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Particle-in-cell/Monte Carlo collisional simulation of negative streamer formation and branching between planar electrodes
摘要： In this paper, the mechanism of negative streamer formation and branching between planar electrodes is studied using a two dimensional particle-in-cell/Monte Carlo collisional model. Super-particles, each one of which represents many physical particles, are used in our model to reduce the calculation and memory usage. The electric field applied to the gap between electrodes is constant and is above the breakdown value already before the streamer formation. For nitrogen at standard temperature and pressure, the simulation is first performed in the background field of 10 MV/m. The results show that prior to branching, the streamer head is flattened, and the non-uniform distribution of electrons accelerated to energies above 50 eV is obtained at the flat streamer head, which then leads to streamer branching. Then, additional simulations in the background fields of 9 MV/m, 11 MV/m, and 12 MV/m are performed to investigate the effects of background field. The results show that the streamer propagates faster, has larger plasma density, and branches more rapidly in a stronger field. The simulation results of argon are also given at a standard temperature and pressure in the background field of 10 MV/m. The results show that more branches can be obtained in argon compared with nitrogen, which agrees well with the experimental results.
关键词： particle-in-cell,nitrogen,argon,Monte Carlo collisional model,negative streamer,branching
更新于2025-09-10 09:29:36
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Strongly interacting Weyl semimetals: Stability of the semimetallic phase and emergence of almost free fermions
摘要： Using a combination of analytical arguments and state-of-the-art diagrammatic Monte Carlo simulations, we show that the corrections to the dispersion in interacting Weyl semimetals are determined by the ultraviolet cutoff and the inverse screening length. If both of these are finite, then the diagrammatic series is convergent even in the low-temperature limit, which implies that the semimetallic phase remains stable. Meanwhile, the absence of a UV cutoff or screening results in logarithmic divergences at zero temperature. These results highlight the crucial impact of Coulomb interactions and screening, mediated, e.g., through the presence of parasitic bands, which are ubiquitous effects in real-world materials. Also, despite sizable corrections from Coulomb forces, the contribution from the frequency-dependent part of the self-energy remains extremely small, thus giving rise to a system of effectively almost free fermions with a strongly renormalized dispersion.
关键词： Weyl semimetals,Coulomb interactions,diagrammatic Monte Carlo,ultraviolet cutoff,screening
更新于2025-09-09 09:28:46
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Pattern recognition of messily grown nanowire morphologies applying multi-layer connected self-organized feature maps
摘要： Multi-layer connected self-organizing feature maps (SOFMs) and the associated learning procedure were proposed to achieve efficient recognition and clustering of messily grown nanowire morphologies. The network is made up by several paratactic 2-D SOFMs with inter-layer connections. By means of Monte Carlo simulations, virtual morphologies were generated to be the training samples. With the unsupervised inner-layer and inter-layer learning, the neural network can cluster different morphologies of messily grown nanowires and build connections between the morphological microstructure and geometrical features of nanowires within. Then, the as-proposed networks were applied on recognitions and quantitative estimations of the experimental morphologies. Results show that the as-trained SOFMs are able to cluster the morphologies and recognize the average length and quantity of the messily grown nanowires within. The inter-layer connections between winning neurons on each competitive layer have significant influence on the relations between the microstructure of the morphology and physical parameters of the nanowires within.
关键词： Messily grown nanowire morphologies,Artificial neural networks,Monte Carlo simulation,Pattern recognition,Self-organizing feature maps
更新于2025-09-09 09:28:46
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Nanometer-Sized Crystalline Clusters of IGZO Films Determined from the Grazing Incidence X-ray Scattering and Anomalous X-ray Scattering Data Combined with Reverse Monte Carlo Simulations
摘要： Grazing incidence X-ray scattering measurements have been carried out on c-axis aligned crystalline-indium gallium zinc oxide (CAAC-IGZO) ?lm and nanocrystalline category-indium gallium zinc oxide (NC-IGZO) ?lm and the following results were obtained: (1) the characteristic layered structure of the IGZO crystal did not hold its shape and the X-ray scattering pro?le showed only a relatively sharp ?rst peak at the wave vector (Q) = 21.8 for CAAC ?lm and 23.1 nm11 for NC ?lm, respectively, and additional weak broad peaks were observed at a higher angle. (2) In the case of the CAAC ?lm, tiny peaks were observed at Q = 7 and 14 nm11, corresponding to the positions of the 003 and 006 re?ections, respectively, of the IGZO crystal. Such tiny peaks were not detected in the case of NC ?lm but the asymmetry of the ?rst peak at the low angle side was clearly observed. (3) These structural features implied that more than three polyhedral units, such as InOx (x = 4-6), GaOy (y = 4-6), and ZnOz (z = 4-6), were likely to coexist. It is appropriate to call this structural feature as cluster-1. (4) A composite-type structure formed by combining these polyhedral units is also likely to exist and leads to middle-range ordering. This structure is called cluster-2. The size of such cluster-2 has been estimated to be 2.2 nm for CAAC ?lm and 1.8 nm for NC ?lm using the measured pair distribution function. To gain insights into the structural features of IGZO ?lms, realistic atomic-scale models were obtained to ?t not only the ordinary interference function of grazing incidence X-ray scattering but also the environmental interference function of the anomalous X-ray scattering (AXS) with Zn-absorption edge using reverse Monte Carlo (RMC) simulation. (5) The resultant models indicated the complex and irregular atomic arrangements of two types of IGZO ?lms, which are well characterized by nanometer-sized crystalline clusters. This characteristic feature may be referred to as crystalline-cluster-composite (triple C) structure.
关键词： anomalous X-ray scattering,nanometer-sized crystalline clusters,IGZO ?lm,grazing incidence X-ray scattering,reverse Monte Carlo (RMC) simulation
更新于2025-09-09 09:28:46
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Modeling Light Propagation through the Tissues of the Head Taking Account of Scattering Anisotropy to Optimize the Positioning of Irradiation Detectors and Sources in a Brain–Computer Interface Based on Near Infrared Spectroscopy
摘要： We describe here use of the Monte Carlo modeling method to specify the parameters of near infrared light propagation though the tissues of the head, which is needed for optimizing the operation of brain–computer interfaces. The studies used a four-layer spherical model of the head consisting of skin, bone, gray matter, and white matter. The relationship between the parameters of the radiation recorded and the distance between the source and detector were obtained.
关键词： foci of hemodynamic activity,near infrared spectrometry,brain–computer interface,Monte Carlo modeling
更新于2025-09-09 09:28:46
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[IEEE 2017 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC) - Atlanta, GA (2017.10.21-2017.10.28)] 2017 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC) - Deep residual learning in CT physics: scatter correction for spectral CT
摘要： Recently, spectral CT has been drawing a lot of attention in a variety of clinical applications primarily due to its capability of providing quantitative information about material properties. The quantitative integrity of the reconstructed data depends on the accuracy of the data corrections applied to the measurements. Scatter correction is a particularly sensitive correction in spectral CT as it depends on system effects as well as the object being imaged and any residual scatter is amplified during the non-linear material decomposition. An accurate way of removing scatter is subtracting the scatter estimated by Monte Carlo simulation. However, to get sufficiently good scatter estimates, extremely large numbers of photons are required, which may lead to unexpectedly high computational costs. Other approaches model scatter as a convolution operation using kernels derived using empirical methods. These techniques have been found to be insufficient in spectral CT due to their inability to sufficiently capture object dependence. In this work, we develop a deep residual learning framework to address both issues of computation simplicity and object dependency. A deep convolution neural network is trained to determine the scatter distribution from the projection content in training sets. In test cases of a digital anthropomorphic phantom and real water phantom, we demonstrate that with much lower computing costs, the proposed network provides sufficiently accurate scatter estimation.
关键词： convolutional neural network,deep residual learning,Monte Carlo simulation,scatter correction,spectral CT
更新于2025-09-09 09:28:46
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[IEEE 2017 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC) - Atlanta, GA (2017.10.21-2017.10.28)] 2017 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC) - Deep Learning Models for PET Scatter Estimations
摘要： Projection data acquired from a positron emission tomography (PET) scanner consist of true, scattered and random events. Scattered events can cause severe artifacts and quantitation errors in reconstructed PET images unless corrected for properly. A scatter correction algorithm is required to predict scattered events from the measurement. Scatter correction requires estimation of both single scatter and multiple scatter profiles. Usually, single scatter profiles are calculated by model-based simulation and multiple scatter profiles are estimated by a kernel-based convolution method. However, design of the convolution kernels for multiple scatter estimation is sophisticated and requires fine parameter tuning. In this work, we adopt deep learning techniques for scatter estimation. We propose two convolutional neural networks. The first network estimates multiple scatter profiles from single scatter profiles, replacing the kernel-based convolution method. The second network is designed to predict the total scatter profiles (including single and multiple scatters) directly from the input of emission and attenuation sinograms. Initial results from both networks show a promise with the potential for more accurate and faster scatter correction for PET.
关键词： Monte Carlo Simulation,Deep Learning,Scatter Estimation,Convolutional Neural Networks,PET
更新于2025-09-09 09:28:46
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[IEEE 2018 40th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC) - Honolulu, HI, USA (2018.7.18-2018.7.21)] 2018 40th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC) - Study of the photoneutron generation caused by a LinAc Beryllium window with a 6 MeV treatment beam
摘要： In most conventional radiation therapy treatments, special attention is payed for neutron contamination when working with energy beams above 8 MeV and generally it is only considered for shielding requirements, not for dose study in patients or employees. The present work is focused on studying the unwanted generated photoneutrons in a Medical Linear Accelerator (LinAc) Varian TrueBeam using a 6 MeV radiation treatment beam. To that, Monte Carlo (MC) simulation code MCNP6.1.1 was used. This version of the code allows the use of unstructured mesh geometries as a novelty, offering more reliable results and higher speed computation. The particularity of the studied LinAc is the presence of a beryllium filter at the treatment head. Since Beryllium causes photonuclear reactions (γ, n) at energies much lower than other LinAc composing materials, this work aims to analyze if this type of units, when using low energy treatment beams (6 MeV), produce neutron pollution and to ensure that this unwanted radiation can be considered negligible.
关键词： Monte Carlo simulation,Medical Linear Accelerator,radiation therapy,photoneutrons,Beryllium filter
更新于2025-09-09 09:28:46
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Phase velocity of drifting spin wave packets in semiconductor two-dimensional electron gas
摘要： We investigated the drift and diffusion dynamics of spin wave packets under spin–orbit effective magnetic fields in two-dimensional electron systems. A simplified model with a spin drift-diffusion equation predicted that the spin phase velocity will change over time from a large negative value to a small positive value. Kerr rotation microscopy revealed the trend we expected for the phase velocity in spin wave packets in a GaAs quantum well. Monte-Carlo simulations agreed with the experiment and showed that the phase velocity can be simply characterized by the spin–orbit parameters and the size ratio between the instantaneous and initial wave packets.
关键词： two-dimensional electron systems,spin wave packets,Kerr rotation microscopy,Monte-Carlo simulations,spin–orbit effective magnetic fields
更新于2025-09-09 09:28:46
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A model for charge transport in semicrystalline polymer thin films
摘要： A model for simulating the charge transport properties of semicrystalline polymer (SCrP) using Monte Carlo simulation is reinvented. The model is validated by reproducing the experimentally observed ?eld and temperature dependence of mobility in Poly(3-hexylthiophene-2,5-diyl) (P3HT) thin ?lms. This study also provides a new physical insight to the origin of much debated negative ?eld dependence of mobility (NFDM) observed at low electric ?eld strengths in P3HT thin ?lms. The observed NFDM, which is not explainable with the mechanisms proposed earlier, is attributed to the weak dependence of transit time on the applied electric ?eld strengths. In the semicrystalline ?lms, the charge transport takes place mostly through the crystalline regions, in which the charge transport is weakly dependent on the strength of the applied electric ?eld. In addition, a possible explanation for the origin of Arrhenius temperature dependence of mobility (lnμ / 1/T) commonly observed in SCrP thin ?lms is also proposed.
关键词： Monte Carlo simulation,charge transport,poly(3-hexylthiophene-2,5-diyl),semicrystalline polymers,negative ?eld dependence of mobility
更新于2025-09-09 09:28:46