Monte‐Carlo simulation of spatial distribution of x‐rays in multi‐film targets. II: Spatial distribution of continuous x‐rays and temperature elevation in W/Al film targets

A Monte‐Carlo simulation written in C++ has been developed to describe spatial distributions of energy dissipation and x‐rays generated by penetrating kilovolt electrons in a multi‐film target. To evaluate the x‐ray source, especially its size and intensity, the use of brightness is proposed as a figure of merit. The Monte‐Carlo simulation approach was applied to a W (5 µm)/Al (200 µm) film target, which has been in practical use as an x‐ray source of small size with high emissivity. The result demonstrates that the experiment had considerable success. The optimum operating condition for the W (5 µm)/Al (200 µm) target in practical use has also been proposed by considering temperature elevation, spot size of electron beam, x‐ray source size and x‐ray intensity. Copyright © 2005 John Wiley & Sons, Ltd.     

Monte‐Carlo simulation of spatial distribution of characteristic x‐rays in multi‐film targets: source size of Al Kα x‐rays in W/Al film targets

A Monte‐Carlo simulation approach has been applied to describe the spatial distribution of characteristic x‐rays in W/Al film targets of different combinations of film thicknesses for the optimal design of a small‐sized x‐ray source having a high x‐ray intensity. The result has led to optimal combinations of W and Al film targets for 100 kV electrons, e.g. W(1 µm)/Al(20 µm), W(3 µm)/Al(15 µm) and W(5 µm)/Al(8 µm). These Al/W targets could be used as x‐ray sources for a medical instrument currently under development. Copyright © 2004 John Wiley & Sons, Ltd.     

Monte Carlo simulation algorithm for B‐DNA

Understanding the structure–function relationship of biomolecules containing DNA has motivated experiments aimed at determining molecular structure using methods such as small‐angle X‐ray and neutron scattering (SAXS and SANS). SAXS and SANS are useful for determining macromolecular shape in solution, a process which benefits by using atomistic models that reproduce the scattering data. The variety of algorithms available for creating and modifying model DNA structures lack the ability to rapidly modify all‐atom models to generate structure ensembles. This article describes a Monte Carlo algorithm for simulating DNA, not with the goal of predicting an equilibrium structure, but rather to generate an ensemble of plausible structures which can be filtered using experimental results to identify a sub‐ensemble of conformations that reproduce the solution scattering of DNA macromolecules. The algorithm generates an ensemble of atomic structures through an iterative cycle in which B‐DNA is represented using a wormlike bead–rod model, new configurations are generated by sampling bend and twist moves, then atomic detail is recovered by back mapping from the final coarse‐grained configuration. Using this algorithm on commodity computing hardware, one can rapidly generate an ensemble of atomic level models, each model representing a physically realistic configuration that could be further studied using molecular dynamics. © 2016 Wiley Periodicals, Inc.     

Origin of miscibility‐induced sequential reordering and crystallization‐induced sequential reordering in binary copolyesters: a Monte Carlo simulation

The effect of the repulsive interaction between the components of binary copolyesters on their sequence order was investigated with the Monte Carlo simulation method. The phase separation and ester‐interchange reactions were implemented simultaneously with a kind of one‐site bond fluctuation model. When the repulsive interaction energy was applied to the binary copolyesters, miscibility‐induced sequential reordering (MISR) was induced. The more repulsive the pair interaction was, the higher the sequence order was. During the MISR process, homoester‐interchange reactions became more favorable because of the repulsive interaction, accompanying the decrease of the interactional free energy. The sequence order resulting from MISR was independent of the relative trial ratio of phase separation to ester‐interchange reaction at a given value of interaction energy. Restoration of the sequence distribution was also simulated with and without the repulsive interaction between the components of the binary copolyesters to investigate the effect of MISR on the crystallization‐induced sequential reordering (CISR) process in binary copolyesters, where sequences with lengths longer than 6 were assumed to crystallize and could not take part in ester‐interchange reactions. The sequence distribution in the amorphous phase was restored via ester‐interchange reactions. When the repulsive interaction was applied to binary copolyesters during the CISR process, restoration of the sequence distribution was accelerated, indicating that MISR can accelerate the CISR process when a polyester blend shows upper critical solution temperature behavior. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1337–1347, 2001     

Simultaneous long‐chain branching and random scission: I. Monte Carlo simulation

In free‐radical olefin polymerizations, the polymer transfer reactions could lead to chain scission as well as forming long‐chain branches. For the random scission of branched polymers, it is virtually impossible to apply usual differential population balance equations because the number of possible scission points is dependent on the complex molecular architecture. On the other hand, the present problem can be solved on the basis of the probability theory by considering the history of each primary polymer molecule in a straightforward manner. The random sampling technique is used to solve this problem and a Monte Carlo simulation method is proposed. In this simulation method, one can observe the structure of each polymer molecule formed in this complex reaction system, and virtually any structural information can be obtained. In the illustrative calculations, the full molecular weight distribution development, the gel point determination, and examples of two‐ and three‐dimensional polymer structure are shown. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 391–403, 2001     

Monte‐Carlo simulation of x‐ray photoelectron emission from silver

Silver 3d x‐ray photoelectron spectroscopy (XPS) spectra were simulated with the Monte‐Carlo method using an effective energy‐loss function that was derived from a reflected electron energy‐loss spectroscopy (REELS) analysis based on an extended Landau approach. After confirming that Monte‐Carlo simulation based on the use of the effective energy‐loss function can successfully describe the experimental REELS spectrum and Ag 3d XPS spectrum, we applied Monte‐Carlo simulation to predict the angular distribution of Ag 3d x‐ray photoelectrons for different x‐ray incidence angles and different photoelectron take‐off angles. The experimental photoelectron emission microscope that we are constructing was confirmed as being close to the optimum configuration, in which the x‐ray incident angle as measured from the surface normal direction is 74° and the photoelectron take‐off angle is set normal to the surface. The depth distribution functions of the Ag 3d X‐ray photoelectrons for different energy windows suggest that the photoelectron emission microscope will exhibit greater surface sensitivity for narrower photoelectron energy windows. Copyright © 2003 John Wiley & Sons, Ltd.     

Homogenization process caused by competition between phase separation and ester‐interchange reactions in immiscible polyester blends: A Monte Carlo simulation

The homogenization process caused by competition between phase separation and ester‐interchange reactions in immiscible polyester blends was investigated via the Monte Carlo simulation method. Phase separation and ester‐interchange reactions were performed simultaneously with the one‐site bond fluctuation model on a homogeneous blend of immiscible polyesters. Three different values of the repulsive pair‐interaction energy (E_AB) between segments A and B and two trial ratios of phase separation to ester‐interchange reactions at a given E_AB were introduced to examine the competition between them. Phase separation was monitored by the calculation of the collective structure factor, and copolymerization was traced by the calculation of the degree of randomness (DR). In all cases, as the homogenization proceeded, the maximum intensity of the collective structure factor initially increased, reached a maximum, and finally decreased, whereas the peak position where the structure factor had a maximum shifted downward in the early stage and then remained unchanged after the intensity of the collective structure factor reached the maximum. This indicates that during the homogenization process, the domain size did not change significantly after phase‐separated structures were developed distinctly. In this simulation, phase‐separated structures were traced until the DR was above 0.8. This result indicates that homogenization can be accomplished via homogeneous ester‐interchange reactions over most of the polyester chains because copolyesters resulting from ester‐interchange reactions do not act as an efficient compatibilizer. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 590–598, 2000     

A study of theoretical depth resolution of GaAs/AlAs reference material with low energy inert‐gas ions by Monte Carlo simulation

A Monte Carlo (MC) simulation program written in C++ has been newly developed to describe the dynamic processes of depth profiling with low energy ions. This MC simulation was applied to the depth profiling of GaAs/AlAs reference material for Ne⁺, Ar⁺, and Xe⁺ ions to elucidate the depth resolution attained by surface analytical techniques. The result clearly predicts that there is a considerable difference between the depth resolutions estimated from the leading and trailing edges of Ne⁺ and Xe⁺ ions, whereas the difference is quite small for Ar⁺ ions. Systematic investigation of the dependence of theoretical depth resolution on primary ion energy has revealed that the preferential sputtering primarily caused by the difference in energy transfer to target atoms through elastic collisions between incident ions and target atoms results in the difference between the leading and trailing edges. The inclusion of other factors, e.g. preferential sputtering effect caused by the metallization of Al atoms on the topmost surface, etc. for further improvement of the MC simulation modeling before accommodating quantitative arguments on the depth resolution is strongly recommended. Copyright © 2006 John Wiley & Sons, Ltd.     

Thermo‐responsive gels in the presence of monovalent salt at physiological concentrations: A Monte Carlo simulation study

In this work, slightly charged thermo‐responsive gels in the presence of salt at concentrations close to physiological conditions have been simulated within a coarse‐grained model widely used in the last decade. These simulations allow differentiate charge and salt effects, which are antagonist and coupled in many real systems because the degree of ionization might depend on the electrolyte concentration. An analysis in terms of the different contributions to osmotic pressure is also presented, which highlights the role played by excluded volume effects. In addition, our results also permit us to test some predictions based on the ideal Donnan equilibrium, a common assumption made to justify the swelling behavior of gels and microgels in the presence of salt. More specifically, simulations show that, for the slightly charged gels simulated here, such an assumption overestimates the concentration of salt inside collapsed gels and underestimates the excess of osmotic pressure associated to the additional electrolyte. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1403–1411     

Surface excitations in electron spectroscopy. Part I: dielectric formalism and Monte Carlo algorithm

The theory describing energy losses of charged non‐relativistic projectiles crossing a planar interface is derived on the basis of the Maxwell equations, outlining the physical assumptions of the model in great detail. The employed approach is very general in that various common models for surface excitations (such as the specular reflection model) can be obtained by an appropriate choice of parameter values. The dynamics of charged projectiles near surfaces is examined by calculations of the induced surface charge and the depth‐ and direction‐dependent differential inelastic inverse mean free path (DIIMFP) and stopping power. The effect of several simplifications frequently encountered in the literature is investigated: differences of up to 100% are found in heights, widths, and positions of peaks in the DIIMFP. The presented model is implemented in a Monte Carlo algorithm for the simulation of the electron transport relevant for surface electron spectroscopy. Simulated reflection electron energy loss spectra are in good agreement with experiment on an absolute scale. Copyright © 2012 John Wiley & Sons, Ltd.