1,139 research outputs found
Lifetimes of 26Al and 34Cl in an astrophysical plasma
We study here the onset of thermal equilibrium affecting the lifetimes of
26Al and 34Cl nuclei within a hot astrophysical photon gas. The 26Al isotope is
of prime interest for gamma ray astronomy with the observation of its delayed
(t_1\2=0.74 My) 1.809MeV gamma-ray line. Its nucleosynthesis is complicated by
the presence of a short lived (t_1\2=6.34s) spin isomer. A similar
configuration is found in 34Cl where the decay of its isomer (34mCl, t_1\2=32m)
is followed by delayed gamma-ray emission with characteristic energies. The
lifetimes of such nuclei are reduced at high temperature by the thermal
population of shorter lived levels. However, thermal equilibrium within 26Al
and 34Cl levels is delayed by the presence of the isomer. We study here the
transition to thermal equilibrium where branching ratios for radiative
transitions are needed in order to calculate lifetimes. Since some of these
very small branching ratios are not known experimentally, we use results of
shell model calculations.Comment: 11 pages, 5 figures, Latex, accepted for publication in Phys. Rev.
Experimental study of the cyclic visco-elasto-plastic behaviour of a polyamide fibre strap
WOSInternational audienceExperimental tensile tests were performed on polyamide-based (PA66) woven strap samples. A strain measuring device was used to measure the strain in the middle and effective part of the woven tensile sample. The tests were performed, on the one hand under monotonous tension at different strain rates and on the other hand under sophisticated cyclic loading histories, including relaxation and creep sequences. The analysis of experimental results was made through a visco-elasto-hysteresis model, based on the superimposition of three stress components. The proposed method allows for characterizing the steady state viscous stress as a function of strain and strain rate, the time-independent irreversible behaviour and the instantaneous modulus increasing with the strain. Based on the visco-elasto-hysteresis model, an analysis enabled us to understand and predict the change in relaxation and creep orientations during complex loading histories
Backbending in 50Cr
The collective yrast band and the high spin states of the nucleus 50Cr are
studied using the spherical shell model and the HFB method. The two
descriptions lead to nearly the same values for the relevant observables. A
first backbending is predicted at I=10\hbar corresponding to a collective to
non-collective transition. At I=16\hbar a second backbending occurs, associated
to a configuration change that can also be interpreted as an spherical to
triaxial transition.Comment: ReVTeX v 3.0 epsf.sty, 5 pages, 5 figures included. Full Postscript
version available at http://www.ft.uam.es/~gabriel/Cr50art.ps.g
Effect of culture in simulated microgravity on the development of mouse embryonic testes
BACKGROUND All known organisms develop and evolve in the presence of gravitational force, and it is evident that gravity has a significant influence on organism physiology and development. Microgravity is known to affect gene expression, enzyme activity, cytoskeleton organization, mitotic proliferation and intracellular signaling.
OBJECTIVES:
The aim of the present study was to study some aspects of the development in vitro of mouse embryonic testes in simulated microgravity.
MATERIAL AND METHODS:
Testes from mouse embryos (12.5-16.5 days post coitum, d.p.c.) were cultured in simulated microgravity and standard static culture conditions. The microgravity condition was provided by a Rotary Cell Culture System (RWV) bioreactor, an apparatus designated for 3D tissue and small organ cultures. After 48 h of the culture in the RWV, testis morphology and size was evaluated.
RESULTS:
The first observation was that the culture in the RWV bioreactor had a beneficial effect on the testis growth and on the survival of germ cells in comparison to static 2D culture methods. Moreover, we found, that RWV culture caused disorganization the gonadal tissues, namely of the testis cords.
CONCLUSIONS:
The results suggest that the maintenance of testis cord could be sensitive to microgravity. We hypothesize that while the effect on testis growth is due to a better nutrient and oxygen supply, the testis cord's disorganization might depend on the microgravity conditions simulated by the bioreactor. Considering the complexity of the processes involved in the formation of the testis cords and their dynamic changes during the embryo fetal period, further studies are needed to identify the causes of such effect
The method of fundamental solutions for problems in static thermo-elasticity with incomplete boundary data
An inverse problem in static thermo-elasticity is investigated. The aim is to reconstruct the unspecified boundary data, as well as the temperature and displacement inside a body from over-specified boundary data measured on an accessible portion of its boundary. The problem is linear but ill-posed. The uniqueness of the solution is established but the continuous dependence on the input data is violated. In order to reconstruct a stable and accurate solution, the method of fundamental solutions is combined with Tikhonov regularization where the regularization parameter is selected based on the L-curve criterion. Numerical results are presented in both two and three dimensions showing the feasibility and ease of implementation of the proposed technique
Hidden breakpoints in genome alignments
During the course of evolution, an organism's genome can undergo changes that
affect the large-scale structure of the genome. These changes include gene
gain, loss, duplication, chromosome fusion, fission, and rearrangement. When
gene gain and loss occurs in addition to other types of rearrangement,
breakpoints of rearrangement can exist that are only detectable by comparison
of three or more genomes. An arbitrarily large number of these "hidden"
breakpoints can exist among genomes that exhibit no rearrangements in pairwise
comparisons.
We present an extension of the multichromosomal breakpoint median problem to
genomes that have undergone gene gain and loss. We then demonstrate that the
median distance among three genomes can be used to calculate a lower bound on
the number of hidden breakpoints present. We provide an implementation of this
calculation including the median distance, along with some practical
improvements on the time complexity of the underlying algorithm.
We apply our approach to measure the abundance of hidden breakpoints in
simulated data sets under a wide range of evolutionary scenarios. We
demonstrate that in simulations the hidden breakpoint counts depend strongly on
relative rates of inversion and gene gain/loss. Finally we apply current
multiple genome aligners to the simulated genomes, and show that all aligners
introduce a high degree of error in hidden breakpoint counts, and that this
error grows with evolutionary distance in the simulation. Our results suggest
that hidden breakpoint error may be pervasive in genome alignments.Comment: 13 pages, 4 figure
Caractérisation du comportement cyclique d'un matériau tissé en traction ondulée
Des résultats expérimentaux sur un matériau tissé en fils de polyamide 6-6 (PA66) sont présentés. Les essais ont été effectués d'une part en traction monotone à différentes vitesses de déformation et d'autre part en sollicitations cycliques sophistiqués de type traction ondulée avec des séquences de fluage ou de relaxation
A gauge theoretic approach to elasticity with microrotations
We formulate elasticity theory with microrotations using the framework of
gauge theories, which has been developed and successfully applied in various
areas of gravitation and cosmology. Following this approach, we demonstrate the
existence of particle-like solutions. Mathematically this is due to the fact
that our equations of motion are of Sine-Gordon type and thus have soliton type
solutions. Similar to Skyrmions and Kinks in classical field theory, we can
show explicitly that these solutions have a topological origin.Comment: 15 pages, 1 figure; revised and extended version, one extra page;
revised and extended versio
Dynamics of thermoelastic thin plates: A comparison of four theories
Four distinct theories describing the flexural motion of thermoelastic thin
plates are compared. The theories are due to Chadwick, Lagnese and Lions,
Simmonds, and Norris. Chadwick's theory requires a 3D spatial equation for the
temperature but is considered the most accurate as the others are derivable
from it by different approximations. Attention is given to the damping of
flexural waves. Analytical and quantitative comparisons indicate that the
Lagnese and Lions model with a 2D temperature equation captures the essential
features of the thermoelastic damping, but contains systematic inaccuracies.
These are attributable to the approximation for the first moment of the
temperature used in deriving the Lagnese and Lions equation. Simmonds' model
with an explicit formula for temperature in terms of plate deflection is the
simplest of all but is accurate only at low frequency, where the damping is
linearly proportional to the frequency. It is shown that the Norris model,
which is almost as simple as Simmond's, is as accurate as the more precise but
involved theory of Chadwick.Comment: 2 figures, 1 tabl
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