中能重离子碰撞过程中同位旋分馏的特征和机制

利用同位旋依赖的QMD模型主要对中能重离子碰撞中同位旋分馏机制和主要特征进行了讨论和分析， 得到了一些有趣的结果， 并建议将同位旋分馏强度作为提取同位旋相关平均场和建立同位旋不对称核物质状态方程的探针. The degree of isospin fractionation is measured by (N/Z)gas / (N/Z)liq，where (N/Z)gas and (N/Z)liq are the saturated neutron proton ratio of nucleon emissions ( gas phase) and that of fragment emitted (liquid phase) in heavy ion collision at intermediate energy. The calculated results by using the isospin dependent quantum molecular dynamics model show that the degree of isospin fractionation is sensitive to the neutron proton ratio of colliding system. In particular， the degree of isospin fractionation sensitively depends on the symmetry potential and weakly on the in medium nucleon nucleon cross section for the neutron rich system. In this case， we propose that the degree of isospin fractionation can be directly compared with the experimental data so that the information about symmetry potential can be obtained.     

Proton radioactivity half-lives with nuclear asymmetry factor

The dependence of proton emission half-lives on the nuclear asymmetry parameter is investigated using the WKB method and two types of empirical formula.Using the single-folding formalism with asymmetry-depend-ent nuclear radius and surface diffuseness of nuclear matter,the nuclear potential and consequently the half-life are functions of the asymmetry factor.Despite small values of asymmetry in neutron-deficient proton emitters,notice-able changes in the half-lives are observed.The addition of an asymmetry parameter term to the two forms of empir-ical formulas leads to a reduction in the rms error for ground state and isomeric transitions.A noticeable reduction of about 43%is obtained for isomeric transitions in the second form of the empirical formula.Considering ground state transitions in two categories,odd-even and odd-odd emitters,and adopting deformation and asymmetry-dependent empirical formulas,the rms decreases remarkably.The low est values of rms errors,viz.0.1492,0.2312,and 0.1999,are obtained for the aforementioned empirical formulas for ground state transitions of odd-even and odd-odd emit-ters and for all isomeric transitions,respectively.     

Structure and reaction dynamics of SHE Z = 130

This study investigates the structural properties of super-heavy nuclei with Z = 130 by adopting the relativistic mean-field(RMF) theory within an axially deformed oscillator basis with the NL3 force parameter set. We study the binding energies, quadrupole deformation, nuclear radii, neutron separation energies, and other bulk properties.Moreover, we analyze the favorable decay modes for clear cognitive content of nuclei, such as alpha decay, using different formulae including the Viola-Seaberg, analytical formula of Royer, universal curve formula, and universal decay law. We compare these with the corresponding fission process. The spontaneous fission of super-heavy nuclei is studied with Z = 130 within the mass region 310 ≤A≤340. The results exhibit good agreement with finite range droplet model(FRDM) data. This formalism presents a significant step forward in the study of the structure and decay modes of the isotopes of Z = 130. With this appraisal, we investigate the possible shell/sub-shell closure for super-heavy nuclei adjacent by decay chains of alpha and other radioactive decay particles.     

Exploring nuclear symmetry energy with isospin dependence in neutron skin thickness of nuclei

We propose an alternative way to constrain the density dependence of the symmetry energy from the neutron skin thickness of nuclei which shows a linear relation to both the isospin asymmetry and the nuclear charge with a form of Z^2/3. The relation of the neutron skin thickness to the nuclear charge and isospin asymmetry is systematically studied with the data from antiprotonic atom measurement, and with the extended Thomas-Fermi approach incorporating the Skyrme energy density functional. An obviously linear relationship between the slope parameter L of the nuclear symmetry energy and the isospin asymmetry dependent parameter of the neutron skin thickness can be found, by adopting 70 Skyrme interactions in the calculations. Combining the available experimental data, the constraint of -20 MeV L 82 MeV on the slope parameter of the symmetry energy is obtained. The Skyrme interactions satisfying the constraint are selected.     

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.     

Mass Fraction of 13C-Pocket in Metal-Poor AGB Stars and the Primary Nature of Neutron Source

Chemical abundances of very metal-poor s-rich stars contain excellent information to set new constraints on models of neutron-capture processes at low metallicity. Using the parametric approach based on the radiative s-process nucleosynthesis model, we obtain the mass fraction q of 13 C-pocket, the overlap factor r, the neutron exposure per interpulse △τ, and the component coefficients of the s-process and the r-process for 25 s-rich stars, respectively. We find that q deduced for the lead stars is comparable to the overlap factor r, which is larger than the standard case （hereafter ST case） of the AGB model （q - 0.05） about 10 times, and AT are about 10 times smaller than the ST case （AT = 7.0 mbarn^-1）. Although the two parameters obtained for the lead stars are very different from the ST case of the AGB stellar model, it is worth noting that the total amounts of 13 C in metal-poor condition are close to the ST case. The above relation is a significant evidence for the primary nature of the neutron source and the lead stars could be polluted by low-mass AGB stars. Because interpulse period declines with increasing stellar mass, for high-mass AGB star, the neutron irradiation may be terminated due to their shorter interpulse period. Thus the neutron exposure per interpulse of the larger AGB stars should be about 10 times smaller than the ST case. In this ease, the primary nature of the neutron source also exists.     

Isospin splitting of nucleon effective mass and symmetry energy in isotopic nuclear reactions

Within an isospin and momentum dependent transport model, the dynamics of isospin particles(nucleons and light clusters) in Fermi-energy heavy-ion collisions are investigated for constraining the isospin splitting of nucleon effective mass and the symmetry energy at subsaturation densities. The impacts of the isoscalar and isovector parts of the momentum dependent interaction on the emissions of isospin particles are explored, i.e., the mass splittings of m_n~*=m_p~* and m_n~* m_p~*(m_n~* m_p~*). The single and double neutron to proton ratios of free nucleons and light particles are thoroughly investigated in the isotopic nuclear reactions of ~(112)Sn+~(112)Sn and ~(124)Sn+~(124)Sn at incident energies of 50 and 120 MeV/nucleon, respectively. It is found that both the effective mass splitting and symmetry energy impact the kinetic energy spectra of the single ratios, in particular at the high energy tail(larger than 20 Me V). The isospin splitting of nucleon effective mass slightly impacts the double ratio spectra at the energy of 50 MeV/nucleon. A soft symmetry energy with stiffness coefficient of γ_s =0.5 is constrained from the experimental data with the Fermi-energy heavy-ion collisions.     

Isospin and F-spin symmetry structure in low-lying levels of 48，50Cr isotopes

The low-energy level structure and electromagnetic transitions of ^48.50Cr nuclei have been studied using the interacting boson model with isospin (IBM-3). A sequence of isospin excitation bands with isospin T = Tz, Tz 1and Tz 2 has been assigned, and compared with available data. According to this study, the 2^3 and 2^2 states are the lowest mixed symmetry states in ^48Cr and ^50Cr, respectively. In particular, the present calculations suggest that a combination of isospin and F-spin excitation can explain the structure in these nuclei. The transition probabilities between the levels are analysed in terms of isoscalar and isovector decompositions which reveal the detailed nature of the energy levels. The results obtained are in good agreement with recent experimental data.     

Prompt fission neutron spectra of n+235U above the (n,nf) fission threshold

Calculations of prompt fission neutron spectra (PFNS) from the ²³⁵U(n, f) reaction were performed with a semi-empirical method for E_n=7.0 and 14.7 MeV neutron energies. The total PFNS were obtained as a superposition of (n,xnf) pre-fission neutron spectra and post-fission spectra of neutrons which were evaporated from fission fragments, and these two kinds of spectra were taken as an expression of the evaporation spectrum. The contributions of (n,xnf) fission neutron spectra on the calculated PFNS were discussed. The results show that emission of one or two neutrons in the (n,nf) or (n,2nf) reactions influences the PFNS shape, and the neutron spectra of the (n,xnf) fission-channel are soft compared with the neutron spectra of the (n,f) fission channel. In addition, analysis of the multiple-chance fission component showed that second-chance fission dominates the PFNS with an incident neutron energy of 14.7 MeV whereas first-chance fission dominates the 7 MeV case.     

Analysis of prompt fission neutron spectrum and multiplicity for 237Np（n,f） in the frame of multi-modal Los Alamos model

The improved version of Los Alamos model with the multi-modal fission approach is used to analyse the prompt fission neutron spectrum and multiplicity for the neutron-induced fission of 237Np. The spectra of neutrons emitted from fragments for the three most dominant fission modes (standard Ⅰ, standard Ⅱ and superlong) are calculated separately and the total spectrum is synthesized. The multi-modal parameters contained in the spectrum model are determined on the basis of experimental data of fission fragment mass distributions. The calculated total prompt fission neutron spectrum and multiplicity are better agreement with the experimental data than those obtained from the conventional treatment of the Los Alamos model.     

A New Decomposition Approach of Dirac Brueckner Hartree—Fock G Matrix for Asymmetric Nuclear Matter

Asymmetric nuclear matter is investigated by the Dirac Brueckner Hartree-Fock (DBHF) approach with a new decomposition of the Dirac structure of nucleon self-energy from the G matrix.It is found that the isospin dependence of the scalar and vector potentials is relatively weak,although boty potentials for neutron (proton) become deep (shallow) in the neutron-rich nuclear matter.The results in asymmetric nuclear matter are rather different from those obtained by a simple method,where the nucleon self-energy is deduced from the single-particle energy.The nuclear binding energy as a function of the asymmetry parameter fulfils the empirical parabolic law up to very extreme isospin asymmetric nuclear matter in the DBHF approach.The behaviour of the density depandence of the asymmetry energy is different from that obtained by non-relativistic approaches,although both give similar asymmetry energy at the nuclear saturation density.     

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

We investigate the isospin effect of Coulomb interaction on the momentum dissipation or nuclear stopping in the intermediate energy heavy ion collisions by using the isospin-dependent quantum molecular dynamics model. The calculated results show that the Coulomb interaction induces obviously the reductions of the momentum dissipation. We also find that the variation amplitude of momentum dissipation induced by the Coulomb interaction depends sensitively on the form and strength of symmetry potential. However, the isospin effect of Coulomb interaction on the momentum dissipation is less than that induced by the in-medium nucleon-nucleon cross section.In this case, Coulomb interaction does not change obviously the isospin effect of momentum dissipation induced by the in-medium two-body collision. In particular, the Coulomb interaction is preferable for standing up the isospin effect of in-medium nucleon-nucleon cross section on the momentum dissipation and reducing the isospin effect of symmetry potential on it, which is important for obtaining the feature about the sensitive dependence of momentum dissipation on the in-medium nucleon-nucleon cross section and weakly on the symmetry potential.     

Chiral Extrapolation of Lattice Data of Λ_b→Λ_c Form Factors

An extrapolation to the physical limit for the lattice data of Λ_b →Λ_c form factors computed in the nonphysical region is made in this work through a class of fitting functions proposed by us with nonlinear dependence on m2/π derived in the chiral perturbative theory(ChPT) and the heavy quark effective theory(HQET) framework. Then the results are applied to calculate the differential and integrated Λ_b →Λ_c semileptonic decay rates. Meanwhile, a comparison between our results and those obtained through the extrapolation functions with naive linear dependenceon m2/π is made.It is shown that the difference between the extrapolated central values of these two cases is about 5%.The total uncertainties(depending on the momentum transfer q~2) in the linear case are about 5% ～10%(caused by the uncertainties of lattice data) and those in the nonlinear case are about 10% ～ 20%(caused by the uncertainties of both lattice data and input parameters in Ch PT and HQET). More accurate lattice data and parameters in ChPT and HQET are needed to reduce the uncertainties of the extrapolated results.     

Effect of System Mass on Isospin Fractionation in Heavy Ion Collisions at Intermediate Energies

Based on the isospin dependent quantum molecular dynamics model(IQMD) we investigated the dependence of isospin fractionation degree (N/Z)n / (N/Z)Nfrag on the mass of system. The (N/Z)n and (N/Z)Nfrag are the neutron proton ratio of nucleon emission (gas phase) and that of fragment emission (liquid phase) respectively. We found that the isospin fractionation degree is a scnsitive function of system mass. The (N/Z)n/ (N/Z)Nfrag, reduces with increasing the mass of colliding system when neutron proton ratio of colliding system is fixed.     

Constraints on neutron skin thickness and symmetry energy of ~(208)Pb through Skyrme forces and cluster model

We used the cluster structure properties of the 212Po to estimate the neutron skin thickness of 208Pb.For this purpose,we considered two important components:(a)alpha decay is a low energy phenomenon;therefore,one can expect that the mean-field,which can explain the ground state properties of 212Po,does not change during the alpha decay process.(b)212Po has a high alpha cluster-like structure,two protons and two neutrons outside its core nucleus with a double magic closed-shell,and the cluster model is a powerful formalism for the estimation of alpha decay preformation factor of such nuclei.The slope of the symmetry energy of 208Pb is estimated to be 75±25 MeV within the selected same mean-fields and Skyrme forces,which can simultaneously satisfy the ground-state properties of parent and daughter nuclei,as their neutron skin thicknesses are consistent with experimental data.     

Bifurcation phenomena and control for magnetohydrodynamic flows in a smooth expanded channel

This work reports the effects of magnetic field on an electrically conducting fluid with low electrical conductivity flowing in a smooth expanded channel. The governing nonlinear magnetohydrodynamic （MHD） equations in induction- free situations are derived in the framework of MHD approximations and solved numerically using the finite-difference technique. The critical values of Reynolds number （based on upstream mean velocity and channel height） for symmetry breaking bifurcation for a sudden expansion channel （1：4） is about 36, whereas the value in the case of the smooth expansion geometry used in this work is obtained as 298, approximately （non-magnetic case）. The flow of an electrically conducting fluid in the presence of an externally applied constant magnetic field perpendicular to the plane of the flow is reduced significantly depending on the magnetic parameter （M）. It is expansion （1：4） is about 475 for the magnetic parameter M found that the critical value of Reynolds number for smooth = 2. The separating regions developed behind the smooth symmetric expansion are decreased in length for increasing values of the magnetic parameter. The bifurcation diagram is shown for a symmetric smoothly expanding channel. It is noted that the critical values of Reynolds number increase with increasing magnetic field strength.     

POSITRONIUM EMISSION FROM SURFACE OF Si SUBSTRATES PASSIVATED BY HF ETCHING

In order to develop a low-temperature heat-treatment technique for the prepa-ration of Si substrates to be used in-situ in molecular beam epitaxy (MBE), surface defects of the HF etched and passivated substrates were investegated with a slow positron beam analysis, the analysis system was equipped on-line with the molecu-lar beam epitaxy device. The fraction of positronium (Ps) emission from Si surface at different temperatures was estimated by a "peak-method", after comparing with the reflection high energy electron diffraction patterns, it was concluded that for a HF treated specimen a clean and stable passivated surface suitable to MBE can be obtained by an in-situ low-temperature (about 550℃) treatment. However, the pa-rameters of HF treatment (etching time, HF concentration, etc.) should be precisely adjusted in order to avoid the production of excessive damages on Si surfaces.     

Study on scattering correction in fast neutron tomography at NECTAR facility

Fast neutron tomography has been established as an inspection and detection tool at the NECTAR facility at the FRM-II reactor.Scattered neutrons from the object become a major disturbance and bring in artifacts and deviations in reconstruction results,especially for hydrogenous material object.In this article,an iterative scattering correction method for fast neutron tomography was proposed.In each loop of iteration the scattering component of the projections will be simulated by Monte-Carlo program MCNPX based on the previous reconstruction result and then it will be subtracted from original projections.The differences between scattering components at different perspectives were quantitatively evaluated and an average scattering component image was used for all projections finally.Smooth and uniform slices with more clear edges were obtained and the new reconstructed attenuation coefficients are quite close to the real one compared to the results without scattering correction,in which case the relative error of the reconstructed attenuation coefficients is about 10%–30%.     

Chiral symmetry restoration and deconfinement in the contact interaction model of quarks with parallel electric and magnetic fields

We study the impact of steady,homogeneous,and external parallel electric and magnetic field strengths(eE ‖ eB) on the chiral symmetry breaking-restoration and confinement-deconfinement phase transition.We also sketch the phase diagram of quantum chromodynamics(QCD) at a finite temperature T and in the presence of background fields.The unified formalism for this study is based on the Schwinger-Dyson equations,symmetry preserving vector-vector contact interaction model of quarks,and an optimal time regularization scheme.At T=0,in the purely magnetic case(i.e.,eE→0),we observe the well-known magnetic catalysis effect.However,in a pure electric field background(eB→0),the electric field tends to restore the chiral symmetry and deconfinement above the pseudo-critical electric field eE_c~(χ,C).In the presence of both eE and eB,we determine the magnetic catalysis effect in the particular region where eB dominates over eE,whereas we observe the chiral inhibition(or electric chiral rotation) effect when eE overshadows eB.At finite T,in the pure electric field case,the phenomenon of inverse electric catalysis appears to exist in the proposed model.Conversely,for a pure magnetic field background,we observe the magnetic catalysis effect in the mean-field approximation and inverse magnetic catalysis with eB-dependent coupling.The combined effects of eE and eB on the pseudo-critical T_c~(χ,C) yields an inverse electromagnetic catalysis,with and without an eB-dependent effective coupling of the model.The findings of this study agree well with the already predicted results obtained via lattice simulations and other reliable effective models of QCD.     

Proposal for muon and white neutron sources at CSNS

The China Spallation Neutron Source (CSNS) is a large scientific facility with the main purpose of serving multidisciplinary research on material characterization using neutron scattering techniques. The accelerator system is to provide a proton beam of 120 kW with a repetition rate of 25 Hz initially (CSNSⅠ), progressively upgradeable to 240 kW (CSNS-Ⅱ) and 500 kW (CSNS-Ⅱ'). In addition to serving as a driving source for the spallation target, the proton beam can be exploited for serving additional functions both in fundamental and applied research. The expanded scientific application based on pulsed muons and fast neutrons is especially attractive in the overall consideration of CSNS upgrade options. A second target station that houses a muon-generating target and a fast-neutron-generating target in tandem, intercepting and removing a small part of the proton beam for the spallation target, is proposed. The muon and white neutron sources are operated principally in parasitic mode, leaving the main part of the beam directed to the spallation target. However, it is also possible to deliver the proton beam to the second target station in a dedicated mode for some special applications. Within the dual target configuration, the thin muon target placed upstream of the fast-neutron target will consume only about 5% of the beam traversed; the majority of the beam is used for fast-neutron production. A proton beam with a beam power of about 60 kW, an energy of 1.6 GeV and a repetition rate of 12.5 Hz will make the muon source and the white neutron source very attractive to multidisciplinary researchers.