3,553 research outputs found
Linear Boltzmann dynamics in a strip with large reflective obstacles: stationary state and residence time
The presence of obstacles modify the way in which particles diffuse. In
cells, for instance, it is observed that, due to the presence of macromolecules
playing the role of obstacles, the mean square displacement ofbiomolecules
scales as a power law with exponent smaller than one. On the other hand,
different situations in grain and pedestrian dynamics in which the presence of
an obstacle accelerate the dynamics are known. We focus on the time, called
residence time, needed by particles to cross a strip assuming that the dynamics
inside the strip follows the linear Boltzmann dynamics. We find that the
residence time is not monotonic with the sizeand the location of the obstacles,
since the obstacle can force those particles that eventually cross the strip to
spend a smaller time in the strip itself. We focus on the case of a rectangular
strip with two open sides and two reflective sides and we consider reflective
obstaclea into the strip
Does communication enhance pedestrians transport in the dark?
We study the motion of pedestrians through an obscure tunnel where the lack
of visibility hides the exits. Using a lattice model, we explore the effects of
communication on the effective transport properties of the crowd of
pedestrians. More precisely, we study the effect of two thresholds on the
structure of the effective nonlinear diffusion coefficient. One threshold
models pedestrians's communication efficiency in the dark, while the other one
describes the tunnel capacity. Essentially, we note that if the evacuees show a
maximum trust (leading to a fast communication), they tend to quickly find the
exit and hence the collective action tends to prevent the occurrence of
disasters
Metastability for reversible probabilistic cellular automata with self--interaction
The problem of metastability for a stochastic dynamics with a parallel
updating rule is addressed in the Freidlin--Wentzel regime, namely, finite
volume, small magnetic field, and small temperature. The model is characterized
by the existence of many fixed points and cyclic pairs of the zero temperature
dynamics, in which the system can be trapped in its way to the stable phase.
%The characterization of the metastable behavior %of a system in the context of
parallel dynamics is a very difficult task, %since all the jumps in the
configuration space are allowed. Our strategy is based on recent powerful
approaches, not needing a complete description of the fixed points of the
dynamics, but relying on few model dependent results. We compute the exit time,
in the sense of logarithmic equivalence, and characterize the critical droplet
that is necessarily visited by the system during its excursion from the
metastable to the stable state. We need to supply two model dependent inputs:
(1) the communication energy, that is the minimal energy barrier that the
system must overcome to reach the stable state starting from the metastable
one; (2) a recurrence property stating that for any configuration different
from the metastable state there exists a path, starting from such a
configuration and reaching a lower energy state, such that its maximal energy
is lower than the communication energy
Relaxation Height in Energy Landscapes: an Application to Multiple Metastable States
The study of systems with multiple (not necessarily degenerate) metastable
states presents subtle difficulties from the mathematical point of view related
to the variational problem that has to be solved in these cases. We introduce
the notion of relaxation height in a general energy landscape and we prove
sufficient conditions which are valid even in presence of multiple metastable
states. We show how these results can be used to approach the problem of
multiple metastable states via the use of the modern theories of metastability.
We finally apply these general results to the Blume--Capel model for a
particular choice of the parameters ensuring the existence of two multiple, and
not degenerate in energy, metastable states
Correlation functions by Cluster Variation Method for Ising model with NN, NNN and Plaquette interactions
We consider the procedure for calculating the pair correlation function in
the context of the Cluster Variation Methods. As specific cases, we study the
pair correlation function in the paramagnetic phase of the Ising model with
nearest neighbors, next to the nearest neighbors and plaquette interactions in
two and three dimensions. In presence of competing interactions, the so called
disorder line separates in the paramagnetic phase a region where the
correlation function has the usual exponential behavior from a region where the
correlation has an oscillating exponentially damped behavior. In two
dimensions, using the plaquette as the maximal cluster of the CVM
approximation, we calculate the phase diagram and the disorder line for a case
where a comparison is possible with results known in literature for the
eight-vertex model. In three dimensions, in the CVM cube approximation, we
calculate the phase diagram and the disorder line in some cases of particular
interest. The relevance of our results for experimental systems like mixtures
of oil, water and surfactant is also discussed.Comment: 31 pages, LaTeX file, 7 figure
Experimental characterization of the LUPIN Rem counter in monoenergetic neutron fields
The LUPIN is a Rem counter for neutron dosimetry in pulsed radiation fields, i.e., those fields whose intensity varies greatly on short timescales with respect to the characteristic time of the utilized detector. This work describes the characterization of the energy response of the instrument. The response function was calculated with the Monte Carlo code MCNP6, representing the geometry and material composition of the LUPIN and simulating an irradiation in expanded and aligned monoenergetic neutron fields. The calculated response was validated in the monoenergetic fields of the National Physical Laboratory. The agreement between the calculated and measured responses is satisfactory, with a maximum discrepancy of 5%
Effect of metal clusters on the swelling of gold-fluorocarbon-polymer composite films
We have investigated the phenomenon of swelling due to acetone diffusion in
fluorocarbon polymer films doped with different gold concentrations below the
percolation threshold. The presence of the gold clusters in the polymer is
shown to improve the mixing between the fluorocarbon polymer and the acetone,
which is not a good solvent for this kind of polymers. In order to explain the
experimental results the stoichiometry and the morphology of the polymer--metal
system have been studied and a modified version of the Flory--Huggins model has
been developed
Hybrid Superconducting Neutron Detectors
A new neutron detection concept is presented that is based on superconductive
niobium (Nb) strips coated by a boron (B) layer. The working principle of the
detector relies on the nuclear reaction 10B+n + 7Li ,
with and Li ions generating a hot spot on the current-biased Nb strip
which in turn induces a superconducting-normal state transition. The latter is
recognized as a voltage signal which is the evidence of the incident neutron.
The above described detection principle has been experimentally assessed and
verified by irradiating the samples with a pulsed neutron beam at the ISIS
spallation neutron source (UK). It is found that the boron coated
superconducting strips, kept at a temperature T = 8 K and current-biased below
the critical current Ic, are driven into the normal state upon thermal neutron
irradiation. As a result of the transition, voltage pulses in excess of 40 mV
are measured while the bias current can be properly modulated to bring the
strip back to the superconducting state, thus resetting the detector.
Measurements on the counting rate of the device are presented and the future
perspectives leading to neutron detectors with unprecedented spatial
resolutions and efficiency are highlighted.Comment: 8 pages 6 figure
Monte Carlo study of gating and selection in potassium channels
The study of selection and gating in potassium channels is a very important
issue in modern biology. Indeed such structures are known in all types of cells
in all organisms where they play many important functional roles. The mechanism
of gating and selection of ionic species is not clearly understood. In this
paper we study a model in which gating is obtained via an affinity-switching
selectivity filter. We discuss the dependence of selectivity and efficiency on
the cytosolic ionic concentration and on the typical pore open state duration.
We demonstrate that a simple modification of the way in which the selectivity
filter is modeled yields larger channel efficiency
Conditional expectation of the duration of the classical gambler problem with defects
The effect of space inhomogeneities on a diffusing particle is studied in the
framework of the 1D random walk. The typical time needed by a particle to cross
a one--dimensional finite lane, the so--called residence time, is computed
possibly in presence of a drift. A local inhomogeneity is introduced as a
single defect site with jumping probabilities differing from those at all the
other regular sites of the system. We find complex behaviors in the sense that
the residence time is not monotonic as a function of some parameters of the
model, such as the position of the defect site. In particular we show that
introducing at suitable positions a defect opposing to the motion of the
particles decreases the residence time, i.e., favors the flow of faster
particles. The problem we study in this paper is strictly connected to the
classical gambler's ruin problem, indeed, it can be thought as that problem in
which the rules of the game are changed when the gambler's fortune reaches a
particular a priori fixed value. The problem is approached both numerically,
via Monte Carlo simulations, and analytically with two different techniques
yielding different representations of the exact result
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