1,454 research outputs found
Anomalous population of He states in reactions with Li
Structure with the lowest energy observed in the He spectrum populated
in the proton knockout reaction with Li beam has a peak at
MeV. This peak is usually interpreted as a resonant ground state of
He. Our theoretical calculations indicate that this peak is likely to be
a pileup of , , and excitations with very similar shapes. %We
predict a very specific nature of the excitation in He. Moreover,
the ``soft'' excitation appears to be the lowest one in energy. Such an
anomalous continuum response is traced to the halo structure of Li
providing extreme low energy shift to all the expected continuum excitations.
Competitions of the initial state structure (ISS) and the final state
interaction (FSI) effects on the spectrum and three-body correlations in
He are discussed. Analogous effect of the extreme low-energy shift could
also be expected in other cases of emitters populated in reactions with
halo nuclei. Simplified example of the He spectrum in knockout
from Be, is given. We also discuss limits on the properties of He
stemming from the observed He spectrum.Comment: 10 pages, 13 figure
Two-proton radioactivity and three-body decay. V. Improved momentum distributions
Nowadays quantum-mechanical theory allows one to reliably calculate the
processes of 2p radioactivity (true three-body decays) and the corresponding
energy and angular correlations up to distances of the order of 1000 fm.
However, the precision of modern experiments has now become sufficient to
indicate some deficiency of the predicted theoretical distributions. In this
paper we discuss the extrapolation along the classical trajectories as a method
to improve the convergence of the theoretical energy and angular correlations
at very large distances (of the order of atomic distances), where only the
long-range Coulomb forces are still operating. The precision of this approach
is demonstrated using the "exactly" solvable semianalytical models with
simplified three-body Hamiltonians. It is also demonstrated that for heavy 2p
emitters, the 2p decay momentum distributions can be sensitive to the effect of
the screening by atomic electrons. We compare theoretical results with
available experimental data.Comment: 13 pages, 18 figure
Observable effects caused by vacuum pair creation in the field of high-power optical lasers
We consider the possibility of an experimental proof of vacuum e+e- pair
creation in the focus of two counter-propagating optical laser beams with an
intensity of the order of 10^20 - 10^22 W/cm^2. Our approach is based on the
collisionless kinetic equation for the distribution function of the e+e- pairs
with the source term for particle production. As a possible experimental signal
of vacuum pair production we consider the refraction of a high-frequency probe
laser beam by the produced e+e- plasma to be observed by an interference
filter. The generation of higher harmonics of the laser frequency in the
self-consistent electric field is also investigated.Comment: 7 pages, 7 figures; typos corrected, Eq.(16) corrected, reference
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Wildland fire propagation modeling: fire-spotting parametrisation and energy balance
Present research concerns the physical background of a wild-fire propagation model
based on the split of the front motion into two parts - drifting and fluctuating. The drifting part is solved by the level set method and the fluctuating part describes turbulence
and fire-spotting. These phenomena have a random nature and can be modeled as a
stochastic process with the appropriate probability density function. Thus, wildland fire
propagation results to be described by a nonlinear partial differential equation (PDE) of
the reaction-diffusion type. A numerical study of the effects of the atmospheric stability
on wildfire propagation is performed through its effects on fire-spotting. Moreover, it
is shown that the solution of the PDE as an indicator function allows to construct the
energy balance equation in terms of the temperature.PhD Grant "La Caixa 2014
From Coulomb excitation cross sections to non-resonant astrophysical rates in three-body systems: Ne case
Coulomb and nuclear dissociation of Ne on light and heavy targets are
studied theoretically. The dipole E1 strength function is determined in a broad
energy range including energies of astrophysical interest. Dependence of the
strength function on different parameters of the Ne ground state
structure and continuum dynamics is analyzed in a three-body model. The
discovered dependence plays an important role for studies of the strength
functions for the three-body E1 dissociation and radiative capture. The
constraints on the configuration mixing in Ne and on
-wave interaction in the O+ channel are imposed based on
experimental data for Ne Coulomb dissociation on heavy target.Comment: 12 pages, 13 figure
Concurent multi-scale physical parametrization of fire-spotting: A study on the role of macro- and meso-scale characteristics of the system
The strong impact of wildfires in terms of lives and homes lost and of damage to ecosystems, calls
for an urgent improvement in the risk management. The aim of the present research is the
improvement of these software codes by proposing a complete physical characterization of fire-spotting within an approach that is ready to be implemented as a post-processing routine of standard
outputs. The main feature of the proposed method is that the effects of random fluctuations are
included in a way that preserves the existing structure of the operational and industrial codes and can
be implemented directly. The operational code WRF-SFIRE have been used to test the proposed post-processing routine. Results show the suitability of the approach for simulating random effects due to
turbulent convection and fire-spotting, which are cases not resolved by standard operational codes.
Results of simulations including response analysis with test cases are shown and discussed.PhD grant "La Caixa 2014
Wildland fire propagation modelling
Wildfire propagation modelling is a challenging problem due to its complex
multi-scale multi-physics nature. This process can be described by a reaction-
diffusion equation based on the energy balance principle. Alternative technique is the so-called
level-set method (LSM), used
in wildfire modelling as well as in many other fields. In the present study a
methodology for fire propagation modelling that reconciles these approaches
is proposed. This methodology is distinguishable and significant from both
academical and industrial point of view because of the inclusion of the ran-
dom effects by preserving the existing algorithms and direct implementation
as a post-processing numerical routine.
The random behaviour of the fire front is caused, for example, by the
turbulence and the fire-spotting phenomenon. A probability density function
(PDF) is employed in order to describe the random process. In earlier studies
it has been shown that new independent ignitions can increase the rate of
spread (ROS) of fire and therefore should be carefully studied. In this respect,
a physical parametrization of the fire-spotting distribution was proposed.
Special attention in the present study is paid to the atmospheric stability
conditions. The parametrization proposed in previous works is completed by the
multiple fire-spotting modelling. Afterwards special attention is paid to the
study of uniqueness of the PDF and consistency with the energy balance
equation. Numerical results and discussions complete the study.PhD grant ”La Caixa 2014
Numerical valuation of two-asset options under jump diffusion models using Gauss-Hermite quadrature
In this work a finite difference approach together with a bivariate Gauss–Hermite quadrature technique is developed for partial integro-differential equations related to option pricing problems on two underlying asset driven by jump-diffusion models. Firstly,
the mixed derivative term is removed using a suitable transformation avoiding numerical drawbacks such as slow convergence and inaccuracy due to the appearance of spurious oscillations. Unlike the more traditional truncation approach we use 2D Gauss–Hermite quadrature with the additional advantage of saving computational cost. The explicit finite difference scheme becomes consistent, conditionally stable and positive. European and American option cases are treated. Numerical results are illustrated and analysed with experiments and comparisons with other well recognized methods.FP7-PEOPLE-2012-ITN program under Grant Agreement Number 304617 (FP7 Marie Curie Action, Project Multi-ITN STRIKE-Novel Methods in Computational Finance) Ministerio de Economía y Competitividad Spanish grant MTM2013-41765-
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