Dynamical aspects of isotopic scaling

Investigation of the effect of the dynamical stage of heavy-ion collisions indicates that the increasing width of the initial isospin distributions is reflected by a significant modification of the isoscaling slope for the final isotopic distributions after de-excitation. For narrow initial distributions, the isoscaling slope assumes the limiting value of the two individual initial nuclei while for wide initial isotopic distributions the slope for hot fragments approaches the initial value. The isoscaling slopes for final cold fragments increase due to secondary emissions. The experimentally observed evolution of the isoscaling parameter in multifragmentation of hot quasiprojectiles at E$_{inc}$=50 AMeV, fragmentation of $^{86}$Kr projectiles at E$_{inc}$=25 AMeV and multifragmentation of target spectators at relativistic energies was reproduced by a simulation with the dynamical stage described using the appropriate model (deep inelastic transfer and incomplete fusion at the Fermi energy domain and spectator-participant model at relativistic energies) and the de-excitation stage described with the statistical multifragmentation model. In all cases the isoscaling behavior was reproduced by a proper description of the dynamical stage and no unambiguous signals of the decrease of the symmetry energy coefficient were observed.Comment: LaTeX, 18 pages, 9 figures, to appear in Phys. Rev.

Effect of nuclear periphery on nucleon transfer in peripheral collisions

A comparison of experimental heavy residue cross sections from the reactions 86Kr+64Ni,112,124Sn with the model of deep-inelastic transfer (DIT) is carried out. A modified expression for nucleon transfer probabilities is used at non-overlapping projectile-target configurations, introducing a dependence on isospin asymmetry at the nuclear periphery. The experimental yields of neutron-rich nuclei close to the projectile are reproduced better and the trend deviating from the bulk isospin equilibration is explained. For the neutron-rich products further from the projectile, originating from hot quasiprojectiles, the statistical multifragmentation model reproduces the mass distributions better than the model of sequential binary decay. In the reaction with proton-rich target 112Sn the nucleon exchange appears to depend on isospin asymmetry of nuclear periphery only when surface separation is larger than 0.8 fm due to the stronger Coulomb interaction at more compact di-nuclear configuration.Comment: LaTeX, 13 pages, 7 figures, to appear in Nuclear Physics

Production of cold fragments in nucleus-nucleus collisions in the Fermi-energy domain

The reaction mechanism of nucleus-nucleus collisions at projectile energies around the Fermi energy is investigated with emphasis on the production of fragmentation-like residues. The results of simulations are compared to experimental mass distributions of elements with Z = 21 - 29 observed in the reactions 86Kr+124,112Sn at 25 AMeV. The model of incomplete fusion is modified and a component of excitation energy of the cold fragment dependent on isospin asymmetry is introduced. The modifications in the model of incomplete fusion appear consistent with both overall model framework and available experimental data. A prediction is provided for the production of very neutron-rich nuclei using a secondary beam of 132Sn where e.g. the reaction 132Sn+238U at 28 AMeV appears as a possible alternative to the use of fragmentation reactions at higher energies.Comment: LaTeX, 15 pages, 5 figures, minor modifications, accepted for publication in Nuclear Physics

Production mechanism of hot nuclei in violent collisions in the Fermi energy domain

A production mechanism of highly excited nuclei formed in violent collisions in the Fermi energy domain is investigated. The collision of two nuclei is decomposed into several stages which are treated separately. Simplified exciton concept is used for the description of pre-equilibrium emission. A modified spectator-participant scenario is used where motion along classical Coulomb trajectories is assumed. The participant and one of the spectator zones undergo incomplete fusion. Excitation energies of both cold and hot fragment are determined. Results of the calculation are compared to recent experimental data in the Fermi energy domain. Data on hot projectile-like, mid-velocity and fusion-like sources are described consistently. Geometric aspects of pre-equilibrium emission are revealed. Explanations to previously unexplained experimental phenomena are given. Energy deposited into non-thermal degrees of freedom is estimated.Comment: To appear in Nuclear Physics A, 27 pages, 19 figures, LaTe

Isoscaling Studies of Fission - a Sensitive Probe into the Dynamics of Scission

The fragment yield ratios were investigated in the fission of 238,233U targets induced by 14 MeV neutrons. The isoscaling behavior was typically observed for the isotopic chains of fragments ranging from the proton-rich to the most neutron-rich ones. The observed high sensitivity of neutron-rich heavy fragments to the target neutron content suggests fission as a source of neutron-rich heavy nuclei for present and future rare ion beam facilities, allowing studies of nuclear properties towards the neutron drip-line and investigations of the conditions for nucleosynthesis of heavy nuclei. The breakdowns of the isoscaling behavior around N=62 and N=80 manifest the effect of two shell closures on the dynamics of scission. The shell closure around N=64 can be explained by the deformed shell. The investigation of isoscaling in the spontaneous fission of 248,244Cm further supports such conclusion. The Z-dependence of the isoscaling parameter exhibits a structure which can be possibly related to details of scission dynamics. The fission isoscaling studies can be a suitable tool for the investigation of possible new pathways to synthesize still heavier nuclei.Comment: 7 pages, 3 figures, RevTex, final version, to appear in Phys. Rev. C as a regular articl