Molecular dynamic analysis of the structure of dendrimers

We present main results of molecular dynamics simulations that we have carried out in order to investigate structural properties of polyamidoamine (PAMAM) dendrimers. Obtained data confirm the PAMAM dendrimer structure proposed by experiments, performed by means of X-ray scattering (SAXS) and quasi-elastic light scattering (QELS) techniques.     

On the use of thresholds in damage energy calculations

It is pointed out that there is an inconsistency in the use of a displacement threshold with the damage energy model that is part of the standard procedure for expressing radiation effects exposures. A simple correction to the model is presented and its consequences are discussed briefly.     

Neutron dosimetry and damage calculations for the TRIGA MARK-II reactor in Vienna

In order to improve the source characterization of the reactor, especially for recent irradiation experiments in the central irradiation thimble, neutron activation experiments were made on 16 nuclides and the neutron flux spectrum was adjusted using the computer code STAY'SL. The results for the total, thermal and fast neutron flux density at a reactor power of 250 kW are as follows: 2.1 × 10¹⁷, 6.1 × 10¹⁶ (E < 0.55 eV), 7.6 × 10¹⁶ (E > 0.1 MeV) and 4.0 × 10¹⁶ (E > 1 MeV) m⁻² s⁻¹. respectively. Calculated damage energy cross sections and gas production rates are presented for selected elements.     

The role of the Boltzmann transport equation in radiation damage calculations

The purpose of this article is to discuss in some detail the uses of the Boltzmann equation in assessing displacement damage due to fast atoms in solids. We derive the appropriate equations for describing atomic motion in solids directly from the non-linear Boltzmann equation for mixtures. The basic assumption is that binary collisions are valid and that the solid can be regarded as amorphous or as a random lattice of atoms. Moreover, we assume that the density of fast atoms (i.e. those with energy greater than about 25 eV) is so small that they do not interact with each other. With these assumptions a set of linear, coupled Boltzmann transport equations are derived which describe the projectile and recoil atom distributions in space, time and velocity. Additionally, we show how electronic interactions may be accounted for in an accurate and analytically useful manner.The Boltzmann equations thus derived are in the so-called forward form, that is the final co-ordinates are the ones operated upon rather than the initial ones. We consider the corresponding adjoint equations and from the mathematical properties of the operators prove a reciprocity relation between the solutions of the forward equations and the adjoint ones. This enables either equation, forward or backward (adjoint), to be used to calculate the distribution function, a fact which is extremely useful in simplifying calculations for certain types of problem. It also demonstrates the inter-relationships between the works of various groups.The essentials of the scattering cross-sections are discussed and it is seen that two distinct problems arise: scattering in the C.M. system, which is determined by the interparticle force law, and scattering in the L-system which is governed only by the laws of conservation of energy and momentum. In both cases we discuss analytical simplifications which lead to useful approximations for solving the transport equation in closed form.One of the basic problems of radiation damage theory is that of sputtering and we spend some time explaining how binding forces at a vacuum-solid interface affect the solution of the transport equation.Several infinite medium problems are solved using backward and forward equations, thus we calculate the collision density of recoiling atoms due to a high energy source and also the time dependent behaviour of the energy spectrum following a pulse of fast atoms. The energy deposition and total number of moving particles at time t is calculated for various scattering models. The importance of electronic stopping in calculating the amount of energy eventually ending up in atomic motion is assessed via a time dependent forward equation. Its connection with the work of Lindhard is pointed out.We discuss displacement damage and show how the number of displacements per primary particle may be calculated via the collision density approach or through the backward equation with suitably modified energy transfer terms. The ideas of energy partitioning are discussed and methods for assessing its accuracy outlined.The spatial distribution of radiation damage receives considerable effort and we look at the various methods of dealing with the anisotropy of scattering, from Legendre polynomial expansions to the straight-ahead, or path-length, approximation. Analytical solutions are given for a variety of simple problems and in particular the vexed question of ion implantation in heterogeneous layered structures is discussed and various solutions proposed. For infinite, homogeneous media we discuss the method of moments using forward and backward equations and conclude that in many situations the forward form has much to commend it although in some situations the backward equation is not without merit. They are indeed complementary techniques.Sigmund's method for calculating the sputtering yield and efficiency is described via the backward equation and the approximations introduced by his “infinite medium boundary condition” are examined. We propose a more accurate theory which includes the half space nature of the system but which, because of this, leads to difficult mathematical problems. Certain simplified problems are solved and ideas for future progress are put forward.The concept of channelling is discussed and methods for including it directly into the Boltzmann equation are outlined. Essentially, these suggest the introduction of an angularly dependent cross-section which accounts in a phenomenological manner for the anisotropy of the microstructure of the solid.A final section is devoted to a stochastic formulation of the particle distribution function through a probability density. An equation is obtained for this density by probability balance and is then simplified by the introduction of a generating function. Successive differentiations of the generating function enable equations for the average value to be obtained, i.e. the conventional Boltzmann equation, and also higher order correlation functions which give a measure of the fluctuations in the number of particles in a cascade. Equations for vacancies and interstitials and their correlations are discussed in this respect.Finally, it should be noted that this paper shows a very personal and possibly biased view of radiation damage calculation. Indeed, it is not intended to discuss radiation damage as such, but rather to outline the difficulties and particularly to show how the Boltzmann equation has beeb and can be used in its forward and backward forms to understand atomic displacement problems.     

Neutron damage calculations in Cu,Nb and Au to 32 MeV: Application to sputtering and deuteron-breakup neutron sources

Primary recoil distributions and specific damage energies have been computed for high energy deuteron-breakup neutrons in Cu, Nb and Au. The calculations are based on theoretical neutron cross sections and consider in particular a d-Be spectrum broadly peaked at 15 MeV with some neutrons above 30 MeV. The theoretical results are similar to corresponding calculations for monoenergetic 15-MeV neutrons and are in good agreement with range measurements of (n, 2n) recoils generated by high energy d-Be neutrons in Nb and Au. The calculations are also consistent with recent d-Be neutron sputtering experiments in Nb and Au and demonstrate the usefulness of deuteron-breakup neutron sources for simulating fusion neutron effects.     

STIM evaluation in GeoPIXE to complement the quantitative dynamic analysis

The GeoPIXE software for quantitative PIXE trace element imaging and analysis is a well established package for evaluation of characteristic X-ray data for both PIXE and SXRF. For the case of microbeam applications on semi-thick samples knowledge of the local areal density distribution is important for precise quantification. A technique is reported to achieve this using the measurement of beam particle energy loss as it traverses the sample, as in scanning transmission ion microscopy (STIM). New functionality is added to the GeoPIXE code through integration of routines for STIM sorting of event-by-event data to create elemental maps of the mean energy after traversing the sample. Integration of stopping powers for a given sample matrix then permits the measured energy loss to be related to the local areal density. In a further step, this information is used for X-ray absorption corrections made directly to the PIXE analysis results.As a complement, user-written plugins operating on single STIM spectra have been used to compare the estimated areal density from chosen spots with the corresponding values calculated with the new GeoPIXE routines. The additions made to the code allow a more precise quantification to be done on inhomogeneous, semi-thick samples.     

Kinematic and dynamic analysis of a serial-link robot for inspection process in EAST vacuum vessel

The present paper introduces a serial-link robot which is named flexible in-vessel inspection robot (FIVIR) and developed for Experimental Advanced Superconducting Tokamak (EAST). The task of the robot is to carry process tools, such as viewing camera and leakage detector, to inspect the components installed inside of EAST vacuum vessel. The FIVIR can help to understand the physical phenomena which could be happened in the vacuum vessel during plasma operation and could be one part of EAST remote handling system if needed. The FIVIR was designed with the consideration of having easy control and a good mechanics property which drives it resulted in function modular design. The workspace simulation and kinematic analysis are given in this paper. The dynamic behavior of the FIVIR is studied by multi-body system simulation using ADAMS software. The study result shows the FIVIR has ascendant kinematic and dynamic performance and can fulfill the design requirement for inspection process in EAST vacuum vessel.     

ICP—AES techniques applied to analysis of precious metals in ores

A method for the simultaneous determination of precious metals and matrix elements in Fe and Cu ores is studied.The instrument used is ICAP-9000 polychromator and its accessory monochromator(N 1),Interferences of matrix are eliminated by matrix matching method,i.e.the influence of matrix was corrected by minus equivalent concentration method,The experiments show that the method is simple,sensitive and reliable;especially,suitable for determination of Au in Cu-Au ores.     

Improved Quantum Molecular Dynamics Model and its Application to Fusion Reaction Near Barrier

An improved quantum molecular dynamics model is proposed. By using this model, the properties of ground state of nuclei from 6Li to 208Pb can be described very well with one set of parameters. The fusion reactions for 40Ca 90Zr, 40Ca 96Zr and 48Ca 90Zr at the energy near the barrier are studied by this model. The experimental data of the fusion cross sections for 40Ca 90,96Zr at the energy near the barrier can be reproduced remarkably well without introducing any new parameters. The mechanism     

Proton induced X-ray fluorescence analysis and its application to the measurement of trace elements in hair

This paper describes the use of proton-induced X-rays as the exciting source in X-ray fluorescence analysis. The possible advantages arising from the virtually mono-energetic nature of this radiation are discussed. A system has been developed using this approach for measuring trace element levels in biological samples and the performance characteristics have been evaluated. The system has been used to analyse a range of specimens and the results of analysis of a set of hair samples are presented as an illustration.     

基于分子力场的镧系和锕系元素 分子动力学研究进展

简要概述了镧系和锕系元素关键种态的力场发展及其应用。早期主要采用非极化力场研究镧系和锕系离子的溶剂化结构,取得了与实验吻合较好的结果。近年来发展起来的极化力场因考虑了对极化效应的处理而可获得更精确的结果,并在镧系和锕系溶液动力学研究中得到了初步的应用。本文从水溶液动力学、f区元素萃取相关的配位动力学、环境与健康相关的动力学等三个方面简要总结了近年来部分在分子力场水平上研究镧系和锕系溶液动力学和生物无机化学的工作。     

A Molecular Dynamics Study on the Dust-Plasma/Wall Interactions in the EAST Tokamak

The interactions between the W nano-dust and deuterium plasma at different locations of the EAST tokamak are simulated using a molecular dynamics code. It is shown that nano-dust particles, with the radius, R d , ～5 nm, can exist for at least several nano-seconds under the interactions from the ions without being ablated in some specific places of the tokamak edge plasma, while those with R d ≥25 nm may be ablated if the plasma temperature T～ 50 eV and density n～10 19 m 3 . In addition, the collisions of tungsten nano-dust grains with a tungsten wall at 100 m/s or 1000 m/s impinging speeds are simulated. It is demonstrated that the dust will stick to the wall, and the collision will not cause substantial damage to the wall, but it may be able to cause partial destruction of the dust grains themselves depending on their incident speeds.     

BNCT中载能粒子对肿瘤细胞损伤效应的Monte Carlo模拟及分析

为更好地了解硼中子俘获治疗(Boron neutron capture therapy,BNCT)中载能粒子穿过单个细胞引起的生物学效应以及理解在细胞或亚细胞水平上的微观剂量分布,采用Monte Carlo程序模拟载能粒子在人体细胞中的输运过程,给出了α粒子垂直细胞表面入射时的射程分布、径迹结构及靶损伤情况,同时模拟分析了^10B位于细胞中不同位置时主要载能粒子(α粒子和^7Li离子)在细胞中能量沉积情况和相应的细胞损伤与存活情况,为BNCT的微剂量研究提供了初步的理论依据。     

Molecular Dynamics Study on the Diffusion Properties of Hydrogen Atoms in Bulk Tungsten

Molecular dynamics simulations were performed to study the diffusion behavior of hydrogen atoms in body-centered cubic(bcc) tungsten(W). The energy distribution of a single hydrogen atom in the (001) plane of tungsten lattice was computed. The values of diffusion barriers agree well with other theoretical and experimental results. The interaction between an H atom and a vacancy was simulated, which shows evidence of strong binding effect. The temperature effect on the diffusion behavior of hydrogen atoms was investigated. The critical temperature for an H atom to diffuse in bulk W with and without vacancies were calculated to be 950 K and 450 K, respectively, which is supported by several experimental results. In addition, the diffusion coefficient of hydrogen atoms in tungsten was evaluated and analyzed.     

Molecular Dynamics Simulations of the Interactions Between Tungsten Dust and Beryllium Plasma-Facing Material

In the present research,molecular dynamics simulation is applied to study the interactions between tungsten dusts and a beryllium plasma-facing material surface.Calculation results show that it is quite difficult for nanometer-size dust particles to damage the plasma-facing material surface,which is different from the micrometer-size ones.The reason may be the size difference between dust and crystal grains.The depth of dust penetration into plasma-facing materials is closely related to the incident velocity,and the impacting angle also plays an important role.Dust and material surface damage is also investigated.Results show that both incident velocity and angle can significantly influence the damage.     

Nanometer site analysis of electron tracks and dose localization in bio-cells exposed to X-ray irradiation

This paper reports on a Monte Carlo simulation analysis of the ionization and excitation clusters in electron tracks, which may contribute to radiation damage of biological cells with high probabilities. The study is aimed at investigating the energy transfer to the cell nucleus exposed to X-rays from low doses in the environment to high doses in radiation therapy. As an example, we adopt a water phantom exposed to X-rays from a 6 MV linac to calculate the expected energy transfer to electrons in liquid water. A track simulation of the electrons produced by the photon interactions was performed. The behavior of low energy electrons below 1 keV is of particular importance in forming clusters of ionization and excitation events within a nanometer scale (<100 nm). We describe the clustering pattern in terms of aggregation index, the distribution of point-to-point distance below several nanometers between the events. By comparing the distribution with that of the Poisson configuration, the clustering effects of the events including successive radical processes are evaluated.