Positron backscattering from solid targets: Modeling of scattering processes via various approaches

Monte Carlo simulation of 1–4 keV positron backscattering from semi-infinite solid targets ranging from Be (z = 4) to Au (z = 79) with normal angle of incidence is here reported. In our study, the elastic and inelastic scattering cross sections are modeled by using various approaches based on either a classical or a quantum mechanical treatment. Calculations of positron backscattering coefficient are then reported for the solid targets of interest. The results obtained show a fairly good agreement with the data available in the literature. The dependence of the positron backscattering coefficient versus the atomic number of the solid target of interest has been investigated. In this respect, polynomial functions are proposed which does not require any recourse to Monte Carlo calculations.     

Kinetic Monte Carlo simulation of the growth of metal clusters on regular array of defects on insulator

A Kinetic Monte Carlo simulation of the nucleation and growth of Pd clusters on a nanostructured alumina substrate is presented. The new Monte Carlo simulation program allows to derive the 3D shape of the growing clusters without performing a full all atoms simulation. The simulation shows, like in previous pure 2D simulations, that clusters nucleate exclusively on the defects of the nanostructure in a limited range of substrate temperature. Around 300 K, the clusters have a compact faceted shape and they grow, at not too large coverage, layer by layer. These results are in agreement with previous studies of the nucleation and growth of Pd clusters on an ultrathin alumina film on Ni₃Al (1 1 1).     

One-dimensional confinement in heterogeneous catalysis: Trapped oxygen on RuO2(110) model catalysts

With temperature programmed reaction (TPR) experiments and kinetic Monte Carlo (kMC) simulations of coadsorbed oxygen and HCl on the RuO₂(110) surface we studied the thermal stabilization of dissociatively adsorbed oxygen. Due to one-dimensional confinement single surface O atoms can be trapped by surface chlorine atoms so that surface oxygen is not able to desorb from the RuO₂(110) surface at the expected temperature of 420 K. Trapped oxygen needs desorption temperatures as high as 700 K where it recombines with bridging O from RuO₂(110) to form O₂. Kinetic modeling of catalytic reactions with dimensional confinement of their reaction intermediates on the catalyst's surface requires the application of kinetic Monte Carlo simulations which are beyond the mean field approach.     

The effect of phonon anisotropic scattering on the thermal conductivity of silicon thin films at 300K and 400K

Previous studies have shown that anisotropy in phonon transport exist because of the difference in phonon dispersion relation due to different lattice directions, as observed by a difference in in-plane and cross-plane thermal conductivities. Our current work intends to study the effect of anisotropy scattering on silicon thermal conductivity at 300 K and 400 K. We adopt the Henyey and Greenstein probability density function in our phonon Monte Carlo simulation to investigate the effect of highly forward and backward scattering events. The impact of applying the anisotropy scattering using this approach is discussed in detail. While the forward and backward scattering will increase and decrease thermal conductivity respectively, the extent of the effect is non-linear such that forward scattering has a more obvious effect on thermal conductivity than backward scattering.     

Monte Carlo study of electron velocity variance in GaAlAs/GaAs/GaAlAs quantum well

The ensemble Monte Carlo method is used to study electron transport in a GaAlAs/GaAs/GaAlAs 1000 Å long quantum well. The rejection method is applied to calculate electron scattering probabilities between 2D and 3D states. The concentration of 2D electrons, the valleys' occupancies and the electron velocity variance along the simulated structure are calculated.     

Examination of the concept of degree of rate control by first-principles kinetic Monte Carlo simulations

The conceptual idea of degree of rate control (DRC) approaches is to identify the “rate limiting step” in a complex reaction network by evaluating how the overall rate of product formation changes when a small change is made in one of the kinetic parameters. We examine two definitions of this concept by applying it to first-principles kinetic Monte Carlo simulations of the CO oxidation at RuO₂(1 1 0). Instead of studying experimental data we examine simulations, because in them we know the surface structure, reaction mechanism, the rate constants, the coverage of the surface and the turn-over frequency at steady-state. We can test whether the insights provided by the DRC are in agreement with the results of the simulations thus avoiding the uncertainties inherent in a comparison with experiment. We find that the information provided by using the DRC is non-trivial: It could not have been obtained from the knowledge of the reaction mechanism and of the magnitude of the rate constants alone. For the simulations the DRC provides furthermore guidance as to which aspects of the reaction mechanism should be treated accurately and which can be studied by less accurate and more efficient methods. We therefore conclude that a sensitivity analysis based on the DRC is a useful tool for understanding the propagation of errors from the electronic structure calculations to the statistical simulations in first-principles kinetic Monte Carlo simulations.     

Structural characterization of Pb nanoislands in SiO2/Si interface synthesized by ion implantation through MEIS analysis

Medium energy ion scattering (MEIS) measurements and transmission electron microscopy (TEM) observations are applied to characterize a buried Pb nanoparticle (NP) system synthesized by ion implantation. The NPs are located at the SiO₂/Si film interface, forming a dense two-dimensional array. Full 2D (energy and angle) experimental MEIS spectra are compared with Monte Carlo simulated ones. The results demonstrate that MEIS measurements provide microstructural information (mean NP volume of about 150 nm³ and areal density of about 4 × 10¹¹ NP/cm²), but no accurate information on the NP geometrical shape.     

Analysis of the spin polarization of secondary electrons emitted from Au/Fe

The spin polarization of secondary electrons emitted from Au thin films on an Fe substrate is studied with a Monte Carlo simulation of electron scattering. The magnetic information depth of the secondary electrons is estimated by fitting the exponential function to the calculated data and we found that the magnetic information depth increases with the primary electron energy from 11.5 at 0.3 to 26.3 Å at 4 keV. This result agrees fairly with the experimental one for the same nonmagnetic/magnetic bilayer system and it is much larger than that reported so far for bulk magnetic materials.     

The miscibility of CuxAg1 − xI using a Tersoff potential

Simulation methods have been used to study the miscibility ofCu_xAg_1 − xI based on a Tersoff potential. Monte Carlo calculations show that Cu_xAg_1 − xI is a complete solid solution. This result agrees well with experiments using NMR and X-ray diffractions methods. Structural, elastic and thermodynamic properties are also predicted at 0.25, 0.5 and 0.75 using molecular dynamics simulations.     

Simulation study using GEANT4 Monte Carlo code for a Gd-coated resistive plate chamber as a thermal neutron detector

The Resistive Plate Chamber (RPC) has been developed in many application areas ever since its introduction, from high energy physics experiments to positron emission tomography. Such detectors can be coated with a Gd layer that enables them to detect thermal neutrons. Consequently these RPCs can be utilized for industrial and medical purposes. Here, we present the configuration of a resistive plate chamber which is utilized to detect thermal neutrons by employing GEANT4 Monte Carlo code. The response of the RPC was evaluated as a function of neutron energy in the GEANT4 Monte Carlo code. The simulation results are taken for incident neutron energy in the energy range from 25 meV to 100 meV. The detection efficiency was found to be between 10% and 20%, depending on the detector configuration, for incident thermal neutrons of 25 meV energy.     

Lateral interactions and zigzag chains of Ho on the Mo(110) surface

The structure of Ho adsorbed layers on the Mo(110) surface has been studied by low energy electron diffraction (LEED) and scanning tunneling microscope (STM). It has been found that at submonolayer coverages Ho atoms, similarly to studied earlier Gd and Nd, form a rich amount of dilute (n × 2) commensurate structures built from zigzag chains oriented along the [11?0] direction. The formation of zigzag chain structures is initiated by the indirect lateral interaction between adsorbed Ho atoms, which, as is illustrated by Monte Carlo simulations, can be well approximated by a screened Coulomb potential superimposed with Friedel oscillations.     

Surface and frustration evidence in Co–Ni–B nanoparticles by FMR measurements

We present ferromagnetic resonance (FMR) measurements on ∼3 nm amorphous magnetic nanoparticles of Co–Ni–B as a function of temperature (T). The particles were studied in powder form and dispersed in a polymer matrix to study the interparticle interaction effect. In both samples the FMR responses are similar down to T∼10 K, where the powder sample shows an intensity increase not followed by the dispersed sample. We argue that the general characteristics are compatible with previous magnetization measurements and Monte Carlo simulations indicating large surface contributions to the effective anisotropy. In this case the frustration of the single-particle behavior observed in the powder sample at very low T (T ⩽ 10 K) is due to interparticle interactions.     

A linear stability analysis for nonlinear,grey, thermal radiative transfer problems

We present a new linear stability analysis of three time discretizations and Monte Carlo interpretations of the nonlinear, grey thermal radiative transfer (TRT) equations: the widely used “Implicit Monte Carlo” (IMC) equations, the Carter Forest (CF) equations, and the Ahrens–Larsen or “Semi-Analog Monte Carlo” (SMC) equations. Using a spatial Fourier analysis of the 1-D Implicit Monte Carlo (IMC) equations that are linearized about an equilibrium solution, we show that the IMC equations are unconditionally stable (undamped perturbations do not exist) if α, the IMC time-discretization parameter, satisfies 0.5 < α ? 1. This is consistent with conventional wisdom. However, we also show that for sufficiently large time steps, unphysical damped oscillations can exist that correspond to the lowest-frequency Fourier modes. After numerically confirming this result, we develop a method to assess the stability of any time discretization of the 0-D, nonlinear, grey, thermal radiative transfer problem. Subsequent analyses of the CF and SMC methods then demonstrate that the CF method is unconditionally stable and monotonic, but the SMC method is conditionally stable and permits unphysical oscillatory solutions that can prevent it from reaching equilibrium. This stability theory provides new conditions on the time step to guarantee monotonicity of the IMC solution, although they are likely too conservative to be used in practice. Theoretical predictions are tested and confirmed with numerical experiments.     

TPD study of NO decomposition on Rh(111) by dynamic Monte Carlo simulation

The kinetics of NO desorption and its decomposition on Rh(111) surfaces have been simulated by using a dynamic Monte Carlo method. During the simulations, we used a triangular lattice that mimics the Rh(111) phase. NO decomposition was studied at low pressure and temperatures ranging from 120 to 1000 K. The present analysis incorporates recent experimental evidence showing that N₂ production occurs either from the classical N + N recombination step or by the formation and successive decay of an (N–NO)* intermediate species. Moreover, N₂ and NO desorption rates are enhanced and the NO dissociation rate is inhibited by coadsorbed NO, N, and O species as nearest neighbors. These effects are taken into account in this study, along with the experimental adsorption, desorption, dissociation, and diffusion rates of the reactants. Our simulations are consistent with the experimental results of TPD spectra and can explain the formation of two peaks, δ-N₂ and β-N₂, as a natural consequence of the reaction mechanism herein proposed. Comparisons with different mechanisms used by other authors are also made.     

Room-temperature deposition of group III metals on Si(1 0 0): A comparative study of nucleation behavior

In this paper, we compare and contrast the processes of nucleation and subsequent growth of single-atom wide metal chains formed when group III metals (Al, Ga, In) are deposited onto Si(1 0 0) at room-temperature (RT). Employing Density Functional Theory (DFT) calculations, diffusion pathways on Si(1 0 0) surface are identified and their associated activation barriers are calculated. Then, the relative stabilities of various C-defect-pinned chains are examined by comparing the relevant adsorption energies. We also account for the observation that defect-nucleated chains grow on only one side of a C-defect by showing that the latter’s presence breaks the symmetry between the two previously equivalent binding sites on either side and rendering one much more stable than the other. Next, a growth model tailored for each group III metal/Si(1 0 0) system and incorporating the above results was simulated using Kinetic Monte Carlo (KMC) techniques to show that the surface morphologies generated by this model accurately reflect the observed ratio of homogeneously to heterogeneously nucleated chains. Finally, we examine through KMC simulations the consequences of the contrasting roles of a defect on In/Si(1 0 0) and Al/Si(1 0 0) – it captures adatoms in the former while it merely blocks direct adatom diffusion in the latter – on key quantities such as the mean island density.     

Enhanced photoacoustic imaging in tissue-mimicking phantoms using polydopamine-shelled perfluorocarbon emulsion droplets

The current work features process parameters for the ultrasound (25 kHz)-assisted fabrication of polydopamine-shelled perfluorocarbon (PDA/PFC) emulsion droplets with bimodal (modes at 100–600 nm and 1–6 µm) and unimodal (200–600 nm) size distributions. Initial screening of these materials revealed that only PDA/PFC emulsion droplets with bimodal distributions showed photoacoustic signal enhancement due to large size of their optically absorbing PDA shells. Performance of this particular type of emulsion droplets as photoacoustic agents were evaluated in Intralipid®–India ink media, mimicking the optical scattering and absorbance of various tissue types. From these measurements, it was observed that PDA/PFC droplets with bimodal size distributions can enhance the photoacoustic signal of blood-mimicking phantom by up to five folds in various tissue-mimicking phantoms with absorption coefficients from 0.1 to 1.0 cm⁻¹. Furthermore, using the information from enhanced photoacoustic images at 750 nm, the ultimate imaging depth was explored for polydopamine-shelled, perfluorohexane (PDA/PFH) emulsion droplets by photon trajectory simulations in 3D using a Monte Carlo approach. Based on these simulations, maximal tissue imaging depths for PDA/PFH emulsion droplets range from 10 to 40 mm, depending on the tissue type. These results demonstrate for the first time that ultrasonically fabricated PDA/PFC emulsion droplets have great potential as photoacoustic imaging agents that can be complemented with other reported characteristics of PDA/PFC emulsion droplets for extended applications in theranostics and other imaging modalities.     

X-ray and gamma-ray absorbed dose profiles in teeth: An EPR and modeling study

Dose profiles in teeth have been experimentally and theoretically studied for different energies and geometries of incident X- and gamma-rays. The experiments were conducted with teeth inside of an Alderson phantom using monodirectional radiation beams at selected energies; they revealed two effects: an apparent lack of dose attenuation between the buccal and the lingual sides of the teeth for energies higher than 120 keV and an attenuation between first and last tooth layers for low-energy beams in the range from 0.28 to 0.57. Monte Carlo simulations confirmed the experimental data and provided dose profiles for other energies and geometries. In particular, exposure in the rotational radiation field produces pronounced dose profiles only for energies lower than 60 keV. The usefulness of these data to estimate the average energy of accidental radiation field is discussed.     

First track-structure measurements of 20 MeV protons with the STARTRACK apparatus

The STARTRACK experimental set-up, mounted on the +50° beam line of the Tandem-Alpi particle accelerator of Legnaro National Laboratories, has been conceived to give an experimental basis to nanodosimetric calculations. STARTRACK is based on a detection system able to measure ionization cluster distributions in a 20 nm propane site with a resolution of one ionization. The experimental layout has been designed to minimize pile-up distortions. The background noise is filtered by off-line dedicated software. Electron cluster distribution of 20 MeV protons has been measured. Monte Carlo data and experimental data are pretty well consistent.     

Application of low energy ion blocking for adsorption site determination of Na Atoms on a Cu(111) surface

Ion blocking in the low keV energy range is demonstrated to be a sensitive method for probing surface adsorption sites by means of the technique of time-of-flight scattering and recoiling spectroscopy (TOF-SARS). Adsorbed atoms can block the nearly isotropic backscattering of primary ions from surface atoms in the outmost layers of a crystal. The relative adsorption site position can be derived unambiguously by simple geometrical constructs between the adsorbed atom site and the surface atom sites. Classical ion trajectory simulations using the scattering and recoiling imaging code (SARIC) and molecular dynamics (MD) simulations provide the detailed ion trajectories. Herein we present a quantitative analysis of the blocking effects produced by sub-monolayer Na adsorbed on a Cu(111) surface at room temperature. The results show that the Na adsorption site preferences are different at different Na coverages. At a coverage θ = 0.25 monolayer, Na atoms preferentially populate the fcc threefold surface sites with a height of 2.7 ± 0.1 Å above the 1st layer Cu atoms. At a lower coverage of θ = 0.10 monolayer, there is no adsorption site preference for the Na atoms on the Cu(111) surface.     

A fully discrete Galerkin method for high frequency exterior acoustic scattering in three dimensions

Standard Galerkin discretization techniques (with locally- or globally-supported basis functions) for boundary integral equations are inefficient for high frequency three dimensional exterior scattering simulations because they require a fixed number of unknowns per wavelength in each dimension, leading to large CPU time and memory requirements to set up the dense Galerkin matrix, with each entry requiring evaluation of multi-dimensional highly oscillatory integrals. In this work, using globally-supported basis functions, we describe an efficient fully discrete Galerkin surface integral equation algorithm for simulating high frequency acoustic scattering by three dimensional convex obstacles that includes a powerful integration scheme for evaluation of four dimensional Galerkin integrals with high-order accuracy. Such high-order order accuracy for various practically relevant frequencies (k ∈ [1, 100,000]) substantially improves on approximations based on standard asymptotic techniques. We demonstrate the efficiency of our algorithm for spherical and non-spherical convex scattering for several wavenumbers 1 ? k ? 100,000 for low to high order prescribed tolerance. Our fully discrete algorithm requires only mild growth in the number of unknowns and CPU time as the frequency increases.