990 resultados para Gregorio X, Papa-


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Nonlinear Thomson backscattering of an intense Gaussian laser pulse by a counterpropagating energetic electron is investigated by numerically solving the electron equation of motion taking into account the radiative damping force. The backscattered radiation characteristics are different for linearly and circularly polarized lasers because of a difference in their ponderomotive forces acting on the electron. The radiative electron energy loss weakens the backscattered power, breaks the symmetry of the backscattered-pulse profile, and prolongs the duration of the backscattered radiation. With the circularly polarized laser, an adjustable double-peaked backscattered pulse can be obtained. Such a profile has potential applications as a subfemtosecond x-ray pump and probe with adjustable time delay and power ratio. (c) 2006 American Institute of Physics.

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Linear Thomson scattering of a short pulse laser by relativistic electron lids been investigated using computer simulations. It is shown that scattering of an intense laser pulse of similar to 33 fs full width at half maximum, with an electron of gamma(o) = 10 initial energy, generates an ultrashort, pulsed radiation of 76 attoseconds, with a photon wavelength of 2.5 nm in the backward direction. The scattered radiation generated by a highly relativistic electron has superior quality in terms of its pulse width and angular distribution in comparison to the one generated by lower relativistic energy electron.

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An analytical fluid model for JxB heating during the normal incidence by a short ultraintense linearly polarized laser on a solid-density plasma is proposed. The steepening of an originally smooth electron density profile as the electrons are pushed inward by the laser is included self-consistently. It is shown that the JxB heating includes two distinct coupling processes depending on the initial laser and plasma conditions: for a moderate intensity (a <= 1), the ponderomotive force of the laser light can drive a large plasma wave at the point n(e)=4 gamma(0)n(c) resonantly. When this plasma wave is damped, the energy is transferred to the plasma. At higher intensity, the electron density is steepened to a high level by the time-independent ponderomotive force, n(e)> 4 gamma(0)n(c), so that no 2 omega resonance will occur, but the longitudinal component of the oscillating ponderomotive field can lead to an absorption mechanism similar to "vacuum heating." (c) 2006 American Institute of Physics.

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XWolter100cm^3

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Theoretical analyses of x-ray diffraction phase contrast imaging and near field phase retrieval method are presented. A new variant of the near field intensity distribution is derived with the optimal phase imaging distance and spatial frequency of object taken into account. Numerical examples of phase retrieval using simulated data are also given. On the above basis, the influence of detecting distance and polychroism of radiation on the phase contrast image and the retrieved phase distribution are discussed. The present results should be useful in the practical application of in-line phase contrast imaging.

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X()X008860nm,X,X

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XCCDX

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Linear Thomson scattering by a relativistic electron of a short pulse laser has been investigated by computer simulation. Under a laser field with a pulse of 33.3-fs full-width at half-maximum, and the initial energy of an electron of gamma(0) = 10, the motion of the electron is relativistic and generates an ultrashort radiation of 76-as with a photon wave length of 2.5-nm in the backward scattering. The radiation under a high relativistic energy electron has better characteristic than under a low relativistic energy electron in terms of the pulse width and the angular distribution. (c) 2005 Elsevier GrnbH. All rights reserved.

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X

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XX

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XX

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Nonlinear X-wave formation at different pulse powers in water is simulated using the standard model of nonlinear Schrodinger equation (NLSE). It is shown that in near field X-shape originally emerges from the interplay between radial diffraction and optical Kerr effect. At relatively low power group-velocity dispersion (GVD) arrests the collapse and leads to pulse splitting on axis. With high enough power, multi-photon ionization (NIPI) and multi-photon absorption (MPA) play great importance in arresting the collapse. The tailing part of pulse is first defocused by MPI and then refocuses. Pulse splitting on axis is a manifestation of this process. Double X-wave forms when the split sub-pulses are self-focusing. In the far field, the character of the central X structure of conical emission (CE) is directly related to the single or double X-shape in the near field. (c) 2007 Elsevier B.V. All rights reserved.

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A fim de se encontrar as evidncias sobre a ocorrncia de catarata em pacientes submetidos a PUVA terapia, realizou-se uma reviso sistemtica da literatura publicada, utilizando-se as bases de dados MEDLINE, LILACS, Cochrane Library, Excerpta Medica e o programa MICROMEDEX. Foram elaboradas estratgias de busca para cada base de dados. Aps aplicao dos critrios de seleo definidos, incluram-se 30 artigos que representavam 21 estudos que abordavam o tema. Os dados de cada estudo foram recolhidos a partir de um formulrio de coleta criado para este fim. A avaliao da qualidade dos estudos se deu por classificao das evidncias em categorias e posterior relao destas categorias com a fora de recomendao das evidncias para a prtica clnica, de acordo com critrios previamente publicados. Ainda, elaborou-se um esquema de classificao de qualidade especfico para os estudos includos. Realizou-se anlise estatstica dos dados apresentados atravs de anlise de sobrevida, utilizando-se o modelo de Cox de hazards proporcionais. As evidncias publicadas das bases de dados, capturadas pelas estratgias de busca e selecionadas pelos critrios de incluso e excluso no so suficientes para se afirmar se a catarata pode ou no ser uma reao adversa a PUVA terapia. No foi possvel, tambm, encontrar um modelo matemtico que pudesse indicar relao entre as variveis explicativas e a funo de hazards.

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Com a introduo do flor como o principal agente anticariognico e, talvez, um aumento do flor na nossa cadeia alimentar, a fluorose dentria tornou-se um problema mundial. Os mecanismos que conduzem formao do esmalte fluortico so desconhecidos, mas devem envolver modificaes nas reaes fsico-qumicas bsicas de desmineralizao e remineralizao do esmalte dentrio. O aumento daquantidade de flor no cristal apatita resulta no aumento dos parmetros de rede. O objetivo deste trabalho caracterizar o esmalte dentrio humano saudvel e fluortico usando difrao de raios X com luz sncrotron. Todos os perfis de espalhamento foram medidos na linha de difrao de raios X (XRD1) do Laboratrio Nacional de Luz Sncrotron, Campinas SP. Os experimentos foram realizados usando amostras em p e em lminas polidas. As amostras em p foram analisadas a fim de obter a caracterizao do esmalte dentrio saudvel. As lminas foram analisadas em reas do esmalte especficas identificadas como fluorticas. Todos os perfis foram comparados com amostras de esmalte de controle e tambm com a literatura. A evidente similaridade entre os perfis de difrao mostraram a analogia entre as estruturas do esmalte dentrio e a hidroxiapatita padro. Fica evidente que os perfis de difrao do esmalte dentrio das amostras em lmina so diferentes daqueles obtidos para o esmalte em p. As diferenas encontradas incluem variao na cristalinidade e orientao preferencial. Os valores encontrados para as distncias interplanares para o esmalte de controle e fluortico das amostras em lmina no apresentaram diferenas estatisticamente significativas. Isto pode ser explicado pelo fato que a hidroxiapatita e a fluoropatita formam cristais com a mesma estrutura hexagonal, mesmo grupo de simetria e tm parmetros de rede muito prximos, os quais a habilidade do sistema no foi suficiente para resolver. Finalmente, este trabalho mostra que a difrao de raios X usando radiao sncrotron uma tcnica poderosa para o estudo da cristalografia e microestrutura do esmalte dentrio e, ainda, pode ser igualmente aplicada no estudo de outros tecidos biolgicos duros e de biomateriais sintticos.