混浊介质多光子激发荧光显微成像的蒙特卡罗模拟

将蒙特卡罗方法与几何光线追踪技术相结合用于研究样品混浊特性对多光子激发蒌光显微成像的影响。推导了模拟多层样品显微成像的数学模型,给出了用程序框图表示的基本仿真系统骨架,对传统蒙特卡罗方法的改进有效地增强了对聚焦光束的模拟能力,初步的模拟结果说明了混浊介质散射特性对显微成像的显著影响。     

Electronic Excitation Energy Migration in Partly Ordered Polymeric Films

Multistep intermolecular energy migration between elongated fluorophores (carbocyanines) in uniaxially oriented polymer films is studied based on fluorescence depolarization and Monte-Carlo simulations. Contrary to disordered systems it is found experimentally that the concentration depolarization of fluorescence in uniaxially oriented films is extremely weak despite effective energy migration. Based on the concentration depolarization experiment in the ordered matrix it is possible to estimate the angle between absorption and fluorescence transition moments of carbocyanines. The values of that angle are very close to those obtained from other methods.     

蒙特卡罗模拟四极阱中原子的参变激发

在中性原子的磁囚禁实验中,磁阱线圈的电流噪声会激发磁阱中的原子运动,势必对原子团的温度和寿命产生不可忽视的影响。对于非简谐阱,这种激发具有能量选择特性,它又取决于电流噪声的频谱分布。选择了实验中常用的四极阱为研究对象,用直接模拟蒙特卡罗方法来模拟四极阱中原子运动的参变激发现象,得到了原子温度与原子数损失随激发频率的变化关系,并进一步计算了两个共振峰处原子温度随调制时间和调制深度的变化曲线。此外,还研究了弹性碰撞速率对参变激发过程中原子温度上升的影响。这些结果对四极阱参变激发的实验有较好的参考价值。     

闪烁光纤时间特性的蒙特卡罗模拟

用蒙特卡罗模拟的方法，研究了影响闪烁光纤时间分辨的物理因素，包括光子在光纤中的发射，传输，吸收和最后在光电倍增管输出端形成电泳冲等过程，并和实验结果进行了比较。     

几种不同材料降能器对200 MeV质子放疗特性的蒙特卡罗模拟

能量选择系统是医用质子回旋加速器放疗系统中极其重要的部件,主要用来调节质子束能量,使得从加速器出来的质子束能量满足临床治疗计划的要求。能量选择系统的核心部件是降能器,目前降能器大多采用石墨作为降能材料。近些年提出用铍或者碳化硼替代石墨作为降能器材料的设想,以期望提高质子束流的传输效率。利用蒙特卡罗软件TOPAS模拟200 MeV质子在石墨、铍和碳化硼三种不同材料多楔形结构降能器中的输运过程,统计了穿过降能器后质子以及产生的次级中子的能量通量,并计算了质子束流在穿过不同降能器后的能量分散,据此得到了三种材料降能器厚度与质子能量之间的关系曲线,同时也分析了不同降能器对质子束传输效率的影响以及次级中子产额的情况。通过对比发现,三种材料降能器对束流的能量发散效果相当,而使用铍或者碳化硼都能提高束流传输效率,尤其铍降能器的性能较优。但铍和碳化硼与质子相互作用会产生更多的次级中子,因此在实际应用时需要更多地考虑次级中子对设备的辐射防护。Energy selection system (ESS) is an important component for medical proton cyclotron system. It has been widely used to modulate the proton energy in accordance with treatment requirements. ESS consists of the energy degrader which was mostly made of graphite. Recent years, to improve the transmission efficiency of the proton beams, beryllium and boron carbide have been proposed to substitute the graphite. In this work, the Monte Carlo code, TOPAS, was used to simulate the transport process of 200 MeV proton beams traversing the multi-wedge energy degrader made of graphite, beryllium and boron carbide, respectively. Energy fluxes of the protons and secondary neutrons after degrader, as well as the energy dispersion of the degraded proton beams, were calculated. It is found that the energy dissipation effect is nearly identical for all three kinds of degrader material, but using the beryllium or even boron carbide can improve the proton transmission efficiency. However, more secondary neutrons would be produced when proton beams interact with the beryllium and boron carbide, suggesting the need of additional consideration for radiation shielding to devices.     

表面单层组装多环芳烃荧光行为的Monte Carlo模拟

用MonteCarlo方法模拟了惰性基片表面单层组装多环芳烃 (传感元素 )的荧光行为 ,考察了基片表面传感元素固定化百分率、传感元素激发百分率 (光吸收效率 )以及传感元素分子在介质中相互缔合形成激基缔合物趋势大小 (用P表示 ,P值介于 0到 1)等因素对传感元素激基缔合物荧光发射强度与单体荧光发射强度之比所产生的影响 .结果表明 ,具有中等光吸收效率的多环芳烃适宜于作为传感元素 ,中等固定化密度的基片可能具有比较理想的传感性能     

用蒙特卡罗方法计算棒状放大器内的泵浦能量分布

建立了用蒙特卡罗和光线跟踪法计算高功率固体激光系统棒状放大器能量沉积分布的模拟程序。在本文的模型中正确考虑了光线在反射器激光棒上的多次反射，氙灯的辐射光谱以及钕玻璃的吸收光谱。计算结果与作者用阈值法测量的增益均匀性以及用哈特曼网格法测量的氙灯泵浦对激光波面畸变的影响是一致的。     

磁性纳米颗粒系统偶极相互作用的Monte Carlo研究

采用局域Monte Carlo方法模拟不同易轴分布的简单立方排列单分散单畴Fe纳米颗粒系统的ZFC-FC曲线及磁滞回线.结果表明：随着偶极相互作用的增强，系统的阻塞温度T_B逐渐增大，且ZFC曲线的峰变宽.说明偶极相互作用使得系统的有效能垒提高，分布宽度增加.研究FC曲线磁化强度的倒数与温度关系，发现偶极相互作用系统中存在反铁磁有序.系统的阻塞态及超顺磁态的磁滞回线表明，极低低温下，随着偶极相互作用的增强，系统的矫顽力和剩磁减小，偶极相互作用阻碍系统的磁化；系统处于超顺磁态，各向异性作用及偶极相互作用使得系统的磁化曲线偏离Langevin曲线且偶极相互作用展现出退磁相互作用效应.偶极相互作用增强，系统磁化曲线与Langevin曲线偏差量的最大值向低场移动.在偶极相互作用下，易轴与外场夹角为45°的磁性纳米颗粒系统的平均有效能垒和有效能垒分布宽度较易轴随机分布系统的大.     

Study on photon migration path of ultrashort light pulse in turbid media by Monte Carlo method

The photon density and the photon weight density are obtained by a Monte Carlo method. Based on these two concepts the Gaussian peak value photon paths and the weight mean photon paths of ultrashort light pulse in turbid media are defined and studied. The width of the Gaussian peak value photon path is also given. The influence of the exit angle and time on the photon path and its width are discussed. The relative probability of the photon path is given by the sum of the photon weight densities along the photon path, which could be used to calculate the normalized diffusive intensity approximately. The diffusive reflective intensities will arrive at the maximum at some instant at the place where the photon path reaches on the entrance surface at the same instant. The absorption coefficient has small effect on the photon path and its width in the case of the photon weight density.     

高能强子–强子碰撞中喷注内部动力学起伏的蒙特卡洛研究

用蒙特卡洛方法对CERN-SppS对撞机质子–反质子对撞中由圆锥法判定的(微)喷注内部的动力学起伏进行了研究.结果表明,喷注内部的动力学起伏近似地和SpS静止靶强子–强子碰撞中的动力学起伏一样,呈现为纵–横各向异性,而在横向平面内各向同性的特征.     

A net-exchange Monte Carlo approach to radiation in optically thick systems

A Monte Carlo approach to radiative transfer in participating media is described and tested. It solves to a large extent the well known problem of Monte Carlo simulation of optically thick absorption configurations. The approach which is based on a net-exchange formulation and on adapted optical path sampling procedures is carefully designed to insure satisfactory convergence for all types of optical thicknesses. The need for such adapted algorithms is mainly related to the problem of gaseous line spectra representation in which extremely large ranges of optical thicknesses may be simultaneously encountered. The algorithm is tested against various band average computations for simple geometries using the Malkmus statistical narrow band model.     

Structural Transition in the Micellar Assembly: A Fluorescence Study

Structural transition can be induced in charged micelles by increasing the ionic strength of the medium. Thus, spherical micelles of sodium dodecyl sulfate (SDS) that exist in water at concentrations higher than the critical micelle concentration assume an elongated rod-like structure in the presence of an increased electrolyte concentration. This is known as sphere-to-rod transition. In this paper, we characterize the change in organization and dynamics that is accompanied by the salt-induced sphere-to-rod transition in SDS micelles using wavelength-selective fluorescence and other steady-state and time-resolved fluorescence parameters. Since the change in micelle organization during such structural transition may not be limited to one region of the micelle, we have selectively introduced fluorophores in two distinct regions of the micelle. We used two probes, N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (NBD-PE) and 25-[N-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-methyl]amino]-27-norcholesterol (NBD-cholesterol), for monitoring the two regions of the micelle. NBD-PE monitors the interfacial region of the micellar assembly, while NBD-cholesterol acts as a reporter for the deeper regions of the micellar interior. Our results show that wavelength-selective fluorescence, in combination with other fluorescence parameters, offers a powerful way to monitor structural transitions induced in charged micelles. These results could be significant to changes in membrane morphology observed under certain physiological conditions.     

Diffusion and Growth of Metal Clusters in Nanocomposites: A Kinetic Monte Carlo Study

Noble metals that are deposited on a polymer surface exhibit surface diffusion and diffusion into the bulk. At the same time the metal atoms tend to form clusters because their cohesive energy is about two orders of magnitude higher than the cohesive energy of polymers. To selfconsistently simulate these coupled processes, we present in this paper a Kinetic Monte Carlo approach. Using a simple model with diffusion coefficients taken as input parameters allows us to perform a systematic study of the behavior of a large ensemble of metal atoms on a polymer surface eventually leading to polymer nanocomposites. Special emphasis is placed on the cluster growth, cluster size distribution and the penetration of clusters into the substrate. We also study the influence of surface defects and analyze how the properties of the resulting material can be controlled by variation of the deposition rate (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Quantification of surface effects: Monte Carlo simulation of REELS spectra to obtain surface excitation parameter

The surface excitation effect is investigated by using the quantum mechanical frame work of complex self-energy of electrons which interact with a bounded semi-infinite medium. In the self-energy formalism, differential inverse inelastic mean free path (DIIMFP) has contributions from bulk and surface plasmons. Monte Carlo simulation of the interaction of electrons with a solid medium and surface has been performed. The surface excitation parameter (SEP) is then obtained from the simulated reflection electron energy loss spectroscopy (REELS) spectra. The calculated SEP results by Monte Carlo simulation are compared with the previous calculations of total surface excitation probability, which was estimated by a numerical integration of surface term of DIIMFP. The contribution merely due to surface excitations towards REELS spectra is extracted by subtracting the two Monte Carlo simulated REELS spectra that based on the two models of electron inelastic scattering, i.e. a full surface model (SM) and a pure bulk model (BM). The surface excitations found to be significant at low energy losses and diminish at higher energy losses whereas the bulk plasmon contributions show opposite behavior and are negligible at lower energy losses. The average number of surface excitations is then evaluated by the computation of ratio of the integrated surface contribution to the elastic peak. The calculated results for Ag are found to be reasonably in agreement with our previous results for total probability of surface excitations and other reported experimental data for SEP. Surface correction factor (SCF) is calculated using SEP for several metals and is compared with the reported ratio of SCF with Ni sample as reference.     

铁磁球/反铁磁纳米体系磁滞回线的Monte Carlo模拟

利用经典Heisenberg模型和Monte Carlo方法研究外磁场和反铁磁磁晶各向异性、交换相互作用对铁磁球均匀嵌入到反铁磁基体中的铁磁/反铁磁纳米体系磁滞回线的影响.模拟结果显示,外加反向最大磁场不同时,磁滞回线形状不同.当磁场正向增加时,体系的磁化强度会产生一个跃变,但跃变高度与反向场最大值无关.反铁磁磁晶各向异性越大,体系的交换偏置现象越明显,且磁化强度回到饱和值所需的外磁场越大.随着反铁磁基体交换相互作用的增大,在正向和负向磁场区域还可能出现新的磁滞现象.     

Surface reconstruction in reactive dynamics: A kinetic Monte Carlo approach

The oscillatory CO oxidation reaction on the restructuring surface of Pt(1 0 0) is studied through a mesoscopic kinetic Monte Carlo (KMC) approach. The present model is an extension of the standard ZGB model with specific attention to the emergence of oscillations in surface reactions. A square and a purely hexagonal lattice are used as substrates on which the CO oxidation reaction steps take place. The dynamics of the reaction on the two substrates exhibit the ZGB kinetic phase transitions, at different kinetic parameter values for each substrate. Surface reconstruction is modelled through switching between the two lattice types. Oscillations are produced in those parametric areas where the steady state concentrations on the two substrates are considerably different. The parametric area where notable oscillations are observed is narrow, but is greatly enhanced when different sticking coefficients of oxygen are taken into account. CO diffusion introduced microscopically to the model on the hexagonal lattice shifts the kinetic transition points and increases considerably the time needed to reach the steady state.     

Ferrimagnetic to Paramagnetic Transition in Magnetite: Mössbauer versus Monte Carlo

Powder magnetite was analyzed in situ via Mössbauer with temperatures ranging from 170 K up to 900 K. Hyperfine fields of the tetrahedral and octahedral sites of magnetite as well as the corresponding average field were followed as a function of temperature in order to elucidate the critical behavior of magnetite at around the Curie temperature. Results evidence a progressive collapse of the Mössbauer spectra onto a singlet-type line at a critical temperature of around 870 K characterized by a critical exponent β = 0.28(2) for the hyperfine field. In order to describe such temperature dependence of the hyperfine field, a Monte Carlo-Metropolis simulation based on a stoichiometric magnetite and an Ising model with nearest magnetic neighbor interactions was also carried out. In the model, we have taken into account antiferromagnetic and ferromagnetic interactions depending on the involved ions. A discussion about the critical behavior of magnetite and a comparison between the hyperfine field obtained via Mössbauer and the magnetization obtained via Monte Carlo is finally presented.     

Effect of exchange interaction in ferromagnetic superlattices:A Monte Carlo study

The Monte Carlo simulation is used to investigate the magnetic properties of ferromagnetic superlattices through the Ising model. The reduced critical temperatures of the ferromagnetic superlattices are studied each as a function of layer thickness for different values of exchange interaction. The exchange interaction in each layer within the interface and the crystal field in the unit cell are studied. The magnetic coercive fields and magnetization remnants are obtained for different values of exchange interaction, different values of temperature and crystal field with fixed values of physical parameters.     

6H-SiC高场输运特性的多粒子蒙特卡罗研究

采用非抛物性能带模型,对6H-SiC高场电子输运特性进行了多粒子蒙特卡罗(Ensemble Monte Carlo)研究.研究表明：温度为296 K时,电子横向漂移速度在电场为2.0×104 V/cm处偏离线性区,5.0×105 V/cm处达到饱和.由EMC方法得到的电子横向饱和漂移速度为1.95×107 cm/s,纵向为6.0×106 cm/s,各向异性较为显著.当电场小于1.0×106 V/cm时,碰撞电离效应对高场电子漂移速度影响较小.另一方面,高场下电子平均能量的各向异性非常明显.电场大于2.0×105 V/cm时,极化光学声子散射对电子横向能量驰豫时间影响较大.当电场一定时,c轴方向的电子碰撞电离率随着温度的上升而增大.对非稳态高场输运特性的分析表明：阶跃电场强度为1.0×106 V/cm时,电子横向瞬态速度峰值接近3.0×107 cm/s,反应时间仅为百分之几皮秒量级.