239 research outputs found
Counting statistics: a Feynman-Kac perspective
By building upon a Feynman-Kac formalism, we assess the distribution of the
number of hits in a given region for a broad class of discrete-time random
walks with scattering and absorption. We derive the evolution equation for the
generating function of the number of hits, and complete our analysis by
examining the moments of the distribution, and their relation to the walker
equilibrium density. Some significant applications are discussed in detail: in
particular, we revisit the gambler's ruin problem and generalize to random
walks with absorption the arcsine law for the number of hits on the half-line.Comment: 10 pages, 6 figure
The critical catastrophe revisited
The neutron population in a prototype model of nuclear reactor can be
described in terms of a collection of particles confined in a box and
undergoing three key random mechanisms: diffusion, reproduction due to
fissions, and death due to absorption events. When the reactor is operated at
the critical point, and fissions are exactly compensated by absorptions, the
whole neutron population might in principle go to extinction because of the
wild fluctuations induced by births and deaths. This phenomenon, which has been
named critical catastrophe, is nonetheless never observed in practice: feedback
mechanisms acting on the total population, such as human intervention, have a
stabilizing effect. In this work, we revisit the critical catastrophe by
investigating the spatial behaviour of the fluctuations in a confined geometry.
When the system is free to evolve, the neutrons may display a wild patchiness
(clustering). On the contrary, imposing a population control on the total
population acts also against the local fluctuations, and may thus inhibit the
spatial clustering. The effectiveness of population control in quenching
spatial fluctuations will be shown to depend on the competition between the
mixing time of the neutrons (i.e., the average time taken for a particle to
explore the finite viable space) and the extinction time.Comment: 16 pages, 6 figure
Properties of branching exponential flights in bounded domains
Branching random flights are key to describing the evolution of many physical
and biological systems, ranging from neutron multiplication to gene mutations.
When their paths evolve in bounded regions, we establish a relation between the
properties of trajectories starting on the boundary and those starting inside
the domain. Within this context, we show that the total length travelled by the
walker and the number of performed collisions in bounded volumes can be
assessed by resorting to the Feynman-Kac formalism. Other physical observables
related to the branching trajectories, such as the survival and escape
probability, are derived as well.Comment: 5 pages, 2 figure
Percolation properties of the neutron population in nuclear reactors
Reactor physics aims at studying the neutron population in a reactor core
under the influence of feedback mechanisms, such as the Doppler temperature
effect. Numerical schemes to calculate macroscopic properties emerging from
such coupled stochastic systems however require to define intermediate
quantities (e.g. the temperature field), which are bridging the gap between the
stochastic neutron field and the deterministic feedback. By interpreting the
branching random walk of neutrons in fissile media under the influence of a
feedback mechanism as a directed percolation process and by leveraging on the
statistical field theory of birth death processes, we will build a stochastic
model of neutron transport theory and of reactor physics. The critical
exponents of this model, combined to the analysis of the resulting field
equation involving a fractional Laplacian will show that the critical diffusion
equation cannot adequately describe the spatial distribution of the neutron
population and shifts instead to a critical super-diffusion equation. The
analysis of this equation will reveal that non-negligible departure from mean
field behavior might develop in reactor cores, questioning the attainable
accuracy of the numerical schemes currently used by the nuclear industry.Comment: 15 pages, 2 figures, 1 tabl
More than a Hundred Years in the Search for an Accurate Diagnosis for Chagas Disease: Current Panorama and Expectations
Chagas disease, or American trypanosomiasis, is a parasitic disease of the Americas. In nature, Trypanosoma cruzi is transmitted through various species of triatomine bugs. However, non-vectorial transmission can also occur, such as transmission through blood products or by transplanting infected organs, by vertical transmission, and lately by oral route. Currently, Chagas disease affects approximately 6–7 million people worldwide, and the process of urbanization in Latin America and migratory movements from endemic countries have led to Chagas disease being diagnosed in areas where the infection is not endemic. There are several methods for diagnosing Chagas disease. Some of these are mostly used for research purposes, while others are used in routine diagnostic laboratories. According to the World Health Organization (WHO), chronic Chagas disease diagnosis is based on two serological techniques. To establish a definitive diagnosis, the results must be concordant. In the case of discordances, the WHO proposes repeating serology in a new sample, and if results remain inconclusive, a confirmatory test should be performed. This chapter shows aspects of the diagnosis of Chagas disease, which varies in its sensitivity and specificity, and its use depends on the geographical location, the available resources, and the purpose of the diagnosis
Production optimisation of a DNA vaccine candidate against leishmaniasis in flask culture
Plasmid DNA (pDNA) vaccines are promising means to prevent and treat infectious diseases, such as leishmaniasis, but immunisation protocols require large amounts of supercoiled plasmid DNA (scpDNA). Although pDNA can be produced at a reasonable cost in bioreactors; this scale of production may not be the best method at the initial step of a vaccine development when many antigens need to be tested. Then, with the goal of improving the production of VR1012-NH36 and pVAX-NH36 pDNA vaccines against leishmaniasis, the effect of the culture medium and temperature on the pDNA yield was studied in flask cultures. The results indicate that the plasmid volumetric yield increased up to 65 mg/l in flask cultures by using a semi-defined medium, and shifting the culture temperature from 37 to 42°C at the late exponential growth phase. This pDNA production, with at least 80% of sc-pDNA at a laboratory scale seems sufficient to evaluate this and other pDNA vaccine candidates in the initial steps of vaccine development.Keywords: Plasmid DNA vaccine, plasmid DNA production, growth medium, flask culture, trace metal optimisation, leishmaniasisAfrican Journal of Biotechnology Vol. 12(31), pp. 4874-488
Development and experimental evaluation of a complete solar thermophotovoltaic system
We present a practical implementation of a solar thermophotovoltaic (TPV) system. The system presented in this paper comprises a sunlight concentrator system, a cylindrical cup-shaped absorber/emitter (made of tungsten coated with HfO2), and an hexagonal-shaped water-cooled TPV generator comprising 24 germanium TPV cells, which is surrounding the cylindrical absorber/emitter. This paper focuses on the development of shingled TPV cell arrays, the characterization of the sunlight concentrator system, the estimation of the temperature achieved by the cylindrical emitters operated under concentrated sunlight, and the evaluation of the full system performance under real outdoor irradiance conditions. From the system characterization, we have measured short-circuit current densities up to 0.95 A/cm2, electric power densities of 67 mW/cm2, and a global conversion efficiency of about 0.8%. To our knowledge, this is the first overall solar-to-electricity efficiency reported for a complete solar thermophotovoltaic system. The very low efficiency is mainly due to the overheating of the cells (up to 120 °C) and to the high optical concentrator losses, which prevent the achievement of the optimum emitter temperature. The loss analysis shows that by improving both aspects, efficiencies above 5% could be achievable in the very short term and efficiencies above 10% could be achieved with further improvements
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