Computer simulation of proton channelling in beryllium

The channelling of protons through a thin beryllium crystal is simulated in a computer. The angular dependence of the momentum density is computed using particle trajectory approximation and is reported as the transmission spectra. In obtaining the spectra, the energy loss suffered by protons due to electron multiple scattering is considered and the effect of thermal vibrations treated separately. The spectra obtained are characteristic of the hcp structure of Be. Positions of the major dips in the transmission spectra are found to correlate well with the directions of neighbouring strings. Variations in the angle of incidence of the beam and in its initial azimuthal angle bring about modifications in the spectra depending on the transverse kinetic energy of the incident particles and the crystalline structure of Be. Thermal vibration of the lattice does not modify the spectra appreciably.     

Effect of hydrogen introduced by channelling implantation on the microstructures in silicon wafers

High-resolution transmission electron microscopy has been employed to study the platelet defects before annealing and the extended defects generated by annealing in the channelling-implanted silicon wafers. It has been found that there apparently appear platelet defects of quite great size and spacing at the maximum projected range of ions (R _max). Additionally, the cracks induced by annealing at 550 °C are generated around R _max instead of the average projected range of ions (R _p) as it is in the non-channelling-implanted samples. Moreover, after annealing at 1000 °C, cracks without branches and cavities arranging in a single array, different from the forked cracks and cavities arranged in several arrays in the non-channelling-implanted samples, are observed in the channelling-implanted silicon wafers. It is suggested that those special microstructure characteristics are ascribed to the channelling effect of implanted hydrogen ions.     

Computer simulation of the collision frequency of two particles in optical tweezers

Optical tweezers have been successfully used in the study of colloid science. In most applications people are concerned with the behaviour of a single particle held in the optical tweezers. Recently, the ability of the optical tweezers to simultaneously hold two particles has been used to determine the stability ratio of colloidal dispersion. This new development stimulates the efforts to explore the characteristics of a two-particle system in the optical tweezers.An infinite spherical potential well has been used to estimate the collision frequency for two particles in the optical trap based on a Monte Carlo simulation. In this article, a more reasonable harmonic potential, commonly accepted for the optical tweezers, is adopted in a Monte Carlo simulation of the collision frequency. The effect of hydrodynamic interaction of particles in the trap is also considered. The simulation results based on this improved model show quantitatively that the collision frequency drops down sharply at first and then decreases slowly as the distance between the two particles increases. The simulation also shows how the collision frequency is related to the stiffness of the optical tweezers.     

低能中子在锆中产生的辐照损伤的计算机模拟研究

以GEANT4为基础采用蒙特卡罗方法对能量为1 MeV的中子在锆 材料中的输运过程进行了模拟分析. 首先计算得出: 反冲核的能量主要分布在1 keV和15 keV之间, 中子和靶核发生两次弹性碰撞的平均空间距离为29.47 mm. 由于中子和靶核在发生连续两次弹性碰撞过程中产生的两个反冲核能量较低, 它们的空间距离又比较大, 由此可以推测出: 由初级离位原子产生的后续级联碰撞可以看做是一系列独立的子级联碰撞过程, 同时也计算了中子在靶材的不同深度区域内产生的反冲核数目和平均能量. 其次, 利用蒙卡方法计算得到的结果, 采用分子动力学方法, 分别计算了五种不同能量下的初级离位原子产生的级联碰撞情况, 给出了初级离位原子的能量与其产生的次级离位原子数目之间的关系以 及不同能量下的初级离位原子产生的损伤区域范围等情况, 通过蒙特卡罗方法和分子动力学方法的结合, 给出了能量为1 MeV的中子在锆材料中产生的初级辐照损伤分布图像. 关键词： 辐照损伤 级联碰撞 蒙特卡洛模拟 分子动力学     

Computer simulation of low-energy displacement events in pure HCP metals

Abstract

Molecular dynamics simulations of low-energy atomic recoils have been carried out for α-Ti (HCP) with a view to investigating the displacement threshold energy, E_d, in detail. These have been undertaken at 0 K and 100 K, using a many-body interatomic potential modified to reflect the dominant two-body interaction at short range. This is the first systematic study of atomic displacement events in the HCP structure using such a potential. The mechanisms of these threshold events have been investigated and the strong orientation dependence of the threshold energy has been interpreted in terms of the HCP crystal structure. Also, a correlation has been found between the magnitude of the threshold displacement energy and the maximum number of atoms temporarily displaced into interstitial positions during a cascade.     

Computer simulation of high energy displacement cascades

Abstract

A methodology developed for modeling many aspects of high energy displacement cascades with molecular level computer simulations is reviewed. The initial damage state is modeled in the binary collision approximation (using the MARLOWE computer code), and the subsequent disposition of the defects within a cascade is modeled with a Monte Carlo annealing simulation (the ALSOME code). There are few adjustable parameters, and none are set to physically unreasonable values. The basic configurations of the simulated high energy cascades in copper, i.e., the number, size and shape of damage regions, compare well with observations, as do the measured numbers of residual defects and the fractions of freely migrating defects. The success of these simulations is somewhat remarkable, given the relatively simple models of defects and their interactions that are employed. The reason for this success is that the behavior of the defects is very strongly influenced by their initial spatial distributions, which the binary collision approximation adequately models. The MARLOWE/ALSOME system, with input from molecular dynamics and experiments, provides a framework for investigating the influence of high energy cascades on microstructure evolution.     

Computer simulation of the vacancy defects interaction with shuffle dislocation in silicon

The interactions between the 60^° shuffle dislocation and two different types of vacancy defects in silicon are separately studied via the molecular dynamics simulation method. The Stillinger–Weber potential is used to describe the atomic interactions. The results show that the dislocation slip velocity will decrease due to the interaction with the vacancy cluster (V ₆). The simulation also reveals that the divacancy will be absorbed by the dislocation. Meanwhile, a climbing of the dislocation occurs during their interactions. However, the divacancy has little effect on the dislocation slip velocity. Based on the above results, the decrease in threading dislocation density in SiGe/Si heterostructures with the use of low-temperature Si buffer layer may be explained.     

Computer simulation of the two-stage ion polishing of a silicon surface

Comparative results of the computer simulation of surface sputtering by a focused ion beam with Gaussian density distribution and a uniform ion flux are presented. The experimental angular dependences of the yield of silicon sputtering by nitrogen and argon ions are used. The possibility of the optimization of ion polishing via methods of computer simulation is demonstrated.     

Computer simulation of phase-contrast images in white synchrotron radiation using micropipes in silicon carbide

A method of computer simulation of phase-contrast images in white synchrotron radiation has been developed to determine the section parameters of micropipes in silicon carbide. The experiments have been carried out using the third-generation synchrotron radiation source the Pohang Light Source (South Korea). The effective spectrum of the synchrotron radiation that forms of an image has been shown to have a relatively sharp maximum at an energy of 16 keV, which makes it possible to conserve coherency within the required limits. A computer program has been developed that automatically determines the diameters of an elliptic section of a micropipe from the condition of coincidence of calculated and experimental profiles. It has been shown that the studied micropipes have a strongly stretched elliptic section that can twist when moving along the pipe axis.     

Cross sections for rotational excitation in He－H2 collisions

Close coupling calculations have been carried out for rotational excitations in He－H₂ collisions with symmetric isotopic substitution (He－H₂, D₂, T₂) and asymmetric isotopic substitution (He－HD, HT, DT). Cross sections have been obtained at the incidence energy of 0.3eV. Based on the calculations, the effect of isotopic substitution on atom－diatom collisions is discussed.     

Site-specific ionisation edge fine-structure of Rutile in the electron microscope

Combined Bloch-wave and density functional theory simulations are performed to investigate the effects of different channelling conditions on the fine-structure of electron energy-loss spectra. The simulated spectra compare well with experiments. Furthermore, we demonstrate that using this technique, the site-specific investigation of atomic orbitals is possible. This opens new possibilities for chemical analyses.     

Computer simulation of charging the silicon dioxide surface and subsurface layers by electron bombardment

A physical model and program of calculating the parameters of charging dielectrics by electron bombardment is described. A method of computer simulation is used to investigate the main processes of charging the subsurface silicon dioxide layers. Dependences of the current density, volume charge density, and electric field strength on the material layer depth are calculated for variable electron beam parameters, irradiation time, and grid potential near the sample surface.     

Close coupling calculations for excitation partial cross sections in Ne--HF collisions

This paper reports that an exact quantum close coupling calculation is carried out for rotational excitation in Ne HF collisions on the available anisotropic potential. Partial cross sections are obtained separately at the incident energies of 48.35, 75, 120 and 150meV. The reliability of the results is demonstrated by comparison with previously published theoretical findings. Based on the calculations, the effect of the potential energy surface on the excitation partial cross sections is discussed in detail.     

非晶/微晶两相硅薄膜电池的计算机模拟

在对不同晶相比硅薄膜的实验研究的基础上，利用有效介质理论估算了这种两相材料的光吸 收系数、迁移率寿命乘积及带隙宽度等参量，计算机模拟了不同结晶比硅薄膜电池的伏安特 性及光谱响应；结果为随着本征层微晶成分的增多，电池的开路电压逐渐减小，短路电流逐 渐增大，本征层的最佳厚度逐渐增大，填充因子有降低的趋势，电池的效率随晶相比的增大 而减小. 电池的光谱响应曲线表明，随晶相比的增大电池的长波响应明显提高. 根据这些模 拟结果，分析讨论了在考虑Lambertian背反射的情况下，非晶/微晶叠层电池的底电池采用 晶相比为40％—50％的两相硅薄膜材料做本征层是最佳选择. 关键词： 两相硅薄膜 太阳能电池 计算机模拟     

Quantum state of muon and proton in crystalline silicon

Quantum state and dynamics of muon and proton in crystalline silicon have been studied by solving one‐particle Schrödinger equations for the impurities. The ground state wavefunctions and energies are determined as a function of local distortion of the host Si lattice, where the Si–H interaction we used is that parameterized by Ramírez and Herrero following the results of earlier pseudopotential‐density‐functional calculation. It is shown that quantum zero‐point motion of muon induces an effective potential barrier between the tetrahedral‐symmetry (T) site and the bond‐center (BC) site, which ensures bistability of muonium states observed in experiments. It is also shown that, if we fully consider the quantum effect with the present model potential, the BC site becomes less stable than the T site on the contrary to the experiments and former theoretical calculations.     

再谈"小球与均质自由杆的碰撞"

讨论了小球在杆上任一点的碰撞问题,并指出《大学物理》2006年第5期《小球与均质自由杆的碰撞》一文中"完全非弹性碰撞后两物体不一定粘在一起共同运动"的说法是错误的.     

High resolution measurements of energy spectra of protons scattered from silicon crystals in the case of planar channelling

The energy spectrum of backscattered protons in the case of incidence along several planar directions shows a fine structure near the high-energy edge. This structure, an oscillatory dependence of the probability of backscattering vs. depth in the crystal, offers a possibility to study the proton trajectory in the lattice and also to obtain the stopping power of protons near planes in silicon.

Application of a simple model for the proton trajectory yields a stopping power near the planes 4 to 5 times higher than for random incidence. These effects have been observed using primary energies in the range 40–140 keV and for incidence along (110). (111), (100) and (112) planes.     

Single source emission of proton projectile fragments in nucleus-emulsion interactions

The projected angular distribution and transverse momentum distribution of proton projectile fragments produced in 3.7A GeV 16O,500A MeV 56Fe,and 1.7A GeV 84Kr induced by different kinds of emulsion target(H,CNO,and AgBr) interactions are investigated.It is found that the projected angular distribution and transverse momentum distribution can be well represented by a single Gaussian distribution.Comparison of transverse momentum distribution with the Maxwell-Boltzmann distribution reveals that proton projectile fragments are emitted from a single-temperature emission source.The temperature is different for different colliding systems,and linearly depends on the target size.