The 23Mg(pγ)24Al and 26Al(pγ)27Si astrophysical reactions are expected to be of considerable importance in the nucleosynthesis of A≥20 nuclei in classical novae. Previous studies have indicated that both reactions are dominated by resonant capture to excited states, above the proton-emission thresholds, in the proton-rich nuclei 24Al and 27Si, respectively. Consequently, by determining the nuclear properties of such resonant states it is possible to estimate the 23Mg(pγ)24Al and 26Al(pγ)27Si stellar reaction rates. In this thesis work, excited states in 24Al and 27Si were populated via the 10B(16O, 2n) and 12C(16O, n) heavy-ion fusion-evaporation reactions, respectively. The beams of 16O ions were produced by the Argonne Tandem Linear Accelerator System and prompt electromagnetic radiation was detected using the GAMMASPHERE detector array, which, in the case of the 24Al experiment, was used in coincidence with recoil selection provided by the Argonne Fragment Mass Analyzer. The two γray spectroscopy studies performed in this work allowed level structure determinations below the respective proton-emission thresholds of 24Al and 27Si nuclei, with improved precision on previous work. In addition to this, these studies also allowed a determination of the nuclear properties of proton-unbound astrophysically important γ decaying states, which, in turn, were used to re-evaluate the 23Mg(pγ)24Al and 26Al(pγ)27Si stellar reaction rates. The improved precision of the level energies and unambiguous assignments of resonant states has reduced the relative uncertainties in both the 23Mg(pγ)24Al and 26Al(pγ)27Si stellar reaction rates, constraining the production of A≥20 nuclei in classical novae.

Philosophiae Doctor - PhD / Low- and medium-spin bands of 187Os have been studied using the AFRODITE array, following the 186W(4He,3n)187Os reaction at a beam energy of 37 MeV. The measurements of 􀀀 coincidences, angular distribution ratios (RAD), polarization and -intensities were performed using eleven High Purity Germanium (HPGe) clover detectors. In the current work, all the previously known bands have been signi cantly extended and ve new bands have been added to the level scheme. The observed bands are interpreted within the cranked shell model (CSM), cranked Nilsson-Strutinsky- Bogoliubov (CNSB) formalism and Quasiparticle-plus-Triaxial-Rotor (QTR) model. Systematic comparison of bands with the neighbouring isotopes has also been made. Comparison of the models with experimental data shows good agreement.

Obtemos propriedades de um modelo de rede para soluções diluídas de polímeros anfifílicos pequenos a partir do estudo das isotermas de potencial químico. Os resultados obtidos na rede de Bethe e em simulações de Monte Carla são apresentados. Introduzimos os cálculos na rede de Bethe através da mistura simétrica e estendemos o tratamento para dímeros, trímeros e tetrâmeros. O tratamento analítico também é generalizado para copolímeros com grau de polimerização arbitrário. As isotermas de potencial químico apresentam laços típicos de uma separação de fase macroscópica, muito embora se trate de um sistema em que duas densidades (anfifílicas livres e anfifílica na micela) coexistem em um sistema homogêneo. A partir destas isotermas, diagramas de fase são construídos. Nas simulações, propomos uma versão eficiente e revisada do \"teste de inserção\" de Widom. O método é inicialmente aplicado à mistura simétrica. Devido a problemas de tamanho finito, as isotermas de potencial químico indicam laços. Os laços podem ser associados a uma distribuição bimodal na curva distribuição de probabilidades no ensemble grande-canônico. Uma \"construção de Maxwell\" é aplicada nos dados, fornecendo uma curva de coexistência que converge rapidamente para a conhecida solução exata. A presença da coexistência de duas densidades é indicada por laços nas isotermas de potencial químico de soluções de anfifílicas pequenas. / We have obtained the properties of a lattice model for dilute solutions of short amphiphilic polymers from the study of chemical potential isotherms. Bethe lattice and Monte Carlo simulation results are presented. We introduce the Bethe lattice calculations for the symmetric mixture and apply them to dimers, trimers and tetramers. The analytic treatment is also generalized for a copolymer of arbitrary degree of polymerization. The chemical isotherms present loops typical of macroscopic phase separation, albeit for a system in which two densities (free amphiphile and amphiphile in micelle) coexist in a homogeneous system. Phase diagrams are presented. For the simulations, we propose a revised and efficient version of Widom\'s insertion test for mixtures. The method is tried on the symmetric mixture. The chemical potential isotherms present loops due to finite size effects. Loops can be associated with the double peak density probability in the grand-canonical ensemble. A \"Maxwell construction\" procedure yields a coexistence curve which converges rapidly to the known exact solution. Loops are present also in the case of short amphiphile solutions, indicating the presence of two densities.

Obtemos propriedades de um modelo de rede para soluções diluídas de polímeros anfifílicos pequenos a partir do estudo das isotermas de potencial químico. Os resultados obtidos na rede de Bethe e em simulações de Monte Carla são apresentados. Introduzimos os cálculos na rede de Bethe através da mistura simétrica e estendemos o tratamento para dímeros, trímeros e tetrâmeros. O tratamento analítico também é generalizado para copolímeros com grau de polimerização arbitrário. As isotermas de potencial químico apresentam laços típicos de uma separação de fase macroscópica, muito embora se trate de um sistema em que duas densidades (anfifílicas livres e anfifílica na micela) coexistem em um sistema homogêneo. A partir destas isotermas, diagramas de fase são construídos. Nas simulações, propomos uma versão eficiente e revisada do \"teste de inserção\" de Widom. O método é inicialmente aplicado à mistura simétrica. Devido a problemas de tamanho finito, as isotermas de potencial químico indicam laços. Os laços podem ser associados a uma distribuição bimodal na curva distribuição de probabilidades no ensemble grande-canônico. Uma \"construção de Maxwell\" é aplicada nos dados, fornecendo uma curva de coexistência que converge rapidamente para a conhecida solução exata. A presença da coexistência de duas densidades é indicada por laços nas isotermas de potencial químico de soluções de anfifílicas pequenas. / We have obtained the properties of a lattice model for dilute solutions of short amphiphilic polymers from the study of chemical potential isotherms. Bethe lattice and Monte Carlo simulation results are presented. We introduce the Bethe lattice calculations for the symmetric mixture and apply them to dimers, trimers and tetramers. The analytic treatment is also generalized for a copolymer of arbitrary degree of polymerization. The chemical isotherms present loops typical of macroscopic phase separation, albeit for a system in which two densities (free amphiphile and amphiphile in micelle) coexist in a homogeneous system. Phase diagrams are presented. For the simulations, we propose a revised and efficient version of Widom\'s insertion test for mixtures. The method is tried on the symmetric mixture. The chemical potential isotherms present loops due to finite size effects. Loops can be associated with the double peak density probability in the grand-canonical ensemble. A \"Maxwell construction\" procedure yields a coexistence curve which converges rapidly to the known exact solution. Loops are present also in the case of short amphiphile solutions, indicating the presence of two densities.

Philosophiae Doctor - PhD / Lying in a rapidly-changing transitional region between nuclei that may behave as spherical vibrators (N ≤ 88) or good rotors (N ≥ 92), the N = 90 isotones present a unique testing ground where new nuclear collective phenomena may be uncovered. As part of a general investigation of collective structures and the coupling of single-particle states to these structures, the 150Sm(12C,4nγγ) and 150Sm(13C,4nγγ) reactions at a beam energy of Elab = 65 MeV were used to study the low and medium spin structure of 158,159Er nuclei below spin 20~. The γ-γ coincidence events were detected in the nine escape-suppressed HPGe Clover detectors using the AFRODITE γ-ray spectrometer at the iThemba Laboratory for Accelerator Based Sciences. DCO ratios and γ-ray polarization measurements were used to establish the spins and parities of newly observed and confirm previously established rotational bands. The observed bands will be discussed in terms of both traditional quadrupole rotational-vibration models and recent predictions of octupole correlations in the N = 90 isotones and neighbouring nuclei. The data obtained will be compared with the spectroscopic systematics of neighbouring nuclei.

Philosophiae Doctor - PhD / Low- and medium-spin bands of 187Os have been studied using the AFRODITE array, following the 186W(4He,3n)187Os reaction at a beam energy of 37 MeV. The measurements of γ − γ coincidences, angular distribution ratios (RAD), polarization and γ-intensities were performed using eleven High Purity Germanium (HPGe) clover detectors. In the current work, all the previously known bands have been significantly extended and five new bands have been added to the level scheme. The observed bands are interpreted within the cranked shell model (CSM), cranked Nilsson-StrutinskyBogoliubov (CNSB) formalism and Quasiparticle-plus-Triaxial-Rotor (QTR) model. Systematic comparison of bands with the neighbouring isotopes has also been made. Comparison of the models with experimental data shows good agreement. The configurations of some of the previously observed bands have been modified. Most importantly, the coupling of 2+ γ band to the 11/2+[615] neutron configuration is observed for the first time.

Thesis (MSc)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: See full text for abstract / AFRIKAANSE OPSOMMING: Sien volteks vir opsomming

The role of the neutron-proton asymmetry (isospin) in the de-excitation of excited nuclei is investigated by studying the isospin dependence of the Ar+Ni fusion-evaporation cross sections. Compound nuclei of Pd, with mass number varying from 92 (close to the limit of stability for proton emission) to 104 (stable), are produced with the same excitation energy (~3 MeV/nucleon), impinging stable and radioactive Ar beams with energies around 13 MeV/nucleon on to isotopically enriched Ni targets. The de-excitation products are collected by the VAMOS magnetic spectrometer and the 4pi charged particle array INDRA. The study shows a decrease of the Ar+Ni fusion-evaporation cross section as the isospin of the system decreases, suggesting that the deexcitation of excited Pd nuclei is influenced by their neutron-proton asymmetry. Also, a larger decrease is observed for the N = Z 92Pd nucleus, explained as the opening of a new de-excitation channel (multifragmentation).

Utilizando tecnicas de espectroscopia-GAMA em linha e reacoes do tipo (hi, xnypGAMA) que populam estados de alto spin estudou-se a estrutura do nucleo duplamente impar ANTPOT.136 PR atraves das reacoes ANTPOT.123 SB (ANTPOT.16 O, 3n)ANTPOT.136 PR e ANTPOT.126 TE (ANTPOT.14 N 4n)ANTPOT.136 PR. Foram realizadas medidas de funcoes de excitacao em quatro energias, e coincidencias GAMA-GAMA-t e distribuicoes angulares em 69mev e 56mev, respectivamente, utilizando detetores hpge de alta resolucao (2kev) e alvos de isotopos enriquecidos de ANTPOT.123 SB (99%) e ANTPOT.126 TE (94%) com espessuras de aproximadamente 9mg/CENTIMETROS QUADRADOS. O esquema de niveis de energia do ANTPOT.136 PR evidencia duas bandas rotacionais que populam um estado isomerico com meia-vida de 92ns. A banda yrast e caracterizada por um staggering em seus niveis de energia, com transicoes m1 intensas e e2 fracas baseada na configuracao PI (h ind.11/2) IND.1/2 TETA NI (h ind.11/2) IND.9/2, de acordo com a sistematica da regiao de massa a=130, mostrando um pequeno energy splitting de 40kev. Na banda secundaria pressupoe-se a configuracao PI (d ind.5/2) IND.3/2 TETA NI (h ind.11/2) IND.11/2. Os resultados experimentais foram interpretados com base no cranked shell model que predizem deformacoes GAMA pequenas e negativas na regiao dos LA e PR / The structure of the doubly odd nucleus 136pr has been studied using techniques of on-line -spectroscopy with the reactions 123Sb (16O,3n) 136 Pr and 126Te(14 N,4n)136 Pr populating high spin states. The excitation functions were measured at four energies, and - t coincidences and angular distributions at 69MeV and 56MeV , respectively, using high resolution HPGe detectors (2keV) and targets of enriched isotopes of 123 Sb (99%) and 126Te (94%) with thicknesses of approximately 9mg/cm². The energy level scheme of 136Pr shows two rotational bands which populate one isomeric state with a half-life of 92s. The Yrast band is characterized by the staggering in energy levels with intense M1 transitions and weak E2 transitions, based on a configuration (h11/2)1/2 v(h11/2)9/2 in agreemment with the sistematics of the mass region A = 130. There is a small energy splitting of 40keV. For the secondary band a configuration n(d5/2)3/2 (h11/2)11/2 is possible. Experimental results were interpreted within the framework of the Cranked Shell Model which predicts small negative deformations in the La and Pr region.

Dans le cadre de l'installation GANIL / SPIRAL-1, un système innovant de source d'ions cible (TISS) a été conçu pour produire des nouveaux faisceaux d'ions radioactifs compétitifs et déterminer dans quelle mesure il est possible d'améliorer les performances de production par l’optimisation de l’ensemble cible-source (ECS) d’ions dédié à la production d'un faisceau d'ions radioactifs spécifique.L'isotope d'alcalin de courte durée de 74Rb ( demi-vie = 64.8 ms) a été choisi comme cas d’école.En étudiant et en améliorant les processus impliqués dans le fonctionnement de l’ECS, i.e. production dans la cible par réaction fusion-évaporation, implantation dans- et sortie du graphite, effusion et ionisation, une efficacité de transformation atome-ion de 75% est attendue, menant à un taux d'ions de l'ordre de 104 pps à la sortie de l’ECS.Les performances estimées permettraient à l'installation GANIL / SPIRAL-1 de délivrer des faisceaux de 74Rb et 114Cs avec des intensités compétitives, jamais atteintes dans d'autres installations.Le dispositif de production a été développé sur la base de mon travail, pour répondre aux objectifs de physique et aux conditions d'opération de SPIRAL-1. L'efficacité, la fiabilité et donc la simplicité étaient essentielles. Le système répond à ces exigences. / In the frame of GANIL/SPIRAL-1 facility, an innovative target ion source system (TISS) has been designed in order to produce new competitive radioactive ion beams, and to determine to which extent it is possible to improve the production performances by optimizing a target ion source system dedicated to the production of a specific radioactive ion beam.The short-lived alkali isotope of 74Rb (half-life = 64.8 ms) has been chosen as a challenging case study.By studying and improving the processes involved in the TISS designed, i.e. in-target production by fusion-evaporation, implantation in and release out of graphite, effusion, and ionization, an atom-to-ion transformation efficiency of 75% was predicted, leading to an ion rate of the order of 104 pps at the exit of the TISS.The estimated performances would allow the GANIL / SPIRAL-1 installation to deliver beams of 74Rb and 114Cs with competitive intensities never reached in other installations.The production device was developed based on my work, to cope with physics objectives and with the conditions of the SPIRAL-1 operation. Efficiency, reliability and thus simplicity were essential. The system meets these requirements.The instrumentation needed for tests existed or have been specifically designed. The TISS has been built, assembled and partly tested.In parallel, issues have been addressed to answer related questions about sticking times, resistivity, and emissivity, to get inputs for the design of the TISS.Expected results with alkali allow thinking that the transposition of the principle to the production of neutron-deficient short-lived isotopes of noble gases and eventually metals could be performing.