Monte Carlo Particle Simulation Research Articles

Environmental dependence of geometrical efficiency for the scintillation cell in radon and thoron measurement.

222Rn is recognized as a matter of international concern for human health risk. Because 220Rn as well as 222Rn coexist in the natural environment, thoron sometimes influences the experiment for radon measurement. It is important to measure radon and thoron separately to evaluate the risk of the exposure to 222Rn. As a discriminative measurement method for 222Rn and 220Rn, a simple technique with a single scintillation cell is well known. However, in recent years, the influence of atmospheric environment on the geometrical efficiency of the scintillation cell has not yet been investigated. In this study, environmental dependence of geometrical efficiency for the scintillation cell in 222Rn and 220Rn measurement was investigated using the Lucas type scintillation cell and Monte Carlo particle simulation. It was found that the influence of temperature and pressure on the geometrical efficiencies were larger than that of relative humidity.

Phase composition of sputter deposited tungsten thin films

Sputter deposited tungsten thin films are studied by X-ray diffraction. Two phases can be identified: α-W and β-W based on the observed (110), and (200) and (210) Bragg reflections, respectively. With increasing film thickness (50 to 200 nm), the phase composition shifts from β-W towards α-W. No influence of the base pressure (3 × 10−3 – 3 × 10−5 Pa) on the phase composition is observed. Also the influence of the argon pressure (0.3 to 0.7 Pa) is rather weak. The strongest shift towards α-W composed thin films is obtained by increasing the discharge power (50 to 250 W). This trend is further studied by energy flux measurements using a calorimetric probe. These measurements rule out a strong change of the substrate temperature, and an impact of the energy flux scaled by the deposition rate (total energy per deposited atom). Test particle Monte Carlo simulations reveal the importance of the momentum of the reflected argon neutrals on the phase composition. The maximum energy of these species is mainly defined by the discharge voltage, and is higher than the directional dependent displacement energy of W. Despite the significant correlation between phase composition and the number of displacement per deposited atom, there is a strong scatter of the phase composition. As the deposition conditions were varied in random way, changes of the target erosion profile, and the changing discharge voltage over each series are probably partially responsible for the observed scatter. This scatter is also enhanced by the long term changes in the phase composition towards the more thermodynamic stable α-W phase.

Assessment of Metal Foil Pump Configurations for EU-DEMO

It is a challenging but key task to reduce the tritium inventory in EU-DEMO to levels that are acceptable for a nuclear regulator. As solution to this issue, a smart fuel cycle architecture is proposed based on the concept of Direct Internal Recycling (DIR), in which the Metal Foil Pump (MFP) will play an important role to separate the unburnt hydrogen isotopes coming from the divertor by exploiting the superpermeation phenomenon. In this study, we will present the assessment of the performance of the lower port of EU-DEMO after the integration of the MFP. For the first time, a thorough comparison of three different MFP (parallel long tubes, sandwich and halo) designs is performed regarding conductance for helium molecules, the pumping speed and the separation factor for deuterium molecules under different physical and geometric parameters. All simulations were carried out in supercomputer Marconi-Fusion with our in-house Test Particle Monte Carlo (TPMC) simulation code ProVac3D because the code had been parallelized with high efficiency. These results are essential for the development of a suitable MFP design in the vacuum-pumping train of EU-DEMO.

Diffusion-Limited Kinetics in Reactive Systems.

A proper representation of chemical kinetics is vital to understanding, modeling, and optimizing many important chemical processes. In liquid and surface phases, where diffusion is slow, the rate at which the reactants diffuse together limits the overall rate of many elementary reactions. Commonly, the textbook Smoluchowski theory is utilized to estimate effective rate coefficients in the liquid phase. On surfaces, modelers commonly resort to much more complex and expensive Kinetic Monte Carlo (KMC) simulations. Here, we extend the Smoluchowski model to allow the diffusing species to undergo chemical reactions and derive analytical formulas for the diffusion-limited rate coefficients for 3D, 2D, and 2D/3D interface cases. With these equations, we are able to demonstrate that when species react faster than they diffuse they can react orders of magnitude faster than predicted by Smoluchowski theory, through what we term "the reactive transport effect". We validate the derived steady-state equations against particle Monte Carlo (PMC) simulations, KMC simulations, and non-steady-state solutions. Furthermore, using PMC and KMC simulations, we propose corrections that agree with all limits and the computed data for the 2D and 2D/3D interface steady-state equations, accounting for unique limitations in the associated derived equations. Additionally, we derive equations to handle couplings between diffusion-limited rate coefficients in reaction networks. We believe these equations should make it possible to run much more accurate mean-field simulations of liquids, surfaces, and liquid-surface interfaces accounting for diffusion limitations and the reactive transport effect.

Inderdigitation, double twist, and topological defects in a system of hard lollipops

ABSTRACT Using hard particle Monte Carlo simulations, we studied a three-dimensional system consisting of identical, lollipop-like particles. Each lollipop was built of five identical, tangent balls placed along a line and one larger ball at one side of the particle and modelled the RM734 molecule, for which ferroelectric and splay nematics were recently discovered in the experiment. Although our model did not recreate these phases, we observed inherently polar type A and C interdigitated smectics. Moreover, an intriguing, isotropic phase consisting of double-twisted clusters joined by planar defects was formed for a moderate packing fraction and ball diameter ratio.

Radiation attenuation and elemental composition of locally available ceramic tiles as potential radiation shielding materials for diagnostic X-ray rooms

Ceramic materials are being explored as alternatives to toxic lead sheets for radiation shielding due to their favorable properties like durability, thermal stability, and aesthetic appeal. However, crafting effective ceramics for radiation shielding entails complex processes, raising production costs. To investigate local viability, this study evaluated Malaysian ceramic tiles for shielding in diagnostic X-ray rooms. Different ceramics in terms of density and thickness were selected from local manufacturers. Energy Dispersive X-ray Fluorescence (EDXRF) and X-ray Fluorescence (XRF) characterized ceramic compositions, while Monte Carlo Particle and Heavy Ion Transport code System (MC PHITS) simulations determined Linear Attenuation Coefficient (LAC), Half-value Layer (HVL), Mass Attenuation Coefficient (MAC), and Mean Free Path (MFP) within the 40–150 kV energy range. Comparative analysis between MC PHITS simulations and real setups was conducted. The C3–S9 ceramic sample, known for homogeneous full-color structure, showcased superior shielding attributes, attributed to its high density and iron content. Notably, energy levels considerably impacted radiation penetration. Overall, C3–S9 demonstrated strong shielding performance, underlining Malaysia's potential ceramic tile resources for X-ray room radiation shielding.

Improved compact representation method for Monte Carlo phase spaces

Phase space files used in particle Monte Carlo simulations can be large, and thereby awkward to distribute and slow to load into memory. To circumvent this, compact source models and phase space representations have been developed. In this study, the aim is to increase the versatility of a previously presented method, based on principal component analysis whitening, by evaluating several modifications to the method on simulated cone-beam computed tomography projection images. Scaling the phase space components before whitening was seen to give the largest improvement and allowed for accurate modelling of phase spaces with a shifted detector and/or a bowtie filter present, configurations where the original method struggled. The modified method reproduced results using the original phase space to within a few percent both in energy fluence on the detector and in simulated projection and scatter images of a patient pelvis, without obvious position-dependence of the errors. Hence, the modified method was deemed sufficiently accurate for most applications relying on cone-beam computed tomography simulations. The final aim is to use the more versatile method for a broader range of phase space-based applications such as linear accelerator beam simulations and room shielding calculations.

Mechanism of capture section affecting an intake for atmosphere-breathing electric propulsion

Atmosphere-Breathing Electric Propulsion (ABEP) can compensate for lost momentum of spacecraft operating in Very Low Earth Orbit (VLEO) which has been widely concerned due to its excellent commercial potential. It is a key technology to improve the capture efficiency of intakes, which collect and compress the atmosphere for ABEP. In this paper, the mechanism of the capture section affecting capture efficiency is investigated by Test Particle Monte Carlo (TPMC) simulations with 3D intake models. The inner surface smoothness and average collision number are determined to be key factors affecting capture efficiency, and a negative effect growth model is accordingly established. When the inner surface smoothness is less than 0.2, the highest capture efficiency and its corresponding average collision number interval are independent of the capture section’s geometry and its mesh size. When the inner surface smoothness is higher than 0.2, the capture efficiency will decrease by installing any capture section. Based on the present results, the manufacturing process and material selection are suggested to be prioritized during the intake geometry design in engineering projects. Then, the highest capture efficiency can be achieved by adjusting the length and mesh size of the capture section.

Dose voxel resolution determination for Monte Carlo electron beam simulation in thin targets.

Monte Carlo particle simulation has become the primary tool for designing low-energy miniature x-ray tubes due to the difficulties of physically prototyping these devices and characterizing their radiation fields. Accurate simulation of electronic interactions within their targets is necessary for modelling both photon production and heat transfer. Voxel-averaging can conceal hot spots in the target heat deposition profile that can threaten the integrity of the tube. This research seeks a computationally-efficient method of estimating voxel-averaging error in energy deposition simulations of electron beams penetrating thin targets to inform the appropriate scoring resolution for a desired accuracy level. An analytical model to estimate voxel-averaging along the target depth was developed and compared to results from Geant4 via its wrapper, TOPAS. A 200 keV planar electron beam was simulated to impinge tungsten targets of thicknesses between 1.5- and 12.5- . For each target, the model was used to calculate the energy deposition ratio between voxels of varying sizes centered on the longitudinal midpoint of the target. Model-calculated ratios were compared to simulation outputs to gauge the model's accuracy. Then, the model was used to approximate the error between the point value of electron energy deposition and a voxel-based measurement. The model underestimates error to within 5% for targets less than 7.5- in thickness with increasing error for greater thicknesses. For the 1.5- target, calculations of the point-vs.-voxel energy deposition show an 11% averaging effect between the midpoint and a 1.5- voxel. Energy deposition profiles along the target depth were also calculated in the Monte Carlo for reference. A simple analytical model was developed with reasonable accuracy to guide Monte Carlo users in estimating the appropriate depth-voxel size for thin-target x-ray tube simulations. This methodology can be adapted for other radiological contexts to increase robustness in point-value estimations. This article is protected by copyright. All rights reserved.

Burnup Characteristics of Transuranic Fuel With Burnable Poison in Dual Cooled Annular Rod of VVER-1000

Abstract The potentiality of Transuranic (TRU) fuel as dual cooled annular fuel rods with different types of burnable absorbers (BAs)- integrated burnable absorbers (IBAs), burnable poison rods (BPRs), coated absorbers, etc. in the hexagonal assembly of VVER-1000 was studied. Annular 7, Annular 8, and Annular 9 models were taken for various combinations of TRU fuel and BAs. Planned models were simulated in Monte Carlo particle simulation code OpenMC and lattice physics deterministic code Dragon Version5. Burnup-dependent multiplication factors (MFs) were simulated for 3000 effective full power days (EFPDs). Reactivity was calculated by taking 3% neutron leakage. The study showed that both the MF and reactivity for TRU fuel are significantly higher than conventional UO2 fuel. Linear reactivity model (LRM) was applied to find out cycle burnup, discharge cycle burnup, and cycle length. High cycle burnup, discharge burnup, and cycle length have been observed for TRU fuel compared to UO2 fuel. Burnup-dependent atomic concentration graphs showed that slight burn of Np-237, constant concentration for Cm-244, a slight increase for Am-242, and linear burnout of BAs- Gd-155, Gd-155, and Er-167. A lower concentration of Xe-135 has been observed for TRU fuel. Pin power distribution and energy-dependent neutron flux for different models and BAs combinations are also included in this study.

Transmission conductance of a cylindrical tube with wall pumping

The concept of transmission conductance is required to describe phenomena occurring in a tube with wall pumping. This is because the gas flow is not constant along the length of the tube but decreases, which implies that the geometric conductance determined by dimensional parameters of the tube cannot be further used. In one of the previous studies, the transmission conductance of a cylindrical tube was analytically derived using the particle balance equation based on the continuity principle. However, the calculated results well agreed with those of the test particle Monte Carlo (TPMC) simulation only when the sticking coefficient was less than 0.1. This is due to the intrinsic limitation of the analytical method, in which the constant conductance of a tube of unit length with uniform wall pumping was used. In this study, we introduced another analytical method based on the cosine law and finite element analysis using the conductance of the tube of unit length as a function of the position in the tube, considering the beaming effect. Using both methods, the relative difference in transmission conductance with TPMC considerably decreased compared to the previous study when the sticking coefficient was greater than 0.1. The results can be used in the design of a non-evaporable getter coated tube or a cryogenic water pump using an adsorbing wall to determine characteristic dimensions. Furthermore, this can be employed in a windowless vacuum coupling or a distributed pumping system for various light source systems.

Susceptibility of multipactor discharges near a dielectric driven by a Gaussian-type transverse rf electric field

Multipactor discharge near an rf window is a key limiting factor in high power microwave systems. In this work, we report special features of dielectric multipactor susceptibility under a Gaussian-type waveform as a function of the rf power density of the transverse rf electric field (P¯rf) and normal restoring field (Edc) via particle-in-cell (PIC) and multiple particle Monte Carlo (MC) simulations. The MC simulations show that, for a Gaussian waveform of a half peak width (Δτ), larger than Δτ/T=0.15 with T = 1 ns the rf repetition period, the susceptibility boundary is similar to that of the conventional sinusoidal waveform-driven multipactor, i.e., two inclined lines in the plane of (P¯rf,Edc). However, by decreasing Δτ, the susceptibility boundary converts to be a closed curve at Δτ/T=0.11 in the plane of (P¯rf,Edc) and further shrinks at Δτ/T=0.05. PIC simulations with a self-consistent surface and space charge effects also show a reduced Edc with increasing P¯rf when P¯rf exceeds a critical value, resulting in a closed curve in the plane of (P¯rf,Edc), and the maximum time-averaged Edc (multipactor strength) also decreases significantly with further decreasing Δτ in agreement with MC simulations. Accordingly, the fraction of the rf power density absorbed by the multipactor discharges also decreases nonlinearly with Δτ from the order of 10−2 to 10−3 (even 10−4), implying a significant improvement compared to the conventional sinusoidal waveform. The simulations also show that the multipactor susceptibility under a transverse Gaussian-type waveform for different frequencies follows the same scaling law in terms of the ratio of the electric field to the rf repetition rate.

Predicting drift characteristics of life rafts: Case study of field experiments in the South China Sea

Every year, hundreds of people die at sea because of vessel and airplane accidents. As marine emergency evacuation equipment, life rafts are highly used in case of maritime accidents. Accurate predictions of drift trajectories and search and rescue (SAR) areas for life rafts at sea can enhance the efficiency and success rate of maritime SAR operations to save human lives. The objective of this study is to determine the drift characteristics of aviation and marine life rafts in the South China Sea, and develop an accurate drift prediction model. A series of maritime drift experiments were conducted in the South China Sea using the standard 6-person life raft (aviation life raft) and the standard 10-person life raft (marine life raft) to obtain 550 drift samples. This data was used to establish three drift models of life rafts: the AP98 leeway drift model, the dynamics drift model and the improved drift model. Finally, Lagrangian particle tracking method and Monte Carlo simulations were used to compare the drift models for 6-person and 10-person life raft. Results indicate that the probability of positive crosswind leeway (CWL) for 6-person and 10-person life raft is 61% and 31.4%, respectively. The jibing frequency is 2% per hour for 6-person and 4% per hour for 10-person life raft. The maximum divergence angle is 11.3° for 6-person and 20.5° for 10-person life raft, indicating that 10-person life raft is more sensitive to the wind speed in the crosswind direction. The improved drift model is more accurate than AP98 leeway drift model and dynamics drift model for predicting drift trajectories of both 6-person and 10-person life rafts. The results of this work may provide effective guidance for drift prediction of aviation life rafts and marine life rafts, which is of great significance for the maritime search and rescue (SAR) operations in the South China Sea.

Spectral Simulation of Heat Transfer Across Polytype Interfaces

Polytype nanowires fabricated in both silicon and germanium are particularly attractive for thermoelectric engineering. In this work, the transport of phonons across polytype heterojunctions such as Si 3C/Si 2H and Ge 3C/Ge 2H is theoretically studied by using a particle Monte Carlo simulation for phonons. Full-Band dispersions and phonon-phonon scattering rates are calculated by using the density-functional theory. Phonon transmission across interfaces is implemented by using a Full-Band version of the Diffusive Mismatch Model. First, the different transport regimes (diffusive, ballistic, and intermediate) for homogenous 3C and 2H Si and Ge bars are investigated by using the Knudsen number as well as the spectral contributions of the thermal flux. Then, single and double polytype Si and Ge heterostructures are studied. The variation of the interface thermal conductance as a function of the geometric dimension as well as the effects of the spectral distribution of the flux are investigated. This local indicator of the phonon transport regime can be used as a local indicator of the occurrence of the out of equilibrium transport regime. Finally, it is shown that the polytype interfaces exhibit significant thermal resistances and generate an out of equilibrium phonon transport regime around the interface over several nanometers.

Pressure profile calculation using finite element analysis that exploits the continuity of gas flow

We present a numerical method of calculating the pressure profile of a large ultra-high vacuum (UHV) system using finite-element analysis (FEA) that exploits the continuity of gas flow. In this study, we introduce a modified FEA (MFEA) that excludes the redundant count of aperture conductance of the element by the Oatley method and uses a correction factor compensating the beaming effect. Along with the correction and by choosing an appropriate length of the element, we improve the result for a cylindrical tube to show a small difference of 5% or less from that of the test particle Monte Carlo (TPMC) simulation. As an example of a practical application, we calculate the pressure profile of an accelerator vacuum system with a split chamber using the MFEA, and the result is validated by comparison with the TPMC. The MFEA method will be useful to design a large UHV system at an early stage that requires design iteration using a simple and fast calculation procedure.

Reliable and Robust Optimization of the Planetary Gear Train Using Particle Swarm Optimization and Monte Carlo Simulation

Reliable and Robust Optimization of the Planetary Gear Train Using Particle Swarm Optimization and Monte Carlo Simulation

Monte Carlo Analysis of the Performance of the ITER Diagnostic Residual Gas Analyzer

The composition of exhausted gas is a key parameter in long-pulse plasma fusion experiments, and its evolution shall be monitored at timescales relevant to plasma dynamics and plasma-wall interactions. A diagnostic residual gas analyzer (DRGA) is a multisensor instrument particularly suited to these studies, and ITER will adopt DRGAs in the equatorial and in the divertor tokamak regions. In this work, we have revisited the design of the ITER divertor DRGA through simple vacuum analytical considerations supported by simulations conducted with Molflow+, a test particle Monte Carlo (TPMC) simulation code commonly used in the particle accelerator community. Starting with recommendations on the manufacturing of the vacuum piping of the DRGA, this work is followed by a complete vacuum characterization of the diagnostic vacuum setup (pressure profiles at base pressure and during sampling, orifice diameter, and length optimization), and finally, the in-vessel residence time of the most important gas species is simulated. These studies have allowed us to give insights into some experimental results recently found on the prototype DRGA installed in the Wendelstein W7-X stellarator.

The emission of energetic electrons from the complex streamer corona adjacent to leader stepping

We here propose to model the production of energetic electrons serving as a source of x-rays and γ-rays, associated to electric discharges in preionized and perturbed air. During its stepping, the leader tip is accompanied by a corona consisting of multitudinous streamers perturbing the air in its vicinity and leaving residual charge behind. We explore the relative importance of air perturbations and preionization on the production of energetic runaway electrons by 2.5D cylindrical Monte Carlo particle simulations of streamers in ambient fields of 16 and 50 kV cm−1 at ground. We explore preionization levels between 1010 and 1013 m−3, channel widths between 0.5 and 1.5 times the original streamer widths and air perturbation levels between 0% and 50% of ambient air. We observe that streamers in preionized and perturbed air accelerate more efficiently than in non-ionized and uniform air with air perturbation dominating the streamer acceleration. We find that in unperturbed air and in fields above breakdown strength preionization levels of 1011 m−3 are sufficient to explain significant runaway electron rates. In perturbed air, the production rate of runaway electrons varies from 1010 to 1017 s−1 with maximum electron energies from some hundreds of eV up to some hundreds of keV in fields above and below the breakdown strength with only a marginal effect of the channel radius. Conclusively, the complexity of the streamer zone ahead of leader tips allows explaining the emission of energetic electrons and photons from streamer discharges in fields below and above the breakdown magnitudes.

Microdosimetric quantities of an accelerator-based neutron source used for boron neutron capture therapy measured using a gas-filled proportional counter

Boron neutron capture therapy (BNCT) is an emerging radiation treatment modality, exhibiting the potential to selectively destroy cancer cells. Currently, BNCT is conducted using a nuclear reactor. However, the future trend is to move toward an accelerator-based system for use in hospital environments. A typical BNCT radiation field has several different types of radiation. The beam quality should be quantified to accurately determine the dose to be delivered to the target. This study utilized a tissue equivalent proportional counter (TEPC) to measure microdosimetric and macrodosimetric quantities of an accelerator-based neutron source. The micro- and macro-dosimetric quantities measured with the TEPC were compared with those obtained via the the particle and heavy ion transport code system (PHITS) Monte Carlo simulation. The absorbed dose from events >20 keV/μm measured free in air for a 1-h irradiation was calculated as 1.31 ± 0.02 Gy. The simulated result was 1.41 ± 0.07 Gy. The measured and calculated values exhibit good agreement. The relative biological effectiveness (RBE) that was evaluated from the measured microdosimetric spectrum was calculated as 3.7 ± 0.02, similar to the simulated value of 3.8 ± 0.1. These results showed the PHITS Monte Carlo simulation can simulate both micro- and macro-dosimetric quantities accurately. The RBE was calculated using a single-response function, and the results were compared with those of several other institutes that used a similar method. However, care must be taken when using such a single-response function for clinical application, as it is only valid for low doses. For clinical dose ranges (i.e., high doses), multievent distribution inside the target needs to be considered.

HOOMD-blue: A Python package for high-performance molecular dynamics and hard particle Monte Carlo simulations

Abstract HOOMD-blue is a particle simulation engine designed for nano- and colloidal-scale molecular dynamics and hard particle Monte Carlo simulations. It has been actively developed since March 2007 and available open source since August 2008. HOOMD-blue is a Python package with a high performance C++/CUDA backend that we built from the ground up for GPU acceleration. The Python interface allows users to combine HOOMD-blue with other packages in the Python ecosystem to create simulation and analysis workflows. We employ software engineering practices to develop, test, maintain, and expand the code.