By considering momentum transfer in the Fermi constraint procedure, the stability of the initial nuclei and fragments produced in heavy-ion collisions can be further improved in quantum molecular dynamics simulations. The case of a phase-space occupation probability larger than one is effectively reduced with the proposed procedure. Simultaneously, the energy conservation can be better described for both individual nuclei and heavy-ion reactions. With the revised version of the improved quantum molecular dynamics model, the fusion excitation functions of 16O+186W and the central collisions of Au+Au at 35 AMeV are re-examined. The fusion cross sections at sub-barrier energies and the charge distribution of fragments are relatively better reproduced due to the reduction of spurious nucleon emission. The charge and isotope distribution of fragments in Xe+Sn, U+U and Zr+Sn at intermediate energies are also predicted. More unmeasured extremely neutron-rich fragments with Z=16–28 are observed in the central collisions of 238U+238U than that of 96Zr+124Sn, which indicates that multi-fragmentation of U+U may offer a fruitful pathway to new neutron-rich isotopes.
The dynamic mechanics in the multinucleon transfer reaction 136Xe+208Pb at an incident energy of Ec.m.=450 MeV is investigated by using the improved quantum molecular dynamics model (ImQMD). The lifetime of the neck directly influences the nucleon exchange and energy dissipation between the projectile and the target. The total-kinetic-energy–mass distributions and excitation energy division of primary binary fragments and the mass distributions of primary fragments at different impact parameters are calculated. The thermal equilibrium between two reaction partners has been observed at the lifetime of a neck larger than 480 fm/c. By using the statistical decay code GEMINI to describe the de-excitation process of the primary fragments, the isotope production cross sections from Pt to At are compared with the prediction by the dinuclear system and GRAZING model. The calculations indicate that the GRAZING model is suitable for estimating the isotope production cross sections only for Z = −1 to +2; the DNS + GEMINI calculations underestimate the cross sections in the neutron-rich and neutron-deficient regions; and the ImQMD + GEMINI calculations give reasonable predictions of the isotope production cross sections for Z = −3 to 0.
We present the calculations on a novel reorientation effect of deuteron attributed to isovector interaction in the nuclear field of heavy target nuclei. The correlation angle determined by the relative momentum vector of the proton and the neutron originating from the breakup deuteron, which is experimentally detectable, exhibits significant dependence on the isovector nuclear potential but is robust against the variation of the isoscaler sector. In terms of sensitivity and cleanness, the breakup reactions induced by the polarized deuteron beam at about 100 MeV/u provide a more stringent constraint to the symmetry energy at subsaturation densities.
Some nearly-symmetric fusion reactions are systematically investigated with the improved quantum molecular dynamics (ImQMD) model. By introducing two-body inelastic scattering in the Fermi constraint procedure, the stability of an individual nucleus and the description of fusion cross sections at energies near the Coulomb barrier can be further improved. Simultaneously, the quasifission process in 154Sm+160Gd is also investigated with the microscopic dynamics model for the first time. We find that at energies above the Bass barrier, the fusion probability is smaller than 10-5 for this reaction, and the nuclear contact time is generally smaller than 1500 fm/c. From the central collisions of Sm+Gd, the neutron-rich fragments such as 164,165Gd, 192W can be produced in the ImQMD simulations, which implies that the quasi-fission reaction could be an alternative way to synthesize new neutron-rich heavy nuclei.
The improved quantum molecular dynamics (ImQMD) model incorporated with the statistical evaporation model is applied to study the production mechanism of transuranium nuclei in the reaction of ^{238}U+^{238}U at 7.0 MeV/nucleon. The production of primary fragments in the dynamical process is simulated by the ImQMD model, and the decays of them are described by the statistical evaporation model (hivap code). The calculated isotope distributions of the residual fragments and the most probable mass number of fragments are generally in agreement with experimental data. By tracking residual fragments back to their original primary fragments with this approach, we find different mechanisms for the production of the residues: the most probable light uraniumlike residues mainly come from the decay of the most probable primary fragments, while the most probable transuranium residues mainly originate from the decay of more neutron-rich primary fragments rather than from the most probable primary ones. For neutron-rich transuranium isotopes ^{254−256}Cf, the decay channel of neutron evaporation is suppressed due to the quick drop of the fission barrier height with the increase of neutron number, which leads to the quick drop of the production cross sections for these residues.
Using covariance analysis, we quantify the correlations between the interaction parameters in a transport model and the observables commonly used to extract information of the Equation of State of Asymmetric Nuclear Matter in experiments. By simulating 124Sn + 124Sn, 124Sn + 112Sn and 112Sn + 112Sn reactions at beam energies of 50 and 120 MeV per nucleon, we have identified that the nucleon effective mass splitting is most strongly correlated to the neutrons and protons yield ratios with high kinetic energy from central collisions especially at high incident energy. The best observable to determine the slope of the symmetry energy, L , at saturation density is the isospin diffusion observable even though the correlation is not very strong (∼0.7). Similar magnitude of correlation but opposite in sign exists for isospin diffusion and nucleon isoscalar effective mass. At 120 MeV/u, the effective mass splitting and the isoscalar effective mass also have opposite correlation for the double n/pn/p and isoscaling p/pp/p yield ratios. By combining data and simulations at different beam energies, it should be possible to place constraints on the slope of symmetry energy (L) and effective mass splitting with reasonable uncertainties.
To explore the roles of the dynamical model and statistical model in the description of multifragmentation in heavy-ion collisions at intermediate energies, the fragments charge distributions of ^{197}Au+^{197}Au at 35 MeV/u are analyzed by using the hybrid model of improved quantum molecular dynamics (ImQMD) model plus the statistical model GEMINI. We find that, the ImQMD model can well describe the charge distributions of fragments produced in central and semicentral collisions. But for the peripheral collisions of ^{197}Au+^{197}Au at 35 MeV/u, the statistical model is required at the end of the ImQMD simulations for the better description of the charge distribution of fragments. By using the hybrid model of ImQMD+GEMINI, the fragment charge distribution of ^{197}Au+^{197}Au at 35 MeV/u can be reproduced reasonably well. The time evolution of the excitation energies of primary fragments is simultaneously investigated.
The Improved Quantum Molecular Dynamics model incorporated with the statistical decay model is used to investigate the intermediate energy nucleon-induced reactions. In our last work, by introducing a phenomenological mechanism called surface coalescence and emission into ImQMD model, the description on the light complex particle emission has been great improved. In this work, taking account of different specific binding energies and separation energies for various light complex particles, the phase space parameters in surface coalescence model are readjusted. By using the new phase space parameters set with better physical fundament, the double differential cross sections of light complex particles are found to be in better agreement with experimental data.
Applying a macroscopic reduction procedure to the improved quantum molecular dynamics (ImQMD) model, the energy dependences of the nucleus-nucleus potential, the friction parameter, and the random force characterizing a one-dimensional Langevin-type description of the heavy-ion fusion process are investigated. Systematic calculations with the ImQMD model show that the fluctuation-dissipation relation found in symmetric head-on fusion reactions at energies just above the Coulomb barrier fades out when the incident energy increases. It turns out that this dynamical change with increasing incident energy is caused by a specific behavior of the friction parameter which directly depends on the microscopic dynamical process, i.e., on how the collective energy of the relative motion is transferred into the intrinsic excitation energy. It is shown microscopically that the energy dissipation in the fusion process is governed by two mechanisms: One is caused by the nucleon exchanges between two fusing nuclei, and the other is due to a rearrangement of nucleons in the intrinsic system. The former mechanism monotonically increases the dissipative energy and shows a weak dependence on the incident energy, while the latter depends on both the relative distance between two fusing nuclei and the incident energy. It is shown that the latter mechanism is responsible for the energy dependence of the fusion potential and explains the fading out of the fluctuation-dissipation relation.
The ternary breakup mechanism of ^{238}U+^{197}Au at 15A MeV has been investigated by a hybrid model that combines the improved quantum molecular dynamics (ImQMD) model together with a statistical code gemini++. The results are in good agreement with the experimental data and indicate that in peripheral reactions, ternary breakup in this reaction results from quasi-U statistical fission while for central and semicentral collisions it can be understood by a two-step mechanism: deep-inelastic collision (DIC) followed by a sequential binary breakup of one of the DIC products. In the process of DIC, there is a large mass transfer from Au to U to form transuranium. Due to the low fission barrier, such transuranium nuclei will decay into stable light nuclei through various fission modes. An event-by-event analysis shows that the second breakup mainly occurs in the deexcitation process and most of the ternary breakup events are from semicentral and peripheral collisions that correspond to deep inelastic and quasi-elastic reactions, respectively.
Fission fragments resulting from the fission of target-like nuclei produced in the ^{40}Ar+^{197}Au at 35 MeV/u are measured in coincidence with the emitted light charged particles (LCPs). Comparison of the N/Z composition of the LCPs at middle and large angles in the laboratory frame shows that particles emitted at smaller angles, which contain a larger contribution from dynamical emission, are more neutron rich. A moving-source model is used to fit the energy spectra of the hydrogen isotopes. A hierarchy from proton to deuteron and triton is observed in the multiplicity ratio between the intermediate velocity source and the compound nucleus source. This ratio is sensitive to the dynamical emission at early stages of the reaction and to statistical emission lasting up to the scission point. Calculations with the improved quantum molecular dynamics (ImQMD) transport-model qualitatively support the picture that more free and bound neutrons are emitted during the early stage, showing a clear dependence of N/Z on the parametrization of the symmetry energy. The time-dependent isospin composition of the emitted particles thus may be used to probe the symmetry energy at subsaturation densities.
The heavy-ion fusion reactions induced by neutron-rich nuclei are investigated with the improved quantum molecular dynamics (ImQMD) model. With a subtle consideration of the neutron skin thickness of nuclei and the symmetry potential, the stability of nuclei and the fusion excitation functions of heavy-ion fusion reactions ^{16}O+^{76}Ge, ^{16}O+^{154}Sm, ^{40}Ca+^{96}Zr, and ^{132}Sn+^{40}Ca are systematically studied. The fusion cross sections of these reactions at energies around the Coulomb barrier can be well reproduced by using the ImQMD model. The corresponding slope parameter of the symmetry energy adopted in the calculations is L≈78 MeV and the surface energy coefficient is g_{sur}=18±1.5 MeV fm^{2}. In addition, it is found that the surface-symmetry term significantly influences the fusion cross sections of neutron-rich fusion systems. For sub-barrier fusion, the dynamical fluctuations in the densities of the reaction partners and the enhanced surface diffuseness at neck side result in the lowering of the fusion barrier.
With the aid of our recent experiment, the fragmentation of 56Fe at 471 A MeV interacting with C and Al targets has been systematically studied by the improved quantum molecular dynamics model together with the statistical GEMINI model. The fragment distributions in heavy-ion collisions at intermediate energies can be well reproduced by using this combination of the two models. It is found that the odd–even effect of the partial cross sections observed in experiments appears in the de-excitation process of the excited primary fragments as a result of pairing effect and is mainly formed in the grazing collisions. The peaked angular distributions of primary ions and their fragments are dominantly due to the heavier fragments produced in the grazing collisions and reveal the nonequilibrium property of collisions and the memory effects of outgoing fragments on the entrance channel.
The dynamics of energy dissipation in head-on fusion reactions of mass-symmetric systems at low bombarding energies is studied by exploiting the improved quantum molecular dynamics model. The results indicate that the form and magnitude of the mass parameter and friction coefficient show strong dependence on system size, bombarding energy, and relative distance. The dynamical mass parameter has almost no effect on the friction coefficient. Two-body collisions play an important role in energy dissipation even when the incident energy is much lower than the Fermi energy. The nucleon-nucleon collisions not only attenuate the energy dissipation but also hinder the nucleon transfer process.
The dynamical effects of initial orientation and deformation (with and without β_{4} deformation) of projectile and target on the ^{238}U+^{238}U reaction have been investigated by using the improved quantum molecular dynamics model. The deformation of colliding nuclei at touching configuration is distributed with a wide width, especially for the nose-nose orientation case. The influence of different orientations and deformations on the average lifetime of the transient composite system and the production probability of superheavy fragments (SHF) with Z>110 at incident energies 7–12 A MeV is studied. The average lifetime of the composite system as a function of incident energy peaks at about 9–10 A MeV for the side-side orientation and 11–12 A MeV for the nose-nose orientation, respectively. The inclusion of β_{4} deformation in prolate uranium seems to enhance the production probability of SHF in the side-side case as the more stable structure of initial uranium effectively reduces the excitation energy of the composite system. It is suggested that the optimal initial condition for production of SHF in ^{238}U+^{238}U could be the side-side orientation and prolate deformation with a small β_{4} deformation (the ground state of uranium). Considering that the symmetry axis of the initial nuclei is oriented in an arbitrary direction with an equal probability, the lifetime of composite system and the SHF production probability are also studied with randomly selected orientation direction of initial nuclei.
Macroscopic parameters as well as precise information on the random force characterizing the Langevin-type description of the nuclear fusion process around the Coulomb barrier are extracted from the microscopic dynamics of individual nucleons by exploiting the numerical simulation of the improved quantum molecular dynamics. It turns out that the dissipation dynamics of the relative motion between two fusing nuclei is caused by a non-Gaussian distribution of the random force. We find that the friction coefficient as well as the time correlation function of the random force takes particularly large values in a region a little bit inside of the Coulomb barrier. A clear non-Markovian effect is observed in the time correlation function of the random force. It is further shown that an emergent dynamics of the fusion process can be described by the generalized Langevin equation with memory effects by appropriately incorporating the microscopic information of individual nucleons through the random force and its time correlation function.
The mechanism of the ternary breakup of the very heavy system 197Au+197Au at an energy of 15A MeV has been studied by using the improved quantum molecular dynamics model. The calculation results reproduce the characteristic features in ternary breakup events explored in a series of experiments; i.e., the masses of three fragments are comparable in size and the very fast, nearly collinear breakup of the colliding system is dominant in the ternary breakup events. Further, the evolution of the time scales of different ternary reaction modes and the behavior of mass distributions of three fragments with impact parameters are studied. The time evolution of the configurations of the composite reaction systems is also studied. We find that for most of the ternary breakup events with the features found in the experiments, the configuration of the composite system has two-preformed-neck shape. The study shows that those ternary breakup events having the characteristic features found in the experiments happen at relatively small impact parameter reactions, but not at peripheral reactions. The ternary breakup reaction at peripheral reactions belongs to binary breakup with a neck emission.
Collisions of very heavy nuclei 197Au + 197Au at the energy of 15A MeV has been studied with the improved quantum molecular dynamics model. The experimental mass distributions of ternary fission fragments for the system 197Au + 197 Au are reproduced well. The direct and sequential ternary fission modes are studied by the time dependent snapshots of typical ternary events. The analysis of deviation from Viola systematics indicates the nonstatistical feature of the ternary fission in these reactions.
The dynamical nucleus-nucleus potentials for fusion reactions 40Ca+40Ca, 48Ca+208Pb and 126Sn+130Te are studied with the improved quantum molecular dynamics (ImQMD) model together with the extended Thomas-Fermi approximation for the kinetic energies of nuclei. The obtained fusion barrier for 40Ca+40Ca is in good agreement with the extracted fusion barrier from the measured fusion excitation function, and the depth of the fusion pockets are close to the results of time-dependent Hartree-Fock calculations. The energy dependence of fusion barrier is also investigated. For heavy fusion system, the fusion pocket becomes shallow and almost disappears for symmetric systems and the obtained potential at short distances is higher than the adiabatic potential.
Within the improved quantum molecular dynamics (ImQMD) model incorporating the statistical decay model, the reactions of 238U+238U at an energy of 7.0A?MeV have been studied. The charge, mass, and excitation energy distributions of primary fragments are investigated within the ImQMD model, and de-excitation processes of these primary fragments are described by a statistical decay model. The mass distribution of the final products in 238U+238U collisions is obtained and compared with the recently obtained experimental data.
Mass parameters for the relative and neck motions in fusion reactions of symmetric systems 90Zr+90Zr, 110Pd+110Pd, and 138Ba+138Ba are studied by means of a microscopic transport model. The shape of the nuclear system is determined by an equidensity surface obtained from the density distribution of the system. The relative and neck motions are then studied and the mass parameters for these two motions are deduced. The mass parameter for the relative motion is around the reduced mass when the reaction partners are at the separated configuration and increases with a decrease of the distance between two reaction partners after the touching configuration. The mass parameter for the neck motion first decreases slightly up to the touching configuration and then increases with the neck width, and its magnitude is from less than a tenth to several times more than the total mass of the system. The mass parameters obtained from the microscopic transport model are larger than the ones obtained from the hydrodynamic model and smaller than those obtained from the linear response function theory. The mass parameters for both motions depend on the reaction systems, but the one for the relative motion depends on the incompressibility of the EoS more obviously than that for neck motion.
Collisions involving 112Sn and 124Sn nuclei have been simulated with the improved Quantum Molecular Dynamics transport model. The results of the calculations reproduce isospin diffusion data from two different observables and the ratios of neutron and proton spectra. By comparing these data to calculations performed over a range of symmetry energies at saturation density and different representations of the density dependence of the symmetry energy, constraints on the density dependence of the symmetry energy at sub-normal density are obtained. Results from present work are compared to constraints put forward in other recent analysis.
Based on the improved quantum molecular-dynamics (ImQMD) model, the incident energy dependence of dynamic potential barriers is investigated in the entrance channel of fusion reactions. The height of the dynamic barrier increases with the incident energy at energies around the Coulomb barrier. The calculated lowest dynamic barrier approaches to the adiabatic barrier, while the highest one goes up to the sudden potential barrier. To understand the energy dependence of the dynamical barrier we study the neck formation and shape evolution of the system which causes the dynamic lowering of the barrier.
The collision of very heavy nuclei 197Au+197Au at 15 A MeV has been studied within the improved quantum molecular dynamics model. A class of ternary events satisfying nearly complete balance of mass numbers is selected. The experimental mass distributions for the system 197Au+197Au ternary fission fragments, the heaviest (A1), the intermediate (A2) and the lightest (A3), are reproduced well. The mean free path of nucleons in the reaction system is studied and the shorter mean free path is responsible for the ternary fission with three mass comparable fragments, in which the two-body dissipation mechanism plays a dominant role.
The collision of very heavy nuclei 197Au+197Au at 15 A MeV has been studied within the improved quantum molecular dynamics model. A class of ternary events satisfying nearly complete balance of mass numbers is selected. The experimental mass distributions for the system 197Au+197Au ternary fission fragments, the heaviest (A1), the intermediate (A2) and the lightest (A3), are reproduced well. The mean free path of nucleons in the reaction system is studied and the shorter mean free path is responsible for the ternary fission with three mass comparable fragments, in which the two-body dissipation mechanism plays a dominant role.
The mass number distributions of three fragments from the ternary fission of the system 197Au+197Au are reproduced rather well by using the improved quantum molecular dynamics (ImQMD) model without any adjusting parameter. It is found that the probability of ternary fission evidently depends on the incident energy and the impact parameter, and the two-body dissipation is the main mechanism responsible for the formation of the third fragment with comparable mass.
The influence of isospin dependence of in-medium nucleon-nucleon cross sections on the n/p ratios for emitted nucleons in reactions 96Zr+96Zr and 96Ru+96Ru at Eb=400AMeV is investigated by means of an improved quantum molecular dynamics model. Our results show that the high energy part of the spectra of the n/p ratios for emitted nucleons is sensitive to the isospin dependence of in-medium nucleon-nucleon cross sections for neutron-rich reaction systems. Therefore, we propose that the n/p ratio of emitted high energy nucleons in a very neutron-rich reaction system at several hundreds of AMeV can be taken as sensitive observables to constrain the isospindependence of in-medium nucleon-nucleon cross sections.
The recent GSI data for proton-induced spallation reactions by using inverse kinematics are analyzed by the improved quantum molecular dynamics model (ImQMD05) merged with the generalized evaporation model (GEM2) and GEMINI model. We find that the model of ImQMD05+GEM2 reproduces the experimental data of mass and charge distributions for proton-induced spallation reactions on heavy targets (208Pb, 238U and 197Au) well and the model of ImQMD05+GEMINI reproduces the ones on light targets (56Fe) well. The experimental data for double differential cross sections of emitted neutrons and protons in intermediate energy proton-induced spallation reactions can also be reproduced well with the same models and this shows that they are not very sensitive to the merged statistical model.
The emissions of neutrons, protons and bound clusters from central 124Sn + 124Sn and 112Sn + 112Sn collisions are simulated using the Improved Quantum Molecular Dynamics model for two different density-dependent symmetry-energy functions. The calculated neutron–proton spectral double ratios for these two systems are sensitive to the density dependence of the symmetry energy, consistent with previous work. Cluster emission increases the double ratios in the low energy region relative to values calculated in a coalescence-invariant approach. To circumvent uncertainties in cluster production and secondary decays, it is important to have more accurate measurements of the neutron–proton ratios at higher energies in the center of mass system, where the influence of such effects is reduced.
The isospin effects in proton-induced reactions on isotopes of 112-132Sn and the corresponding β-stable isobars are studied by means of the improved quantum molecular dynamics model and some sensitive probes for the density dependence of the symmetry energy at subnormal densities are proposed. The beam energy range is chosen to be 100–300 MeV. Our study shows that the system size dependence of the reaction cross sections for p+112-132Sn deviates from the Carlson's empirical expression obtained by fitting the reaction cross sections for proton on nuclei along the β-stability line and sensitively depends on the stiffness of the symmetry energy. We also find that the angular distribution of elastic scattering for p+132Sn at large impact parameters is very sensitive to the density dependence of the symmetry energy, which is uniquely due to the effect of the symmetry potential with no mixture of the effect from the isospin dependence of the nucleon-nucleon cross sections. The isospin effects in neutron-induced reactions are also studied and it is found that the effects are just opposite to that in proton-induced reactions. We find that the difference between the peaks of the angular distributions of elastic scattering for p+132Sn and n+132Sn at Ep,n=100 MeV and b=7.5 fm is positive for soft symmetry energy Usymsf and negative for super-stiff symmetry energy Usymnlin and close to zero for linear density dependent symmetry energy Usymlin, which seems very useful for constraining the density dependence of the symmetry energy at subnormal densities.
Five sets of isospin-dependent in-medium nucleon–nucleon cross sections obtained by means of many-body theories with different approaches or calculation details are tested by comparing the calculated reaction cross sections of proton-induced reactions on various targets with the experimental data. The calculations are performed by using the improved quantum molecular dynamics model. The comparison indicates that these theoretically predicted isospin-dependent in-medium nucleon–nucleon cross sections can reasonably describe the medium suppression effect on the nucleon–nucleon cross sections when the medium densities are at the range of ρ < 0.5ρ0. But it seems that the medium suppression effect in-medium density range of 0.5ρ0 < ρ < ρ0 provided by these in-medium nucleon–nucleon cross sections is too weak to reproduce the experimental data.
The dynamic, adiabatic and diabatic entrance potentials in strongly damped reactions of 238U+238U, 232Th+250Cf are calculated and compared. The feature of the dynamical potential implies that it is possible for the composite systems to stick together for a period of time. By means of the improved quantum molecular dynamics model the time evolution of the density and charge distributions of giant composite systems and their fragments for reactions 238U+238U, 232Th+250Cf are investigated, from which the lifetimes of giant composite systems at different energies are obtained. The longest average lifetime of 238U+238U is found when the incident energy is about
The elliptic flow for Z≤2 particles in heavy ion collisions at energies from several tens to several hundreds MeV per nucleon is investigated by means of a transport model, i.e., a new version of the improved quantum molecular dynamics model (ImQMD05). This model employs a complete Skyrme potential energy density functional. The influence of different effective interactions and medium corrections of nucleon-nucleon cross sections on the elliptic flow are studied. Our results show that a soft nuclear equation of state and incident energy dependent in-medium nucleon-nucleon cross sections are required to describe the excitation function of the elliptic flow at intermediate energies. The size dependence of transition energies for the elliptic flow at intermediate energies is also studied. The system size dependence of transition energies fits a power of system size with an exponent of 0.223.
By using the Improved Quantum Molecular Dynamics model, the ^{244}Pu+^{244}Pu, ^{238}U+^{238}U and ^{197}Au+^{197}Au reactions at the energy range of E_{c.m.}=800 MeV to 2000 MeV are studied. We find that the production probability of superheavy fragments (SHFs) with Z≥114 for the ^{244}Pu+^{244}Pu reaction is much higher compared with that for the ^{238}U+^{238}U reaction and no product of SHF is found for the ^{197}Au+^{197}Au. The production probability of SHFs strongly depends on the incident energy and a narrowly peaked energy dependence of production probability is found. The decay mechanism of the composite system of projectile and target is studied and the time scale of decay process is explored. The binding energies and the shapes of SHFs are studied. The binding energies of SHFs are broadly distributed and the shapes of SHFs are strongly deformed.
The peripheral heavy-ion collisions of 112,124Sn+86Kr at Eb=25 A MeV are studied by means of the improved quantum molecular dynamics model (ImQMD). It is shown that the slope of the average N/Z ratio of emitted nucleons versus impact parameters for these reactions is very sensitive to the density dependence of the symmetry energy. Our study also shows that the yields of 3H and 3He decrease with impact parameters and that the slope of the yield of 3H versus impact parameters as well as the ratio of Y(3H)/Y(3He) depends strongly on the symmetry potential for peripheral heavy-ion collisions.
The improved quantum molecular dynamics model is further developed by introducing new parameters in interaction potential energy functional based on Skyrme interaction of SkM* and SLy series. The properties of ground states of selected nuclei can be reproduced very well. The Coulomb barriers for a series of reaction systems are studied and compared with the results of the proximity potential. The fusion excitation functions for a series of fusion reactions are calculated and the results are in good agreement with experimental data.
By using the updated improved quantum molecular dynamics model in which a surface-symmetry potential term has been introduced, the excitation functions for fusion reactions of 40,48Ca+90,96Zr at energies around the Coulomb barrier have been studied. The experimental data of the fusion cross sections for 40Ca+90,96Zr have been reproduced remarkably well without introducing any new parameters. The fusion cross sections for the neutron-rich fusion reactions of 48Ca+90,96Zr around the Coulomb barrier are predicted to be enhanced compared with a non-neutron-rich fusion reaction. In order to clarify the mechanism of the enhancement of the fusion cross sections for neutron-rich nuclear fusions, we pay great attention to studying the dynamic lowering of the Coulomb barrier during a neck formation. The isospin effect on the barrier lowering is investigated. It is interesting that the effect of the projectile and target nuclear structure on fusion dynamics can be revealed to a certain extent in our approach. The time evolution of the N/Z ratio at the neck region has been firstly illustrated. A large enhancement of the N/Z ratio at neck region for neutron-rich nuclear fusion reactions is found.
The neck dynamics and nucleon transfer through the neck in fusion reactions 40Ca+90,96Zr are studied by applying the improved quantum molecular dynamics model. A special attention is paid to the dynamic behaviour of the neck development at touching point and to the contribution of excess neutrons in a neutron-rich target (or projectile) to neck formation and nucleon transfer.
An improved quantum molecular dynamics model is developed. By using this model, the properties of ground state of nuclei from 6Li to 208Pb are described very well with one set of parameters. The fusion reactions for 40Ca+90Zr, 40Ca+96Zr, and 48Ca+90Zr at energies near the barrier are studied by this model. The experimental data of the fusion cross sections for 40Ca+90,96Zr at energies near the barrier are reproduced remarkably well without introducing any new parameters. The mechanism for the enhancement of the fusion cross sections for neutron-rich nuclear reaction near barrier is analyzed.