Skyrme-Hartree-Fock方法对Ca同位素基态性质的研究 (Ⅰ)结合能、壳结构、半径与密度分布

用Skyrme Hartree Fock模型系统地研究了40 Ca到70 Ca到Ca同位素偶偶核的基态性质及其同位旋依赖性 .讨论了结合能、均方根半径、密度分布随同位旋的变化 ,研究了壳效应对结合能、双中子分离能S2n及和表面弥散厚度的影响 .结果表明壳效应在S2n、表面弥散厚度中表现得非常明显 .质子均方根半径Rrms随相对中子数I=(N -Z) /A的变化满足Rrms=35(1 +αI + βI2 )rpZ1/ 3.离心位垒对核表面以外的密度分布有很大影响 .     

近垒熔合机制的动力学研究

通过考虑动力学中的壳效应, 对同位旋相关的量子分子动力学模型做了进一步改进, 并利用改进的量子分子动力学模型对近垒熔合机制做了动力学研究. 对轻系统的熔合反应, 改进的模型能够给出与实验结果一致的熔合激发函数, 同时, 对重系统的俘获截面也能给出与实验测量值基本一致的计算结果, 尤其是对近垒或垒下区域也能作出很好的描述. 作为对模型的进一步检验, 对核--核相互作用势做了静态研究, 并与亲近势的结果做了比较.     

Skyrme–Hartree–Fock方法对Ca同位素基态性质的研究(Ⅰ)结合能、壳结构、半径与密度分布

用Skyrme Hartree Fock模型系统地研究了⁴⁰Ca到⁷⁰Ca到Ca同位素偶偶核的基态性质及其同位旋依赖性.讨论了结合能、均方根半径、密度分布随同位旋的变化,研究了壳效应对结合能、双中子分离能S2n及和表面弥散厚度的影响.结果表明壳效应在S2n、表面弥散厚度中表现得非常明显.质子均方根半径R_rms随相对中子数I=(N-Z)/A的变化满足R_rms=3/5(1+αI+βI²)r_pZ^1/3.离心位垒对核表面以外的密度分布有很大影响.     

张量力在~(16)O+~(16)O熔合动力学中的效应

利用时间相关Hartree-Fock理论和完整Skyrme有效相互作用研究了~(16)O+~(16)O碰撞在库仑位垒附近的熔合动力学。数值计算是在没有任何对称性约束的三维笛卡尔基下完成。将时间相关Hartree-Fock理论和冻结密度近似下的能量密度泛函方法给出的库仑位垒与实验结果进行了比较,发现同位旋标量的张量项能降低自旋饱和体系~(16)O+~(16)O的库仑位垒,而库仑位垒高度随着同位旋矢量的张量项的耦合常数减小而降低。并计算了包含和不包含张量力的~(16)O+~(16)O熔合截面,发现张量力对~(16)O+~(16)O碰撞在库仑位垒附近的熔合动力学影响较小。     

40, 48Ca+90, 96Zr近垒熔合反应的动力学研究

发展了一种改进的量子分子动力学模型,并用这一模型研究了40,48Ca+90,96Zr的近垒熔合反应.改进的量子分子动力学模型能很好地描述一系列核,从6Li到208Pb,的基态性质及它们的时间演化.在相同参数下计算得到的40Ca+90Zr以及40Ca+96Zr这两个熔合反应的激发函数与实验结果都符合得相当好.在分析丰中子核熔合截面增强机制中,发现丰中子核熔合反应初始阶段颈部的N/Z值明显偏大,促使反应中动态位垒降低,从而引起了丰中子核反应熔合截面的增强. An improved Quantum Molecular Dynamics (QMD) model is proposed and the fusion reactions 40, 48Ca+90, 96Zr are studied by using this model. With our improved QMD model, the ground state properties and their time evolution of nuclei from 6Li to 208Pb can be reproduced reasonably well and the excitation functions of fusion cross section for reactions40Ca+96Zr and 40Ca+96Zr at near barrier can be reproduced remarkably well with the same set of parameters...     

重离子位垒下熔合——多维位垒透射及动力学过程的研究

前言入射能量接近和低于库仑位垒能区的重离子熔合反应是当前重离子反应研究中一个十分活跃的前沿领域。实验和理论的探索已经证实并描述了高于位垒的低能区重离子(轻核到中重核)反应主要经历的全熔合反应机制。两个硬球在一维(径向距离)库仑+核势相互作用下碰撞、并越过熔合位垒熔合的图像给出了对于垒上低能区全熔合截面的相当好的拟合。然而,相当简化的一维位垒模型的成功是由于垒上低能区全熔合截面一般说来并不灵敏于其它自由度。     

从晕中子核引起核反应中提取对称势

利用同位旋相关量子分子动力学理论研究了中子晕核引起核反应机制中重要的同位旋效应以提取对称势. 因为同位旋相关量子分子动力学理论中的相互作用和介质中核子-核子碰撞截面都灵敏地依赖于碰撞系统的密度分布, 本项研究工作基于中子晕核扩展的密度分布. 该密度分布包含了反应机制中同位旋效应和疏散内部结构的平均特征. 为了弄清楚晕核引起核反应机制的同位旋效应, 在完全相同的入射道条件下, 比较了由中子晕核炮弹引起的同位旋效应和由相等质量的稳定核炮弹引起同位旋效应. 结果发现中子晕核炮弹引起的发射中子-质子比和同位旋分馏比明显大于相等质量稳定弹核产生的结果. 因而可以通过理论结果与实验数据的系统比较提取对称势.     

同位旋效应对中能重离子反应中轻粒子产物的影响

利用同位旋相关的量子分子动力学模型,研究了¹¹²Sn+¹¹²Sn和¹²⁴Sn+¹²⁴Sn两反应系统在不同入射能量、不同碰撞参数、不同势场和不同核子-核子碰撞截面下的粒子发射特征.阐述了发射体系的同位旋对轻粒子产额比的影响.发现轻粒子产额比是同位旋的敏感观测量.另外,还发现中快度区发射的粒子有更高的丰中子程度.同时,轻粒子的比不敏感于核子核子碰撞截面,而敏感于核态方程,这使得从轻粒子比提取同位旋相关的核态方程变成一种可能. 关键词： 同位旋相关的量子分子动力学 同位旋效应 轻粒子产额比     

QMD模型的改进及其在低能重离子反应中的应用

丰中子核以及重核熔合机制的研究以及中能重离子碰撞中多重碎裂的研究都迫切需要一个统一的、自洽的微观动力学模型.经过对量子分子动力学模型进行根本的改进,发展了一个新的、适用于低能以及中能重离子反应的统一描述的微观动力学模型.改进的量子分子动力学(ImQMD)模型能够将整个熔合反应过程中的动力学效应、同位旋效应以及弹靶质量不对称效应等比较全面地、自洽地考虑进来,从而给熔合反应的研究提供了一个新的途径.ImQMD模型能够很好地再现一系列核的基态性质以及10多个熔合反应的激发函数(包括丰中子核熔合体系以及实验最新测量的132Sn+64Ni熔合体系).此外还运用该模型初步探索了重核熔合过程中复合体系的寿命与体系的入射能量、体系大小以及体系的中子质子比的依赖关系. We have developed a new microscopic dynamical model called improved quantum molecular dynamical model (ImQMD). This model can describe the fusion process at energies near the Coulomb barrier as well as the multifragmentation process at intermediate energies in heavy-ion collision (HIC) uniformly. By using this model, fusion cross sections (including some of neutron-rich nuclei reactions and that of newly measured~(132)Sn+~(64)Ni fusion reaction) of tens reactions can be reproduced remarkably well. In fusi...     

超重核合成的同位素依赖探讨

利用二参量Smoluchowski方程计算了54Fe+204Pb,56Fe+206Pb,58Fe+208Pb冷熔合和32,34,36S+238U热熔合的复合核形成截面和蒸发残余截面, 结果清楚地显示出超重核合成截面随同位素的变化。 由于较低的入射道库仑位垒、 较低的不对称裂变谷中的条件鞍点和较小的中子分离能, 一般地说, 丰中子同位素的超重核形成截面明显增强。 The cross sections of the compound nucleus formation and e vaporation residue for the 54Fe+204Pb, 56Fe+206Pb, 58Fe+208Pb cold fusion and 32,34, 36S+238U hot fusion have been calculated by using a two parameter Smoluchowski equation. Our results clearly show the isotope dependence of superheavy nucleus production. The formation cross sections of the neutron rich isotope are, generally speaking, obviously enhanced due to the lower Coloumb barrier , lower height of the conditional saddle point, and smaller neutron separation energy.     

Entrance channel mass asymmetry dependence of compound nucleus formation

The fusion reactions ⁴⁸Ca + ¹⁵⁴Sm and ¹⁶O + ¹⁸⁶W leading to the same compound nucleus ²⁰²Pb are studied within the framework of an improved isospin dependent quantum molecular dynamics model. The entrance channel mass asymmetry dependence of compound nucleus formation is found by analyzing the shell correction energies, Coulomb barriers and fusion cross sections. The calculated fusion cross sections agree quantitatively with the experimental data. We conclude that the compound nucleus formation is favorable for the system with larger mass asymmetry.     

Double folding model calculation applied to fusion reactions

The interaction potential between a spherical and a deformed nucleus is calculated within the double-folding model for deformed nuclei.We solve the double folding potential numerically by using the truncated multipole expansion method.The shape,separation and orientation dependence of the interaction potential,fusion cross section and barrier distribution of the system 16O+154Sm are investigated by considering the quadrupole and hexadecapole deformations of 154Sm.It is shown that the height and the position of the barrier depend strongly on the deformation and the orientation angles of the deformed nucleus.These are quite important quantities for heavy-ion fusion reactions,and hence produce great effects on the fusion cross section and barrier distribution.     

Double folding model calculation applied to fusion reactions

The interaction potential between a spherical and a deformed nucleus is calculated within the double-folding model for deformed nuclei. We solve the double folding potential numerically by using the truncated multipole expansion method. The shape, separation and orientation dependence of the interaction potential, fusion cross section and barrier distribution of the system ¹⁶O+¹⁵⁴Sm are investigated by considering the quadrupole and hexadecapole deformations of ¹⁵⁴Sm. It is shown that the height and the position of the barrier depend strongly on the deformation and the orientation angles of the deformed nucleus. These are quite important quantities for heavy-ion fusion reactions, and hence produce great effects on the fusion cross section and barrier distribution.     

Fusion-evaporation cross-sections for 48Ca + 154Sm near the Coulomb barrier

Measurements of fusion-evaporation cross-sections for the system ⁴⁸Ca + ¹⁵⁴Sm have been performed in the sub- and near-barrier energy range. Barrier-passing cross-sections have been obtained by adding recently measured capture-fission cross-sections at the same energies, and the barrier distribution for capture has been extracted. The data have been analyzed within a coupled-channel model, and a large subbarrier cross-section enhancement is observed, due to the ground-state prolate deformation of ¹⁵⁴Sm. The ⁴⁸Ca + ¹⁵⁴Sm capture cross-sections are compared to existing data on ¹⁶O + ¹⁸⁶W fusion, leading to the same CN, where a few higher-energy points have also been measured. The evaporation residue cross-sections for the two systems above the barrier indicate that complete fusion is inhibited for ⁴⁸Ca + ¹⁵⁴Sm by 40% in that energy region, with respect to ¹⁶O + ¹⁸⁶W.     

Alpha-induced reaction cross-section for Sm,U, Np targets: influence of hexadecapole deformation and deformed surface diffuseness

Alpha-induced reactions on~(154)Sm,~(233,235,236,238)U, and ~(237)Np deformed nuclei are studied theoretically.The effects of hexadecapole deformation, deformed surface diffuseness parameter, and orientation on barrier height and position, fusion cross-section at any angle, and fusion cross-section have been investigated. Both hexadecapole deformation and deformed surface diffuseness can affect barrier characteristics and enhance fusion cross-section. Good agreement between experimental data and theoretical calculations with quadrupole and hexadecapole deformation and deformed surface diffuseness were observed for the ~4He+~(154)Sm,~(235)U,~(237)Np reactions.     

Calculation of Interaction Potentials between Spherical and Deformed Nuclei

The interaction potential for spherical-deformed reaction partners is calculated. The shape, separation and orientation dependence of the interaction potential and fusion cross section of the system ^32S＋^154Sm are investigated within the double-folding model of the deformed nuclei. The effective nucleon-nucleon interaction is taken to be the M3Y-Reid potential. The density is considered for three terms of the expansion using the truncated multipole expansion method, which is a deformed Fermi shape With quadrupole and hexadecapole for the density distribution of ^154Sm. It is found for the interaction potential that the height and the position of barrier strongly depend on the deformations, the orientation angle of the deformed nucleus, and hence produce great effects on fusion cross section. The integrated fusion cross section is in good agreement with the experimental data.     

Systematic study of deformation effects on fusion cross-sections using various proximity potentials

The influence of static quadrupole and hexadecapole (positive & negative) deformation of targets are studied using eleven different versions of nuclear potentials. The height and position of the interaction barrier for the reactions induced by spherical projectile (¹⁶O) on the deformed targets such as ¹⁶⁶Er, ¹⁵⁴Sm and ¹⁷⁶Yb have been estimated. It is found that the nucleus-nucleus potential strongly depends on the value of the deformation parameters and orientation of the target. The experimental fusion cross-section of the reactions ¹⁶O + ¹⁷⁶Yb, ¹⁶O +¹⁶⁶Er and ¹⁶O +¹⁵⁴Sm are investigated by applyingWong’s formula using various parameterizations of the proximity potential as well as an assessment of the results of a multi-dimensional barrier penetration model (BPM). The fusion cross-sections by Prox 77, Prox 88, Prox 00, Prox 00DP, Denisov DP, Bass 80, CW 76 and AW 95 potentials are found to be better than the rest in comparison to experimental data.     

INFLUENCE OF SHELL STRUCTURE ON DEEP INELASTIC COLLISIONS

A kinematically complete study of the symmetric systems ¹⁵⁴Sm+¹⁵⁴Sm and ¹⁴⁴Sm+¹⁴⁴Sm has been performed at energies 30% in excess of the interaction barrier. They have been chosen because of their different internal structure: ¹⁴⁴Sm has a closed N=82 neutron shell and hence a spherical ground state confighration; ¹⁵⁴Sm with ten neutrons outside this shell is strongly deformed. Over the whole range of kinetic energy loss the variances of the measured mass distributions were found to be similar in both reactions, whereas the variances of the element distributions are considerably larger at small energy losses in the ¹⁴⁴Sm system. Based on the shell-corrected potential energy surface these observations are attributed to the closed N=82 neutron shell which, for ¹⁴⁴Sm, hinders the neutron exchange and leads to a preferential transfer of protons.     

Effects of the entrance channel mass asymmetry in fusion reactions

The symmetric and asymmetric fusion reaction systems forming the same compound nuclei 26Al,30Si,38Ar and 170Hf are investigated with the frame of improved isospin dependent quantum molecular dynamics model.The entrance channel mass asymmetry dependence of compound nucleus formation is found by analyzing the shell correction energies,the Coulomb barriers and the fusion cross sections.The calculated fusion cross sections agree quantitatively with the experimental data.The results indicate that compound nucleus formation is favorable for the systems with larger mass asymmetry because of the smaller Coulomb contribution to the fusion barrier.     

On the influence of shell structure in dissipative collisions

A kinematically complete study of the symmetric systems ¹⁴⁴Sm+¹⁴⁴Sm and ¹⁵⁴Sn+¹⁵⁴Sm has been performed at energies 30% in excess of the interaction barrier. They have been chosen because of their different internal structure: ¹⁴⁴Sm has a closed N = 82 neutron shell and a spherical ground-state configuration; ¹⁵⁴Sm with ten neutrons outside this shell is strongly deformed. The observed gross features of both reactions like energy, angular and total mass or element distributions are very similar; the ratio of the mass variances as function of the total kinetic energy loss indicates the number of exchanged nucleons to be comparable in all stages of the reactions. At small energy losses, however, the element distributions of the ¹⁴⁴Sm + ¹⁴⁴Sm reaction are considerably broader, pointing at an enhanced proton transfer at the cost of the number of exchanged neutrons in this system. This observation is attributed to the influence of the closed shell which seems to block the transfer of neutrons at low excitation energies. These results can be explained quantitatively by the different gradients of the shell-corrected potential energy surfaces of the two systems.