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    李庆峰博士的博士毕业论文

    题目:重离子碰撞中的同位旋及近阈K产生

    目录

    第一章 序言

    第二章 研究基础
    2.1 量子分子动力学模型(QMD)
    2.1.1 QMD理论
    2.1.2 QMD输运过程   
    2.1.3 QMD模型的发展
    2.2 相对论Boltzmann-Uehling-Uhlenbeck (RBUU)
    2.2.1 传统BUU理论的发展历史
    2.2.2 自洽RBUU理论
    2.2.3 RBUU输运过程及与QMD模型的比较


    第三章 中能重离子碰撞中的同位旋
    3.1 对称势
    3.2 介质中两体弹性散射截面的同位旋依赖
    3.3 同位旋相关性在QMD程序中的实现
    3.4 关于同位旋问题的一些模型计算及讨论
    3.4.1 对称势对多重碎裂产物的同位旋分布影响
    3.4.2 系统平衡程度探索
    3.4.3 两体截面的同位旋效应对核阻止本领的影响


    第四章 中高能重离子碰撞中的K产生
    4.1 K介子势
    4.2 K介子产生基本截面
    4.3 计算结果及讨论
    4.3.1 SIS能区重离子碰撞的K+产生及集体流
    4.3.2 SIS能区重离子碰撞的K-吸收对其产生的影响
    4.4 K介子研究进展


    第五章 总结及展望

     

    Abstract

    The isospin effect and K production in intermediate and high energy Heavy Ion Collisions (HICs) are hot topics in the nuclear physics. Based on the Isospin-dependent Quantum Molecular Dyanmics (IQMD) model and self-consistent Relativistic Boltzmann-Uehling-Uhlenbeck (RBUU) model, we have studied them and obtained some interesting results.
     
    As for the study of isospin in intermediate energy HICs, we’ve investigated how stength (CS) affects many measurable observables, such as the yield, phase-space, and isospin distributions of fragments. The aim is to find sensitive observables for strength of SP. Our results show the rapidity distribution of differential neutron-proton counting (Nnp) from more neutron-rich reactions can be favorable to extract the information of SP. In the study of isospin-dependent nucleon-nucleon elastic cross sections in nuclear medium we find the medium correction of nucleon-nucleon scattering cross sections is also isospin dependent, depends on the baryon density weakly and  depends on the baryon density significantly. And based on these findings, we further study the dependence of nuclear stopping on system size, initial N/Z, SP and the medium correction of two-body cross sections. It’s found that the effect of initial N/Z ratio and isospin SP on stopping is weak, and the excitation function of stopping depends on the form of medium correction of two-body cross sections and the equation of state of nuclear matter strongly. For probing the isospin dependence of the medium correction of two-body cross sections, the results show that the behavior of the excitation function of stopping for medium size nuclear collisions can provide clearer information than that for heavier systems. We have further studied equilibration with respect to isospin degree of freedom and proposed that the neutron-proton differential rapidity distribution is a sensitive probe to the energy dependence of the degree of equilibration: there exists memory effect in multifragmentation process; the degree of equilibration at E=400A MeV is higher than that at E=100A MeV; the average N/Z ratio of IMF reduces largely as beam energy increases from 100A MeV to 400A MeV.
     
    As for the K production in high energy HICs, we firstly study K+ production. The results show that the kaon flow is sensitive to both the kaon-nucleon sigma term (SKN) and the equation of state of nuclear matter. The collective flows of both nucleons and K+ mesons need a “soft” EoS with compressibility K»200-300 MeV, and SKN=200-400 MeV seems suitable to explain the measured K+ flow. We further find that anti-flow appears at certain large impact parameter, which seems to be more sensitive to SKN and might provide more definite information of the magnitude of SKN. The absorption effect of  mesons is also studied, we find although there exists strong  absorption,  spectrum can still contain the messages of  in nuclear medium. Our investigations show the ratio of the yield of K+ to K depends on the size of the nuclear systems, which is mainly caused by the absorption effect of the K in the nuclear medium. By detailed studies on both mean-absorptions path of the K in different nuclear systems and the individual effect of each of elementary production channels, we find both K production and K absorption effect depend on the size of nuclear system and the elementary production processes. It should be emphasized that the channels  and  play important roles in theK production.
     
     
    Key words: heavy ion collisions, isospin, K production

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