DQ（方德清） Fang

Isospin influences on particle emission and critical phenomena in nuclear dissociation

Features of particle emission and critical point behavior are investigated as functions of the isospin of disassembling sources and temperature at a moderate freeze-out density far medium-size Xe isotopes in the framework of the isospin-dependent lattice gas model. Multiplicities of emitted light particles, isotopic, and isobaric ratios of light particles show the strong dependence on the isospin of the dissociation source, but double ratios of light isotope pairs and the critical temperature determined by the extreme values of some critical observables are insensitive to the isospin of the systems. Values of the power law parameter of cluster mass distribution, mean multiplicity of intermediate mass fragments (IMF), information entropy, and Campis second moment also show a minor dependence on the isospin of Xe isotopes at the critical point. In addition, the slopes of the average multiplicities of the neutrons (N-n), protons (N-p), charged particles (N-CP), and IMFs (N-IMF), slopes of the largest fragment mass number (A(max)), and the excitation energy per nucleon of the disassembling source (E*/A) to temperature are investigated as well as variances of the distributions of N-n, N-p, N-CP, N-IMF, A(max), and E*/A. It is found that they can be taken as additional judgements to the critical phenomena. [S0556-2813(99)302108-1].

Isospin effect in statistical sequential decay

Isospin effect of the statistical emission fragments from the equilibrated source is investigated in the frame of statistical binary decay implemented into the GEMINI code, isoscaling behavior is observed and the dependences of isoscaling parameters alpha and beta on emission fragment size, source size, source isospin asymmetry and excitation energies are studied. Results show that alpha and beta neither depends on light fragment size nor on source size. A good linear dependence of alpha and beta on the inverse of temperature T is manifested and the relationship of alpha=4C(sym)[(Z(s)/A(s))(1)(2)-(Z(s)/A(s))(2)(2)]/T and beta=4C(sym)[(N-s/A(s))(1)(2)-(N-s/A(s))(2)(2)]/T from different isospin asymmetry sources is satisfied. The symmetry energy coefficient C-sym extracted from simulation results is similar to 23 MeV which includes both the volume and surface term contributions, of which the surface effect seems to play a significant role in the symmetry energy.

RMF calculation and phenomenological formulas for the rms radii of light nuclei

The RMF (Relativistic Mean Field) calculations are performed systematically for light isotopes (A < 40) with NL-SH parameter set. The calculated binding energies and root mean square (rms) radii of light nuclei are discussed. On the basis of the theoretical calculations and the experimental data, we have suggested a group of phenomenological formulas for the neutron, proton and matter rms radii for nuclei with A < 40, including the drip-line nuclei. In these formulas, the effects of binding energies, isospin and separation energies are considered. These formulas not only agree to the experimental data for stable nuclei well, but also reproduce the abnormal large radii of nuclei near the drip-line that the previous empirical models fail to reproduce them

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.

Azimuthal asymmetry of direct photons in intermediate energy heavy-ion collisions

Hard photon emitted from energetic heavy ion collisions is of very interesting since it does not experience the late-stage nuclear interaction, therefore it is useful to explore the early-stage information of matter phase. In this work, we have presented a first calculation of azimuthal asymmetry, characterized by directed transverse flow parameter F and elliptic asymmetry coefficient v(2), for proton-neutron bremsstrahlung hard photons in intermediate energy heavy-ion collisions. The positive F and negative v(2) of direct photons are illustrated and they seem to be anti-correlated to the corresponding free protons flow. (c) 2008 Elsevier B.V. All rights reserved.

Neutron removal cross section as a measure of neutron skin

We study the relation between neutron removal cross section (sigma(-N)) and neutron skin thickness for finite neutron-rich nuclei using the statistical abrasion ablation model. Different sizes of neutron skin are obtained by adjusting the diffuseness parameter of neutrons in the Fermi distribution. It is demonstrated that there is a good linear correlation between sigma(-N) and the neutron skin thickness for neutron-rich nuclei. Further analysis suggests that the relative increase of neutron removal cross section could be used as a quantitative measure for neutron skin thickness in neutron-rich nuclei.

Roles of deformation and orientation in heavy-ion collisions induced by light deformed nuclei at intermediate energy

The reaction dynamics of axisymmetric deformed (24)Mg+(24)Mg collisions has been investigated systematically by an isospin-dependent quantum molecular dynamics model. It is found that different deformations and orientations result in apparently different properties of reaction dynamics. We reveal that some observables such as nuclear stopping power (R), multiplicity of fragments, and elliptic flow are very sensitive to the initial deformations and orientations. There exists an eccentricity scaling of elliptic flow in central body-body collisions with different deformations. In addition, the tip-tip and body-body configurations turn out to be two extreme cases in central reaction dynamical process.

Measurement of One- and Two-Neutron Transfers in Reaction of 6He+9Be at 25 MeV/u

The cross sections of one- and two-neutron transfers induced by 6He at 25 MeV/u on a 9Be target were measured in RIKEN. Clear identification of the recoiled Be isotopes was achieved. In total five 11Be and 371 10Be events, the corresponding two- and one-neutron transfers were obtained and analysed for transfer reaction cross sections. The results are useful to determine the spectroscopic factors of the internal halo structure of the 6He nucleus.

Nucleon-nucleon momentum-correlation function as a probe of the density distribution of valence neutrons in neutron-rich nuclei

Proton-neutron, neutron-neutron, and proton-proton momentum-correlation functions (C-pn, C-nn, and C-pp) are systematically investigated for C-15 and other C-isotope-induced collisions at different entrance channel conditions within the framework of the isospin-dependent quantum-molecular-dynamics model complemented by the correlation after burner (CRAB) computation code. C-15 is a prime exotic nucleus candidate due to the weakly bound valence neutron coupling with closed-neutron-shell nucleus C-14. To study density dependence of the correlation function by removing the isospin effect, the initialized C-15 projectiles are sampled from two kinds of density distribution from the relativistic mean-field (RMF) model in which the valence neutron of C-15 is populated in both 1d5/2 and 2s1/2 states, respectively. The results show that the density distributions of the valence neutron significantly influence the nucleon-nucleon momentum-correlation function at large impact parameters and high incident energies. The extended density distribution of the valence neutron largely weakens the strength of the correlation function. The size of the emission source is extracted by fitting the correlation function by using the Gaussian source method. The emission source size as well as the size of the final-state phase space are larger for projectile samplings from more extended density distributions of the valence neutron, which corresponds to the 2s1/2 state in the RMF model. Therefore, the nucleon-nucleon momentum-correlation function can be considered as a potentially valuable tool to diagnose exotic nuclear structures, such as the skin and halo.

Measurement of the reaction cross section of (18)C and observations of fragments from (17)C and (18)C at 80A MeV

The one- and two-neutron removal reactions from (17)C and (18)C as well as the reaction cross section of (18)C have been studied using a carbon target at 80A MeV. The longitudinal momentum distributions of (15,16)C fragments from (17)C and (16,17)C fragments from (18)C were measured by a direct time-of-flight method. The width of (15)C fragments from (17)C is fairly smaller than that from other C isotopes. The experimental data are discussed within the framework of the Glauber model.