Intermediate Energy Heavy Ion Collisions Research Articles

In-mediumNNcross sections determined from the nuclear stopping and collective flow in heavy-ion collisions at intermediate energies

In-medium nucleon-nucleon scattering cross sections are explored by comparing results of quantum molecular dynamics simulations to data on stopping and on elliptic and directed flow in intermediate-energy heavy-ion collisions. The comparison points to in-medium cross sections which are suppressed at low energies but not at higher energies. Positive correlations are found between the degree of stopping and the magnitudes of elliptic and directed flows.

Isospin effect in the nuclear reaction induced by neutron-halo nuclei

We study the average property of the isospin effect of reaction induced by halo-neutron nuclei 8He and 10He in the intermediate energy heavy ion collisions using the isospin-dependent quantum molecular dynamics model (IQMD). This study is based on the extended neutron density distribution for the halo-neutron nuclei, which includes the average property of the isospin effect of reaction mechanism and loose inner structure. The extended neutron density distribution brings an important isospin effect into the average property of reaction mechanism because the interaction potential and nucleon-nucleon(N-N)cross section in IQMD model depend sensitively on the density distribution of colliding system. In order to see clearly the average properties of reaction mechanism induced by halo-neutron nuclei we also compare the results for the neutron-halo colliding systems with those for the corresponding stable colliding systems under the same incident channel condition. We found that the extended density distribution for the neutron-halo projectile brings an important isospin effect to the reaction mechanism, which leads to the decrease of nuclear stopping R, yet induces obvious increase of the neutron-proton ratio of nucleon emissions and isospin fractionation ratio for all beam energies studied in this work, compared to the corresponding stable colliding system. In this case, nuclear stopping, the neutron-proton ratio of nucleon emissions and isospin fractionation ratio induced by halo-neutron nuclei can be used as possible probes for studying the average property of the isospin effect of reaction mechanism and extracting the information of symmetry potential and in-medium N-N cross section by the neutron-halo nuclei in heavy ion collisions.

Scaling of anisotropy flows in intermediate energy heavy ion collisions

Anisotropic flows ( v 1 , v 2 and v 4 ) of light nuclear clusters are studied by a nucleonic transport model in intermediate energy heavy ion collisions. The number-of-nucleon scalings of the directed flow ( v 1 ) and elliptic flow ( v 2 ) are demonstrated for light nuclear clusters. Moreover, the ratios of v 4 / v 2 2 of nuclear clusters show a constant value of 1/2 regardless of the transverse momentum. The above phenomena can be understood by the coalescence mechanism in nucleonic level and are worthy to be explored in experiments.

Analysis of the Nuclear Level-Density Parameters of Some Large Deformed Odd-A and Odd-Odd Nuclei in the Region of Rare Earth Elements

Nuclear level density plays an important role in estimation of nuclear reaction rates, statistical calculations of astrophysics, spallations neutrons measurements, and studies of intermediate-energy heavy-ion collisions. In particular, level densities have been used successfully in calculation of the neutron-capture cross-section basic data required for both design and nuclear model calculations in nuclear science and technologies.In this study the nuclear level density based on nuclear low-lying collective level bands at excitations near neutron binding energy is analyzed in terms of collective excitation modes. The nuclear level-density parameters of some large deformed odd-A and odd-odd nuclei in the region of rare earth elements have been calculated by considering different excitation bands of observed nuclear spectra. The method used assumes equidistant spacing of the collective coupled state bands of nuclei considered. The values of calculated nuclear level-density parameters have been compared with those of the compiled values for s-wave neutron resonance data.

Isospin- and momentum-dependent interaction in isospin-dependent quantum molecular dynamics

An isospin-dependent quantum molecular dynamical model (IQMD) is developed, with the isospin degree of freedom in the momentum-dependent interaction(MDI) included in IQMD, to obtain an isospin- and momentum-dependent interaction (IMDI) in IQMD. We investigate the effect of IMDI on the isospin fractionation ratio and its dynamical mechanism in the intermediate energy heavy ion collisions. It is found that the IMDI induces the significant reductions in the isospin fractionation ratio for all of beam energies, impact parameters, neutron–proton ratios and mass number of colliding systems. However, the strong dependence of isospin fractionation ratio on the symmetrical potential is preserved, with the isospin degree of freedom included in the MDI, i.e. the isospin fractionation ratio is still a good probe for extracting the information about the equation of state of isospin asymmetrical nuclear matter.

AVERAGE PROPERTY OF FRAGMENTATION REACTION AND MOMENTUM DISSIPATION INDUCED BY HALO-NUCLEI IN HEAVY ION COLLISIONS

We study systematically the average property of fragmentation reaction and momentum dissipation induced by halo-nuclei in intermediate energy heavy ion collisions for different colliding systems and different beam energies within the isospin dependent quantum molecular dynamics model (IQMD). This study is based on the extended halo-nucleus density distributions, which indicates the average property of loosely inner halo nucleus structure, because the interaction potential and in-medium nucleon-nucleon cross section in IQMD model depend on the density distribution. In order to study the average properties of fragmentation reaction and momentum dissipation induced by halo-nuclei we also compare the results for the halo-nuclear colliding systems with those for corresponding stable colliding systems with same mass under the same incident channel condition. We find that the effect of extended halo density distribution on the fragment multiplicity and nuclear stopping (momentum dissipation) are important for the different beam energies and different colliding systems. For example the extended halo density distributions increase the fragment multiplicity but decrease the nuclear stopping for all of incident channel conditions in this paper.

Scaling of anisotropic flow and momentum-space densities for light particles in intermediate energy heavy ion collisions

Abstract Anisotropic flows ( v 2 and v 4 ) of light nuclear clusters are studied by isospin-dependent quantum molecular dynamics model for the system of 86 Kr + 124 Sn at intermediate energy and large impact parameters. Number-of-nucleon scaling of the elliptic flow ( v 2 ) are demonstrated for the light fragments up to A = 4 , and the ratio of v 4 / v 2 2 shows a constant value of 1/2. In addition, the momentum-space densities of different clusters are also surveyed as functions of transverse momentum, in-plane transverse momentum and azimuth angle relative to the reaction plane. The results can be essentially described by momentum-space power law. All the above phenomena indicate that there exists a number-of-nucleon scaling for both anisotropic flow and momentum-space densities for light clusters, which can be understood by the coalescence mechanism in nucleonic degree of freedom for the cluster formation.

Medium modifications of the nucleon–nucleon elastic cross section in neutron-rich intermediate energy HICs

Several observables of unbound nucleons which are to some extent sensitive to the medium modifications of nucleon–nucleon elastic cross sections in neutron-rich intermediate energy heavy ion collisions are investigated. The splitting effect of neutron and proton effective masses on cross sections is discussed. It is found that the transverse flow as a function of rapidity, the Qzz as a function of momentum, and the ratio of halfwidths of the transverse to that of longitudinal rapidity distribution Rt/l are very sensitive to the medium modifications of the cross sections. The transverse momentum distribution of correlation functions of two nucleons does not yield information on the in-medium cross section.

Entrance channel effects on the role of isospin-dependent momentum interaction in isospin fractionation in heavy ion collisions

An isospin degree of freedom is inserted into the momentum dependent interaction in the quantum molecular dynamics model to obtain an isospin dependent momentum interaction given in a form practically usable in isospin dependent quantum molecular dynamics model. We investigate the entrance channel effects for the role of isospin momentum dependent interaction on the isospin fractionation ratio and its dynamical mechanism in the intermediate energy heavy ion collisions. It is found that the isospin dependent momentum interaction induces a significant reduction of isospin fractionation ratio under all entrance channel conditions. However the strong dependence of isospin fractionation ratio on the symmetry potential is preserved after considering the isospin degree of freedom in the momentum dependent interaction.

Nuclear reaction dynamics induced by halo-nuclei at intermediate energy heavy ion collisions

We studied systematically the reaction dynamics induced by neutron-halo nuclei and proton-halo nuclei within the isospin dependent quantum molecular dynamics,such as the effects of loose bound halo-nuclei on the fragmentation reaction and momentum dissipation for different colliding systems with different beam energies and different impact parameters. In order to emphasize the roles of neutron-halo nucleus 19B and proton-halo nucleus 23Al on the reaction dynamics wealso calculated the the reaction dynamics induced by the stable nuclei 19F and 23Na with equal mass under identical incident channel conditions. Based on the comparison of results of reaction dynamics induced by halo-nucleus colliding systems and stable nucleus collidinmg systems we found that the roles of loose bound halo-nucleus structure on the fragmentation multiplicity and nuclear stopping (momentum dissipation) are important for all of colliding systems with different beam energies and minor impact parameters, such as, the loose bound halo-nuclei structure increases the fragmentation multiplicity, but reduces the nuclear stopping.

Probing the equation of state with pions

The influence of the isospin-independent and isospin-dependent equation of state (EoS) with momentum dependence, as well as the Coulomb interaction on the pion production in intermediate energy heavy-ion collisions (HICs), is studied for both isospin-symmetric and neutron-rich systems. The Coulomb interaction plays an important role in the reaction dynamics, and strongly influences the rapidity and transverse momentum distributions of charged pions. It even leads to the π−/π+ ratio deviating slightly from unity for isospin-symmetric systems. The Coulomb interaction between mesons and baryons is also crucial for reproducing the proper pion flow since it changes the behaviour of the directed and the elliptic flow components of pions visibly. The EoS can be better investigated in neutron-rich system if multiple probes are measured simultaneously. For example, the rapidity and the transverse momentum distributions of the charged pions, the π−/π+ ratio, the various pion flow components, as well as the difference of π+ − π− flows. A new sensitive observable is proposed to probe the symmetry potential energy at high densities, namely the transverse momentum distribution of the elliptic flow difference .

Probing the density dependence of the symmetry potential in intermediate-energy heavy ion collisions

Based on the ultrarelativistic quantum molecular dynamics model, the effects of the density-dependent symmetry potential for baryons and of the Coulomb potential for produced mesons are investigated for neutron-rich heavy ion collisions at intermediate energies. The calculated results of the Δ−/Δ++ and π−/π+ production ratios show a clear beam-energy dependence on the density-dependent symmetry potential, which is stronger for the π−/π+ ratio close to the pion production threshold. The Coulomb potential of the mesons changes the transverse momentum distribution of the π−/π+ ratio significantly, though it alters only slightly the π− and π+ total yields. The π− yields, especially at midrapidity or at low transverse momenta and the π−/π+ ratios at low transverse momenta are shown to be sensitive probes of the density-dependent symmetry potential in dense nuclear matter. The effect of the density-dependent symmetry potential on the production of both K0 and K+ mesons is also investigated.

COLLECTIVE MOTION IN NUCLEAR COLLISIONS AND SUPERNOVA EXPLOSIONS

Motivated by the success of kinetic theory in the description of observables in intermediate and high energy heavy-ion collisions, we apply kinetic theory to the physics of supernova explosions. The algorithmic implementation for the high-density phase of the iron core collapse is discussed.

Isospin Momentum-Dependent Interaction and Its Role on theIsospin Fractionation Ratio in Intermediate Energy Heavy Ion Collisions

We investigate the role of isospin momentum-dependent interaction on theisospin fractionation ratio and its dynamical mechanism in theintermediate energy heavy ion collisions, by inserting an isospin degreeof freedom into the momentum-dependent interaction to obtain an isospinmomentum-dependent interaction given in a form practically usable in theisospin-dependent quantum molecular dynamics model. It is found that theisospin momentum-dependent interaction brings an important isospineffect into the isospin fractionation ratio. In particular, the isospinmomentum-dependent interaction reduces obviously the reduction ofisospin fractionation ratio. Thus the isospin dependence ofmomentum-dependent interaction is thus important for studying accuratelythe equation of state of isospin asymmetry nuclear matter.

Radial flow has little effect on clusterization at intermediate energies in the framework of the lattice gas model

The Lattice Gas Model was extended to incorporate the effect of radial flow. Contrary to popular belief, radial flow has little effect on the clusterization process in intermediate energy heavy-ion collisions except adding an ordered motion to the particles in the fragmentation source. We compared the results from the lattice gas model with and without radial flow to experimental data. We found that charge yields from central collisions are not significantly affected by inclusion of any reasonable radial flow.

Multiplicity distributions of intermediate mass fragments in the thermodynamic model

Multiplicity distributions of intermediate mass fragments (IMF) seen in intermediate energy heavy ion collisions have been of great interest in the last ten years. The distributions of single intermediate sized element are close to Poissonian and have been subjected to widely different physical interpretations. We calculate the multiplicity distributions in a thermodynamic model whose physical basis is equal population in phase space which implies equilibrium statistics. We found distributions of single element are to a good approximation Poissonian, and such result is attributed to the small emission probability of a given IMF among all the possible channels. We also found significant deviation from Poisson distribution for a range of IMFs, and suggest strong correlations as an explanation. Systematics of the multiplicity distributions are explored in both one component and two component models. In one component model, sudden changes for the multiplicity distributions are found at the phase transition region. In two component model, the multiplicity distributions have more subtleties. Results from the current model are compared with available data, and good agreements are found. New signals for the multi-particle correlations are also suggested.

Isospin Effect of Coulomb Interaction on Momentum Dissipationin Intermediate Energy Heavy Ion Collisions

We investigate the isospin effect of Coulomb interaction on the momentumdissipation or nuclear stopping in the intermediate energy heavy ioncollisions by using the isospin-dependent quantum molecular dynamics model.The calculated results show that the Coulomb interaction inducesobviously the reductions of the momentum dissipation. We also find thatthe variation amplitude of momentum dissipation induced by the Coulombinteraction depends sensitively on the form and strength of symmetrypotential. However, the isospin effect of Coulomb interaction on themomentum dissipation is less than that induced by the in-medium nucleon–nucleon cross section. In this case, Coulomb interaction does not changeobviously the isospin effect of momentum dissipation induced by thein-medium two-body collision. In particular, the Coulomb interaction ispreferable for standing up the isospin effect of in-medium nucleon–nucleon cross section on the momentum dissipation and reducing theisospin effect of symmetry potential on it, which is important forobtaining the feature about the sensitive dependence of momentumdissipation on the in-medium nucleon–nucleon cross section and weakly onthe symmetry potential.

Isospin effect of Coulomb interaction on the dissipation and fragmentation in intermediate energy heavy ion collisions

We investigate separately the isospin effects of Coulomb interaction and symmetry potential on the dissipation and fragmentation in the intermediate energy heavy ion collisions by using isospin-dependent quantum molecular dynamics model. The calculated results show that the Coulomb interaction induces the reductions of both isospin fractionation ratio and nuclear stopping (momentum dissipation). However, the Coulomb interaction not only does not change obviously the strong isospin effect of the symmetry potential on the isospin fractionation ratio but also does not change obviously that of in-medium two-body collision on the nuclear stopping. On the contrary, the symmetry potential induces the enhancement of the isospin fractionation ratio but it is insensitive to the nuclear stopping. Finally, the competition between the Coulomb interaction and symmetry potential induces the reductions of both isospin fractionation ratio and nuclear stopping for two forms of symmetry potentials in this paper.

Effect of Coulomb Interaction on the Isospin FractionationProcess

We studied the effect of Coulomb interaction on the isospinfractionation in the intermediate energy heavy ion collisions by usingthe isospin-dependent quantum molecular dynamics model. The calculatedresults show that Coulomb interaction induces the reduction of the isospinfractionation process with the evolutions of neutron–proton ratio andmass of system. Because Coulomb interaction is repulsive for the proton,more binding protons become free, which produces the neutron-poor gasphase and neutron-rich liquid phase, compared to the neutron–protonratio of the system. The isospin fractionation degree is weakened by theCoulomb term. In contrast, the symmetry potential is repulsive forneutrons and attractive for protons in the neutron-rich system, and thenthe binding neutrons more than the protons become free, which produces aneutron-rich gas phase and neutron-poor liquid phase, so that theisospin fractionation degree is increased. The competition between theeffects from the Coulomb interaction and the symmetry potential inducesthe reduction of the isospin fractionation degree for all the systemmasses. The properties for the sensitive dependence of isospinfractionation degree on the symmetry potential and weak dependence onthe nucleon–nucleon cross section are preserved for all theneutron-rich systems.

Effects of momentum-dependent nuclear potential on two-nucleon correlation functions and light cluster production in intermediate energy heavy-ion collisions

Using an isospin- and momentum-dependent transport model, we study the effects due to the momentum dependence of isoscalar nuclear potential as well as that of symmetry potential on two-nucleon correlation functions and light cluster production in intermediate energy heavy-ion collisions induced by neutron-rich nuclei. It is found that both observables are affected significantly by the momentum dependence of nuclear potential, leading to a reduction of their sensitivity to the stiffness of nuclear symmetry energy. However, the t/$^{3}$He ratio remains a sensitive probe of the density dependence of nuclear symmetry energy.