Universiẗat, Diss., 2001--München.

The asymptotic behavior of the wave functions of three charged particles has been investigated. There are two different types of three-body scattering wave functions. The first type of scattering wave function evolves from the incident three-body wave of three charged particles in the continuum. The second type of scattering wave function evolves from the initial two-body incident wave. In this work the asymptotic three-body incident wave has been derived in the asymptotic regions where two particles are close to each other and far away from the third particle. This wave function satisfies the Schrodinger equation up to terms O(1/3pa), where pa is the distance between the center of mass of two particles and the third particle. The derived asymptotic three-body incident wave transforms smoothly into Redmond’s asymptotic incident wave in the asymptotic region where all three particles are well separated. For the scattering wave function of the second type the asymptotic threebody scattered wave has been derived in all the asymptotic regions. In the asymptotic region where all three particles well separated, the derived asymptotic scattered wave coincides with the Peterkop asymptotic wave. In the asymptotic regions where two particles are close to each other and far away from the third one, this is a new expression which is free of the logarithmically diverging phase factors that appeared in the Peterkop approach. The derived asymptotic scattered wave resolves a long-standing phase-amplitude ambiguity. Based on these results the expressions for the exact prior and post breakup amplitudes have been obtained. The post breakup amplitude for charged particles has not been known and has been derived for the first time directly from the prior form. It turns out that the post form of the breakup amplitude is given by a surface integral in the six dimensional hyperspace, rather than a volume integral, with the transition operator expressed in terms of the interaction potentials. We also show how to derive a generalized distorted-wave-Born approximation amplitude (DWBA) from the exact prior form of the breakup amplitude. It is impossible to derive the DWBA amplitude from the post form. The three-body Coulomb incident wave is used to calculate the reaction rates of 7Be(ep, e)8B and 7Be(pp, p)8B nonradiative triple collisions in stellar environments.

Techn. Universiẗat, Diss., 2005--Berlin.

The research initiatives addressed in this thesis are geared towards improving the performance of passive optical networks through the development of advanced dynamic bandwidth allocation protocols. In particular, the aim of the research undertaken is to enhance the quality of service offered by standard passive optical networks with reduced network costs. To that extent, a dynamic multi-wavelength protocol has been developed to increase the network upstream bandwidth and introduce multiple service levels to a fibre to the home-based giga-bit passive optical network. Simulation results have confirmed the reduction of the mean packet delay by adjusting the ITU-T standard G984 giga-bit passive optical network frame format by means of the introduction of extended wavelength band overlay based on the ITU-T Coarse- Wavelength Division Multiplexing grid to support the multi-wavelength functionality. To evaluate the multi-wavelength upstream operation of the newly implemented models in OPNET, 2-dimensional Dynamic Bandwidth Allocation algorithms have been introduced to manage the network resources in both the time and wavelength domains. Furthermore, the enhanced traffic allocation among the supported wavelengths in new protocol confirmed a performance improvement in the network total capacity and the mean packet delay, which demonstrates the network reliability and improves the quality of the provided service according to the subscriber service level agreement, with a minimum guaranteed bandwidth of 100 Mbit/s to fulfil applications and associated bandwidth requirements for the next generation access network.

With new radioactive isotope accelerators coming online in the next decade, the problem of extracting reliable nuclear structure information from reactions with unstable nuclei deserves considerable attention. A method has been proposed to determine spectroscopic factors (SFs) using the asymptotic normalization coefficient (ANC) to fix the external contribution of a nonperipheral reaction, reducing the uncertainty in the SF. The ¹⁵C[left right arrow]¹⁴C+n system was chosen as a test case for this new method. The direct neutron capture rate on ¹⁴C is important for a variety of topics of interest in astrophysics, and the ANC for ¹⁵C[left right arrow]¹⁴C+n was also used to calculate this reaction rate. The objective of the first part of this work was to find the ANC for ¹⁵C[left right arrow]¹⁴C+n. This was done in two independent experiments. First, the heavy ion neutron transfer reaction ¹³C(¹⁴C,¹⁵C)¹²C was measured at 12 MeV/nucleon. Second, the inverse kinematics reaction d(¹⁴C,p)¹⁵C was measured using the new Texas Edinburgh Catania Silicon Array (TECSA). The next phase of the experimental program was to measure a reaction with a non-negligible interior contribution, for which ¹⁴C(d,p)¹⁵C at 60 MeV deuteron energy was used. This reaction turned out to be more peripheral than anticipated, and as a result, the ANC for the ground state was extracted from this measurement as well. The final results for the three measurements are C²2s1/2 = 1.96±0.16 fm⁻¹ for the ground state and C²1d5/2 = (4.23±0.38)·10⁻³ fm⁻¹ for the first excited state. Because the 60 MeV ¹⁴C(d,p)¹⁵C reaction turned out to have a very weak dependence on the interior, the SF could not be determined for the ¹⁴C+n ground state in ¹⁵C using the new method. A lower limit of 1.05 was found for the first excited state. It is possible that other reactions might turn out to be more suitable for this method, however, the difficulty encountered at this relatively high deuteron energy highlights a substantial problem likely to be seen in other applications. Using the ANCs determined in this work, the astrophysical ¹⁴C(n,γ)¹⁵C reaction rate was calculated. The resulting value for the cross section for capture to the ground state at 23 keV was σgs(23 keV)=5.1±0.4 μb and to the first excited state was σexc(23 keV)=0.2±0.02 μb.

Componentes de buraco de nêutrons nos isótopos ANTPOT 99, 101, 103 Ru foram investigadas atrav6s de reações (d,t), induzidas por dêuterons de 15,5 MeV em ANTPOT 100Ru e 16 MeV em ANTPOT 102 Ru e ANTPOT 101 Ru. Os grupos de trítons emergentes, analisados em momento por um espectrógrafo magnético, foram detectados em placas de emulsão nuclear. Resolução em energia melhor que 8 keV permitiu a identificação de 14, 36 e 46 níveis, até as energias de excitação de 1,4; 2,1 e 2,5 MeV, respectivamente, nos ANTPOT 99, 101, 103 Ru. Os momentos angulares orbitais transferidos, l, e as intensidades espectroscópicas foram obtidos a partir da comparação entre distribuições angulares experimentais, medidas cm ângulos judiriosamente escolhidos entre 8º e 46º, e as previstas pela Aproximação de Born com Ondas Distorcidas. A análise das distribuições das intensidades espectroscópicas, associadas a cada l, nos três isótopos revela um padrão de semelhança, embora haja um deslocamento das maiores intensidades espectroscópicas para menores energias, quando se eleva o número de nêutrons, que indica, juntamente com um aumento da densidade de níveis, crescente deformação nuclear. Especial atenção é dirigida a transições para estados de baixa energia de excitação com l = 3 e l = 1, associadas a orbitais não pertencentes à camada de valência, cuja descrição é discutida em termos de um modelo quase-partícula-rotor prolato, não-rígido, com o tratamento completo da interação de Coriolis, e do Modelo de Bósons e Férmions Interatuantes. / Neutron-hole components in 99, 101, 103 Ru Isotopes were Investigated by (d,t) reactions at incident deuteron energies of 15.5 MeV and 16 MeV on, respectively, 100 Ru and 102,104 Ru. Outgoing triton groups were momentum analyzed by a magnetic spectrograph and dctectcd in nuclear emulsion plates with an energy resolution better than 8 keV. A total of 14, 36 and 46 leveis up to 1.4, 2.1 and 2.5 MeV excitation energy were identified, respectively, 99, 101, 103 Ru. The transferred orbital angular momenta, l, and the spectroscopic strengths were obtained by con1paring experimental angular distributions, measured at carefully chosen scattering angles between 8º and 46º, with Distorted Wave Born Approximation predictions. The analysis of the spectroscopic strength distributions corresponding to each l-value reveals a similar pattern among the three isotopes, although there is a shift of the highest strengths towards low energy, for increasing neutron number, indicating increasing deform at ion. Special attention is drawn to transitions to low-lying states with l=3 and l=1 character, associated with the next major shell, whose description is discussed in terms of a quasiparticle-prolate non-rigid rotor model with the Coriolis effect fully treated, and the Interacting Boson-Fermion Model.

Componentes de buraco de nêutrons nos isótopos ANTPOT 99, 101, 103 Ru foram investigadas atrav6s de reações (d,t), induzidas por dêuterons de 15,5 MeV em ANTPOT 100Ru e 16 MeV em ANTPOT 102 Ru e ANTPOT 101 Ru. Os grupos de trítons emergentes, analisados em momento por um espectrógrafo magnético, foram detectados em placas de emulsão nuclear. Resolução em energia melhor que 8 keV permitiu a identificação de 14, 36 e 46 níveis, até as energias de excitação de 1,4; 2,1 e 2,5 MeV, respectivamente, nos ANTPOT 99, 101, 103 Ru. Os momentos angulares orbitais transferidos, l, e as intensidades espectroscópicas foram obtidos a partir da comparação entre distribuições angulares experimentais, medidas cm ângulos judiriosamente escolhidos entre 8º e 46º, e as previstas pela Aproximação de Born com Ondas Distorcidas. A análise das distribuições das intensidades espectroscópicas, associadas a cada l, nos três isótopos revela um padrão de semelhança, embora haja um deslocamento das maiores intensidades espectroscópicas para menores energias, quando se eleva o número de nêutrons, que indica, juntamente com um aumento da densidade de níveis, crescente deformação nuclear. Especial atenção é dirigida a transições para estados de baixa energia de excitação com l = 3 e l = 1, associadas a orbitais não pertencentes à camada de valência, cuja descrição é discutida em termos de um modelo quase-partícula-rotor prolato, não-rígido, com o tratamento completo da interação de Coriolis, e do Modelo de Bósons e Férmions Interatuantes. / Neutron-hole components in 99, 101, 103 Ru Isotopes were Investigated by (d,t) reactions at incident deuteron energies of 15.5 MeV and 16 MeV on, respectively, 100 Ru and 102,104 Ru. Outgoing triton groups were momentum analyzed by a magnetic spectrograph and dctectcd in nuclear emulsion plates with an energy resolution better than 8 keV. A total of 14, 36 and 46 leveis up to 1.4, 2.1 and 2.5 MeV excitation energy were identified, respectively, 99, 101, 103 Ru. The transferred orbital angular momenta, l, and the spectroscopic strengths were obtained by con1paring experimental angular distributions, measured at carefully chosen scattering angles between 8º and 46º, with Distorted Wave Born Approximation predictions. The analysis of the spectroscopic strength distributions corresponding to each l-value reveals a similar pattern among the three isotopes, although there is a shift of the highest strengths towards low energy, for increasing neutron number, indicating increasing deform at ion. Special attention is drawn to transitions to low-lying states with l=3 and l=1 character, associated with the next major shell, whose description is discussed in terms of a quasiparticle-prolate non-rigid rotor model with the Coriolis effect fully treated, and the Interacting Boson-Fermion Model.

Os processos de quebra e fusão que ocorrem em núcleos fracamente ligados tem apresentado grande interesse teórico e experimental recentemente. Para sistemas simples, como o dêuteron, essas reações tem sido estudado por décadas. O importante papel desempenhado pelo processo de quebra em reações induzidas por dêuterons, levou ao desenvolvimento da técnica CDCC que descreve simultaneamente o espalhamento elástico. Entretanto, quando os processos de quebra e fusão são consideradas separadamente, a aproximação DWBA provém razoável concordância com os dados experimentais para espectros e distribuições angulares. Nesse trabalho, usamos o potencial de alcance zero da aproximação DWBA na forma post para estimar as seções de choque total de quebra elástica e Coulomb-induzida como função da energia para diversos núcleos-alvo. Usamos o modelo de folding para estimar a seção de choque de absorção do dêuteron e comparamos as seções de choque de fusão completa e quebra elástica com a seção de choque de reação do dêuteron, com o objetivo de caracterizar a dependência da energia e da carga/massa dessas contribuições para a seção de choque de reação.

Techn. University, Diss., 2006--Darmstadt.

La nucléosynthèse du fluor n'est pas encore clairement expliquée. Plusieurs scénarios prédisent que la réaction de capture alpha radiative sur l'azote 15 est la principale réaction de production de fluor. Dans l'expression du taux de cette réaction un paramètre essentiel est manquant, la largeur partielle alpha de la résonance sur le niveau d'énergie E = 4,377 MeV du fluor 19. Une mesure directe est exclue du fait de la très faible valeur attendue de la section efficace. Nous avons déterminé cette largeur alpha par le biais d'une réaction de transfert et une analyse en FR-DWBA (Finite Range Distorted Wave Born Approximation) dans un modèle simplifié de cluster alpha. Cette expérience a été effectuée avec un faisceau de lithium 7 accéléré à 28 MeV sur une cible gazeuse d'azote 15. Les 16 premiers niveaux du fluor ont été étudiés. Les facteurs spectroscopiques ont été extraits pour la majorité de ces niveaux. Les largeurs alpha des niveaux au-dessus du seuil ont été déterminées. Plusieurs largeurs alpha ont été comparées avec des valeurs publiées de mesures directes et l'écart reste dans la plage d'incertitude que nous avons estimée (facteur 2). La largeur alpha du niveau d'énergie E = 4,377 MeV a été déterminée, sa valeur est environ 60 fois plus faible que la valeur utilisée jusqu'à ce jour. L'influence de ce nouveau taux est observée principalement dans les étoiles dites AGB (Asymptotic Giant Branch) lors de phénomènes de pulsations thermiques. Dans ce modèle l'impact de nos mesures est sensible.