732 research outputs found
The effect of pressure on statics, dynamics and stability of multielectron bubbles
The effect of pressure and negative pressure on the modes of oscillation of a
multi-electron bubble in liquid helium is calculated. Already at low pressures
of the order of 10-100 mbar, these effects are found to significantly modify
the frequencies of oscillation of the bubble. Stabilization of the bubble is
shown to occur in the presence of a small negative pressure, which expands the
bubble radius. Above a threshold negative pressure, the bubble is unstable.Comment: 4 pages, 2 figures, accepted for publication in Physical Review
Letter
MODELO BIDIMENSIONAL DE RIESGOS DEL MANTENIMIENTO DE SISTEMAS INTEGRADOS DE GESTIÓN (ERP)
La adopción y expansión de las TecnologÃas de la Información y la Comunicación en el ámbito empresarial se está produciendo a gran velocidad. De la mano de las más innovadoras TIC y de los sistemas informáticos, surgen y se desarrollan los sistemas ERP. Éstos han sido implantados por empresas de todo el mundo. Tras su implantación, comienza su mantenimiento. Para que el resultado de estos proyectos sea satisfactorio, los riesgos que lo afectan tienen que ser gestionados. Una pobre gestión de estos riesgos, con frecuencia origina fallos en el sistema, lo que hace que las compañÃas tengan que asumir altas pérdidas. Para gestionar adecuadamente los riesgos, los profesionales deben comenzar identificándolos y clasificándolos. Para apoyar su labor, hemos realizado un estudio formal de los riesgos que afectan al mantenimiento de ERPs. La investigación finaliza con la elaboración de un Modelo de dos dimensiones compuesto por los riesgos identificados en la literatura
SÃndrome de TAR y estado de heterocigosis para anemia falciforme
Resumen Presentación de un paciente con el cuadro clÃnico del sÃndrome de TAR, quien presenta además un estado de heterocigosis para anemia falciforme. El sÃndrome de TAR, descrito por primera vez en 1959 por Shaw y Oliver, es un trastorno genético no común que ocurre con una prevalencia aproximada de 1: 500.000 a 1: 1.000.000 de nacidos vivos. En 1969 Hall y col. delinearon los criterios diagnósticos de este sÃndrome que incluyen ausencia bilateral del radio con presencia de ambos pulgares y trombocitopenia. Otras anomalÃas descritas en menor frecuencia son: Alteraciones óseas del húmero y cúbito, en los casos severos, focomelia, mal- formaciones de expresión variable en extremidades inferiores, malformaciones cardiacas e intolerancia a la leche de vaca (1, 2). Los hallazgos encontrados en este caso se comparan con los de la literatura y se discute su posible etiologÃa, además de resaltar la importancia de realizar un diagnóstico y tratamiento precoz y preciso especialmente de la trombocitopenia. Palabras clave: Malformaciones congénitas, aplasia, hipoplasia, trombocitopenia, anemia falciforme. Abstract This paper shows a patient with clinical features of the TAR syndrome, who presents heterozigosis state for sickle cell anemia. The TAR syndrome was described by Shaw and Oliver in 1959, it is a rare genetic disease, occurring with an approximate prevalence of 1 case in 500.000 to 1 million births. Hall et al. described the current diagnostic criteria for TAR syndrome in 1969. These include bilateral absence of the radii in the presence of both thumbs and trombocytopenia. Other anomalies described included ulnar or humeral defects and the most severe cases phocomelia, lower limb involvement is variable, cadiac anomalies and cow’s milk intolerance (1,2) In this paper, we compare the findings of the literature review, discuss the ethiology and the importance of a diagnostic and treatment specially for trombocytopenia Key words: Congenital malformations, aplasia, hypoplasia, trombocytopenia, sickle cell anemia
Atomic Deuterium Adsorbed on the Surface of Liquid Helium
We investigate deuterium atoms adsorbed on the surface of liquid helium in
equilibrium with a vapor of atoms of the same species. These atoms are studied
by a sensitive optical method based on spectroscopy at a wavelength of 122 nm,
exciting the 1S-2P transition. We present a direct measurement of the
adsorption energy of deuterium atoms on helium and show evidence for the
existence of resonantly enhanced recombination of atoms residing on the surface
to molecules.Comment: 6 pages 4 figure
UCN Upscattering rates in a molecular deuterium crystal
A calculation of ultra-cold neutron (UCN) upscattering rates in molecular
deuterium solids has been carried out, taking into account intra-molecular
exictations and phonons. The different moelcular species ortho-D2 (with even
rotational quantum number J) and para-D2 (with odd J) exhibit significantly
different UCN-phonon annihilation cross-sections. Para- to ortho-D2 conversion,
furthermore, couples UCN to an energy bath of excited rotational states without
mediating phonons. This anomalous upscattering mechanism restricts the UCN
lifetime to 4.6 msec in a normal-D2 solid with 33% para content.Comment: 3 pages, one figur
Limits to Sympathetic Evaporative Cooling of a Two-Component Fermi Gas
We find a limit cycle in a quasi-equilibrium model of evaporative cooling of
a two-component fermion gas. The existence of such a limit cycle represents an
obstruction to reaching the quantum ground state evaporatively. We show that
evaporatively the \beta\mu ~ 1. We speculate that one may be able to cool an
atomic fermi gas further by photoassociating dimers near the bottom of the
fermi sea.Comment: Submitted to Phys. Rev
An Apparatus to Control and Monitor the Para-D2 Concentration in a Solid Deuterium, Superthermal Source of Ultra-cold Neutrons
Controlling and measuring the concentration of para-D2 is an essential step
toward realizing solid deuterium as an intense ultra-cold neutron (UCN) source.
To this end, we implemented an experimental technique to convert para- to
ortho-deuterium molecules by flowing D2 gas through a cryogenic cell filled
with paramagnetic hydrous ferric oxide granules. This process efficiently
reduced the para-D2 concentration from 33.3% to 1.5%. Rotational Raman
spectroscopy was applied to measure the residual para-D2 contamination to
better than 2 parts in 10^3, and the hydrogen contamination to 1 part in 10^3.
We also contrast our optical technique to conventional thermal conductivity
measurements of the para-D2 concentration, reporting some of the relevant
strengths and weaknesses of our implementation of each technique.Comment: accepted for publication in NIM
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X-Ray Study of Pressure-Collapsed Fullerite
X-ray-diffraction studies are described for a new phase of carbon called collapsed fullerite (CF) that was produced by application of high pressure to fullerite (C). At 20 GPa there is an irreversible transition to a phase that has neither the (111) Bragg peak of diamond nor any of the Bragg peaks associated with the fcc phase of C. The spectrum of CF is flat and featureless in the range of study.Engineering and Applied Science
Continuous and semicontinuous reaction systems for high-solids enzymatic hydrolysis of lignocellulosics
An attractive operation strategy for the enzymatic hydrolysis of lignocellulosics results from dividing the process into three stages with complementary goals: continuous enzyme adsorption at low-solids loading (5% w/w) with recycling of the liquid phase; continuous liquefaction at high-solids content (up to 20% w/w); and, finally, continuous or semicontinuous hydrolysis with supplementation of fresh enzymes. This paper presents a detailed modeling and simulation framework for the aforementioned operation strategies. The limiting micromixing situations of macrofluid and microfluid are used to predict conversions. The adsorption and liquefaction stages are modeled as a continuous stirred tank and a plug flow reactor, respectively. Two alternatives for the third stage are studied: a train of five cascading stirred tanks and a battery of batch reactors in parallel. Simulation results show that glucose concentrations greater than 100 g L-1 could be reached with both of the alternatives for the third stage324805819sem informaçãosem informaçã
Static Properties of Trapped Bose-Fermi Mixed Condensate of Alkali Atoms
Static properties of a bose-fermi mixture of trapped potassium atoms are
studied in terms of coupled Gross-Pitaevskii and Thomas-Fermi equations for
both repulsive and attractive bose-fermi interatomic potentials. Qualitative
estimates are given for solutions of the coupled equations, and the parameter
regions are obtained analytically for the boson-density profile change and for
the boson/fermion phase separation. Especially, the parameter ratio
is found that discriminates the region of the large boson-profile change. These
estimates are applied for numerical results for the potassium atoms and checked
their consistency. It is suggested that a small fraction of fermions could be
trapped without an external potential for the system with an attractive
boson-fermion interaction.Comment: 8 pages,5 figure
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