基于粗粒化模型对有机溶剂的分子动力学模拟

在基于Boltzmann分布对四种基本构象进行Monte Carlo取样后, 通过与全原子模型的范德华势比较得到了Gay-Berne (GB)参数. 又在对用量化计算得到的分子体系的电势进行电荷、偶极矩和四极矩的拟合后, 得到了电多极展开势(EMP)参数. 利用得到的粗粒化参数, 基于粗粒化模型, 对CHCl₃及四氢呋喃(THF)两种有机溶剂进行了分子动力学模拟(MDS), 并将结果同全原子模拟进行了比较. 计算结果表明用粗粒化模型从整体上能重复全原子模型的模拟结果, 但在某些细节的计算与全原子模型有偏差, 其原因可能是目前工作仅考虑了单位点情况, 为此今后在对具有复杂结构的分子进行粗粒化模拟时还应考虑合理放置及增加相互作用位点.     

Effects of Distorted Wave on Proton Elastic Scattering from Light Nuclei at 200 MeV

The distorted wave is introduced into the relativistic impulse approximation to generate the Dirac optical potentials for proton elastic scattering. Those potentials, produced by folding the target ground state wavefunction with the free nucleon-nucleon interactions, are used to reevaluate scattering observables, such as differential cross section, analysing power and spin rotation function, for proton elastic scattering from ^12C and ^16O at Elab = 200 MeV, respectively. The inclusion of the distorted wave in the original relativistic impulse approximation has brought out better results of the observables, especially at small scattering angles.     

粗粒化模型中的Gay-Berne相互作用势

合理描述非球形粗粒化粒子间的相互作用是提高粗粒化分子动力学模拟速度的关键.为此本文介绍了Gay-Berne势.将之应用于两种有机小分子体系,在合理选择构象集后,由遗传算法得到了粗粒化粒子的GB参数,并通过对粗粒化模型和全原子模型得到的范德瓦耳斯相互作用的对比检验了GB力场参数.最后,指出如何处理作用位点是粗粒化模型发展的一个关键问题.     

曲率能及压缩能对缺中子裂变系统断前中子发射的影响

用改进了的动力学与统计相结合的模型, 研究了曲率能及压缩能对\,4\,个缺中子裂变系统: ¹⁸⁸Pt, ²⁰⁰Pb, ²¹³Fr和²²⁴Th的断前中子发射的影响. 计算结果显示用改进模型得到的断前中子多重性相比原来模型的低估要更靠近实验数据. 对于目前工作考虑压缩能的原因进行了讨论.     

形变相关的壳修正对N=126处断前中子发射的同位素效应的影响

利用动力学加统计模型就形变相关的壳修正（DDSC）对中子126壳层附近209,213,217Fr断前中子发射的影响进行了研究。模拟结果表明,DDSC抬升了复合核的裂变位垒,且213Fr的升高约为209,217Fr的2倍,但却没有改变鞍点位置。尽管位垒升高延缓了系统的裂变,但其动力学过程受热力学驱动力（TDF）和核阻尼间竞争的主导,因此准确提取壳修正的作用还需考虑核耗散的形变关系。在裂变第一阶段,当核耗散取一体耗散（OBD）参数时,壳修正没有改变断前中子发射的同位素效应,然而当核耗散取标准参数设置（SPS）时,由于213Fr的TDF存在着异常增强,故该规律未能展现。在裂变第二阶段,位垒升高引起的断前中子发射的增强受到了TDF与核阻尼间竞争的反制,故断前中子发射的同位素效应仍未能显现。综合两阶段情况,DDSC对N=126处断前中子发射的同位素效应的影响受第一阶段规律的支配。The effect of deformation-dependent shell correction (DDSC) on the emission of prescission neutron (EPN) is studied within a dynamical and statistical model for three isotopes of 209,213,217Fr near the neutron 126 closure-shell. The results show that the fission barriers are enhanced with DDSC, and the increment of 213Fr is almost 2 times those of 209,217Fr, but those saddle points are not changed. Although the enhancement of fission barrier delays nuclear fission, the fission dynamics process is controlled by the competition between thermodynamic driving force (TDF) and nuclear damping, so the deformation-dependence of nuclear dissipation must be considered in order to extract the role of shell correction. The shell correction doesn't alter isotope effect of EPN with OBD nuclear dissipation in the first phase of nuclear fission, but the rule does not been exhibited because that there is abnormal enhancement of TDF using SPS nuclear dissipation. The increment of EPN caused by the rise of fission barrier is countered by the competition between TDF and nuclear damping in the second phase of nuclear fission, hence the effect of EPN cannot exhibit. The effect of DDSC on EPN near the neutron 126 closure-shell is dominated by the rules in the first phase of nuclear fission.     

基于路径分析方法探究核耗散的形变关系

利用动力学加统计模型对200Pb 系统的断前中子的发射进行了模拟。计算结果表明,在核耗散的形变关系(DDND) 确定的情况下,断前中子多重性会随着能级密度坐标关系(DDLDP) 的增强而减少,且该规律主要由第一阶段内断前中子的发射决定。此外,选定DDLDP 而采用核耗散的不同形变关系也都很好地重现了实验数据。为了解释上述现象,提出了研究热核退激的路径分析方法,其主要内容是把核耗散看成是实验粒子在拉伸维度上的阻尼力,而核裂变的动力学过程是该阻尼力和自由能产生的驱动力之间相互竞争的结果。利用该方法对200Pb 系统的断前中子多重性进行了分析,并指出在确定DDND时应考虑DDLDP。Within a certain deformation dependence of nuclear dissipation (DDND), the simulation results about the deexcitation process of 200Pb show that the prescission neutron multiplicities decrease with increasing deformation dependence of level density parameter (DDLDP). Moreover, this pattern is determined by the emission of prescission neutron in the first period of nuclear fission. On the other hand, all of the results from two kinds of different DDNDs can reproduce the experiment data perfectly with a certain DDLDP. In order to illustrate those phenomena, the pathwise analysis method (PAM) is presented in this paper. In the PAM, nuclear dissipation is treated as the damping force on the elongation dimension, whereas the fission dynamics process results from the competition between damping force and the driving force stemming from the nuclear free energy. The prescission neutron multiplicity in the deexcitation process of 200Pb is analyzed, and the results point out that the DDLDP needs to be taken into account in studying the DDND.     

形变对断前粒子发射同位旋效应的影响

用动力学与统计相结合的模型和统计模型分别研究了形变对重同量异位素²⁰⁰Ag,²⁰⁰Tl,²⁰⁰Bi和轻同量异位素¹¹⁰Pd, ¹¹⁰In, ¹¹⁰Te断前粒子发射的影响. 计算表明形变的影响是存在的, 但形变却并没有改变中子和带电粒子发射 对系统同位旋的依赖关系. 对可能的原因也进行了讨论.     

入射质子的能量对质子与晕核弹性散射观测量的影响

用相对论脉冲近似(RIA)对不同的中能质子(200MeV, 400MeV和800MeV)与¹⁴Be, ¹⁶O和¹²C的弹性散射进行了研究, 讨论了不同的入射能量对3种观测量, 即微分散射截面、分析本领和自旋转动函数的影响. 研究发现在小散射角的区域晕中子对观察量的影响趋势保持一致, 不随入射质子能量的改变而改变.     

曲率能对热力学驱动力的影响

为了研究曲率能对核裂变热力学驱动力（TDF）的影响,首先利用包含曲率能的截断版小液滴模型计算了200Pb和224Th的位垒和熵垒,对比液滴模型的计算结果表明:曲率能未改变224Th的位垒鞍点,却将200Pb的位垒鞍点向后推移。能级密度参数的形变关系越强则两系统的熵垒鞍点越靠近基态。为了进一步探究曲率能如何通过位势和熵势影响TDF,以断前中子多重性（PNM）为探针,通过两种方案进行了模拟,结果表明:曲率能降低了两系统的位势驱动力,而增强了其熵势驱动力。结合PNM的计算表明,前一种效应要比后一种效应明显,因此曲率能总体减弱了200Pb和224Th的TDF,进而延缓了两系统的核裂变进程。In order to study the effect of curvature energy on the thermodynamic driving force (TDF) of nuclear fission, the potential and entropy barrier of 200Pb and 224Th systems are calculated by using the truncated droplet model including curvature energy, respectively. Compared with the liquid drop model, the results show that curvature energy does not affect the saddle point of 224Th, but pushes the saddle point of 200Pb backwards the ground state. The stronger the deformation dependence of the level density parameter is, the closer the saddle point of entropy barrier for these systems is to the ground state. In order to further investigate how curvature energy affects TDF through nuclear potential and entropy, respectively, the prescission neutron multiplicity (PNM) is selected as the probe, some simulations based on two schemes are carried out. The results show that curvature energy reduces the potential driving force of 200Pb and 224Th, and enhances the entropy potential driving force. Combined with the calculations and analyses of PNM, the former effect is more obvious than the latter, so curvature energy weakens TDF of two systems on whole, thus delaying the nuclear fission process of two systems.