PREDICTIONS OF THE QMD MODEL

The effect of neutron enrichment of the neck formed during reseparation of two colliding nuclei was studied with the aim to infer an information on the density dependence of the asymmetry energy term in the equation of state. By using a version of the QMD model of Łukasik it is shown that the neutrons-to-protons ratio of the intermediate mass fragments (IMF) emitted from the neck, 〈N/Z〉, is not a sufficiently sensitive observable to discriminate between different assumptions regarding the symmetry term of the equation of state. As an alternative, isotopic ratios of selected pairs of isotopes turned out to be useful observables more sensitive to the assumed form of the symmetry term of the equation of state. A comparison with experimental data on isotopic ratios for beryllium and boron IMFs from the 124Sn + 64Ni reaction at 35 MeV/nucleon tentatively suggests that the ASY-SOFT option of the symmetry energy term in EOS is preferred.

ISOTOPIC EFFECTS IN EMISSION OF INTERMEDIATE MASS FRAGMENTS IN THE 124Sn + 64Ni REACTION AT 35 MeV/NUCLEON

We present results of a new analysis of the experiment carried out by the REVERSE-ISOSPIN collaboration on the 124Sn + 64Ni reaction at the bombarding energy of 35 MeV/nucleon. The mass numbers of the intermediate mass fragments (isotopes of Be, B and C) could be resolved in this experiment. Angular distributions of the neck fragmentation products in the rest frame of the primary projectile-like fragment were studied. The angular dependence of the isotopic ratios of the intermediate mass fragments proves the effect of neutron enrichment of the neck region.

MASS AND ENERGY CORRELATIONS IN FAST TERNARY BREAK-UP OF 197Au + 197Au SYSTEM

Detailed characteristics of a new reaction mechanism of fast ternary break-up in collisions of a heavy nucleus-nucleus system 197Au + 197Au at 15 MeV/nucleon, studied in experiments at the INFN LNS Laboratory in Catania by using the multidetector array CHIMERA are presented. Results of an analysis of mass and energy correlations complement our earlier report that demonstrated the mechanism of aligned re-separation of the Au + Au system into three massive fragments in semi-peripheral collisions.

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. 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 rotating Au+Au system is investigated in the present paper. For this purpose the Los Alamos finite-range macroscopic dynamical model 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.

CALCULATIONS OF FUSION-EVAPORATION CROSS SECTIONS IN THE 48Ca + 206Pband48Ca + 208Pb REACTIONS

By using a unique set of experimental data on (1n, 2n, 3n and 4n) fusion-evaporation cross sections for 48Ca + 206Pb and 48Ca + 208Pb reactions, measured by Oganessian et al., we have tested the conventional Bohr-Wheeler description of the competition between evaporation and fission in these reactions. Level densities in both, the equilibrium and saddle-point configurations were calculated using the Reisdorf-Ignatyuk prescription. Strutinsky shell corrections of Moller et al. were used for the equilibrium, and zero shell corrections were assumed for the saddle-point configuration. The model does not contain adjustable parameters.

IMPACT-PARAMETER DEPENDENCE OF TERNARY AND QUATERNARY RESEPARATION OF HEAVY COLLIDING SYSTEMS

Recently, we reported on the observation of a new reaction mechanism of fast collinear breakup of the 197Au + 197Au system into three and four fragments of comparable size. Here we present results of an additional analysis that shows that these ternary and quaternary breakup reactions occur at extremely deep-inelastic collisions corresponding to small values of the impact parameter, while more peripheral collisions lead to well known binary deep-inelastic reactions.