MC模拟在随钻方位伽马成像正演中的应用

蒙特卡罗模拟是放射性测井一种重要的数值模拟方法,利用通用程序MCNP可方便地建立各种地层模型,为放射性测井研究提供了简便有效的方法。借助于蒙特卡罗方法建立随钻条件下地层模型,正演模拟随钻方位伽马成像特征。结果表明,随钻方位伽马成像图呈现正弦线特征,利用成像图可准确反演井眼相对倾角和放射性地层厚度;井眼相对倾角越大,放射性地层厚度越厚,成像图的正弦线高度值及在井轴方向上的展布高度值越大;井眼尺寸对利用成像图求取井眼相对倾角没有影响,但对计算放射性地层厚度有影响,仪器偏心对利用成像图求取井眼相对倾角和放射性地层厚度都有较大影响,若伽马计数满足测井计数统计性,可忽略井眼流体对成像图的影响。     

Monte Carlo simulations of copper clustering in Fe–Cu alloys under irradiation

We present the computational approach for studying the microstructures of Cu clusters in Fe–Cu alloys by combining the molecular dynamics (MD) simulation and Monte Carlo methods. The MD simulation is used to characterize the primary damage resulting from the displacement cascade in Fe. Then, using the Metropolis Monte Carlo methods, the microstructure of the Cu clusters is predicted under the assumption that the system will evolve towards the equilibrium state. The formation of the Cu clusters is apparent for Fe–Cu alloys of a higher Cu content (1.0 w/o), whereas the degree of Cu clustering is not significant for the lower Cu content (0.1 w/o) alloys. The atomic configuration of the Cu–vacancy complex under irradiation, produced by this simulation, is in a fair agreement with the experiments. The simulation is expected to provide important information on the Cu-cluster morphology, which is useful for experimental data analysis.     

Dosimetric measurements and Monte Carlo simulation for achieving uniform surface dose in pulsed electron beam irradiation facility

A prototype pulsed electron beam irradiation facility for radiation processing of food and medical products is being commissioned at our centre in Indore, India. Analysis of surface dose and uniformity for a pulsed beam facility is of crucial importance because it is influenced by various operating parameters such as beam current, pulse repetition rate (PRR), scanning current profile and frequency, scanning width and product conveying speed. A large number of experiments are required to determine the harmonized setting of these operating parameters for achieving uniform dose. Since there is no readily available tool to set these parameters, use of Monte Carlo methods and computational tools can prove to be the most viable and time saving technique to support the assessment of the dose distribution. In the present study, Monte Carlo code, MCNP, is used to simulate the transport of 10 MeV electron beam through various mediums coming into the beam path and generate an equivalent dose profile in a polystyrene phantom for stationary state. These results have been verified with experimentally measured dose profile, showing that results are in good agreement within 4%. The Monte Carlo simulation further has been used to optimize the overlapping between the successive pulses of a scan to achieve ±5% dose uniformity along the scanning direction. A mathematical model, which uses the stationary state data, is developed to include the effect of conveyor speed. The algorithm of the model is discussed and the results are compared with the experimentally measured values, which show that the agreement is better than 15%. Finally, harmonized setting for operating parameters of the accelerator are derived to deliver uniform surface dose in the range of 1–13 kGy/pass.     

Monte Carlo investigation of the effect of small cutouts on beam profile parameters of 12 and 14 MeV electron beams

Cutouts, which are used as field-shaping shield, affect several electron beam parameters. These effects are more observable for small field sizes and high energy electron beams. Owing to the fact that small fields prevent the lateral scatter equilibrium, at higher energies larger field radius is required for the establishment of lateral equilibrium.The profile curves are derived from circular, triangular, and square cutout shapes and size placed in a 10 × 10 cm² electron applicator. These profile curves are obtained using parallel plane type ion chamber at the R₁₀₀, R₉₀, R₈₀ and R₅₀ depths. Correspondingly, the source surface distance is 100 cm.In this study MCNP Monte Carlo (MC) simulation was used to compare Percentage Depth Dose (PDD) and Profile of electron beams.Monte Carlo and measured results showed a good compliance for PDD and beam profile. The measurements and calculations showed that as the field width decreases, the Flatness and Penumbra Ratio also decreases. In other words, flatter plateau was available for larger fields. Also the Coverage Ratio for each of the profiles is presented. The flatness and symmetry values for triangle shapes were greater than the two other shapes.Knowledge of these changes are significant in radiation therapy. Accordingly, a comparison between the Monte Carlo data and the measured results can be beneficial for treatment simulation and development of treatment planning systems.     

Multi-site kinetic Monte Carlo simulations of thermal desorption spectroscopy data

Presented is a kinetic Monte Carlo simulation (KMCS) algorithm for simulating experimental thermal desorption spectroscopy (TDS) data. The KMCS is based on the Master equation approach and applies a first-passage-time analysis, i.e., the time dependence of the kinetics is matched correctly. The KMCS–TDS scheme used here includes multiple kinetically distinct adsorption sites and the effect of lateral interactions as required for fitting experimental data. After the results of extensive tests of the algorithm by means of synthetic data are discussed, experimental TDS curves are reproduced by the KMCS. Two applications are demonstrated: iso-butane adsorption on ZnO(0 0 0 1)–Zn and n-pentane adsorption on carbon nanotubes.     

Monte Carlo simulation code for confocal 3D micro‐beam X‐ray fluorescence analysis of stratified materials

Stratified materials are of great importance for many branches of modern industry, e.g. electronics or optics and for biomedical applications. Examination of chemical composition of individual layers and determination of their thickness helps to get information on their properties and function. A confocal 3D micro X‐ray fluorescence (3D µXRF) spectroscopy is an analytical method giving the possibility to investigate 3D distribution of chemical elements in a sample with spatial resolution in the micrometer regime in a non‐destructive way. Thin foils of Ti, Cu and Au, a bulk sample of Cu and a three‐layered sandwich sample, made of two thin Fe/Ni alloy foils, separated by polypropylene, were used as test samples. A Monte Carlo (MC) simulation code for the determination of elemental concentrations and thickness of individual layers in stratified materials with the use of confocal 3D µXRF spectroscopy was developed. The X‐ray intensity profiles versus the depth below surface, obtained from 3D µXRF experiments, MC simulation and an analytical approach were compared. Correlation coefficients between experimental versus simulated, and experimental versus analytical model X‐ray profiles were calculated. The correlation coefficients were comparable for both methods and exceeded 99%. The experimental X‐ray intensity profiles were deconvoluted with iterative MC simulation and by using analytical expression. The MC method produced slightly more accurate elemental concentrations and thickness of successive layers as compared to the results of the analytical approach. This MC code is a robust tool for simulation of scanning confocal 3D µXRF experiments on stratified materials and for quantitative interpretation of experimental results. Copyright © 2011 John Wiley & Sons, Ltd.     

井型高纯锗探测器体源样品无源效率刻度

采用蒙特卡罗程序MCNP建立物理模型,对井型高纯锗探测器的效率进行虚拟刻度。模拟计算在密度为0.4,1.2 gcm-3的6种不同成分环境样品中,探测器对射线的探测效率,当能量高于0.10 MeV时,体源样品的探测效率主要与样品密度、射线能量相关。以土壤、水和植物油样品为代表,结合所选取的函数模型,确定了密度范围0.1~1.6 gcm-3的固体源、密度1.0 gcm-3的水溶液和密度0.92 gcm-3的油溶液体源的效率函数及参数。实验采用标准源,对探测效率模拟结果进行了验证,探测效率的模拟值与实验值符合较好,二者误差均在3%以内。说明MCNP程序可以较为准确地模拟计算井型高纯锗探测器对射线的探测效率,验证了该无源效率刻度方法的准确性和可行性。     

Design and gamma sensitivity measurement of a novel dual-emitter vacuum Compton detector

A novel dual-emitter vacuum Compton detector (D-VCD) with higher gamma ray detecting efficiency is proposed. The emitters are made of Ta--Al clad metal. The gamma ray sensitivity is studied by Monte Carlo simulation using the MCNP code. A comparison between calculations and results measured by using the 1.25~MeV gamma ray of Co-60 is also performed. Experimental sensitivities for two sample D-VCDs with the same materials and structures are 1.92×10^ - 20 and 2.02×10^ - 20~C.cm²/MeV separately, which are consistent with the simulation result of 1.98×10^ - 20~C.cm²/MeV and are 4 times higher than that of VCD with a single Fe emitter. According to the simulation results, in a gamma energy range from 0.5 to 3~MeV, the maximum sensitivity variance for the D-VCD is less than 15%, and less than 5% in a range from 1 to 2~MeV in particular. The novel D-VCD is applicable to the detection of intense pulse gamma rays.     

Monte Carlo simulation of quantum statistical lattice models

In this article we review recent developments in computational methods for quantum statistical lattice problems. We begin by giving the necessary mathematical basis, the generalized Trotter formula, and discuss the computational tools, exact summations and Monte Carlo simulation, that will be used to examine explicit examples. To illustrate the general strategy, the method is applied to an analytically solvable, non-trivial, model: the one-dimensional Ising model in a transverse field. Next it is shown how to generalized Trotter formula most naturally leads to different path-integral representations of the partition function by considering one-dimensional fermion lattice models. We show how to analyze the different representations and discuss Monte Carlo simulation results for one-dimensional fermions. Then Monte Carlo work on one- and two-dimensional spin-$\text{1}\text{2}$ models based upon the Trotter formula approach is reviewed and the more dedicated Handscomb Monte Carlo method is discussed. We consider electron-phonon models and discuss Monte Carlo simulation data on the Molecular Crystal Model in one, two and three dimensions and related one-dimensional polaron models. Exact numerical results are presented for free fermions and free bosons in the canonical ensemble. We address the main problem of Monte Carlo simulations of fermions in more than one dimension: the cancellation of large contributions. Free bosons on a lattice are compared with bosons in a box and the effects of finite size on Bose-Einstein condensation are discussed.     

用地面阵列寻找TeV能区γ点源的方法探讨

文中探讨搜寻TeV能区γ点源的方法.讨论利用student变量t判断来自源区及背景区宇宙线事例的统计差别,并由Bayes定理与MonteCarlo模拟相结合计算源区各能段的信号数,推算蟹状星云在TeV能区的γ射线微分能谱     

The NIMO Monte Carlo model for box-air-mass factor and radiance calculations

A new fully spherical multiple scattering Monte Carlo radiative transfer model named NIMO (NIWA Monte Carlo model) is presented. The ray tracing algorithm is described in detail along with the treatment of scattering and absorption, and the simulation of backward adjoint trajectories. The primary application of NIMO is the calculation of box-air-mass factors (box-AMFs), which are used to convert slant column densities (SCDs) of trace gases, derived from UV–visible multiple axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements, into vertical column densities (VCDs). Box-AMFs are also employed as weighting functions for optimal estimation retrievals of vertical trace gas profiles from SCDs. Monte Carlo models are well suited to AMF calculations at high solar zenith angles (SZA) and at low viewing elevation angles where multiple scattering is important. Additionally, the object-oriented structure of NIMO makes it easily extensible to new applications by plugging in objects for new absorbing or scattering species. Box-AMFs and radiances, calculated for various wavelengths, SZAs, viewing elevation and azimuth angles and aerosol scenarios, are compared with results from nine other models using a set of exercises from a recent radiative transfer model intercomparison. NIMO results for these simulations are well within the range of variability of the other models.     

多段阵列式钕玻璃放大器泵浦腔的光线追迹分析

采用光线追迹和蒙特卡罗法以及实验拟合数据，建立了多段钕玻璃放大器闪光灯泵浦腔的数值计算模型，模拟结果表明，光传输效率，储能密度分布和泵浦均匀性与泵浦腔几何形成有关，提出了一种新泵浦腔构形，它具有效率高，均匀性好和造价不很优点，本文所得结果对大尺寸多段阵列式钕玻璃的大器的设计有参考价值。     

Transport and Energy Deposition of hot Electrons through the Shock Ignition pre Compressed Target

Shock ignition as an alternative scheme of the laser fusion has the potential of achieving efficient implosion. However, hot electrons produced in result of ignitor‐corona interaction may penetrate deep into the fuel making the compression less effective. Transport and energy deposition of hot electron beam into the dense pre compressed of HiPER target by means of Monte Carlo approach are discussed considering the influence of real density and electron beam characteristics. The target parameters before igniting the hot spot have been extracted from a fluid code and used as the initial profile for Monte Carlo simulations. In comparison with simplified step like density profile, electrons penetrate slightly deeper in the case of real shaped density profile. In addition, deposition zone of a broad spectrum electron beam is wider while, monoenergetic electrons depose their energy locally resulting more maximum energy deposition value. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Use of the CEARXRF GUI‐Based Monte Carlo–Library Least‐Squares (MCLLS) Code for the Micro‐Focused EDXRF analyzer

The CEARXRF GUI‐Based Monte Carlo–Library Least‐Squares (MCLLS) Code is demonstrated with results from a micro‐focused EDXRF analyzer, which can be used to calculate elemental weight fractions in metal alloys or rock samples accurately by library least‐squares regression of the measured X‐ray spectrum with computer‐generated elemental library spectra. An elemental stratified sampling variance reduction technique has been implemented in the CEARXRF5 code, which equalizes the statistical precision of the elemental libraries within the measured sample independent of the relative elemental amounts that are present. Also, an improved Si(Li) detector response function (DRF) has been obtained for micro‐focused X‐ray fluorescence (XRF) analyzers, and the DRF parameters are obtained based on regression from pure elemental experimental spectra. It is demonstrated that the resulting MCLLS approach can greatly improve the accuracy of elemental XRF analysis results. Copyright © 2011 John Wiley & Sons, Ltd.     

Availability of water glass/Bi2O3 composites in dielectric and gamma-ray screening applications

ABSTRACT

Sodium silicate (Na₂Si₃O₇) also known as water glass is a very low cost material which is used in many industrial applications such as a builder in detergents, as a binder and adhesive etc. But so far the electrical properties of sodium silicate and its ability to screen radiation have never been investigated. In the present study, the frequency dependent electrical properties and gamma-ray shielding performance of water glass based bismuth oxide composites have been studied for the first time. In accordance with this purpose, Na₂Si₃O₇/Bi₂O₃ glassy composites have been prepared for searching their possible applications in electronics and radiation screening. The surface morphology of the samples have been determined by Scanning Electron Microscope (SEM). The frequency dependent electrical properties such as complex impedance, complex dielectric function and conductivity have been analyzed at room temperature between 1 and 40?MHz. As a result of alternative current (ac) electrical analysis, it has been determined that the Na₂Si₃O₇/Bi₂O₃ composites can be utilized as a dielectric layer in capacitors. On the other hand, since bismuth oxide is an anti-radiative material, the gamma-ray screening parameters such as mass attenuation coefficient, half layer and tenth layer values along with mean free path of the composites have been defined experimentally by using NaI(Tl) scintillation detector for the Ba-133 radiation source at 81 and 356?keV. The values of these parameters have also been checked by Monte–Carlo simulation. Since a good agreement has been assigned between experimental and Monte–Carlo simulation results, the related gamma ray shielding parameters have been determined by Monte–Carlo simulation for other gamma photon energies (140?keV, 208?keV, 468?keV, and 661?keV) which are generated from Tc-99, Lu-177, Ir-132, and Cs-137 sources. Ultimately, Na₂Si₃O₇/Bi₂O₃(35%) composite has been suggested as an eco-friendly, lead-free glassy structured material for the gamma radiation shielding in medical applications.     

Influence of laser power density on damage of comb by photodynamic therapy—simulation and validation of mathematical models

Photodynamic therapy （PDT） has poor therapeutic outcomes for the treatment of port-wine stain （PWS） lesions with long drug-light intervals （DLIs）. This letter investigates the possibility of improving the treatment efficacy through increasing the laser power density using a computer simulation and a cock comb model. The computational model includes a Monte Carlo simulation for the laser distribution and a calculation of the singlet oxygen concentrations （102）. Both simulation and experimental results show that increasing the power density from i00 to 140 mW/cm^2 not only improves the PDT efficacy, but also results in the unwanted skin damage.     

Particle-in-Cell/Monte Carlo Collision simulation of planar DC magnetron sputtering

In this paper a numerical simulation of a planar DC magnetron discharge is performed with the Particle-in Cell/Monte Carlo Collision （PIC/MCC） method. The magnetic field used in the simulation is calculated with finite element method according to experimental configuration. The simulation is carried out under the condition of gas pressure of 0.665 Pa and voltage magnitude of 400V. Typical results such as the potential distribution, charged particle densities, the discharge current density and ion flux onto the target are calculated. The erosion profile from the simulation is compared with the experimental data. The maximum erosion position corresponds to the place where the magnetic field lines are parallel to the target surface.     

X‐ray spectra by means of Monte Carlo simulations for imaging applications

X‐ray tubes have a broad range of applications worldwide, including several techniques for atomic physics, like X‐ray fluorescence, as well as for medical imaging, like computed tomography. The performances of X‐ray imaging detectors have shown to be significantly sensitive to the incident beam spectrum. Therefore, an accurate knowledge of the X‐ray beam becomes necessary for the emission source characterization and the whole imaging process comprehension. Direct measurements and suitable Monte Carlo simulations may be used to establish the X‐ray spectra. Dedicated Monte Carlo simulation routines, based on the PENELOPE code, have been developed to determine the Bremsstrahlung X‐ray spectra generated by conventional X‐ray tubes. The simulated spectra have been validated by comparison with the corresponding experimental data showing an overall good agreement. The incorporation of a suitably designed virtual grid allowed to assess the angular distribution of Bremsstrahlung yield, showing a remarkable anisotropy. In addition, a dedicated program has been developed for virtual imaging, which enables to perform suitable X‐ray absorption contrast images. Also, the developed program includes a user‐friendly graphic interface to allow the upload of required input parameters, which include setup arrangement, beam characteristics, sample properties and image simulation parameters (spatial resolution, tracks per run, etc.). The software includes dedicated subroutines which handle the physical process from X‐ray generation up to detector signal acquisition. The aim of the developed program is to perform virtual imaging by means of absorption contrast and using conventional X‐ray sources, which may be a useful tool for the study the X‐ray imaging techniques in several research fields as well as for educational purposes. The performed comparisons with experimental data have shown good agreement. The obtained results for X‐ray imaging may constitute useful information for the comprehension and improvement of X‐ray image quality, like absorption contrast optimization, detail visualization, definition and detectability. Copyright © 2010 John Wiley & Sons, Ltd.     

Calibration of a germanium well-detector using 60Co: The effects of the angular correlation of the two gamma rays emitted in cascade,quantified with Monte Carlo simulations

Radiation sources of ⁶⁰Co are commonly measured by means of HPGe gamma spectrometers, either as unknown sources or as calibration sources. However, the two gamma rays that are emitted by this nuclide, at 1.17 MeV and 1.33 MeV respectively, follow each other so rapidly that in the cases where both photons interact with the crystal, the electronics will record a single additive pulse. This is a cascade or coincidence summing effect. Such effect induces in the gamma spectra a “sum-peak”, appearing at 2.5 MeV on the energy axis, generated by the pair of photons which have both been entirely absorbed by the detector. Also, the second photon is correlated for direction to the first one, i.e., it will not be emitted at random, with an isotropic probability. Then the question arises of to what extent the two effects, cascade and correlation, might affect the count rates for the three peaks. Various answers have already been published, but without practical solutions. In this context, the present work was devoted to further explore the question by means of Monte Carlo simulation, in the case of a welltype detector. As a result, inside the well the sum peak only is not affected and it allows accurate evaluation of the detector efficiency at 1.25 MeV. Outside the well, near the detector, none of the three peaks can be directly used for efficiency evaluation, unless the relevant corrections can be evaluated. At a distance from the detector, the two single peaks can be used, but with the drawback of a low efficiency.