3,489 research outputs found
Generation of High-Energy Photons with Large Orbital Angular Momentum by Compton Backscattering
Usually, photons are described by plane waves with a definite 4-momentum. In
addition to plane-wave photons, "twisted photons" have recently entered the
field of modern laser optics; these are coherent superpositions of plane waves
with a defined projection hbar*m of the orbital angular momentum onto the
propagation axis, where m is integer. In this paper, we show that it is
possible to produce high-energy twisted photons by Compton backscattering of
twisted laser photons off ultra-relativistic electrons. Such photons may be of
interest for experiments related to the excitation and disintegration of atoms
and nuclei, and for studying the photo-effect and pair production off nuclei in
previously unexplored experimental regimes.Comment: 4 pages; RevTe
Neural networks in geophysical applications
Neural networks are increasingly popular in geophysics.
Because they are universal approximators, these
tools can approximate any continuous function with an
arbitrary precision. Hence, they may yield important
contributions to finding solutions to a variety of geophysical applications.
However, knowledge of many methods and techniques
recently developed to increase the performance
and to facilitate the use of neural networks does not seem
to be widespread in the geophysical community. Therefore,
the power of these tools has not yet been explored to
their full extent. In this paper, techniques are described
for faster training, better overall performance, i.e., generalization,and the automatic estimation of network size
and architecture
Investigación pesquera: perspectiva actual, tensiones y aspectos emergentes. El futuro y su aproximación
[EN] The current development of fishery research can be considered as much a technical development as a scientific-conceptual one. In relation to the technical development we analyse the evolution of the concepts fishing effort and fishing power, as well as vulnerability, availability and accessibility. In the conceptual analysis of the basic parameters we consider new contributions with regards to recruitment and evolution of populations based on concepts such as the system’s carrying capacity and the effect of inverse density dependence. The impact of the available space is analysed as well as the effect of the prey-predator relationship in the context of the flows between the different levels in the trophic web. We point out that fishery analysis strategies need to consider that, from both the biological and socio-economic points of view, the system is never balanced but rather is at the very limit or even over the limit. On the whole, fishing (human action on the resource) can be understood within the context of the ecosystem. This situation implies introducing the concept of uncertainty. Aspects such as ecosystem elasticity are analysed in their broadest sense. In these terms, recovery of an ecosystem and of Large Marine Ecosystems (LME) is still possible, but the result can be different due to the appearance of opportunistic species. Some concepts such as fuzzy sets, and chaos and fractal analysis are important tools for analysing the evolution and management of ecosystems exploited by fisheries[ES] El desarrollo actual de la ciencia de las pesquerías es considerado tanto como desarrollo técnico como desarrollo científico-conceptual. En el primer aspecto se analiza la evolución de los conceptos: esfuerzo de pesca y poder de pesca. Así como la vulnerabilidad disponibilidad y accesibilidad. Se considera el análisis conceptual de los parámetros básicos con nuevas aportaciones sobre el reclutamiento y la evolución de las poblaciones a partir de conceptos como la capacidad de carga del sistema y el efecto de la densodependencia inversa. Se analiza el impacto del espacio disponible así como el efecto de la relación presa-depredador en el contexto de los flujos entre los diversos niveles de la relación trófica. La estrategia se sitúa considerando que tanto desde el punto de vista biológico como socieconómico el sistema no está nunca en equilibrio sino en el límite o fuera del mismo. En conjunto la pesca –acción del hombre sobre el recurso– se sitúa en el contexto del ecosistema. Esta situación implica introducir situaciones de incertidumbre. Se analizan aspectos como la elasticidad de los ecosistemas en su aspecto amplio. En estos términos la recomposición de un ecosistema y también de Large Marine Ecosystems (LME) es posible, pero el resultado puede ser diferente –aparición de especies oportunistas. Algunos conceptos como los conjuntos borrosos, el análisis caótico y la fractalidad son instrumentos importantes en el análisis de la evolución y el control de los ecosistemas depredados por la pescaPeer reviewe
The FIR-absorption of short period quantum wires and the transition from one to two dimensions
We investigate the FIR-absorption of short period parallel quantum wires in a
perpendicular quantizing magnetic field. The external time-dependent electric
field is linearly polarized along the wire modulation. The mutual Coulomb
interaction of the electrons is treated self-consistently in the ground state
and in the absorption calculation within the Hartree approximation. We consider
the effects of a metal gate grating coupler, with the same or with a different
period as the wire modulation, on the absorption. The evolution of the
magnetoplasmon in the nonlocal region where it is split into several Bernstein
modes is discussed in the transition from: narrow to broad wires, and isolated
to overlapping wires. We show that in the case of narrow and not strongly
modulated wires the absorption can be directly correlated with the underlying
electronic bandstructure.Comment: 15 pages, 9 figures, Revtex, to appear in Phys. Rev.
Improved Limits on Spin-Dependent WIMP-Proton Interactions from a Two Liter CFI Bubble Chamber
Data from the operation of a bubble chamber filled with 3.5 kg of CFI
in a shallow underground site are reported. An analysis of ultrasound signals
accompanying bubble nucleations confirms that alpha decays generate a
significantly louder acoustic emission than single nuclear recoils, leading to
an efficient background discrimination. Three dark matter candidate events were
observed during an effective exposure of 28.1 kg-day, consistent with a neutron
background. This observation provides the strongest direct detection constraint
to date on WIMP-proton spin-dependent scattering for WIMP masses
GeV/c.Comment: 4 pages, 4 figures V2 submitted to match journal versio
Профессор С. Д. Заверткин (1951-2013)
22 мая 2013 г. ушел из жизни доктор геолого-минералогических наук, кандидат физико-математических наук, профессор кафедры высшей математики Физико-технического института Томского политехнического университета Сергей Дмитриевич Заверткин
Crotonaldehyde hydrogenation on Rh supported catalysts
The vapor-phase hydrogenation of crotonaldehyde on Rh supported catalysts has been studied. The effect of some variables of preparation in catalysts prepared by the sol-gel and impregnation methods on the surface and catalytic properties were analyzed. It was found, that the porosity of the support has a small effect on the selectivity to the unsaturated alcohol and the presence of partially reducible supports such as ZrO2 and TiO2, may increase the selectivity to crotyl alcohol via an enhancement of the polarization of the C=O bond as a result of surface decoration of the metal component. The presence of chlorine ions in the metal-support interfacial region, also allows, in a certain extension, a polarization of the carbonyl bond.Se estudió la hidrogenación en fase vapor de crotonaldehído sobre catalizadores de Rh soportados. Se analizó la influencia de diferentes variables de preparación de catalizadores obtenidos por el método sol-gel y por impregnación. Se encontró que la porosidad del soporte tiene sólo un pequeño efecto sobre la selectividad hacia el alcohol insaturado y que la presencia de soportes parcialmente reducibles como ZrO2 y TiO2 pueden incrementar la selectividad hacia el alcohol crotílico a través de un aumento de la polarización del enlace C=O como consecuencia de una decoración superficial del componente metálico. Adicionalmente, la presencia de iones cloruro en la interfase metal-soporte, también contribuyen en alguna medida a la polarización del enlace carbonílico.The authors thank CONICYT (Chile, FONDECYT Grants 1980345 and 2990065 and CSIC-CONICYT Collaboration Program) for their financial support.Peer reviewe
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