arXiv subjects

Caiwan Shen

Publications and source records attributed to Caiwan Shen.

36 records · Page 2Linked to original sources

arXiv2016

Helium-3 production from Pb+Pb collisions at SPS energies with the UrQMD model and the traditional coalescence afterburner

A potential version of the UrQMD (UrQMD/M) transport model and a traditional coalescence model are combined to calculate the production of $^3$He fragments in central Pb+Pb collisions at SPS energies 20-80 GeV/nucleon. It is found that the Lorentz transformation in the afterburner influences visibly the $^3$He yield and should be considered in calculations. The rapidity distribution of $^3$He multiplicities (including the concave shape) can be described well with UrQMD/M when it stops during t$_{\rm cut}$=100$\pm$25 fm$/c$ and the coalescence afterburner with one parameter set of ($R_0$,$P_0$)=(3.8 fm, 0.3 GeV$/$c) is taken into use afterwards.

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arXiv2015

Residue cross sections of $^{50}$Ti-induced fusion reactions based on the two-step model

$^{50}$Ti-induced fusion reactions to synthesize superheavy elements are studied systematically with the two-step model developed recently, where fusion process is divided into approaching phase and formation phase. Furthermore, the residue cross sections for different neutron evaporation channels are evaluated with the statistical evaporation model. In general, the calculated cross sections are much smaller than that of $^{48}$Ca-induced fusion reactions, but the results are within the detection capability of experimental facilities nowadays. The maximum calculated residue cross section for producing superheavy element $Z=119$ is in the reaction $^{50}$Ti+$^{247}$Bk in $3n$ channels with $σ_{\rm res}(3n)=0.043$ pb at $E^{*}$ = 37.0 MeV.

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arXiv2015

Influence of clustering and hadron potentials on the rapidity distribution of protons from the UrQMD model

The Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model supplemented by potentials for both pre-formed hadrons and confined baryons (called UrQMD/M) are used to describe rapidity distributions of both the E895 proton data at AGS energies and the NA49 net proton data at SPS energies. With the help of a coalescence afterburner using only one parameter set of ($R_0$, $P_0$)=(3.8 fm, 0.3 GeV$/$c), both sets of experimental data can be described fairly well except for a small discrepancy seen for the net protons at mid-rapidity from heavy ion collisions (HICs) at high SPS energies. Furthermore, in contrast to the logarithmic dependence with beam energy at SIS energies there are still about 10$\%$ of protons in clusters from central HICs at the beam energy of $80$ GeV$/$nucleon.

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arXiv2011

Fusion Hindrance in the Heavy Ion Reactions -- Border Between the Normal and Hindered Fusions

The fusion hindrance in heavy ion collisions is studied in the framework of the two-center liquid drop model. It appears that the neck and the radial degrees of freedom might both be hampered by an inner potential barrier on their path between the contact configuration to the compound nucleus. Heavy ion reactions with and without the two kinds of fusion hindrance are classified through systematic calculations. It is found that the number of reactions without radial fusion hindrance is much smaller than that without neck fusion hindrance, and for both kinds of fusion hindrance the number of reactions without fusion hindrance at small mass-asymmetry parameter $α$ is smaller than that at large $α$. In the formation of a given compound nucleus, if a reaction with $α_c$ is not hindered, then other reactions with $α> α_c$ are also not hindered as it is well known experimentally.

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arXiv2011

Non-equilibrium dynamics in heavy ion collisions at low SIS energies

The Ultrarelativistic Quantum Molecular Dynamics (UrQMD) model, a microscopic transport model, is used to study the directed and elliptic collective flows and the nuclear stopping in Au+Au collisions at incident energies covered by INDRA and lower-energy FOPI experiments. It is seen clearly that these observables are sensitive to both, the potential terms (including iso-scalar and iso-vector parts as well as the momentum dependent term) in the equation of state (EoS) and the collision term (including the Pauli-blocking and the medium-modified nucleon-nucleon elastic cross section (NNECS)). The momentum modifications of both, the mean-field potentials and the density dependent NNECS, are found to be sensitive to the collectivity of heavy-ion collisions. At INDRA energies ($\le 150$~MeV/nucleon), the dynamic transport with a soft EoS with momentum dependence and with the momentum-modified density-dependent NNECS describes the directed flow exhibited by hydrogen isotopes ($Z=1$) emitted at mid-rapidity fairly well.

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arXiv2010

System-size dependence of the pion freeze-out volume as a potential signature for the phase transition to a Quark Gluon Plasma

Hanburry-Brown-Twiss (HBT) correlation functions and radii of negatively charged pions from C+C, Si+Si, Cu+Cu, and In+In at lower RHIC/SPS energies are calculated with the UrQMD transport model and the CRAB analyzing program. We find a minimum in the excitation function of the pion freeze-out volume at low transverse momenta and around $E_{lab}\sim 20-30A$GeV which can be related to the transition from hadronic to string matter (which might be interpreted as a pre-cursor of the QGP). The existence of the minimum is explained by the competition of two mechanisms of the particle production, resonance decays and string formation/fragmentation.

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arXiv2009

Difficulties in probing density dependent symmetry potential with the HBT interferometry

Based on the updated UrQMD transport model, the effect of the symmetry potential energy on the two-nucleon HBT correlation is investigated with the help of the coalescence program for constructing clusters, and the CRAB analyzing program of the two-particle HBT correlation. An obvious non-linear dependence of the neutron-proton (or neutron-neutron) HBT correlation function ($C_{np,nn}$) at small relative momenta on the stiffness factor $γ$ of the symmetry potential energy is found: when $γ\lesssim 0.8$, the $C_{np,nn}$ increases rapidly with increasing $γ$, while it starts to saturate if $γ\gtrsim 0.8$. It is also found that both the symmetry potential energy at low densities and the conditions of constructing clusters at the late stage of the whole process influence the two-nucleon HBT correlation with the same power.

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arXiv2009

Probing the momentum dependence of medium modifications of the nucleon-nucleon elastic cross sections

The momentum dependence of the medium modifications on nucleon-nucleon elastic cross sections is discussed with microscopic transport theories and numerically investigated with an updated UrQMD microscopic transport model. The semi-peripheral Au+Au reaction at beam energy $E_b=400A$ MeV is adopted as an example. It is found that the uncertainties of the momentum dependence on medium modifications of cross sections influence the yields of free nucleons and their collective flows as functions of their transverse momentum and rapidity. Among these observables, the elliptic flow is sensitively dependent on detailed forms of the momentum dependence and more attention should be paid. The elliptic flow is hardly influenced by the probable splitting effect of the neutron-neutron and proton-proton cross sections so that one might pin down the mass splitting effect of the mean-field level at high beam energies and high nuclear densities by exploring the elliptic flow of nucleons or light clusters.

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arXiv2009

Analysis of the Fusion Hindrance in Mass-symmetric Heavy Ion Reactions

The fusion hindrance, which is also denominated by the term extra-push, is studied on mass-symmetric systems by the use of the liquid drop model with the two-center parameterization. Following the idea that the fusion hindrance exists only if the liquid drop barrier (saddle point) is located at the inner side of the contact point after overcoming the outer Coulomb barrier, the reactions in which two barriers are overlapped with each other are determined. It is shown that there are many systems where the fusion hindrance does not exist for the atomic number of projectile or target nucleus $Z\leq43$, while for $Z>43$, all of the mass-symmetric reactions are fusion-hindered.

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arXiv2008

Isospin dependence of reactions $^{48}$Ca+$^{243-251}$Bk

The fusion process of $^{48}$Ca induced reactions is studied with the two-step model. In this model, the fusion process is devided into two stages: first, the sticking stage where projectile and target come to the touching point over the Coulomb barrier from infinite distance, and second, the formation stage where the di-nucleus formed with projectile and target evolve to form the spherical compound nucleus from the touching point. By the use of the statistical evaporation model, the residue cross sections for different neutron evaporation channels are analyzed. From the results, optimum reactions are given to synthesize $Z$ = 117 element with $^{48}$Ca induced reactions.

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arXiv2005

Screening of nuclear pairing in nuclear and neutron matter

The screening potential in the $^1S_0$ and $^3S_1$ pairing channels in neutron and nuclear matter in different approximations is discussed. It is found that the vertex corrections to the potential are much stronger in nuclear matter than in neutron matter.

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arXiv2003

Theory of fusion hindrance and synthesis of the superheavy elements

The two-step model for fusion reactions of massive systems is briefly reviewed.By the use of fusion probabilities obtained by the model and of survival probabilities obtained by the new statistical code, we predict residue cross sections for 48Ca+actinide systems leading to superheavy elements with Z=114, 116 and 118.

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arXiv2003

Neutrino mean free path in neutron stars

The neutrino propagation in neutron stars is studied in the framework of the linear response method. The medium effects are treated in the non-relativistic Brueckner-Hartree-Fock approach either in the mean-field approximation or in the RPA. The residual interaction is expressed in terms of the Landau parameters extracted from the equation of state of spin- and isospin-polarized nuclear matter. The Brueckner theory including three-body forces is used for determining the equation of state. Numerical predictions for the response function of nuclear matter in $β$-equilibrium and the neutrino mean free path are presented in a range of baryonic densities and temperatures. The main results are a dominance of the charge-exchange component over the scattering component and an enhancement of the neutrino mean free path induced by nuclear correlations.

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arXiv2003

Screening Effects on $^1S_0$ Pairing in Neutron Matter

The $^1S_0$ superfluidity of neutron matter is studied in the framework of the generalized Gorkov equation. The vertex corrections to the pairing interaction and the self-energy corrections are introduced and approximated on the same footing in the gap equation. A suppression of the pairing gap by more than 50% with respect to the BCS prediction is found, which deeply changes the scenario for the dynamical and thermal evolution of neutron stars.

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arXiv2003

Landau parameters of nuclear matter in the spin and spin-isospin channels

The equation of state of spin and isospin polarized nuclear matter is determined in the framework of the Brueckner theory including three-body forces. The Landau parameters in the spin and spin-isospin sectors are derived as a function of the baryonic density. The results are compared with the Gamow-Teller collective modes. The relevance of $G_0$ and $G_0'$ for neutron stars is shortly discussed, including the magnetic susceptibility and the neutron star cooling.

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arXiv2003

Prediction and formation mechanism of triaxial superdeformed nuclei for $A\sim 80$

The three dimensional Total Routhian Surface (TRS) calculations are carried out for 64 nuclei between $70\leq A\leq 90$ to find triaxial superdeformed nuclei. Total of 12 nuclei were predicted to have triaxial superdeformation in which the neutron rotational energy plays a key role and the neutron shell energy plays additional role in the formation of triaxial superdeformed nuclei.

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arXiv2002

Neutrino Mean Free Path in Neutron Stars

The Landau parameters of nuclear matter and neutron matter are extracted from the Brueckner theory including three-body forces. The dynamical response function to weak neutrino current is calculated in terms of the Landau Parameters in the RPA limit. Then, the neutrino mean free path in neutron stars is calculated for different conditions of density and temperature.

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arXiv2002

Two-Step Model of Fusion for Synthesis of Superheavy Elements

A new model is proposed for fusion mechanisms of massive nuclear systems where so-called fusion hindrance exists. The model describes two-body collision processes in an approaching phase and shape evolutions of an amalgamated system into the compound nucleus formation. It is applied to $^{48}$Ca-induced reactions and is found to reproduce the experimental fusion cross sections extremely well, without any free parameter. Combined with the statistical decay theory, residue cross sections for the superheavy elements can be readily calculated. Examples are given.

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