Proton Radiography of a Laser-Driven Implosion

Protons accelerated by a picosecond laser pulse have been used to radiograph a 500 mu m diameter capsule, imploded with 300 J of laser light in 6 symmetrically incident beams of wavelength 1.054 mu m and pulse length 1 ns. Point projection proton backlighting was used to characterize the density gradients at discrete times through the implosion. Asymmetries were diagnosed both during the early and stagnation stages of the implosion. Comparison with analytic scattering theory and simple Monte Carlo simulations were consistent with a 3 +/- 1 g/cm(3) core with diameter 85 +/- 10 mu m. Scaling simulations show that protons > 50 MeV are required to diagnose asymmetry in ignition scale conditions.

Dynamical simulation of inelastic quantum transport

A method for correlated quantum electron-ion dynamics is combined with a method for electronic open boundaries to simulate in real time the heating, and eventual equilibration at an elevated vibrational energy, of a quantum ion under current flow in an atomic wire, together with the response of the current to the ionic heating. The method can also be used to extract inelastic current voltage corrections under steady-state conditions. However, in its present form the open-boundary method contains an approximation that limits the resolution of current-voltage features. The results of the simulations are tested against analytical results from scattering theory. Directions for the improvement of the method are summarized at the end.

Optical Cooling Using the Dipole Force

The term `laser cooling' is applied to the use of optical means to cool the motional energies of either atoms and molecules, or micromirrors. In the literature, these two strands are kept largely separate; both, however suffer from severe limitations. Laser cooling of atoms and molecules largely relies on the internal level structure of the species being cooled. As a result, only a small number of elements and a tiny number of molecules can be cooled this way. In the case of micromirrors, the problem lies in the engineering of micromirrors that need to satisfy a large number of constraints---these include a high mechanical Q-factor, high reflectivity and very good optical quality, weak coupling to the substrate, etc.---in order to enable efficient cooling. During the course of this thesis, I will draw these two sides of laser cooling closer together by means of a single, generically applicable scattering theory that can be used to explain the interaction between light and matter at a very general level. I use this `transfer matrix' formalism to explore the use of the retarded dipole--dipole interaction as a means of both enhancing the efficiency of micromirror cooling systems and rendering the laser cooling of atoms and molecules less species selective. In particular, I identify the `external cavity cooling' mechanism, whereby the use of an optical memory in the form of a resonant element (such as a cavity), outside which the object to be cooled sits, can potentially lead to the construction of fully integrated optomechanical systems and even two-dimensional arrays of translationally cold atoms, molecules or even micromirrors.

Electron-induced hydrogen loss in uracil in a water cluster environment

Low-energy electron-impact hydrogen loss due to dissociative electron attachment (DEA) to the uracil and thymine molecules in a water cluster environment is investigated theoretically. Only the A'-resonance contribution, describing the near-threshold behavior of DEA, is incorporated. Calculations are based on the nonlocal complex potential theory and the multiple scattering theory, and are performed for a model target with basic properties of uracil and thymine, surrounded by five water molecules. The DEA cross section is strongly enhanced when the attaching molecule is embedded in a water cluster. This growth is due to two effects: the increase of the resonance lifetime and the negative shift in the resonance position due to interaction of the intermediate negative ion with the surrounding water molecules. A similar effect was earlier found in DEA to chlorofluorocarbons. 

An experimental investigation of dechanneling in copper single crystals

This investigation comprises a comparison of experimental and theoretical dechanneling of MeV protons in copper single crystals. Dechanneling results when an ion's transverse energy increases to the value where the ion can undergo small impact parameter collisions with individual atoms. Depth dependent dechanneling rates were determined as functions of lattice temperature, ion beam energy and crystal axis orientation. Ion beam energies were IMeV and 2MeV,temperatures ranged from 35 K to 280 K and the experiment was carried out along both the (lOa) and <110) axes. Experimental data took the form of aligned and random Rutherford backscattered energy spectra. Dechanneling rates were extracted from these spectra using a single scattering theory that took explicit account of the different stopping powers experienced by channeled and dechanneled ions and also included a correction factor to take into account multiple scattering effects along the ion's trajectory. The assumption of statistical equilibrium and small angle scattering of the channeled ions allows a description of dechanneling in terms of the solution of a diffusion like equation which contains a so called diffusion function. The diffusion function is shown to be related to the increase in average transverse energy. Theoretical treatments of increase in average transverse energy due to collisions of projectiles with channel electrons and thermal perturbations in the lattice potential are reviewed. Using the diffusion equation and the electron density in the channel centre as a fitting parameter dechanneling rates are extracted. Excellent agreement between theory and experiment has been demonstrated. Electron densities determined in the fitting procedure appear to be realistic. The surface parameters show themselves to be good indicators of the quality of the crystal.

Relevé polarimétrique d'étoiles candidates pour des disques de débris

Le relevé DEBRIS est effectué par le télescope spatial Herschel. Il permet d’échantillonner les disques de débris autour d’étoiles de l’environnement solaire. Dans la première partie de ce mémoire, un relevé polarimétrique de 108 étoiles des candidates de DEBRIS est présenté. Utilisant le polarimètre de l’Observatoire du Mont-Mégantic, des observations ont été effectuées afin de détecter la polarisation due à la présence de disques de débris. En raison d’un faible taux de détection d’étoiles polarisées, une analyse statistique a été réalisée dans le but de comparer la polarisation d’étoiles possédant un excès dans l’infrarouge et la polarisation de celles n’en possédant pas. Utilisant la théorie de diffusion de Mie, un modèle a été construit afin de prédire la polarisation due à un disque de débris. Les résultats du modèle sont cohérents avec les observations. La deuxième partie de ce mémoire présente des tests optiques du polarimètre POL-2, construit à l’Université de Montréal. L’imageur du télescope James-Clerk-Maxwell passe de l’instrument SCUBA à l’instrument SCUBA-2, qui sera au moins cent fois plus rapide que son prédécesseur. De même, le polarimètre suit l’amélioration et un nouveau polarimètre, POL-2, a été installé sur SCUBA-2 en juillet 2010. Afin de vérifier les performances optiques de POL-2, des tests ont été exécutés dans les laboratoires sub-millimétriques de l’Université de Western Ontario en juin 2009 et de l’Université de Lethbridge en septembre 2009. Ces tests et leurs implications pour les observations futures sont discutés.

Size-dependent surface plasmon resonance in silver silica nanocomposites

Silver silica nanocomposites were obtained by the sol–gel technique using tetraethyl orthosilicate (TEOS) and silver nitrate (AgNO3) as precursors. The silver nitrate concentration was varied for obtaining composites with different nanoparticle sizes. The structural and microstructural properties were determined by x-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopic (XPS) studies were done for determining the chemical states of silver in the silica matrix. For the lowest AgNO3 concentration, monodispersed and spherical Ag crystallites, with an average diameter of 5 nm, were obtained. Grain growth and an increase in size distribution was observed for higher concentrations. The occurrence of surface plasmon resonance (SPR) bands and their evolution in the size range 5–10 nm is studied. For decreasing nanoparticle size, a redshift and broadening of the plasmon-related absorption peak was observed. The observed redshift and broadening of the SPR band was explained using modified Mie scattering theory

A multiwave range test for obstacle reconstructions with unknown physical properties

We develop a new multiwave version of the range test for shape reconstruction in inverse scattering theory. The range test [R. Potthast, et al., A ‘range test’ for determining scatterers with unknown physical properties, Inverse Problems 19(3) (2003) 533–547] has originally been proposed to obtain knowledge about an unknown scatterer when the far field pattern for only one plane wave is given. Here, we extend the method to the case of multiple waves and show that the full shape of the unknown scatterer can be reconstructed. We further will clarify the relation between the range test methods, the potential method [A. Kirsch, R. Kress, On an integral equation of the first kind in inverse acoustic scattering, in: Inverse Problems (Oberwolfach, 1986), Internationale Schriftenreihe zur Numerischen Mathematik, vol. 77, Birkhäuser, Basel, 1986, pp. 93–102] and the singular sources method [R. Potthast, Point sources and multipoles in inverse scattering theory, Habilitation Thesis, Göttingen, 1999]. In particular, we propose a new version of the Kirsch–Kress method using the range test and a new approach to the singular sources method based on the range test and potential method. Numerical examples of reconstructions for all four methods are provided.

Femtosecond light distribution at skin and liver of rats: analysis for use in optical diagnostics

In this study we investigated the light distribution under femtosecond laser illumination and its correlation with the collected diffuse scattering at the surface of ex-vivo rat skin and liver. The reduced scattering coefficients mu`s for liver and skin due to different scatterers have been determined with Mie-scattering theory for each wavelength (800, 630, and 490 nm). Absorption coefficients mu(a) were determined by diffusion approximation equation in correlation with measured diffused reflectance experimentally for each wavelength (800, 630, and 490 nm). The total attenuation coefficient for each wavelength and type of tissue were determined by linearly fitting the log based normalized intensity. Both tissues are strongly scattering thick tissues. Our results may be relevant when considering the use of femtosecond laser illumination as an optical diagnostic tool. [GRAPHICS] A typical sample of skin exposed to 630 nm laser light (C) 2010 by Astro Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA

Recursive renormalization of the singlet one-pion-exchange plus point-like interactions

The subtracted kernel approach is shown to be a powerful method to be implemented recursively in scattering equations with regular plus point-like interactions. The advantages of the method allows one to recursively renormalize the potentials, with higher derivatives of the Dirac-delta, improving previous results. The applicability of the method is verified in the calculation of the 1 So nucleon-nucleon phase-shifts, when considering a potential with one-pion-exchange plus a contact interaction and its derivatives. The S-1(0) renormalization parameters are fitted to the data. The method can in principle be extended to any derivative order of the contact interaction, to higher partial waves and to coupled channels. (c) 2005 Elsevier B.V. All rights reserved.

Renormalization in few-body nuclear physics

Renormalized fixed-point Hamiltonians are formulated for systems described by interactions that originally contain point-like singularities (as the Dirac-delta and/or its derivatives). They express the renormalization group invariance of quantum mechanics. The present approach for the renormalization scheme relies on a subtracted T-matrix equation.

Renormalization group invariance of quantum mechanics

We propose a framework to renormalize the nonrelativistic quantum mechanics with arbitrary singular interactions. The scattering equation is written to have one or more subtraction in the kernel at a given energy scale. The scattering amplitude is the solution of a nth order derivative equation in respect to the renormalization scale, which is the nonrelativistic counterpart of the Callan-Symanzik formalism, Scaled running potentials for the subtracted equations keep the physics invariant fur a sliding subtraction point. An example of a singular potential, that requires more than one subtraction to renormalize the theory is shown. (C) 2000 Published by Elsevier B.V. B.V. All rights reserved.

Trajectory of neutron-neutron-C-18 excited three-body state

The trajectory of the first excited Efimov state is investigated by using a renormalized zero-range three-body model for a system with two bound and one virtual two-body subsystems. The approach is applied to n-n-C-18, where the n-n virtual energy and the three-body ground state are kept fixed. It is shown that such three-body excited state goes from a bound to a virtual state when the n-C-18 binding energy is increased. Results obtained for the n-C-19 elastic cross-section at low energies also show dominance of an S-matrix pole corresponding to a bound or virtual Efimov state. It is also presented a brief discussion of these findings in the context of ultracold atom physics with tunable scattering lengths. (C) 2008 Elsevier B.V. All rights reserved.

Renormalization of the one-pion-exchange interaction

A renormalization scheme for the nucleon-nucleon (NN) interaction based on a subtracted T-matrix equation is proposed and applied to the one-pion-exchange potential supplemented by contact interactions. The singlet and triplet scattering lengths are given to fix the renormalized strengths of the contact interactions. With only one scaling parameter (μ), the results show an overall very good agreement with neutron-proton data, particularly for the observables related to the triplet channel. The agreement is qualitative in the 1 S0 channel. Between the low-energy NN observables we have examined, the mixing parameter of the 3S1-3D1 states is the most sensitive to the scale. The scheme is renormalization group invariant for μ → ∞. © 1999 Elsevier Science B.V. All rights reserved.

Empilhamento sísmico pela composição de ondas planas

No presente trabalho de tese é apresentada uma nova técnica de empilhamento de dados sísmicos para a obtenção da seção de incidência normal ou afastamento fonte-receptor nulo, aplicável em meios bidimensionais com variações laterais de velocidade. Esta nova técnica denominada Empilhamento Sísmico pela Composição de Ondas Planas (empilhamento PWC) foi desenvolvida tomando como base os conceitos físicos e matemáticos da decomposição do campo de ondas em ondas planas. Este trabalho pode ser dividido em três partes: Uma primeira parte, onde se apresenta uma revisão da técnica de empilhamento sísmico convencional e do processo de decomposição do campo de ondas produzido a partir de fontes pontuais em suas correspondentes ondas planas. Na segunda parte, é apresentada a formulação matemática e o procedimento de aplicação do método de empilhamento sísmico pela composição de ondas planas. Na terceira parte se apresenta a aplicação desta nova técnica de empilhamento na serie de dados Marmousi e uma analise sobre a atenuação de ruído. A formulação matemática desta nova técnica de empilhamento sísmico foi desenvolvida com base na teoria do espalhamento aplicado a ondas sísmicas sob a restrição do modelo de aproximação de Born. Nesse sentido, inicialmente se apresenta a determinação da solução da equação de onda caustica para a configuração com afastamento fonte-receptor finito, que posteriormente é reduzido para a configuração de afastamento fonte-receptor nulo. Por outra parte, com base nessas soluções, a expressão matemática deste novo processo de empilhamento sísmico é resolvida dentro do contexto do modelo de aproximação de Born. Verificou-se que as soluções encontradas por ambos procedimentos, isto é, por meio da solução da equação da onda e pelo processo de empilhamento proposto, são iguais, mostrando-se assim que o processo de empilhamento pela composição de ondas planas produz uma seção com afastamento fonte-receptor nulo. Esta nova técnica de empilhamento basicamente consiste na aplicação de uma dupla decomposição do campo de ondas em onda planas por meio da aplicação de dois empilhamentos oblíquos (slant stack), isto é um ao longo do arranjo das fontes e outro ao longo do arranjo dos detectores; seguido pelo processo de composição das ondas planas por meio do empilhamento obliquo inverso. Portanto, com base nestas operações e com a ajuda de um exemplo de aplicação nos dados gerados a partir de um modelo simples, são descritos os fundamentos e o procedimento de aplicação (ou algoritmo) desta nova técnica de obtenção da seção de afastamento nulo. Como exemplo de aplicação do empilhamento PWC em dados correspondentes a um meio com variações laterais de velocidade, foi aplicado nos dados Marmousi gerados segundo a técnica de cobertura múltipla a partir de um modelo que representa uma situação geológica real. Por comparação da seção resultante com a similar produzida pelo método de empilhamento convencional, observa-se que a seção de afastamento nulo desta nova técnica apresenta melhor definição e continuidade dos reflectores, como também uma melhor caracterização da ocorrência de difrações. Por último, da atenuação de ruído aleatório realizada nos mesmos dados, observa-se que esta técnica de empilhamento também produz uma atenuação do ruído presente no sinal, a qual implica um aumento na relação sinal ruído.