Classical novae are explained as thermonuclear explosions on the surface of white dwarf stars accreting hydrogen-rich material from less evolved companions in binary star systems. These events occur frequently within our galaxy and have been proposed as significant contributors to the galactic abundance of 13C, 15N, 17/18O and 18/19F. The short-lived isotope 18F (t1/2 = 110 min) is of particular importance since it may provide a signature of novae events through the detection of 511 keVγ-ray emission following the β+ decay of a 18F nucleus. During classical novae the 17O(p,γ)18F reaction governs the production of 18F and affects the synthesis of the rare isotopes mentioned above. Prior to the present study, the 17O(p,γ)18F reaction rate was poorly determined owing to a lack of low-energy experimental data. The present work reports on the first accurate measurements of the resonant and non-resonant contributions to the 17O(p,γ)18F reaction cross section in the energy region relevant for classical novae. Measurements were performed at the Laboratory for Underground Nuclear Astrophysics (LUNA) accelerator facility of the Laboratori Nazionali del Gran Sasso (LNGS) in Italy. Here the γ-ray background is suppressed by up to 3 orders of magnitude, thus providing a unique environment for low-energy measurements of reaction cross sections. Prompt γ rays associated with the formation and decay of states in 18F were analysed to determine the resonant and non-resonant contributions to the reaction cross section. The total non-resonant S-factor was determined at energies between Ecm ≈ 200 - 370 keV and the strength of a key resonance at Ecm = 183 keV was obtained with the best precision to date. The uncertainty in the reaction rate is now sufficiently low to place firmer constraints on nucleosynthesis predictions from accurate models of novae.

Type I X-ray bursts (XRB) are highly energetic and explosive astrophysical events, observed as very sudden and intense emissions of X-rays. X-ray bursts are believed to be powered by a thermonuclear runaway on the surface of a neutron star in a binary system. XRB models are dependent on the accurate information of the nuclear reactions involved. The 18Ne(α,p)21Na reaction is considered to be of great importance as a possible breakout route from the Hot-CNO cycle preceding the thermonuclear runaway. In this thesis work, the 18Ne(α,p)21Na reaction cross-section was indirectly measured at Ecm(α,p) = 2568, 1970, 1758, 1683, 1379 and 1194 keV, using the time-reverse 21Na(p,α)18Ne reaction. Since the time-reverse approach only connects the ground states of 21Na and 18Ne, the cross sections measured here represent lower limits of the 18Ne(α,p)21Na cross-section. An experiment was performed using the the ISAC-II facility at TRIUMF, Vancouver, Canada. A beam of 21Na ions was delivered to a polyethylene (CH2)n target placed within the TUDA scattering chamber. The reaction 18Ne and 4He ions were detected using silicon strip detectors, with time-of-flight and ΔE/E particle identification techniques used to distinguish the ions from background. The measurement at Ecm = 1194 keV is the lowest energy measurement to date of the 18Ne(α,p)21Na cross section. The measured cross sections presented in this thesis were compared to the NON-SMOKER Hauser-Feshbach statistical calculations of the cross section and to the unpublished results of another time-reverse investigation performed by a collaboration at the Argonne National Laboratory. A 18Ne(α,p)21Na reaction rate calculation based on the measured cross sections was performed. In comparison with previous reaction rate estimates, our results indicate a rate that is about a factor 2-3 lower than Hauser-Feshbach calculations, suggesting that a statistical approach may not be appropriate for cross section calculations for nuclei in this mass region. The astrophysical consequences of our new results appear to remain nevertheless negligible. These are also presented in this thesis.

Nesse trabalho medimos e analisamos distribuições angulares do espalhamento elástico para os isótopos 7Be, 9Be e 10Be em alvo de 12C. A distribuição do 7Be foi medida a uma energia de 18.8 MeV, em dois laboratórios, com o sistema TWINSOL, na Universidade de Notre dame, e com o sistema RIBRAS, na Universidade de São Paulo, onde foi completada a distribuição angular. As distribuições para o 9Be e 10Be foram medidas completamente no sistema RIBRAS, em energias de 26.0 e 23.2 MeV, respectivamente. Cada uma dessas distribuições angulares foi analisada considerando no modelo ótico e também o formalismo dos canais acoplados. Testamos múltiplos potenciais óticos em cada distribui9ção, com a finalidade de descrever a seção de choque elástica de cada sistema. Para os sistemas que envolvem núcleos fracamente ligados (7Be, 9Be e 8B) foram feitos cálculos com acoplamento do contínuo (CDCC), enquanto que para o núcleo 10Be, que é fortemente ligado, acoplamos os dois primeiros estados ligados. Também, a partir da análise de cada uma das distribuições elásticas, foi possível obter a seção total de reação, que foi comparada sistematicamente com outros núcleos leves espalhados em carbono. Dessa análise foi possível concluir que o canal do break up Coulombiano, nesses sistemas leves, não é fortemente influente na seção de choque total de reação, o que implica que a interação entre alvo e projétil é dominada pelo potencial nuclear. / In the present work, we measured elastic scattering angular distributions for teh isotopes 7Be, 9Be and 10Be on 12C target. The angular distribution of 7Be at 18.8 MeV, was measured in two laboratories, with the TWINSOL system, in Notre Dame University, and the RIBRAS system, in São Paulo University, where the angular distribution was completed. The angular distribution for 9Be and 10Be isotopes measured completely in the RIBRAS system at 26.0 and 23.2 MeV, respectively. All angular distribution were analized considering optical model and coupled channel formalism. We tested multiple optical potentials on each distribution to describe the elastic cross section for these systems. For the weakly bound projectiles (7Be, 9Be and 8B)calculations with continous coupling (CDCC) were performed, while for the 10Be nucleus, which is tghtly bound nucleus, we coupled the first two bound states. We also performed a systematic analysis of the total reaction cross section obtained from elastic scattering distribution of several light particles on 12C. From this analysis, we concluded that the Coulomb break up channel, in these light systems, does not have a strong influence on the total reaction cross section, what implies that the projetile-target interaction is dominated by the nuclear potential.

Nesse trabalho medimos e analisamos distribuições angulares do espalhamento elástico para os isótopos 7Be, 9Be e 10Be em alvo de 12C. A distribuição do 7Be foi medida a uma energia de 18.8 MeV, em dois laboratórios, com o sistema TWINSOL, na Universidade de Notre dame, e com o sistema RIBRAS, na Universidade de São Paulo, onde foi completada a distribuição angular. As distribuições para o 9Be e 10Be foram medidas completamente no sistema RIBRAS, em energias de 26.0 e 23.2 MeV, respectivamente. Cada uma dessas distribuições angulares foi analisada considerando no modelo ótico e também o formalismo dos canais acoplados. Testamos múltiplos potenciais óticos em cada distribui9ção, com a finalidade de descrever a seção de choque elástica de cada sistema. Para os sistemas que envolvem núcleos fracamente ligados (7Be, 9Be e 8B) foram feitos cálculos com acoplamento do contínuo (CDCC), enquanto que para o núcleo 10Be, que é fortemente ligado, acoplamos os dois primeiros estados ligados. Também, a partir da análise de cada uma das distribuições elásticas, foi possível obter a seção total de reação, que foi comparada sistematicamente com outros núcleos leves espalhados em carbono. Dessa análise foi possível concluir que o canal do break up Coulombiano, nesses sistemas leves, não é fortemente influente na seção de choque total de reação, o que implica que a interação entre alvo e projétil é dominada pelo potencial nuclear. / In the present work, we measured elastic scattering angular distributions for teh isotopes 7Be, 9Be and 10Be on 12C target. The angular distribution of 7Be at 18.8 MeV, was measured in two laboratories, with the TWINSOL system, in Notre Dame University, and the RIBRAS system, in São Paulo University, where the angular distribution was completed. The angular distribution for 9Be and 10Be isotopes measured completely in the RIBRAS system at 26.0 and 23.2 MeV, respectively. All angular distribution were analized considering optical model and coupled channel formalism. We tested multiple optical potentials on each distribution to describe the elastic cross section for these systems. For the weakly bound projectiles (7Be, 9Be and 8B)calculations with continous coupling (CDCC) were performed, while for the 10Be nucleus, which is tghtly bound nucleus, we coupled the first two bound states. We also performed a systematic analysis of the total reaction cross section obtained from elastic scattering distribution of several light particles on 12C. From this analysis, we concluded that the Coulomb break up channel, in these light systems, does not have a strong influence on the total reaction cross section, what implies that the projetile-target interaction is dominated by the nuclear potential.

<p>Reaction cross sections were measured for protons at 65.5 MeV, for deuterons at 37.8, 65.5, 97.4 MeV, for ³He at 96.4, 137.8, 167.3 MeV, and for ⁴He at 69.6, 117.2, 163.9 and 192.4 MeV. Targets were ⁹Be, ¹²C, ¹⁶O, ²⁸Si, ^40,48Ca, ^58,60Ni, ^{112,116,118,120,124}Sn and ²⁰⁸Pb. ⁴⁸Ca and ¹¹⁸Sn targets were used only for some of the measurements. Rest of the targets have been measured for all energies and projectiles but ⁴He at 69.6 MeV where reaction cross sections were measured for ⁹Be, ¹²C, ¹⁶O, ²⁸Si, ⁴⁰Ca targets. A modification of a standard attenuation technique was used. Details of the experimental design are presented.</p><p>Experimental uncertainties were 2-3% for p, 3% for d and 3-10% for ^3,4He. </p><p>A strong forward peaking of the reaction products was observed for ^3,4He. Therefore the standard reaction cross section measurement technique was not applicable for these projectiles. The forward peaking is also responsible for the increase of experimental uncertainties for these projectiles. The forward peaking of the reaction products is not known for other projectiles and has also not been observed with ^3,4He at different - both higher and lower - energies. Possible explanations for this phenomenon are discussed.</p><p>Optical model calculations of the reaction cross sections are in good agreement with the measured values.</p><p>The measurements were performed with beams from the Gustav Werner cyclotron at the The Svedberg Laboratory, Uppsala.</p>

<p>The reaction cross section gives the probability that a particle will undergo a nonelastic process when passing through a nuclear medium. Therefore reaction cross section data are of importance both for theoretical studies and for applications in such diverse fields as medicine, biology, astrophysics and accelerator-driven transmutation of nuclear waste.</p><p>There exist many data sets with angular distributions of elastic scattering, but very few measurements of the complementary reaction cross section have been performed. The measurement is in principle simple but has in practice proved to be very difficult to perform, and the relatively limited amount of experimental data displays some serious inconsistencies.</p><p>Results from measurements of reaction cross sections are presented for:</p><p>• ³He on ⁹Be, ¹²C, ¹⁶O, ²⁸Si, ⁴⁰Ca, ^58,60Ni, ^{112,116,118,120,124}Sn and ²⁰⁸Pb at 96, 138 and 167 MeV</p><p>• protons on ¹²C, ⁴⁰Ca, ⁹⁰Zr and ²⁰⁸Pb at six energies in the energy range 80-180 MeV, and on </p><p>⁵⁸Ni at 81 MeV. </p><p>Experimental uncertainties were 3-9% for ³He and 1.5-8% for protons.</p><p>The apparatus and the experimental method used for the measurements of reaction cross sections, using a modified attenuation technique, is described. The detection method enables simultaneous measurements of reaction cross sections for five different sizes of the solid angles in steps from 99.0 to 99.8% of the total solid angle. The final results are obtained by extrapolation to the full solid angle.</p><p>Experimental results are compared with predictions from optical model calculations using phenomenological global optical potentials.</p><p>Phenomenological parametrizations of reaction cross sections for scattering of projectiles on targets are presented. The parametrizations show that reaction cross sections are very sensitive to matter distributions at very large radii of both the projectile and the target. For protons the derived relations makes it possible to predict the reaction cross sections on targets for which no experimental data exist.</p>

Reaction cross sections were measured for protons at 65.5 MeV, for deuterons at 37.8, 65.5, 97.4 MeV, for 3He at 96.4, 137.8, 167.3 MeV, and for 4He at 69.6, 117.2, 163.9 and 192.4 MeV. Targets were 9Be, 12C, 16O, 28Si, 40,48Ca, 58,60Ni, 112,116,118,120,124Sn and 208Pb. 48Ca and 118Sn targets were used only for some of the measurements. Rest of the targets have been measured for all energies and projectiles but 4He at 69.6 MeV where reaction cross sections were measured for 9Be, 12C, 16O, 28Si, 40Ca targets. A modification of a standard attenuation technique was used. Details of the experimental design are presented. Experimental uncertainties were 2-3% for p, 3% for d and 3-10% for 3,4He. A strong forward peaking of the reaction products was observed for 3,4He. Therefore the standard reaction cross section measurement technique was not applicable for these projectiles. The forward peaking is also responsible for the increase of experimental uncertainties for these projectiles. The forward peaking of the reaction products is not known for other projectiles and has also not been observed with 3,4He at different - both higher and lower - energies. Possible explanations for this phenomenon are discussed. Optical model calculations of the reaction cross sections are in good agreement with the measured values. The measurements were performed with beams from the Gustav Werner cyclotron at the The Svedberg Laboratory, Uppsala.

The reaction cross section gives the probability that a particle will undergo a nonelastic process when passing through a nuclear medium. Therefore reaction cross section data are of importance both for theoretical studies and for applications in such diverse fields as medicine, biology, astrophysics and accelerator-driven transmutation of nuclear waste. There exist many data sets with angular distributions of elastic scattering, but very few measurements of the complementary reaction cross section have been performed. The measurement is in principle simple but has in practice proved to be very difficult to perform, and the relatively limited amount of experimental data displays some serious inconsistencies. Results from measurements of reaction cross sections are presented for: • 3He on 9Be, 12C, 16O, 28Si, 40Ca, 58,60Ni, 112,116,118,120,124Sn and 208Pb at 96, 138 and 167 MeV • protons on 12C, 40Ca, 90Zr and 208Pb at six energies in the energy range 80-180 MeV, and on 58Ni at 81 MeV. Experimental uncertainties were 3-9% for 3He and 1.5-8% for protons. The apparatus and the experimental method used for the measurements of reaction cross sections, using a modified attenuation technique, is described. The detection method enables simultaneous measurements of reaction cross sections for five different sizes of the solid angles in steps from 99.0 to 99.8% of the total solid angle. The final results are obtained by extrapolation to the full solid angle. Experimental results are compared with predictions from optical model calculations using phenomenological global optical potentials. Phenomenological parametrizations of reaction cross sections for scattering of projectiles on targets are presented. The parametrizations show that reaction cross sections are very sensitive to matter distributions at very large radii of both the projectile and the target. For protons the derived relations makes it possible to predict the reaction cross sections on targets for which no experimental data exist.

Foi calculado microscopicamente a seção de choque total de reação para os sistemas ANTPOT. 12 C + ANTPOT. 12 C, ANTPOT. 1-2 C+ ANTPOT. 40 Ca, ANTPOT. 12 C + ANTPOT. 90 Zr, ANTPOT. 12 C + ANTPOT. 208 Pb, ANTPOT. 40 Ca + ANTPOT. 40 Ca, ANTPOT. 40 Ca + ANTPOT. 208 Pb, ANTPOT. 90 Zr + ANTPOT. 90 Zr, ANTPOT. 90 Zr + ANTPOT. 208 Pb e ANTPOT. 208 Pb + ANTPOT. 208 Pb numa ampla faixa de energia. Foi usada a expressão WKB para a defasagem imaginária, na representação do parâmetro do impacto. A parte imaginária do potencial óptico foi construída, usando o primeiro termo da teoria de espalhamento múltiplo, incorporando na sua expressão o principio de Pauli. A inclusão da interação nuclear e coulombiana mostrou ser importante. Os resultados teóricos não se apresentam em bom acordo com os poucos dados experimentais existentes, a baixas energias. Este resultado foi atribuído a fraca absorção contida no potencial imaginário, que contém o processo de knock-out quase-livre como mecanismo dominante de reação. / The total reaction cross section of the systems 12C +12C, 12C + 40Ca, 12C + 90Zr, 12C + 208 Pb, 40Ca + 40Ca, 40Ca + 208Pb, 90Zr + 90Zr, 90 Zr + 208 Pb and 20B Pb + 208 Pb for a wide range of energies has been calculated microscopically. A WKB expression for the imaginary part of the optical potential has been constructed by using the first term of multiple scattering theory with the effect of Pauli blocking incorporated into it. The inclusion of the nuclear and Coulomb interactions is shown to be important. The theoretical results do not show very good agreement with the experimental data at lower energies. This is attributed to the weak absorption contained in the imaginary potential of the tpApB interaction, wich contained only quasi-free knock-out as the dominant reaction mechanism.

Foi calculado microscopicamente a seção de choque total de reação para os sistemas ANTPOT. 12 C + ANTPOT. 12 C, ANTPOT. 1-2 C+ ANTPOT. 40 Ca, ANTPOT. 12 C + ANTPOT. 90 Zr, ANTPOT. 12 C + ANTPOT. 208 Pb, ANTPOT. 40 Ca + ANTPOT. 40 Ca, ANTPOT. 40 Ca + ANTPOT. 208 Pb, ANTPOT. 90 Zr + ANTPOT. 90 Zr, ANTPOT. 90 Zr + ANTPOT. 208 Pb e ANTPOT. 208 Pb + ANTPOT. 208 Pb numa ampla faixa de energia. Foi usada a expressão WKB para a defasagem imaginária, na representação do parâmetro do impacto. A parte imaginária do potencial óptico foi construída, usando o primeiro termo da teoria de espalhamento múltiplo, incorporando na sua expressão o principio de Pauli. A inclusão da interação nuclear e coulombiana mostrou ser importante. Os resultados teóricos não se apresentam em bom acordo com os poucos dados experimentais existentes, a baixas energias. Este resultado foi atribuído a fraca absorção contida no potencial imaginário, que contém o processo de knock-out quase-livre como mecanismo dominante de reação. / The total reaction cross section of the systems 12C +12C, 12C + 40Ca, 12C + 90Zr, 12C + 208 Pb, 40Ca + 40Ca, 40Ca + 208Pb, 90Zr + 90Zr, 90 Zr + 208 Pb and 20B Pb + 208 Pb for a wide range of energies has been calculated microscopically. A WKB expression for the imaginary part of the optical potential has been constructed by using the first term of multiple scattering theory with the effect of Pauli blocking incorporated into it. The inclusion of the nuclear and Coulomb interactions is shown to be important. The theoretical results do not show very good agreement with the experimental data at lower energies. This is attributed to the weak absorption contained in the imaginary potential of the tpApB interaction, wich contained only quasi-free knock-out as the dominant reaction mechanism.