对称核物质中核子-核子散射截面的研究

在扩展的Brueckner-Hartree-Fock (BHF)理论框架下, 采用Argonne V₁₄ 两体相互作用势研究了对称核物质中核子-核子散射的总截面和微分截面, 分别讨论了三体核力(TBF)重排效应和基态关联效应对全同和非全同核子散射截面的影响。 计算表明: 低动量区域的基态关联效应会导致介质中核子-核子散射截面的增大; 而随着密度的增加,TBF重排效应的逐渐加强会减小介质中的核子-核子散射截面。 The nucleon nucleon cross sections in symmetric nuclear matter were investigated in the framework of the extended Brueckner-Hartree-Fock(BHF) approach with Argonne V₁₄ two body interaction. The influences of the ground state correlation and the rearrangement contribution of the three body force (TBF) on the cross section have been obtained and discussed separately for identical and non identical nucleon collisions. It is shown that the magnitudes of the cross section are increased by the effects of the ground state correlation in low momentum transfers,and are suppressed in medium with increasing density when the rearrangement contribution of the TBF force is considered.     

从能谱中提取核子-核子碰撞截面的信息

用同位旋相关的量子分子动力学(IQMD)模型计算了入射能量在30~200 MeV/u内, 不同中质比的反应系统(76Zn+76Zn, 76Ge+76Ge,76Se+76Se和76Kr+76Kr)碰撞产生的原子核阻止和反应产物能谱。 发现原子核阻止和不同方向的动能比值有很强的关联性,且可以用垂直方向和平行方向的碎片总动能比值的平方根近似地代替原子核阻止,用来提取同位旋相关介质中的核子-核子碰撞截面的信息。在核反应实验中,很容易测量到该物理量, 它是一个较好的探针。 The collision of different neutron/proton ratio, intermediate mass reaction systems(76Zn+76Zn, 76Ge+76Ge,76Se+76Se and 76Kr+76Kr) at the beam energy of 30~200 MeV/u are studied by using isospin dependent quantum molecular dynamics(IQMD) model. It is found that there exists a strong relationship between nuclear stopping and the ratio of different direction kinetic energy, and nuclear stopping can be replaced by the square root of the ratio of perpendicular and parallel total kinetic energy of fragments, and it can be used to extract information of isospin dependent in medium nucleon nucleon cross section. This physical quantity can be easily measured in nuclear reaction experiments, and it is a better probe.     

α发射数可作为提取同位旋相关平均场的灵敏探针

利用同位旋相关的量子分子动力学, 对中能重离子碰撞过程中的α发射数的同位旋效应进行了分析。 研究表明, α发射数强烈地依赖于同位旋相关平均场的影响, 而与同位旋相关核子 核子碰撞截面的依赖较弱, 可以作为提取同位旋相关平均场信息的灵敏探针。 由于实验上α粒子发射数的观测比较容易, 而且在很宽地能量范围内, 无论丰中子系统还是缺中子系统都较好地满足上述规律, 这样就可以通过实验上对α发射数的探测, 来提取介质中同位旋相关平均场的信息。 同时建议由实验对缺中子系统的质子发射数进行探测, 从中提取同位旋相关核子 核子碰撞截面的信息。 The α emissions in heavy ion collisions at intermediate energies are studied by using isospin dependent quantum molecular dynamics model. The number of α emitted is found to be strongly dependent on the isospin dependent mean field and weakly on the nucleon nucleon cross sections. The number of α emitted is easy to be measured in experiment and the above conclusion is still available no matter in neutron rich systems or in neutron deficient systems in a wide energy region, so it can be used as an excellent probe for extracting information of the isospin dependent mean field. After several years searching, some information of the isospin asymmetry equation of state has been obtained in theoretically. We suggest to investigate the number of α emitted in neutron rich systems and the number of proton emitted in neutron deficient systems experimentally to obtain more information of the isospin dependent mean field and the isospin dependent nucleon nucleon cross section.     

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.     

Analysis of the total activation cross section of all possible reactions producing the same radioactive nuclide for the same element

Firstly, according to the regulation of growth and decay of radioactive nuclides produced in reactions, a formula used to calculate the total activation cross section of all possible reactions producing the same radioactive nuclide for the same element is deduced, and it is pointed out that the activation formula given in two references is incorrect. Then, as an example, the so-called total activation cross section in one of the two references is analyzed and the correct results of the cross sections of ¹⁸²W(n,p)^{182(m m+g)}Ta, ¹⁸³W(n,p)¹⁸³Ta and ²⁰⁶Pb(n,α)²⁰³Hg induced by neutrons around 14 MeV calculated with the data given in the literature, the nuclear parameters and some evaluated values are given. Finally, the correct results are compared with other values collected in the literature.     

Isospin Effects on Anisotropic Flows in Intermediate Energy Heavy Ion Collisions

Anisotropic flows per nucleon （v1/A, v2/A, v3/A and v4/A） of light fragments up to the mass number 4 as a function of transverse momentum per nucleon are studied for 55 Me V/nucleon 58Fe＋58Fe and 58 Ni＋ 58 Ni at large impact parameters by the isospin-dependent quantum molecular dynamics model. The effects of symmetry energy and nucleon-nucleon cross sections, which are both isospin-dependent on anisotropic flows, are studied in detail. In comparison of the two systems with or without symmetry potential term, the results show that the strength of flows is sensitive to symmetry potential and nucleon-nucleon cross sections, which mainly cause a repulsion effect in this energy region.     

Study of the process e^＋e^- →π^＋π^-π^0π^0 at energies √s 〈 1 GeV

In an experiment with the Spherical Neutral Detector at VEPP-2M collider the cross section of the process e^＋e^- →π^＋π^-π^0π^0 was measured. At energies √s 〈 920 MeV this cross section was measured for the first time. The energy dependence of the cross section is well discribed by the vector dominance model with contributions from p, p＇, p＂ mesons. The decay probability ρ→π^＋π^-π^0π^0 was found to be Bρ = （1.60±0.74±0.18） × 10^-5. The upper limit for the decay ω→π^＋π^-π^0π^0 was improved by two orders of magnitude compared to the previous measurements and is Bω 〈 2 × 10^-4 at 90% confidence level.     

Study of the process e~+e~- →π~+π~-π~0π~0 at energies s～(1/2)<1 GeV

In an experiment with the Spherical Neutral Detector at VEPP-2M collider the cross section of the process e+e-→π+π-π0π0 was measured.At energies s～(1/2)920 MeV this cross section was measured for the first time.The energy dependence of the cross section is well discribed by the vector dominance model with contributions from ρ,ρ,ρ mesons.The decay probability ρ→π+π-π0π0 was found to be Bρ =(1.60±0.74±0.18)×10-5.The upper limit for the decay ω→π+π-π0π0 was improved by two orders of magnitude compared to the previous measurements and is Bω 2×10-4 at 90% confidence level.     

What kind of optical model potentials should be used for deuteron stripping reactions?

This paper presents the results of a study that compares CTOM, a microscopic optical model potential(OMP), which is an optical model co-created by the China Nuclear Data Center & Tuebingen University, to CH89, which is a typical phenomenological OMP.The respective OMPs were tested by applying them to the modelling of nucleon elastic scattering and(d,p) transfer reactions involving14C,36S, and58Ni targets at both low and relatively high energies. The results demonstrated that although both potentials successfully accounted for the angular distributions of both the elastic scattering and transfer cross sections, the absolute values of the transfer cross sections calculated using CTOM were approximately 25% larger than those calculated using CH89. This increased transfer cross sections allowed CTOM to produce single particle strength reduction factors for the three reactions that were consistent with those extracted from(e,e′p) reactions as well as with more recent(p,2p) and(p,pn) reactions. Notch tests suggested that nucleon elastic scattering and transfer reactions are sensitive to different regions of the OMP;accordingly,phenomenological OMPs, which are constrained only by elastic scattering cross sections, may not be sufficient for nucleon transfer reactions. Therefore, we suggest that microscopic OMPs, which reflect more theoretical considerations, should be preferred over phenomenological ones in calculations of direct nuclear reactions.     

原子核放射性对核对称能的约束（英文）

核对称能的密度依赖性对于原子核物理和天体物理中的许多问题很重要。基于密度依赖的结团模型,奇特的结团放射性被用来约束核对称能及其斜率的大小。在密度依赖的结团模型中,清楚地给出了结团放射性子核208Pb的中子皮大小与对称能的斜率参数L（ρ0） 之间的关联。发现从M3Y核子-核子相互作用得到的结团-208Pb 同位旋矢量势对于对称能的斜率参数L（ρ0） 非常重要。基于结团放射性实验数据和新的208Pb 的中子皮大小实验数据,成功得到对称能的斜率参数L（ρ0） 的大小。也讨论了利用质子放射性数据提取的斜率参数L（ρ0）。The density-dependence of symmetry energy is of particular importance to many problems in nuclear physics and astrophysics. Exotic cluster radioactivity is proposed to constrain the density slope of symmetry energy L(ρ0) by using the density-dependent cluster model (DDCM) where the cluster radioactivity serves as a link between the neutron skin thickness of 208Pb and the density slope L(ρ0). The isovector part of cluster-208Pb potential constructed from the M3Y nucleon-nucleon interaction is found to be very important in determining the density slope parameter L(ρ0). The correlation between the neutron skin thickness of 208Pb and the density slope parameter are obtained from cluster radioactivities around 208Pb with measured data. The constraint of L(ρ0) from proton radioactivity is also discussed.     

Elastic and inelastic scattering of protons from Li between 25 and 45 MeV

The elastic and inelastic scattering of protons from ⁶Li has been studied at incident energies of 25.9, 29.9, 35.0, 40.1 and 45.4 MeV. The 2.18 MeV (3⁺, T = 0) first excited state of ⁶Li was found to be strongly excited, but the 3.56 MeV (0⁺, T = 1) second excited state was quite weakly excited. Angular distributions for excitation of the 2.18 MeV level were measured at all five energies, while angular distributions for excitation of the 3.56 MeV level were extracted only at 25.9 and 45.4 MeV. To test the applicability of the optical model for the scattering of protons from such a light nucleus the elastic scattering angular distributions have been analyzed using the eleven-parameter search code SEEK. Available polarization angular distributions were included in the analysis. Reasonable fits to the data have been obtained with an average geometry potential. Theoretical estimates of the real part of the optical potential and the inelastic scattering differential cross sections have been made using the microscopic model for proton-nucleus scattering. Both phenomenological and realistic forces have been considered and the necessary nuclear transition densities have been extracted from experimental elastic and inelastic electron scattering data. An estimate of a possible spin-spin term in the optical potential has also been made.     

Calculation of the 6He + p elastic scattering cross sections within the folding approach and the high-energy approximation for the optical potential

Data on cross sections for the ⁶He + p elastic scattering at energies of tens of MeV/nucleon are analyzed by calculating the microscopic optical potential (OP) (its real and imaginary parts). The effect produced on the cross section by the dependence of the nucleon-nucleon potential on the nuclear matter density, the role of the spin-orbit interaction, and the role of nonlinearity and renormalization of the microscopic OP are studied. A comparison with the experimental data allows sensitivity of cross sections to these effects to be tested.     

Optical model description of the elastic scattering of 6He from a polarized proton target at 71 MeV/nucleon

The vector analyzing power and differential cross section for the elastic scattering of ⁶He nucleus from polarized protons at 71 MeV/nucleon have been analyzed using the optical model potentials. Different versions of the nuclear potential are constructed using phenomenological potentials and semimicroscopic potentials based upon four different versions of 6He density and three effective nucleon-nucleon interactions. The effects of density dependence and the sensitivity of the cross sections for interactions have been investigated.     

Investigation of Self-Consistent Relativistic Microscopic Optical Potential

In Dirac-Brueckner calculations for nuclear matter,the average binding energy per nucleon versus density curve is not uniquely defined if coupling to anti-particle is neglected.According to the Hugenholtz-Van Hove theorem,a constraint requires that the nucleon separation energy equals to the fermi energy at saturation density.Choosing saturation energy as empirical value E_B/A=－15.8MeV,the self-consistent calculation leads to the saturation density k_f=1.41fm^－1 and effective mass m*=0.52m,in compressive coefficient k=208MeV.Applying the first law of thermodynamics,self-consistent effective mass (real scalar potential) and the binding energy per nucleon as function of the nuclear density can be obtained.With the realistic nucleon-nucleon interaction (Bonn potential),the vector potential can be obtained from solving the RBBG equation,which weakly depends on the momentum.The cross section and spin observables of the nucleon-nucleus scattering are studied with this new self-consistent relativistic microscopic optical potential.     

Momentum distributions in nuclei measured with relativistic heavy ions

In a peripheral reaction between relativistic heavy ions, where one nucleon is knocked out of the projectile, the momentum distribution of the remaining fragment reflects the momentum distribution of the knocked out nucleon. This has been proven in a previous paper. Here we study how the final-state interaction between the knocked out nucleon and the observed fragment influences the result: The real part of the optical potential which describes the final-state interaction shifts the experimental momentum distribution by a value 〈k_∥〉 of a few tens of MeV/c and the imaginary part reduces the cross sections by a factor 2 roughly. We also derive the cross section for a proton as target.     

Optical potentials in relativistic meson-nucleon model

The relativistic microscopic optical potentials (RMOP) at E < 300 MeV have been derived and investigated based on Walecka's meson-nucleon model. An effective lagrangian including nucleon, σ- and ω-mesons, which is required to reproduce the nuclear matter saturation properties, has been introduced and used to calculate the self-energy of a nucleon in the nuclear medium. Systematical analyses of the scattering data are performed with the RMOP. Finally, several effects, such as the meson-nucleon vertex form factor, isovector meson exchanges, non-linear σ-model are studied.     

用高能核子非弹性散射研究核力有效势

本文用扭曲波玻恩近似法及多体高能近似法,处理了原子核对高能核子的非弹性散射现象。在具有可靠的靶核激发态波函数的情况下,可利用这些理论处理方法研究核内两核子间的有效势,本文具体就碳核对185MeV入射核子的非弹性散射进行了计算。在计算中利用了粒子-空穴模型核波函数。在采用了具有各种交换性质并包含自旋轨道耦合项的有效势后,用一组合理的位阱参数,由多体高能近似法计算的理论值可与几个微分截面及极化实验曲线同时符合。     

Messung der Neutronenpolarisation bei der Reaktion12C(d,n 0)13N mit Hilfe der Mott-Schwinger-Streuung

The polarization of neutrons produced in the reaction¹²C(d, n ⁰)¹³N was measured. Deuterons from the Karlsruhe isochronous cyclotron were used to induce this reaction at 51,5 MeV laboratory energy. The degree of polarization was determined by using the special features of Mott-Schwinger scattering. With an uranium scatterer analysing efficiencies of up to 0.92 can be obtained at very small angles (0.23°). The analysing efficiency can be calculated if the differential cross section at 0° and the total cross section is known. These quantities were experimentally determined. The differential cross section for 49.4 MeV neutrons, scattered by uranium, was measured between 0.88° and 2.10°. By an extrapolation the value 43.4±2.6 b/sr was found for the nuclear differential cross section at zero degree. A total cross section ofσ _t=4.80±0.22 b was obtained. The neutron polarization was measured at a reaction angle of 24.5° and the result isP=?0.45±0.07. This value is fairly above the semiclassical 1/3 limit and can be only explained, if spin orbit forces are taken into account. For (d, n) reactions this is the first neutron-polarization measurement above an energy of 20 MeV.