共查询到20条相似文献,搜索用时 15 毫秒
1.
To study the spectrum reconstruction of the 20 MV X-ray generated by the Dragon-I linear induction accelerator, the Monte Carlo method is applied to simulate the attenuations of the X-ray in the attenuators of different thicknesses and thus provide the transmission data. As is known, the spectrum estimation from transmission data is an ill-conditioned problem. The method based on iterative perturbations is employed to derive the X-ray spectra, where initial guesses are used to start the process. This algorithm takes into account not only the minimization of the differences between the measured and the calculated transmissions but also the smoothness feature of the spectrum function. In this work, various filter materials are put to use as the attenuator, and the condition for an accurate and robust solution of the X-ray spectrum calculation is demonstrated. The influences of the scattering photons within different intervals of emergence angle on the X-ray spectrum reconstruction are also analyzed. 相似文献
2.
X‐ray tubes have a broad range of applications worldwide, including several techniques for atomic physics, like X‐ray fluorescence, as well as for medical imaging, like computed tomography. The performances of X‐ray imaging detectors have shown to be significantly sensitive to the incident beam spectrum. Therefore, an accurate knowledge of the X‐ray beam becomes necessary for the emission source characterization and the whole imaging process comprehension. Direct measurements and suitable Monte Carlo simulations may be used to establish the X‐ray spectra. Dedicated Monte Carlo simulation routines, based on the PENELOPE code, have been developed to determine the Bremsstrahlung X‐ray spectra generated by conventional X‐ray tubes. The simulated spectra have been validated by comparison with the corresponding experimental data showing an overall good agreement. The incorporation of a suitably designed virtual grid allowed to assess the angular distribution of Bremsstrahlung yield, showing a remarkable anisotropy. In addition, a dedicated program has been developed for virtual imaging, which enables to perform suitable X‐ray absorption contrast images. Also, the developed program includes a user‐friendly graphic interface to allow the upload of required input parameters, which include setup arrangement, beam characteristics, sample properties and image simulation parameters (spatial resolution, tracks per run, etc.). The software includes dedicated subroutines which handle the physical process from X‐ray generation up to detector signal acquisition. The aim of the developed program is to perform virtual imaging by means of absorption contrast and using conventional X‐ray sources, which may be a useful tool for the study the X‐ray imaging techniques in several research fields as well as for educational purposes. The performed comparisons with experimental data have shown good agreement. The obtained results for X‐ray imaging may constitute useful information for the comprehension and improvement of X‐ray image quality, like absorption contrast optimization, detail visualization, definition and detectability. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
3.
《Physica A》1996,231(4):504-514
Modifying the Redfield model of sexual reproduction and the Penna model of biological aging, we compare reproduction with and without recombination in age-structured populations. In constrast to Redfield and in agreement with Bernardes we find sexual reproduction to be preferred to asexual one. In particular, the presence of old but still reproducing males helps the survival of younger females beyond their reproductive age. 相似文献
4.
V. Schwämmle A. O. Sousa S. M. de Oliveira 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,51(4):563-570
Parapatric speciation is studied using an individual-based model with sexual
reproduction. We combine the theory of mutation accumulation for biological ageing
with an environmental selection pressure that varies according to the individuals
geographical positions and phenotypic traits. Fluctuations and genetic diversity of
large populations are crucial ingredients to model the features of evolutionary
branching and are intrinsic properties of the model.
Its implementation on a spatial lattice gives interesting insights into the population
dynamics of speciation on a geographical landscape and the disruptive selection that leads
to the divergence of phenotypes.
Our results suggest that assortative mating is not an obligatory ingredient to obtain
speciation in large populations at low gene flow. 相似文献
5.
Mitra Ghergherehchi Junho Ko Narjes ArabyarMohammadi Hossein Afarideh Yoon Sang Kim 《Pramana》2018,90(3):35
The \(\alpha \) decay half-lives of hyper and normal isotopes of Po nuclei are studied in the present work. The inclusion of \(\Lambda \)–N interaction changes the half-life for \(\alpha \) decay. The theoretical predictions on the \(\alpha \) decay half-lives of normal Po isotopes are compared with experimental results and are seen to be matching well with each other. The neutron shell closure at \(N = 126\) is found to be the same for both normal and hypernuclei. The Geiger–Nuttal (G–N) law for \(\alpha \) decay is unaltered in the case of hypernuclei. The hypernuclei will decay into normal nuclei by mesonic or non-mesonic decay modes. Since the half-lives of normal Po nuclei are well within the experimental limits, our theoretical results suggest experimental verification of the \(\alpha \) emission from hyper Po nuclei in a cascade process. 相似文献
6.
A model for opinion formation based on the Theory of Social Impact
is presented and studied by means of numerical simulations.
Individuals with two states of opinion are impacted due to social
interactions with: i) members of the society, ii) a strong leader
with a well-defined opinion and iii) the mass media that could
either support or compete with the leader. Due to that competition,
the average opinion of the social group exhibits phase-transition
like behaviour between different states of opinion. 相似文献
7.
Riccardo Fantoni 《The European Physical Journal B - Condensed Matter and Complex Systems》2016,89(2):36
We perform path integral Monte Carlo simulations to study the imaginary time dynamics of metastable supercooled superfluid states and nearly superglassy states of a one component fluid of spinless bosons square wells. Our study shows that the identity of the particles and the exchange symmetry is crucial for the frustration necessary to obtain metastable states in the quantum regime. Whereas the simulation time has to be chosen to determine whether we are in a metastable state or not, the imaginary time dynamics tells us if we are or not close to an arrested glassy state. 相似文献
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Ensemble Monte Carlo simulations of electron transport through a new aluminum gallium nitride/gallium nitride (AlGaN/GaN) cold cathode emitter are reported. We analyze the energy spectrum of carriers prior to being injected into a low work function slab of Lanthanum Hexaboride (LaB6) as a function of the ramp energy of the carriers at the AlGaN/GaN heterojunction. Plasmon scattering is shown to be the major scattering mechanism in the structure leading to substantial shift towards the low kinetic energy end of the energy spectrum of the carriers injected into the low work function Lanthanum Hexaboride thin film. Intervalley scattering is found to dominate in the depletion layer at the GaN/LaB6interface. Design improvements to increase the efficiency of the cold cathode are suggested. 相似文献
10.
《Physics letters. [Part B]》1987,197(4):548-552
The world sheet of the bosonic string is regularized by triangulation. The corresponding statistical theory of triangulated random surfaces is simulated by Monte Carlo methods and the critical exponent γ for the susceptibility determined. A careful analysis of the systematic and statistical errors reveals that we have to wait for the next generation of computers before a high-precision measurement of γ is possible. 相似文献
11.
Recent experiments measuring the thick-target yield curve of narrow resonances in (p, γ) reactions on23Na,26Mg and27Al nuclei clearly show the Lewis effect. We have studied this effect theoretically on the basis of a Monte Carlo simulation of the nuclear and electronic energy loss curve in the target. We are able to reproduce the experimental data qualitatively. We discuss possible implications of precision measurements of the Lewis effect. 相似文献
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13.
Roger M Guenoun P Muller F Belloni L Delsanti M 《The European physical journal. E, Soft matter》2002,9(4):313-326
The concentration profiles of monomers and counterions in star-branched polyelectrolyte micelles are calculated through Monte
Carlo simulations, using the freely jointed chain model. We have investigated the onset of different regimes corresponding
to the spherical and Manning condensation of counterions as a function of the strength of the Coulomb coupling. The Monte
Carlo results are in fair agreement with the predictions of Self-Consistent-Field analytical models. We have simulated a real
system of diblock copolymer micelles of (sodium-polystyrene-sulfonate)(NaPSS)-(polyethylene-propylene)(PEP) with f = 54 hydrophilic branches of N = 251 monomers at room temperature in salt-free solution. The calculated form factor compares nicely with our neutron scattering
data.
Received 18 July 2002 and Received in final form 11 October 2002
RID="a"
ID="a"e-mail: roger@drecam.saclay.cea.fr 相似文献
14.
We develop an approximation-free diagrammatic Monte Carlo technique to study fermionic particles interacting with each other simultaneously through both an attractive Coulomb potential and bosonic excitations of the underlying medium. Exemplarily we apply the method to the long-standing exciton-polaron problem and present numerically exact results for the wave function, ground-state energy, binding energy and effective mass of this quasiparticle. Focusing on the electron-hole pair bound-state formation, we discuss various limiting cases of a generic exciton-polaron model. The frequently used instantaneous approximation to the retarded interaction due to the exchange of phonons is found to be of very limited applicability. 相似文献
15.
Alfred B. Bortz 《Journal of statistical physics》1974,11(3):181-193
Recently Cahn's generalized diffusion equation theory of spinodal decomposition in binary alloys has been modified to include the effects of thermal fluctuations. This paper reports studies of a one-dimensional binary alloy system in which fluctuations can be observed on an atomic time scale. The system, a computer-simulated linear chain binary alloy which evolves from an initially random atomic arrangement through interchange of unlike nearest neighbors via the Monte Carlo technique, rapidly develops grains of two different concentrations and then slowly experiences coarsening. A numerical solution of the diffusion equation successfully predicts the development of grain structure, but only predicts coarsening to the extent present as fluctuations in the initial atomic arrangement. The simulated alloy coarsens further than the prediction of the diffusion equation because of thermal fluctuations which develop naturally during its evolution. This suggests that thermal fluctuations may play an important role in coarsening in real alloys. 相似文献
16.
The Trotter-Suzuki transformation has been used to obtain the classical representation ford-dimensional lattice systems with boson and fermion degrees of freedom. A Monte Carlo algorithm for the equivalent (d+1)-dimensional classical system is presented. Numerical results are shown for the Heisenberg-spin-glass, the XY model and the spinless fermion lattice gas in two dimensions. 相似文献
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18.
Nonuniversal critical dynamics in Monte Carlo simulations 总被引:1,自引:0,他引:1
19.