Monte Carlo simulation of dose distribution in water around 57Fe3O4 magnetite nanoparticle in the nuclear gamma resonance condition

In this work was proposed a new radiotherapy enhancement method consisting of the administration of magnetic nanoparticles into the cells with further irradiation with a gamma-ray beam. As a result, adjusting the energy distribution of a gamma-ray beam and ⁵⁷Fe abundance it is possible to achieve an extremely intensive electron emission because of a nuclear resonance. The produced conversion and Auger electrons can be used as an effective tool for DNA lesions production. We developed a Monte Carlo model for an electron and gamma emission by ⁵⁷Fe nucleus using the Geant4 program package. The parameters of a resonant absorption were taken from Mössbauer spectra of magnetite nanoparticles synthesized for the administration into live cells. The space distribution of the radiation dose showed an increase in the dose of 2–2.5 times in the case of the natural abundance and more than 50 times in the case of the 66 % enrichment of the nanoparticles.     

Dilution effect on the compensation temperature in a honeycomb nano-lattice: Monte Carlo study

In this work, we use Monte Carlo simulations with the Metropolis algorithm to study the dilution effect on the magnetic properties in a honeycomb nano-lattice. The geometry of the system is formed by two sub-lattices of 45+45 = 90 atoms consisting of the spins σ = 3/2 and S = 5/2. The ground state phase diagrams at zero absolute temperature are presented. Also, the variation of the total magnetization with reduced temperature for several values of the reduced exchange coupling are discussed. It is found that the compensation temperature depends on the reduced exchange coupling parameters. Furthermore, the dilution of the S spins affects the compensation temperature and hysteresis loops of the studied system.     

Thermodynamic properties of water in confined environments: a Monte Carlo study

Monte Carlo simulations of Mercedes-Benz water in a crowded environment were performed. The simulated systems are representative of both composite, porous or sintered materials and living cells with typical matrix packings. We studied the influence of overall temperature as well as the density and size of matrix particles on water density, particle distributions, hydrogen bond formation and thermodynamic quantities. Interestingly, temperature and space occupancy of matrix exhibit a similar effect on water properties following the competition between the kinetic and the potential energy of the system, whereby temperature increases the kinetic and matrix packing decreases the potential contribution. A novel thermodynamic decomposition approach was applied to gain insight into individual contributions of different types of inter-particle interactions. This decomposition proved to be useful and in good agreement with the total thermodynamic quantities especially at higher temperatures and matrix packings, where higher-order potential-energy mixing terms lose their importance.     

The effect of inactive impurities on a surface in NO-CO reaction: A Monte Carlo simulation

The interaction among the reacting species in the NO-CO reaction on a metal catalytic surface that proceeds according to the Langmuir-Hinshelwood thermal mechanism is studied by means of Monte Carlo simulations. The study of this system is essential for the understanding of the influence of impurities on the catalytic oxidation of NO by CO. It is found that this complex system exhibits irreversible phase transitions between active states with sustained reaction and poisoned states without reaction. The same system has also been investigated by non-thermal (Eley-Rideal) mechanism. Both the phase diagrams of the surface coverage and the steady state production of CO₂ and N₂ are evaluated as a function of the partial pressures of the reactants in the gas phase. From this study, it is observed that with the increase of impurities, the production rate reduces and the reaction stops at a certain point. Moreover, the first order transition in the phase diagram converts into second order phase transition that is in accordance with the experimental findings. Therefore, the first order phase transition, which is a characteristic of such catalytic reactions, is eliminated.     

Size effect on magnetic properties of a nano-graphene bilayer structure: A Monte Carlo study

In this paper we use the Monte Carlo simulations to investigate the magnetic properties of an Ising ferromagnetic–antiferromagnetic model. The system is based on a nano-graphene structure-like bilayer with two bloc sizes: N=24 and 42 spins. For each size N, the upper layer A is formed with spin −3/2, whereas the lower layer B is composed of spin −5/2. We only consider the first nearest-neighbor interactions between the sites i and j. The magnetic properties are studied, in the absence as well as in the presence of a crystal magnetic field, and an external magnetic field. The increasing temperature and crystal field as well as the inter-layer coupling constant, are also studied for this system sizes N=24 and 42 spins. The zero-field-cooled and the field cooled magnetization behaviors are investigated for different values of external magnetic field and a fixed value of exchange interaction between the two blocs. The magnetizations as well as the magnetic susceptibilities versus the temperature are used in order to obtain blocking temperature.     

Monte Carlo modelling of human body for dose conversion coefficients of 137Cs in soil of the Eastern Black Sea region of Turkey

Absorbed dose conversion coefficients for a human body standing on the soil surface were calculated for ¹³⁷Cs in the soil of Eastern Black Sea region of Turkey using the Monte Carlo simulation method. The results were given in nGy h^?1 per Bq cm^?2 as a function of depth. Simple numerical integration of this function multiplied with the known specific activity as a function of depth gives the absorbed dose conversion coefficients for any distribution of ¹³⁷Cs in soil. Gaussian distribution of ¹³⁷Cs in the soil was considered for calculations.     

Monte Carlo modelling of human body for dose conversion coefficients of 137Cs in soil of the Eastern Black Sea region of Turkey

Absorbed dose conversion coefficients for a human body standing on the soil surface were calculated for (137)Cs in the soil of Eastern Black Sea region of Turkey using the Monte Carlo simulation method. The results were given in nGy?h(-1) per Bq?cm(-2) as a function of depth. Simple numerical integration of this function multiplied with the known specific activity as a function of depth gives the absorbed dose conversion coefficients for any distribution of (137)Cs in soil. Gaussian distribution of (137)Cs in the soil was considered for calculations.     

The effect of lattice constant on the storage capacity of hydrogen hydrates: a Monte Carlo study

ABSTRACT

We use grand canonical Monte Carlo simulations to examine the effect of the size of hydrate cavities (as reflected through the lattice constant of the hydrate unit cell) on the efficiency of clathrate hydrates in storing hydrogen gas. With this approach, the hydrate lattice is treated as a solid substrate where gas absorption takes place. Of practical interests are cases, where the lattice-size parameters are changed in such a way that they can promote/enhance multiple cavity occupancy, namely the presence of more than one guest-gas molecule in the same hydrate cavity. This phenomenon is commonly observed in the case of hydrogen hydrates and could increase their storage capacity. A parametric analysis is also carried out to quantify the correlation between the change in the lattice constant and the storage capacity since small changes in the size of the crystal unit cell may induce significant changes in the storage capacity especially in cases where multiple cavity occupancy occurs.     

The influence of surface reconstruction on the kinetics of adsorption-desorption processes: A Monte Carlo study of a simple model

The influence of surface reconstruction on the kinetics of adsorption-desorption processes is studied through a simple two-position model by means of Monte Carlo simulation. Effects due to constraints on the translational motion of activated complexes and to heterogeneity are particularly investigated. Heterogeneity emerges as the most important factor to explain the huge variation of the preexponential Arrhenius parameter with coverage observed in the H/W(001) system. In the present model it is conjectured that heterogeneity originates from additional interactions of H with surface or sub-surface W atoms when hydrogen is adsorbed on sites where surface W atoms are farther apart due to reconstruction.     

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Polycrystalline silicon thin film formation from inductively coupled plasma enhanced chemical vapor deposition system was studied. The dilution effect of H₂ on film deposition was discussed. The X-ray diffractometry, Raman spectra and scanning electron microscope measurement were carried out to analyze the influence of H₂ on the microstructures and the topography of polycrystalline silicon thin films. The optimum conditions for polycrystalline silicon thin films deposition were also discussed. The results indicated that polycrystalline silicon thin films with columnar structure crystals were fabricated on glass substrate. The deposition rate exhibited monotonic increase with Silane ratio R, a maximum deposition rate of 0.65nm⋅s^-1 was obtained. However, the crystal volume fraction of polycrystalline silicon thin films initially increased from 60.5% to 67.3%, and then slightly decreased with the increase of R. Therefore, the crystal has a maximum value of 67.3% at R=4.8%. The polycrystalline silicon thin films had a compact and well-arranged structure at this ratio. This structure can also increase carrier mobility and improve the efficiency of solar cells.     

氢气稀释比例对多晶硅薄膜微观结构和沉积特性的影响

利用电感耦合等离子体增强化学气相沉积法(ICP-PECVD)直接在普通玻璃衬底上低温沉积多晶硅薄 膜,主要研究了不同氢气稀释比例对薄膜沉积特性和微观结构的影响。采用 X 射线衍射仪(XRD)、拉曼光谱仪和 扫描电子显微镜(SEM)表征了在不同氢气比例条件下所制备多晶硅薄膜的微结构、形貌,并对不同条件下样品的 沉积速率进行了分析。实验结果表明：随着混合气体中硅烷比例的增加,薄膜的沉积速率不断增加;晶化率先增 加,后减小;当硅烷含量为4.8%时,晶化率达到最大值67.3%。XRD 和 SEM 结果显示多晶硅薄膜在普通玻璃衬 底上呈柱状生长,且晶粒排列整齐、致密,这种结构可提高载流子的纵向迁移率,有利于制备高效多晶硅薄膜太阳能电池。     

Island formation during Al deposition on 5-fold Al-Cu-Fe quasicrystalline surfaces: Kinetic Monte Carlo simulation of a disordered-bond-network lattice-gas model

Scanning tunneling microscopy experiments have previously revealed the formation of pseudomorphic starfish-shaped islands during the initial stages of Al deposition on 5-fold icosohedral Al-Cu-Fe quasicrystal surfaces. To simulate this process, we first identify appropriate 5-fold surface terminations of Al-Cu-Fe from a model for bulk structure, and construct associated potential energy surfaces for the binding of Al adatoms on these terminations. We then identify a ‘disordered-bond-network’ (DBN) connecting neighboring local adsorption sites for Al on Al-Cu-Fe, and determine site binding energies as well as activation barriers for Al adatom hopping between neighboring sites. Al-Al adsorbate interactions, which stabilize islands, are also prescribed. Then, within the framework of a DBN lattice-gas model, we simulate the deposition and diffusion of Al on Al-Cu-Fe. We explore the competition between starfish and incomplete starfish ensembles of sites (which provide traps in the form of deep potential energy wells for diffusing Al) and isolated trap sites, with regard to the heterogeneous nucleation and aggregation of Al into islands.     

Effect of a magnetoelastic interaction on the rate of transient processes in iron borate single crystals

A semiempirical relation between the rate of transient processes in iron borate single crystals and the amplitude of the magnetic field and intensity of the magnetoelastic oscillations is proposed. Experimental data confirming the applicability of the expression to real pulsed magnetization-reversal curves are presented. Fiz. Tverd. Tela (St. Petersburg) 41, 654–655 (April 1999)     

Monte Carlo simulation in the theory of wave propagation in random media: I. Modified and Feynman path integrals

The purpose of this article (comprising parts I and II) is to develop and test the approach of combining a path-integral technique and a complex-valued Monte Carlo method to calculate the highest moments of the Green function of the stochastic wave equation for a random medium against the background of large-scale inhomogeneities. In part I, the new modified path-integral representations of the Green function moments of the stochastic wave equation have been developed. The limiting transition of these representations to the Feynman path integrals corresponding to the parabolic approximation is discussed. Path-integral representations for Green function moments are given for three models: a model of the stochastic wave equation and models of parabolic and Markov approximations. The Metropolis algorithm underlying the Monte Carlo method for calculating real and complex-valued path integrals is discussed in brief. Numerical results are presented in part II of the article.     

Surface tracer diffusion in the presence of a solid barrier: a Monte Carlo study

We investigate, by way of Monte Carlo simulations, the diffusion of adsorbates on a square lattice. The lattice contains inaccessible sites forming linear barriers. The adsorbate atoms do not interact among themselves, except for the hard core. The tracer diffusion coefficients, reduced by their values in the absence of the barrier, are plotted versus the length of the barrier. We find that the diffusion along the barrier is not affected by its presence, whereas that perpendicular to it decreases linearly, for a large interval of the barrier, with its length. The reduced diffusion coefficient does not depend on the concentration of the diffusing particles, but its value increases with the perpendicular distance between the barriers.     

Monte Carlo meets the fishnet: String formation in a gas of Feynman diagrams

It is likely that the quark confinement mechanism at large N should be understood purely in terms of high-order planar Feynman diagrams; in particular, the center of the gauge group can play no role whatever. With this motivation I consider the diagrammatic expansion of loop integrals 〈PTr exp(g∫ø)〉 in planar wrong-sign ø⁴ theory. It is shown that the sum of all fishnet diagrams contributing to the loop can be expressed as the grand partition function of an unusual gas, whose dynamics can be simulated on a computer. The “molecules” of this gas correspond to vertices of the position-space diagrams, the molecular interactions are determined by the propagators, and the coupling constant plays the role of a chemical potential. The most remarkable feature of this gas is the existence of a critical coupling g_c, where string formation takes place. As g → g_c the fishnet vertices tend to cluster around the minimal surface of the loop, thereby forming a string. This is clearly illustrated with the help of computer graphics. The role of asymptotic freedom in bringing the coupling to the critical point, and the connection to the Polyakov string, are also discussed. In the hamiltonian formulation a new and very straightforward explanation of quark confinement is presented.     

Effects of quenched disorder in the two-dimensional Potts model: a Monte Carlo study

Motivated by recent experiments on phase behavior of systems confined in porous media, we have studied the effect of randomness on the nature of the phase transition in the two-dimensional Potts model. To model the effects of the porous matrix we introduce a random distribution of couplings P(J(ij))=pdelta(J(ij)-J1)+(1-p)delta(J(ij)-J2) in the q state Potts Hamiltonian. An extensive Monte Carlo study is made on this system for q=5. We studied two different cases of disorder (a) J(1)/J(2)-->infinity and p=0.8 and (b) J(1)/J(2)=10 and p=0.5. We observed, in both cases, that the weak first order transition that appears in the pure case, changes to a second-order transition. A finite size scaling analysis shows that the correlation length exponent nu is close to 1 and the best fit to the dependence of the specific heat on system size is logarithmic. This suggests that both cases belong to the universality class of the Ising model. In contrast, the magnetic exponents point to a different universality class.     

Effect of a magnetic field on the track structure of low-energy electrons: a Monte Carlo study

The increasing use of MRI-guided radiation therapy evokes the necessity to investigate the potential impact of a magnetic field on the biological effectiveness of therapeutic radiation beams. While it is known that a magnetic field, applied during irradiation, can improve the macroscopic absorbed dose distribution of electrons in the tumor region, effects on the microscopic distribution of energy depositions and ionizations have not yet been investigated. An effect on the number of ionizations in a DNA segment, which is related to initial DNA damage in form of complex strand breaks, could be beneficial in radiation therapy. In this work we studied the effects of a magnetic field on the pattern of ionizations at nanometric level by means of Monte Carlo simulations using the Geant4-DNA toolkit. The track structure of low-energy electrons in the presence of a uniform static magnetic field of strength up to 14 T was calculated for a simplified DNA segment model in form of a water cylinder. In the case that no magnetic field is applied, nanodosimetric results obtained with Geant4-DNA were compared with those from the PTB track structure code. The obtained results suggest that any potential enhancement of complexity of DNA strand breaks induced by irradiation in a magnetic field is not related to modifications of the low-energy secondary electrons track structure.     

Monte Carlo simulation and parameterized treatment on the effect of nuclear elastic scattering in high-energy proton radiography

A version of Geant4 has been developed to treat high-energy proton radiography. This article presents the results of calculations simulating the effects of nuclear elastic scattering for various test step wedges. Comparisons with experimental data are also presented. The traditional expressions of the transmission should be correct if the angle distribution of the scattering is Gaussian multiple Coulomb scattering. The mean free path(which depends on the collimator angle) and the radiation length are treated as empirical parameters, according to transmission as a function of thickness obtained by simulations. The results can be used in density reconstruction, which depends on the transmission expressions.