表面峰强度的Monte Carlo模拟计算

本文介绍用Monte Carlo模拟方法对入射He~+离子能量在0.1—2.OMeV范围内,硅、铝单晶在几种不同表面结构条件下的表面峰强度进行了计算。讨论了离子入射能量、入射角度、晶格原子热振动幅度、相关系数、表面增强因子、原子位移和吸附原子等因素对计算结果的影响。     

Comparison between Monte Carlo and experimental aluminum and silicon electron energy loss spectra

A Monte Carlo (MC) simulation is described and used to calculate the energy distribution spectra of backscattered electrons from Al and Si. For the simulations, elastic scattering cross sections are calculated by numerically solving the Dirac equation in a central field. Inelastic scattering cross sections are computed within the dielectric response theory developed by Ritchie, and by Tung et al. Extension from the optical case to non-zero momentum transfer is done according to Ritchie and Howie. To evaluate surface and bulk contributions to the spectra, the Monte Carlo model treats the surface excitations according to the Werner differential surface and volume excitation probability theory. The Monte Carlo calculations are compared with the experimental reflection electron energy loss (REEL) spectra acquired in our laboratory.     

Evaluation of influence of surface barrier in solid on ion reflection by bipartition model of ion transport

The influence of surface barrier of solid upon ion reflection was studied in a few papers of other authors by using the Monte-Carlo simulation. Based on the bipartition model of ion transport, a new analytical theory has been developed instead of the MC simulation, due to important implication of the effect for fusion research. In the present paper we have calculated the number reflection coefficients of H , D , He and T normally incident on C, Al and Cu for ion energy from several eV to one hundred keV respectively. Our computational results accorded with the MC simulation. The results have shown that the effect of surface barrier on ion reflection becomes evident when the energy of incident ions is lower than one keV. In particular, for the ion energy from several eV to one hundred eV, the discrepancies of number reflection coefficients can increase up to 0.1-0.3, showing this influence to be very important.     

Interpolation formula for particle reflection coefficient of keV light ions obliquely incident on heavy solids

The particle reflection coefficient of light keV ions backscattered from heavy solids as a function of the ion incidence angle has been determined by a suitable interpolation formula. The formula has two fitting parameters which are obtained by using results from two limiting analytic approaches: by the single collision model - in case of nearly perpendicular incidence; and by the small-angle multiple scattering theory - in case of glancing angles of incidence. The obtained interpolation formula is a universal function of the scaled transport cross section and the angle of incidence. Comparison of our calculations with Monte Carlo simulation data and the experimental results of other authors gives good agreement for particle reflection coefficient.     

Development of vectorized Monte Carlo particle transport programs

Special developments of vectorized Monte Carlo simulation codes for particle interaction with matter were performed for applications in medium and high energy physics, namely simulation of particle shower development in particle detectors. Particle showers in such large pieces of matter comprise particle energies from the energy of the incident high energy particle down to thermal energies. Due to high multiplicity of cascade particles with low energy, the Monte Carlo programs for low energy particle transport (mainly neutrons and photons) are the most time consuming ones. Therefore, we developed a vectorized geometry package for use with Monte Carlo programs, since it was found that in many cases this part of the codes is the most time consuming one at all. The question how to gather vectors in Monte Carlo programs was solved by a scheme of particle parallel processing by the code. Recently we developed vectorized versions of a nuclear event generator and a routine for tracking of neutral particles in material. The nuclear event generator code uses tabulated partial cross sections for all reactions of interest and simulates single nuclear collision events in the CM system, conserving energy and momentum in each. The tracking routine and the nuclear event generator together with the geometry package form the main parts of a particle transport MC program. Thus, we were able to assemble such a program for neutral particles with most of the inner loops vectorized. A series of test runs was performed, which showed CPU time gains of about 4.     

Using gamma-gamma coincidence measurements to validate Monte Carlo generated detector response functions

Monte Carlo simulation of gamma-ray transport for the purpose of performing elemental analysis of bulk samples requires the tracking of gamma rays in the sample and also in the detector(s) used. Detector response functions (DRF’s) are an efficient and accurate variance reduction technique that greatly decreases the simulation time by substituting the tracking of gamma rays inside the detector by predefined single energy gamma-ray spectra. These spectra correspond to the average response of the detector for incident gamma rays.DRF’s are generated by Monte Carlo methods and are benchmarked with experimental data. In this work, prompt gamma-gamma coincidence measurements are presented as a way to validate DRF’s for high-energy gamma rays.     

Stopping cross sections of liquid water for MeV energy protons

Cross sections for the stopping of swift protons in liquid water have been measured for the first time by using a liquid water jet target of 50 μm in diameter. The energy loss spectra of incident 2.0 MeV protons were measured at scattering angles of 5.0-50 mrad. Experimental energy loss spectra have been successfully reproduced by Monte Carlo simulation calculations (GEANT4.9.1.p02 toolkit) by taking account of multiple scattering of projectile ions inside the liquid water target. The present stopping cross sections are found to be considerably smaller than other standard stopping power data, revealing e.g. about 11% deviation from those of SRIM2003.     

Microcomputer Monte Carlo electron transport codes for beta skindose calculations

The adaptation of a one-dimensional Monte Carlo election transport program (called ZEBRA) for use on microcomputers in a program called Eltran2 is described. The purpose of this adaptation was to reduce the cost of the Monte Carlo calculations. Eltran2 has, in turn, been modified into a two-dimensional program called Eltran3 for computing the dose from a point or a disk source to a cylindrical target. For Monte Carlo calculations, theoretical beta energy spectra are calculated based on the Fermi beta decay theory. The calculated average energies of spectra agree with the values in related publications to within 6%. An extended study has been done using Eltran2 and Eltran3 to facilitate the design of, a beta/gamma skin dose monitor. The programs calculate the effects of angular distribution of source electrons and the radial distribution of the hot particle dose. It is found that the hot particle dose averaged over a live skin area of 1 cm² significantly underestimates the real dose value at the very small area just under the hot particle by a factor of about 1000     

Spatial distribution of reflected gamma rays by Monte Carlo simulation

In nuclear facilities, the reflection of gamma rays of the walls and metals constitutes an unknown origin of radiation. These reflected gamma rays must be estimated and determined. This study concerns reflected gamma rays on metal slabs. We evaluated the spatial distribution of the reflected gamma rays spectra by using the Monte Carlo method. An appropriate estimator for the double differential albedo is used to determine the energy spectra and the angular distribution of reflected gamma rays by slabs of iron and aluminium. We took into the account the principal interactions of gamma rays with matter: photoelectric, coherent scattering (Rayleigh), incoherent scattering (Compton) and pair creation. The Klein-Nishina differential cross section was used to select direction and energy of scattered photons after each Compton scattering. The obtained spectra show peaks at 0.511^∗ MeV for higher source energy. The Results are in good agreement with those obtained by the TRIPOLI code [J.C. Nimal et al., TRIPOLI02: Programme de Monte Carlo Polycin?etique à Trois dimensions, CEA Rapport, Commissariat à l’Energie Atomique. [1]].     

MONTE CARLO CALCULATION OF ENERGY DEPOSITION BY DELTA RAYS AROUND ION TRACKS

The radial distribution of dose around the path of a heavy ion has been studied by a Monte Carlo transport analysis of the delta rays produced along the track of a heavy ion based on classical binary collision dynamics and a single scattering model for the electron transport process.Result comparisons among this work and semi-empirical expression based delta ray theory of track structure,as well as other Monte Carlo calculations are made for 1,3MeV protons and several heavy ions.The results of the Monte Carlo simulations for energetic heavy ions are in agreement with experimental data and with results of different methods.The characteristic of this Monte Carlo calculation is a simulation of the delta rays theory of track structure.     

基于CAD邻居列表和包围盒的蒙特卡罗粒子输运几何跟踪加速方法研究

几何跟踪主要进行蒙特卡罗粒子输运计算中粒子位置和径迹长度的计算,它是蒙特卡罗粒子输运计算的关键技术之一。由于聚变堆几何结构极其复杂,使得几何跟踪在整个蒙特卡罗粒子输运计算中占据30%~80%的计算时间,因此几何跟踪方法的效率是决定聚变堆蒙特卡罗粒子输运计算效率的重要因素之一。本文提出了基于CAD的邻居列表和包围盒加速方法,并基于FDS团队自主研发的超级蒙特卡罗核计算仿真软件系统SuperMC进行实现。利用聚变堆FDS-Ⅱ和ITER模型对本文方法进行了数值验证,测试结果表明本文方法不影响计算结果,并能使程序计算效率提高50%~60%,证明了本文方法的正确性和有效性。     

An ab initio Monte Carlo study of low energy positron and electron implantation in elemental metals and multilayers

A Monte Carlo code developed at Brookhaven to study positron and electron implantation in solids will be discussed. This code models the trajectories of a large number of positrons or electrons as they move through the material and uses this information to calculate stopping profiles, backscattered fractions and transmitted fractions. The particle-atom interactions are incorporated via elastic and inelastic scattering cross-sections. The incident particle energies vary from 1 to 10 keV and the simulation stops when the particle energy goes down to 25 eV or if the particle is backscattered. Main results of our Monte Carlo simulations of the mean implantation depth and implantation profiles for both semi-infinite metallic elements and multilayers will be discussed.     

Measurements of Lα, Lβ X-ray production cross sections of Bi by 17-40 keV electron impact

We present results of measurements of L_α, L_β X-ray production cross sections for the element Bi (Z = 83) by 17-40 keV electron impact. The target used in the experiment was prepared by evaporating the element Bi to the thick pure carbon substrate. The effects of multiple scattering of electrons when penetrating the target film, of electrons reflected from the thick pure carbon substrate and of bremsstrahlung photons produced by the impact of incident electrons on the target are corrected by means of Monte Carlo simulation. The experimental data, reported here for the first time in the energy region of 17-40 keV, are compared with the DWBA theory and the PWBA-C-Ex theory. They are in good agreement.     

油砂地层中子-伽玛能谱Monte-Carlo模拟

本工作应用自行研制开发的中子 伽玛能谱测井的中子 伽玛射线随空间、能量、时间分布的蒙特卡罗模拟软件包计算了裸眼井、井眼里无下井仪器、油饱和砂岩地层、井眼注油条件下沿井轴和井壁的中子 伽玛射线随能量、时间分布及沿井轴和沿井壁的中子 伽玛射线能谱 ,研究了源距对中子 伽玛射线随能量、时间分布以及井轴与井壁中子 伽玛射线能谱的影响。     

线极化γ射线康普顿散射的蒙特卡罗模拟

本文利用蒙特卡罗程序Geant4模拟了不同能量、不同线极化度γ射线的康普顿散射过程,通过记录散射γ射线方位角上计数的不对称性模拟计算了入射γ射线的极化度。比较了模拟计算的极化度与原入射射线的极化度,发现两者符合很好。结果表明,利用蒙特卡罗方法能模拟极化光子与物质的相互作用,可为γ极化仪的设计提供指导。     

Simulation of energy spectra of slow He ions scattered from a copper surface

Scattering of low energy (2 keV) He⁺ ions from a polycrystalline copper surface is studied. Experimental Time-Of-Flight (TOF) spectra are presented for different values of the angle of incidence α and a fixed scattering angle θ = 129°. Using Monte Carlo simulation, based on the TRIM (TRansport of Ions in Matter [6]) code, energy spectra are calculated and compared to experimental data. The influence of different combinations of interaction potential and inelastic energy loss on the spectrum shape and on the angular distribution of the scattering yield is considered in the simulation. Simulations show very good agreement with the experiment for all investigated geometries. An analysis of the experimental yield of scattered particles in different parts of the spectrum is performed to gain information on the relative contributions of surface single scattering and multiple scattering.     

A new high speed solution for the evaluation of Monte Carlo radiation transport computations

Advancements in parallel and cluster computing have made many complex Monte Carlo simulations possible in the past several years. Unfortunately, cluster computers are large, expensive, and still not fast enough to make the Monte Carlo technique useful for calculations requiring a near real-time evaluation period. For Monte Carlo simulations, a small computational unit called a Field Programmable Gate Array (FPGA) is capable of bringing the power of a large cluster computer into any personal computer (PC). Because an FPGA is capable of executing Monte Carlo simulations with a high degree of parallelism, a simulation run on a large FPGA can be executed at a much higher rate than an equivalent simulation on a modern single-processor desktop PC. In this paper, a simple radiation transport problem involving moderate energy photons incident on a three-dimensional target is discussed. By comparing the evaluation speed of this transport problem on a large FPGA to the evaluation speed of the same transport problem using standard computing techniques, it is shown that it is possible to accelerate Monte Carlo computations significantly using FPGAs. In fact, we have found that our simple photon transport test case can be evaluated in excess of 650 times faster on a large FPGA than on a 3.2 GHz Pentium-4 desktop PC running MCNP5.     

中子-光子输运程序TOPAN的开发与验证

开发的粒子输运蒙特卡罗通用程序TOPAN能模拟中子和光子的耦合输运,可给出中子和光子在介质中输运后的点通量、面通量、体通量、面流量等参数。该程序除具备常用的蒙特卡罗软件功能外,还增加了处理介质温度变化的等效质量热运动模型和非均匀介质中粒子输运模块,具备粒子标识功能,初步具备了进行一些复杂问题中粒子输运模拟的能力。结合具体算例对TOPAN程序的各功能模块进行了比对验证。     

Effect of energy loss in the simulation of slow ion scattering by a solid surface

As previously shown [B. Arezki, Y. Boudouma, P. Benoit-Cattin, A.C. Chami, C. Benazeth, K. Khalal, M. Boudjema, J. Phys.: Condens. Mat. 10 (1998) 741, K. Khalal-Kouache, A.C. Chami, M. Boudjema, P. Benoit-Cattin, C. Benazeth, Y. Boudouma, Nucl. Instr. and Meth. B 183 (2001) 279], energy spectra of scattered particles from amorphous solid surface can be deduced from angular distributions of the total path length of the projectiles. In preceding papers [K. Khalal-Kouache, A.C. Chami, M. Boudjema, P. Benoit-Cattin, C. Benazeth, Y. Boudouma, Nucl. Instr. and Meth. B 183 (2001) 279, K. Khalal-Kouache, A. Mekhtiche, A.C. Chami, M. Boudjema, Surf. Coat. Technol. 201 (2007) 8420], a model based on the transport theory was used to simulate the scattering of slow ions by a solid target. Angular distributions of the total path length were computed and compared, in absolute value, to those simulated using a Monte Carlo code. A good agreement was obtained except for the highest values of the total path length L. In the transport theory, the variation of the scattering cross section versus energy during the slowing down of the particle was not taken into account. The purpose of this contribution is to take into account this effect within the model. The obtained results are found to be in agreement with Monte Carlo computation for angular distributions of the total path length and for energy spectra of the scattered ions.     

Differential neutron albedos for cylindrical water surfaces

Total albedos, as well as albedos differentiated in terms of both energy and direction, were calculated by means of a Monte Carlo program for cylindrical water surfaces of different radii of curvature. It is shown that a cylindrical coordinate system in which the polar angle is measured relative to the normal on the scattering surface is preferable to one in which this angle is measured relative to the axial direction of the surface. For small polar angles in the preferred system, even when the surface radius approaches the mean free path of the incident neutrons, the effect of this radius remains small. With increasing polar angles the effect of the surface radius becomes more important, especially for those neutrons moving more or less perpendicular to the axial direction of the surface. In the latter case, even at radii as large as 10 n mean free paths, the radial effect at a large polar angles is significant.