A. Olmi

Dynamics of the fusion process

Abstract The binary reaction products from the interaction of a 208Pb beam with targets of 26Mg, 27Al, 48Ca, 50Ti, 52Cr, 58Fe and 64Ni have been studied with the aid of a large position-sensitive ring counter, operated in a two-particle coincidence mode. The intensity of γ-rays and X-rays per event was also recorded. Within a broad range around mass symmetry, the center of mass angular distributions, γ-ray multiplicities, total kinetic energy distributions, and absolute mass yields have been determined as a function of the bombarding energy, ranging from 1.0 to 1.8 times the interaction barrier. When analysed on the basis of fusion models, the cross section for the mass equilibration reaction demonstrates that deformations induced at contact influence the fusion of these heavy systems significantly and characteristically. The γ-ray multiplicities appear to be strongly influenced by statistical angular momentum components that remain with the fragments after separation. Their magnitudes indicate that during the reaction, the collision complex becomes at least as compact as the liquid-drop saddle shape appropriate to a non-rotating nucleus with the same total mass and charge. Finally, some angular distributions show that the entire process of coalescence and reseparation can occur before the system has made one-half revolution; i.e. within a time of 5–10 × 10−21 s. No X-ray emission from the combined system 208Pb + 48Ca is observed.

Reactions between 12C and heavy target nuclei at 84 MeV/u

Abstract Using an experimental setup which allows the coincident investigation of the various projectile and target fragments the reactions between 84 MeV/u 12 C projectiles and different targets were studied. Impact parameters have been deduced from the size and multiplicity of projectile fragments. Using a “minimum bias” triqqer relative cross sections have been determined for the different reaction channels. New results on the production of medium-heavy fragments are presented.

The PbPb collision

Abstract The reaction 208 Pb on 208 Pb was studied at bombarding energies of 7.0 and 7.57 MeV/u. One-particle inclusive measurements using a large-area position-sensitive ionisation chamber delivered the kinetic energy, charge and scattering angle of the reaction products. A precise calibration of the stopping power for very heavy ions in the detector gas was performed. The measured Wilczynski diagrams show, for increasing loss of kinetic energy, an increase of the mean scattering angle. It is attributed to the dominance of the repulsive Coulomb forces with respect to the attractive nuclear forces. The element distribution for the 208 Pb on 238 U reaction at 7.5 MeV/u was also measured and compared to the PbPb and UU reactions. Fission probabilities are derived as a function of charge and total kinetic energy loss. The most striking result is seen in the σ z 2 versus TKEL correlation: the average rate of energy loss per nucleon exchange is abnormally large. It is shown that this behaviour is associated with the double magic closed shell character of the colliding nuclei. Nuclear structure information is extracted through a simple parametrisation.

Energy loss and nucleon exchange in the reaction of 86Kr with 166Er

Abstract Projectile-like products from the reaction 86 Kr on 166 Er at 8.18 MeV/amu have been measured using a large-area position-sensitive ionization chamber. The observed element distributions for a given energy loss are found to be asymmetric. An analysis of the data in terms of the residual kinetic energy above the Coulomb barrier, however, yields symmetric and Gaussian-shaped element distributions. At large energy losses, the centroids of these Gaussian distributions exhibit a drift towards symmetric mass splits. The correlation between the variance of the element distribution σ z 2 and the number of exchanged nucleons N ex is discussed. The exchange mechanism, as described by a simple one-body dissipation model, can account for a large portion of the measured energy loss.

Mass drift in reactions between a heavy and a light nucleus

Abstract The dynamics of heavy-ion collisions is studied experimentally for metastable composite systems with interaction times between the collision time of quasi- and deep-inelastic processes and the capture time of fast-fission and compound-nucleus-fission reactions. Detailed angular, total kinetic energy and mass distributions are measured for reaction products with masses between the target and projectile masses including the symmetric masses. The mass drift is observed as a function of total kinetic energy and scattering angle.

Fast dissipative collisions

Abstract At a bombarding energy of 14.7 MeV/u the charge, mass and angular distributions have been measured as a function of energy loss for reaction products induced by 98 Mo and 92 Mo projectiles on targets of 98 Mo, 147 Sm, 154 Sm and 92 Mo, 154 Sm, 238 U, respectively. An analysis of the product distributions shows the presence of a non-equilibrium sharing of the excitation energy, an exponential growth of the charge and mass variances, a complete N Z equilibration and a large yield of non-binary complex fragments. Monte Carlo simulations are found to be an indispensable tool of the analysis.

Angular dependence of the products in the 86Kr + 166Er collision

Abstract The reaction 86 Kr + 166 Er is studied at a bombarding energy of 8.2 MeV/u. The results obtained in a one-particle inclusive measurement using a large-area position-sensitive ionisation chamber are studied in detail with respect to the experimental observables: nuclear charge Z , c.m. scattering angle θ and total kinetic energy loss TKEL. It is investigated which quantity is the most relevant in order to follow the evolution of dissipative phenomena. First and second moments of the angular distributions ( θ 0 and Γ θ ) are extracted as a function of energy loss using an unfolding procedure for the positive and negative scattering angles. A linear correlation between θ 0 and σ 2 z is evidenced. Using simplifying assumptions an upper limit of 310 mb is estimated for a long-living symmetric fragmentation component.

Structure effects in dissipative collisions

Abstract An ansatz representing a generalisation of the one-body recoil formula is proposed on the basis of the total kinetic energy loss versus σ Z 2 correlation systematics observed in collisions between very heavy nuclei. It is shown that such correlations are surprisingly sensitive to the shell structure of the colliding nuclei. The mechanism of energy dissipation appears also to be strongly influenced by the structure effects.

On the influence of shell structure in dissipative collisions

Abstract A kinematically complete study of the symmetric systems 144 Sm+ 144 Sm and 154 Sn+ 154 Sm has been performed at energies 30% in excess of the interaction barrier. They have been chosen because of their different internal structure: 144 Sm has a closed N = 82 neutron shell and a spherical ground-state configuration; 154 Sm with ten neutrons outside this shell is strongly deformed. The observed gross features of both reactions like energy, angular and total mass or element distributions are very similar; the ratio of the mass variances as function of the total kinetic energy loss indicates the number of exchanged nucleons to be comparable in all stages of the reactions. At small energy losses, however, the element distributions of the 144 Sm + 144 Sm reaction are considerably broader, pointing at an enhanced proton transfer at the cost of the number of exchanged neutrons in this system. This observation is attributed to the influence of the closed shell which seems to block the transfer of neutrons at low excitation energies. These results can be explained quantitatively by the different gradients of the shell-corrected potential energy surfaces of the two systems.

Influence of shells on the driving potential in dissipative collisions

Abstract The 184 W + 232 Th reaction has been studied at E Lab = 8.2 MeV/u in order to test the influence of shells on the mean value of the element distributions in damped collisions. No significant drift toward the 208 Pb + 208 Pb system has been observed. A comparison with the 184 W + 184 W collision sets an upper limit of one charge unit to the drift.