24,515 research outputs found
Neutrino transport in type II supernovae: Boltzmann solver vs. Monte Carlo method
We have coded a Boltzmann solver based on a finite difference scheme (S_N
method) aiming at calculations of neutrino transport in type II supernovae.
Close comparison between the Boltzmann solver and a Monte Carlo transport code
has been made for realistic atmospheres of post bounce core models under the
assumption of a static background. We have also investigated in detail the
dependence of the results on the numbers of radial, angular, and energy grid
points and the way to discretize the spatial advection term which is used in
the Boltzmann solver. A general relativistic calculation has been done for one
of the models. We find overall good agreement between the two methods. However,
because of a relatively small number of angular grid points (which is
inevitable due to limitations of the computation time) the Boltzmann solver
tends to underestimate the flux factor and the Eddington factor outside the
(mean) ``neutrinosphere'' where the angular distribution of the neutrinos
becomes highly anisotropic. This fact suggests that one has to be cautious in
applying the Boltzmann solver to a calculation of the neutrino heating in the
hot-bubble region because it might tend to overestimate the local energy
deposition rate. A comparison shows that this trend is opposite to the results
obtained with a multi-group flux-limited diffusion approximation of neutrino
transport. The accuracy of the Boltzmann solver can be considerably improved by
using a variable angular mesh to increase the angular resolution in the
semi-transparent regime.Comment: 19 pages, 17 figures, submitted to A&
Reference-free evaluation of thin films mass thickness and composition through energy dispersive x-ray spectroscopy
In this paper we report the development of a new method for the evaluation of
thin films mass thickness and composition based on the Energy Dispersive X-Ray
Spectroscopy (EDS). The method exploits the theoretical calculation of the
in-depth characteristic X-ray generation distribution function, /(
z), in multilayer samples, obtained by the numerical solution of the electron
transport equation, to achieve reliable measurements without the need of a
reference sample and multiple voltages acquisitions. The electron transport
model is derived from the Boltzmann transport equation and it exploits the most
updated and reliable physical parameters in order to obtain an accurate
description of the phenomenon. The method for the calculation of film mass
thickness and composition is validated with benchmarks from standard
techniques. In addition, a model uncertainty and sensitivity analysis is
carried out and it indicates that the mass thickness accuracy is in the order
of 10 g/cm, which is comparable to the nuclear standard techniques
resolution. We show the technique peculiarities in one example measurement:
two-dimensional mass thickness and composition profiles are obtained for a
ultra-low density, high roughness, nanostructured film.Comment: This project has received funding from the European Research Council
(ERC) under the European Union's Horizon 2020 research and innovation
programme (ENSURE grant agreement No. 647554
Monte Carlo techniques for time-dependent radiative transfer in 3-D supernovae
Monte Carlo techniques based on indivisible energy packets are described for
computing light curves and spectra for 3-D supernovae. The radiative transfer
is time-dependent and includes all effects of O(v/c). Monte Carlo quantization
is achieved by discretizing the initial distribution of 56Ni into radioactive
pellets. Each pellet decays with the emission of a single energy packet
comprising gamma-ray photons representing one line from either the 56Ni or the
56Co decay spectrum. Subsequently, these energy packets propagate through the
homologously-expanding ejecta with appropriate changes in the nature of their
contained energy as they undergo Compton scatterings and pure absorptions.
The 3-D code is tested by applying it to a spherically-symmetric SN in which
the transfer of optical radiation is treated with a grey absorption
coefficient. This 1-D problem is separately solved using Castor's co-moving
frame moment equations. Satisfactory agreement is obtained.
The Monte Carlo code is a platform onto which more advanced treatments of the
interactions of matter and radiation can be added. Some of these have already
been developed and tested in previous papers and are summarized here.Comment: 14 pages, 5 figures. Accepted by A&
Spatially hybrid computations for streamer discharges: II. Fully 3D simulations
We recently have presented first physical predictions of a spatially hybrid
model that follows the evolution of a negative streamer discharge in full three
spatial dimensions; our spatially hybrid model couples a particle model in the
high field region ahead of the streamer with a fluid model in the streamer
interior where electron densities are high and fields are low. Therefore the
model is computationally efficient, while it also follows the dynamics of
single electrons including their possible run-away. Here we describe the
technical details of our computations, and present the next step in a
systematic development of the simulation code. First, new sets of transport
coefficients and reaction rates are obtained from particle swarm simulations in
air, nitrogen, oxygen and argon. These coefficients are implemented in an
extended fluid model to make the fluid approximation as consistent as possible
with the particle model, and to avoid discontinuities at the interface between
fluid and particle regions. Then two splitting methods are introduced and
compared for the location and motion of the fluid-particle-interface in three
spatial dimensions. Finally, we present first results of the 3D spatially
hybrid model for a negative streamer in air
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