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[en] The number of tallies performed in a given Monte Carlo calculation is limited in most modern Monte Carlo codes by the amount of memory that can be allocated on a single processor. By using domain decomposition, the calculation is now limited by the total amount of memory available on all processors, allowing for significantly more tallies to be performed. However, decomposing the problem geometry introduces significant issues with the way tally statistics are conventionally calculated. In order to deal with the issue of calculating tally variances in domain decomposed environments for the Shift hybrid Monte Carlo code, this paper presents an alternative approach for reactor scenarios in which an assumption is made that once a particle leaves a domain, it does not reenter the domain. Particles that reenter the domain are instead treated as separate independent histories. This assumption introduces a bias that inevitably leads to under-prediction of the calculated variances for tallies within a few mean free paths of the domain boundaries. However, through the use of different decomposition strategies, primarily overlapping domains, the negative effects of such an assumption can be significantly reduced to within reasonable levels.
[en] We consider an evolution transport equation and show that its solution converges to the solution of a diffusion equation, as the mean free path converges to zero. We suppose the medium to be isotropic and consider perfect reflection as boundary conditions
[fr]Nous demontrons la convergence de la solution d'une equation de transport vers la solution d'une equation de diffusion, quand le parcours libre moyen tend vers zero. On considere des equations d'evolution avec la reflexion speculaire comme condition aux limites, dans un milieu isotrope
[en] Measurements of the electron transparency of a Micromegas mesh are compared to simulations. The flux conservation argument is shown to lead to inaccurate estimates of the transparency, the importance of accurate geometric modelling of the mesh is discussed and the effect of the dipole moment of the mesh is demonstrated. This study provides a validation of the microscopic simulation methods specifically developed for micropattern devices where the characteristic dimensions are of the same order of magnitude as the electron mean free path in the gas.
[en] In this paper we analyze an imaging technique based on intensity speckle correlations over incident field position proposed by Newmann and Webb (2014 Phys. Rev. Lett. 113 263903). Its purpose is to reconstruct a field incident on a strongly scattering random medium. The thickness of the complex medium is much larger than the scattering mean free path so that the wave emerging from the random section forms an incoherent speckle pattern. Our analysis clarifies the conditions under which the method can give a good reconstruction and characterizes its performance. The analysis is carried out in the white-noise paraxial regime, which is relevant for the applications in optics that motivated the original paper. (paper)
[en] In these lectures we describe the bases of angular resolved photoemission: determination of the electronic band structure of solids (bulk), measurements of life-time and mean free path, determination of surfaces states (valence and core) and their relationship with surface reconstruction. (author)
[pt]Nesta palestra descrevem-se as bases da fotoemissao com variacao angular: determinacao da estrutura da banda eletronica dos solidos (bulk), medidas de tempo de vida, livre caminho medio, determinacao de estados de superficies (valencia e caroco) e sua relacao com reconstrucao da superficie. (A.C.A.S.)
[en] Photonic crystals are extremely sensitive to structural disorder even to the point of completely losing their functionalities. While, on one side, this can be detrimental for applications in traditional optical devices, on the other side, it gives also rise to very interesting new physics and maybe even new applications. We propose a route to introduce disorder in photonic crystals in a controlled way by creating a certain percentage of vacancies in the lattice. We show how the method works and what type of materials can be obtained this way. Also, we use this system to probe the role of disorder on the resulting transport properties from various points of view, including measurements of the transport and scattering mean free path and the diffusion constant.
[en] To obtain the mean free path of solar energetic particles (SEPs) for a solar event, one usually has to fit time profiles of both flux and anisotropy from spacecraft observations to numerical simulations of SEPs' transport processes. This method can be called a simulation method. But a reasonably good fitting needs a lot of simulations, which demand a large amount of calculation resources. Sometimes, it is necessary to find an easy way to obtain the mean free path of SEPs quickly, for example, in space weather practice. Recently, Shalchi et al. provided an approximate analytical formula of SEPs' anisotropy time profile as a function of particles' mean free path for impulsive events. In this paper, we determine SEPs' mean free path by fitting the anisotropy time profiles from Shalchi et al.'s analytical formula to spacecraft observations. This new method can be called an analytical method. In addition, we obtain SEPs' mean free path with the traditional simulation methods. Finally, we compare the mean free path obtained with the simulation method to that of the analytical method to show that the analytical method, with some minor modifications, can give us a good, quick approximation of SEPs' mean free path for impulsive events.
[en] In this work, the effects of a laser field on the inelastic scattering of particles in a dense plasma, are studied. In particular, the effects on the mean-free-path and energy loss are calculated. (A.C.A.S.)
[pt]Neste trabalho estudam-se os efeitos de um campo de laser sobre processos de espalhamento inelastico de particulas em plasmas densos. Em particular calculam-se os efeitos sobre o livre caminho medio e a perda de energia. (A.C.A.S.)
[en] classical particles diffuse inside the one dimensional interval [0,L]. There are reflecting walls at the edges of this interval. The diffusion constant of each particle is assumed independent of its energy and position. The particles interact through short range repulsive interaction that prevents them from exchanging positions - they stay ordered in there initial ordering on the line. The probability that at time t, all the particles are back at the points there were at t=0 (up to a distance less than a mean free path), and the configuration space average of this probability, are calculated. The interaction is dominating these quantities, even for times in which it is not expected to affect most of the dynamics of the system. Both results are exact, analytic, and have the form of the sum over terms that represents different kinds of many body processes - describing different ways of clustering of particles in the system. Connections to the Bethe Ansatz method, to quantum chaos and mesoscopic systems, and to random walks in nonequilibrium statistical mechanics, are pointed out
[en] In the last years many experiments with heavy-ion accelerator beams have been performed. Some of these experiments give evidence for anomalously short mean free paths (mfp's) of projectile fragments for the first few centimeters after the point of emission. We give the possible explanation of this effect based on the statistical bias of the estimate for the mfp used by experimenters. The unbiased estimate for the reciprocal of the mfp is presented. (orig.)