初始溅射粒子密度对烧蚀粒子密度和速度分布的影响

采用蒙特-卡罗(Monte Carlo)模拟方法,研究了初始溅射粒子密度对其传输中的密度和速度分布以及环境气体密度分布的影响.结果表明,随着初始溅射粒子密度增大,烧蚀粒子和环境气体高密度峰的交叠区离开靶的最大距离减小,被衬底反弹后,距靶的最小距离减小,烧蚀粒子的速度分布随初始溅射粒子密度增大而变宽,当初始溅射粒子密度大于8.33×10²⁵ m^-3时,出现速度劈裂现象.所得结论为进一步定量研究纳米晶粒的成核机理提供了基础. 关键词： Monte Carlo模拟 烧蚀粒子 密度分布 速度分布     

采用加权直方图分析和蒙特卡洛重采样法对自由能级的校正研究

本文采用加权直方图分析方法和蒙特卡洛重采样方法进行自由基校正研究. 生成的自由能表面几乎可以收敛到具有足够采样情况的精准表面,并且比常规的加权直方图分析校正方法能更稳定地处理采样不足的情况,为检测能级校正表面的不确定性提供指南,并且定义了明确的标准用以确定能改善其视觉效果的自由能表面平滑程度. 本文通过水中的丙氨酸二肽和KillerRed蛋白质子转移的自由能图证明该方法的优势,说明蒙特卡洛重采样法可以作为在产生自由能表面实际的系统中的实用工具.     

Systematic for parity distribution in nuclear level density near neutron separation energies

The correct form of nuclear level density function ρ(U,J,π)$\rho (U,J,\pi )$ is required to calculate nuclear cross-sections needed for various applications ranging from reactor designing, nuclear astrophysics, etc., to transmutation of nuclear waste. The asymmetrical statistical distribution of parity of nuclear levels at low energies poses an intriguing problem leading to larger uncertainties in the calculated values of cross-sections. On the basis of high resolution data available on individual resonance parameters (Eo,Jπ,Γn)$(E_o,J^{\pi },{\Gamma }_n)$ for s- and p-wave neutrons, mass and energy dependence formulae for the parity distribution in the nuclear level density have been proposed which supports the fact of equipartition of parities at high excitation energies.     

Microscopic analysis of octupole shape transitions in neutron-rich actinides with relativistic energy density functional

Quadrupole and octupole deformation energy surfaces, low-energy excitation spectra, and electric transition rates in eight neutron-rich isotopic chains – Ra, Th, U, Pu, Cm, Cf, Fm, and No – are systematically analyzed using a quadrupole-octupole collective Hamiltonian model, with parameters determined by constrained reflectionasymmetric and axially-symmetric relativistic mean-field calculations based on the PC-PK1 energy density functional.The theoretical results of low-lying negative-parity bands, odd-even staggering, average octupole deformations β_3,and B(E3;3_1~- →0_1~+) show evidence of a shape transition from nearly spherical to stable octupole-deformed, and finally octupole-soft equilibrium shapes in the neutron-rich actinides. A microscopic mechanism for the onset of stable octupole deformation is also discussed in terms of the evolution of single-nucleon orbitals with deformation.     

改进粒子水平集法及其应用

针对传统粒子水平集方法的缺陷提出一种虚拟粒子对距离函数的修正方法.改进算法针对虚拟粒子与待修正网格点的各种相对位置进行误差修正,可以避免虚拟粒子在非界面法线方向移动而产生的误差.数值求解涡旋流场问题显示:改进算法的界面捕捉性能得到显著提高.在此基础上,结合投影法求解Navier-Stokes方程组,数值模拟存在高密度比(10³量级)及粘性比(10²量级)的不可压缩两相流问题(气泡上升及自由面不稳定性问题).计算结果与已有结果吻合良好,详细分析主要控制参数对瑞利-泰勒不稳定性现象的影响.     

Comparisons of Lagrangian and Eulerian PDF methods in simulations of non-premixed turbulent jet flames with moderate-to-strong turbulence-chemistry interactions

Transported probability density function (PDF) methods have been applied widely and effectively for modelling turbulent reacting flows. In most applications of PDF methods to date, Lagrangian particle Monte Carlo algorithms have been used to solve a modelled PDF transport equation. However, Lagrangian particle PDF methods are computationally intensive and are not readily integrated into conventional Eulerian computational fluid dynamics (CFD) codes. Eulerian field PDF methods have been proposed as an alternative. Here a systematic comparison is performed among three methods for solving the same underlying modelled composition PDF transport equation: a consistent hybrid Lagrangian particle/Eulerian mesh (LPEM) method, a stochastic Eulerian field (SEF) method and a deterministic Eulerian field method with a direct-quadrature-method-of-moments closure (a multi-environment PDF-MEPDF method). The comparisons have been made in simulations of a series of three non-premixed, piloted methane–air turbulent jet flames that exhibit progressively increasing levels of local extinction and turbulence-chemistry interactions: Sandia/TUD flames D, E and F. The three PDF methods have been implemented using the same underlying CFD solver, and results obtained using the three methods have been compared using (to the extent possible) equivalent physical models and numerical parameters. Reasonably converged mean and rms scalar profiles are obtained using 40 particles per cell for the LPEM method or 40 Eulerian fields for the SEF method. Results from these stochastic methods are compared with results obtained using two- and three-environment MEPDF methods. The relative advantages and disadvantages of each method in terms of accuracy and computational requirements are explored and identified. In general, the results obtained from the two stochastic methods (LPEM and SEF) are very similar, and are in closer agreement with experimental measurements than those obtained using the MEPDF method, while MEPDF is the most computationally efficient of the three methods. These and other findings are discussed in detail.     

Particle confinement and density profile behaviour on HL-1M

In this paper the density profile behaviour and the particle confinement operation regime on HL-1M have been studied under the pellet injection (PI), supersonic molecular beam injection (SMBI), gas puffing (GP) and lower hybrid current drive experimental situations. The relationships between density profile, particle confinement time and edge safe factor have been explored. The density profile, which is measured by six-channel far-infrared ray laser interferometer has been analysed by using the peaking coefficient calculation code. Changes of the outward and inward diffusion velocities before and after the peaking of the central density profile have been calculated using the global particle balance equations. The particle confinement operation regimes have been discussed. The peaking density profile can be easily obtained under the condition of efficient fuelling. In ohmic discharges, confinement time increases as the peaking density profile factor rises, and is saturated at a critical value related to the fuelling efficiency. The particle confinement time of SMBI lies between the values of GP and PI, and its value is about 3-5 times of the energy confinement time.     

Modeling of exchange bias in the antiferromagnetic (core)/ferromagnetic (shell) nanoparticles with specialized shapes

Zero-field-cooled (ZFC) and field-cooled (FC) hysteresis loops of egg- and ellipsoid-shaped nanoparticles with inverted ferromagnetic (FM)-antiferromagnetic (AFM) core-shell morphologies are simulated using a modified Monte Carlo method, which takes into account both the thermal fluctuations and energy barriers during the rotation of spin. Pronounced exchange bias (EB) fields and reduced coercivities are obtained in the FC hysteresis loops. The analysis of the microscopic spin configurations allows us to conclude that the magnetization reversal occurs by means of the nucleation process during both the ZFC and FC hysteresis branches. The nucleation takes place in the form of “sparks” resulting from the energy competition and the morphology of the nanoparticle. The appearance of EB in the FC hysteresis loops is only dependent on that the movements of “sparks” driven by magnetic field at both branches of hysteresis loops are not along the same axis, which is independent of the strength of AFM anisotropy. The tilt of “spark” movement with respect to the symmetric axis implies the existence of additional unidirectional anisotropy at the AFM/FM interfaces as a consequence of the surplus magnetization in the AFM core, which is the commonly accepted origin of EB. Our simulations allow us to clarify the microscopic mechanisms of the observed EB behavior, not accessible in experiments.     

MCNP Dose Calculations in a CT Phantom for Therapeutic External Photon Beam

In this paper, we have addressed the problem of the radiation transport with the Monte Carlo N particle(MCNP) code. This is a general purpose Monte Carlo tool designed to transport neutron, photon and electron in three dimensional geometries. To examine the performance of MCNP5 code in the field of external radiotherapy, we performed the modeling of an Electron Density phantom (EDP) irradiated by photons from 60Co source. The model was used to calculate the Percent Depth Dose (PDD) at different depths in an EDP. One field size for PDD has been examined. A 60Co photons source placed at 80 cm source to surface distance (SSD). The results of calculations were compared to TPS data obtained at National Institute of Oncology of Rabat.     

深穿透粒子输运问题的自适应抽样方法

屏蔽计算中的深穿透问题一直是蒙特卡罗计算的一个难题,研究了一种发射点作为驿站的随机游动机制,推导了相应的自适应抽样方法。其主要优势在于,在蒙特卡罗方法求解粒子输运的同时,利用已经获得的信息,自适应地控制各次抽样数,不断完善计算进程。通过对碰撞点引进重要性函数,实现发射点作为驿站的重要性抽样,并结合自适应控制达到最佳抽样状态。数值结果表明:基于发射点作为驿站的自适应抽样方法,在一定程度上克服了深穿透计算中估计值偏低现象。相应的重要函数抽样方法获得了满意的结果。     

Collective octupole vibration and proton single-particle levels in zirconium nuclei

Self-energy correction to the shell model single-particle motion, arising from the excitation of octupole vibration in the intermediate state, accounts quite well for the energy shifts of the 2p _1/2 and 1g _9/2 proton orbits in zirconium nuclei.     

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运用蒙特卡洛程序MCNPX以及欧洲活化计算程序FISPACT-2007,分析了中国聚变工程实验堆(CFETR)满功率运行时产氚包层各分区的核热功率,计算了满功率运行时产氚包层在高能中子的照射下活化产物的放射性活度、典型栅元的停堆剂量率.计算结果表明,当CFETR在满功率运行一年后停堆,停堆时的包层总活度为6.61×1019Bq,停堆十年后总活度降为7.98×1018Bq,为刚停堆时总活度的12.1%.结果显示,CFETR包层设计不存在突出的安全问题.     

非均匀大气中γ射线输运的蒙特卡罗模拟方法

γ射线在高空大气中的输运问题常采用蒙特卡罗方法进行模拟,但由于大气是非均匀连续介质,通常需要根据高度上的密度变化将大气进行分层处理.高空大气密度变化非常剧烈,精细的分层会带来很大的计算量.针对非均匀连续介质中的粒子输运问题,利用质量距离抽样方法代替传统的步长抽样方法,提出一种可直接模拟粒子在非均匀连续介质中输运的蒙特卡罗方法,实现γ射线在非均匀大气中输运的蒙特卡罗模拟,并与MCNP程序的计算结果进行比较,结果符合较好.     

A Kinetic Code System to Study Impurity Deposition and Fuel Retention in Gaps Between Divertor Tiles

For the purpose of investigating impurity deposition and fuel retention in gaps between divertor tiles, we combine a three‐dimensional Monte Carlo impurity transport code ITCD with a Particle‐In‐Cell Monte Carlo collision code plus a plasma‐surface interaction (PSI) code. The simulation results show that a potential hill formed near the entrance of the gap can prevent the ionized eroded carbon species from entering the gap. It is found that carbon sources for the co‐deposition in gaps are mainly located at the plasma‐closest edges of the gap, and this finding indicates that special measures associated with the engineering issues should be taken to minimize fuel retention in gaps. The smaller scale of carbon sources inside the gap and the existence of the potential hill near the entrance of the gap are the reasons why the eroded carbon species spatial density is lower inside the gap than outside the gap. Through the study on several scenarios with different substrate and plasma temperatures, the simulation with the combined code can deepen the understanding of fuel retention in the codeposited layers in gaps (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

输运问题蒙特卡罗模拟方法回顾及展望北大核心CSCD

蒙特卡罗(MC)方法具有复杂几何处理能力强,方法通用灵活,核数据完备,模拟忠实于物理过程等特点,成为中子学数值模拟的首选方法之一。在核能领域,MC方法得益于计算机的快速发展,在辐射屏蔽、反应堆堆芯临界安全分析、乏燃料后处理、放射性废物处置、核设施退役、核事故应急、放射性石油测井、核医学等领域均有广泛应用。对MC方法及软件输运计算做简要回顾,并对未来发展进行展望。     

CFETR氚增殖包层活化计算分析

运用蒙特卡洛程序MCNPX以及欧洲活化计算程序FISPACT-2007,分析了中国聚变工程实验堆(CFETR)满功率运行时产氚包层各分区的核热功率,计算了满功率运行时产氚包层在高能中子的照射下活化产物的放射性活度、典型栅元的停堆剂量率。计算结果表明,当CFETR在满功率运行一年后停堆,停堆时的包层总活度为6.61-10¹⁹Bq,停堆十年后总活度降为7.98-10¹⁸Bq,为刚停堆时总活度的12.1%。结果显示,CFETR包层设计不存在突出的安全问题。     

Cascade-exciton model analysis of level density parameter dependence in proton induced fission cross sections of some sub-actinide nuclei

Fission cross sections strongly depend on the ratio of the level density parameter in fission to neutron emission, a_f/a_n. In this work, a cascade-exciton model implemented in the code CEM95 has been used to observe this effect for proton induced fission cross sections of tungsten, lead and bismuth. The method was employed using different level density parameter ratios for each fission cross section calculation. The calculated fission cross sections are compared with the available experimental data in the literature. It has been observed that a change of the ratio of the level density parameter, a_f/a_n, is necessary with the incident energy of the proton, to best estimate the fission cross sections in CEM95.     

Contact value relations and density functional theory for the electrical double layer

The recently developed contact-corrected density functional theory is applied to an electric double layer containing a valence and size symmetric electrolyte. The restricted primitive model is used to describe the electrolyte, and a non-polarisable, smooth, planar, uniformly charged hard surface is used to model the electrode. Comparison with Monte Carlo simulation results for this system shows that the theory satisfies the first contact value relation in line with the premise of the approach. Additionally, the theory is found to satisfy the semi-empirical second contact value relation quite closely. This is a welcome result, since no reference is made to this second relation nor is the relation used in the theoretical development. The second contact relation is closely related to the anomalous positive temperature derivative of the double layer capacitance at low temperature, low ion concentration, and low electrode charge, and indeed, the contact-corrected density functional theory predicts this effect.     

基于声粒子分布积分的无网格声场计算方法

小尺度封闭空间内部声场的数值计算是声学设计、噪声控制等领域的关键技术。由于波动声学及几何声学方法计算频率上的限制,中频段声场计算问题一直是个难点。本文以声学无网格法为基础,提出了一种基于声粒子分布积分的无网格声场数值计算方法。文中利用声线跟踪理论计算声场中的声粒子分布,并以某个时间点上的声粒子作为蒙特卡罗法中的积分点,将其应用于无网格法中,从而获得声场中的节点声压。利用该方法对一个矩形封闭空间的中低频声场进行了计算,并与模态叠加法、商用声场计算软件、经典无网格法的结果进行了对比,证明基于声粒子分布积分的无网格声场数值计算方法在中低频段相较于传统基于网格的方法具有更高的精度。     

Density and fluctuations in shell model spectra

A simple method, based on the Gram-Charlier expansion, to separate the smooth and fluctuating parts of the level density is developed. Applications to spectra arising from large shell model calculations are considered. Fluctuations in the energy levels are studied. It is found that with this proper decomposition, even long range correlations agree with the predictions of the random matrix theory.