Dissertations / Theses on the topic 'Experimental nuclear astrophysic'

This thesis reports the direct measurements of the 22Ne(α,γ)26Mg and 20Ne(p,γ)21Na reactions at astrophysical energies of interest. The 22Ne(α,γ)26Mg reaction competes with the 22Ne(α,n)25Mg reaction which is the main source of neutrons for the s-process in low-mass Asymptotic Giant Branch and massive stars. At temperatures T < 300 MK where the (α,γ) channel becomes dominant, the rate of the 22Ne(α,γ)26Mg reaction is influenced by several resonances studied only indirectly. The first part of this thesis concerns the direct measurement of one of these resonances, Er = 334 keV, which so far was studied only indirectly leading to six orders of magnitude range of possible values for its resonance strength. The experiment has been performed at LUNA (Laboratory for Underground Nuclear Astrophysics) using the intense alpha beam of the LUNA 400 kV accelerator and a windowless gas target combined with a high-efficiency BGO detector. In the present study, an upper limit of 4.0·10−11 eV has been determined for the resonance strength. Taking into account these results, an up-dated 22Ne(α,γ)26Mg thermonuclear reaction rate was obtained and its role on the predicted 25Mg/26Mg ratio of a 5M⊙ AGBs was investigated. The data show a decrease by a factor of 15 of the intershell 25Mg/26Mg ratio. The 20Ne(p,γ)21Na is the slowest reaction of the NeNa cycle. It determines the velocity of the cycle and defines the final abundances of the isotopes synthesized in this cycle. The uncertainties on the NeNa cycle are affected by the 20Ne(p,γ)21Na reaction rate. The main goal of the second part of this thesis was the direct measurement of the Ecm = 366 keV resonance which dominates the total rate in the temperature range between 0.2 GK and 1 GK. The measurement has been performed at LUNA using the windowless gas target and two high-purity germanium detectors placed at different positions. This measurement allowed to reduce the uncertainty on the strengths of the 366 keV resonance from 18% to 7%. These results were used to update the 20Ne(p,γ)21Na reaction rate.

In the ejecta from ONe novae outbursts nuclei up to A~40 are observed. The 30P(p,γ)31S reaction is thought to be the bottleneck for the production of all elements heavier than sulphur. However, due to uncertainties in the properties of key proton-unbound resonances the reaction rate is not well determined. In this thesis work, excited states in 31S were populated via the 28Si(4He,n) light-ion fusion-evaporation reaction and the prompt electromagnetic radiation was then detected with the GAMMASPHERE detector array. This γ-ray spectroscopy study, and comparisons with the stable mirror nucleus 31P, allowed the determination of the 31S level structure below the proton-emission threshold and also of the key proton-unbound states for the 30P(p,γ)31S reaction. In particular, transitions from key, low-spin states were observed for the first time. This new information was then used for the re-evaluation of the 30P(p,γ)31S reaction in the temperature range relevant for ONe novae. The newly calculated rate is higher than previous estimates implying a greater flux of material processed to high-Z elements in novae. Astrophysical X-ray bursts are thought to be a result of thermonuclear explosions on the atmosphere of an accreting neutron star. Between these bursts, energy is thought to be generated by the hot CNO cycles. The 15O(α,γ)19Ne reaction is one reaction that allows breakout from these CNO cycle and into the rp-process to fuel outbursts. The reaction is expected to be dominated by a single 3/2+ resonance at 4.033 MeV in 19Ne, however, limited information is available on this key state. This thesis work reports on a pioneering study of the 20Ne(p,d)19Ne reaction in inverse kinematics performed at the Experimental Storage Ring (ESR) as a means of accessing the 4.033-MeV state in 19Ne. The unique background free, high luminosity conditions of the ESR were utilised for this, the first transfer reaction performed at the ESR. The results of this pioneering test experiment are presented along with suggestions for future measurements at storage ring facilities.

centenares de keV hasta unos pocos MeV), presenta un extraordinario potencial para comprender los procesos más extremos que tienen lugar en el Universo, como las explosiones estelares o los aceleradores de partículas. No obstante, a pesar de los enormes esfuerzos realizados por los observatorios de rayos gamma (en el pasado y en la actualidad), se requiere una mejora en la sensibilidad instrumental para aprovechar el enorme potencial científico contenido en este rango energético. Durante las dos últimas décadas se han buscado formas de mejorar la sensibilidad de los instrumentos, incrementando la eficiencia de los detectores y reduciendo el ruido instrumental (mediante sofisticados mecanismos de blindaje y técnicas de análisis). Con este objetivo, un enorme esfuerzo en innovación instrumental (construcción de prototipos y estudios numéricos) está siendo realizado por una comunidad creciente de científicos que se enfrentan al reto de preparar la próxima generación de telescopios de rayos gamma. En particular, son especialmente notables los avances logrados en los últimos años en el campo de la focalización de rayos gamma mediante lentes de difracción. Conceptualmente, una lente de rayos gamma reduciría drásticamente el ruido instrumental ya que concentra los fotones en un detector de pequeñas dimensiones (el ruido es proporcional al volumen del instrumento). Una lente de difracción, para observaciones en astrofísica nuclear, no es sólo un concepto teórico, sino una realidad, gracias principalmente al proyecto CLAIRE. Asimismo, el desarrollo de la tecnología para la focalización de rayos gamma ha incentivado el desarrollo de las diferentes tecnologías de detección. Un detector apropiado para el plano focal de una lente gamma, debe disponer de capacidad de imagen, proporcionar espectroscopia de alta resolución y medir la polarización de los fotones incidentes. El trabajo presentado en esta tesis comprende tanto la óptica de focalización como el detector del plano focal. Con respecto a la óptica, se presenta el ensayo realizado con el prototipo CLAIRE, mediante el cual se ha confirmado los principios de una lente de difracción. En cuanto al plano focal, esta investigación se ha desarrollado principalmente en el marco de estudio de las misiones espaciales GRI (2007) y DUAL (2010), propuestas a la ESA dentro del programa “Cosmic Vision 2015-2025”. En el marco de la misión GRI, se presenta una configuración para el detector del plano focal basado en detectores pixelados de Cd(Zn)Te, al tiempo que se investiga y desarrolla un primer prototipo de detector pixelado de CdTe. Cabe destacar que el sistema de detección propuesto fue registrado con éxito mediante una patente europea, y está siendo desarrollado para su aplicación en medicina nuclear. En relación a la propuesta DUAL, se presenta un estudio del ruido instrumental obtenido mediante simulaciones numéricas con el fin de precisar la sensibilidad del instrumento (basado en detectores de Germanio) propuesto en esta misión. Más allá de las tecnología consideradas en GRI y DUAL, una amplia variedad de detectores pueden ser explorados, bien para el plano focal de una lente de difracción o bien como sistemas de detección por sí mismos. En este sentido, se ha ampliado el espectro de tecnologías y se ha incluido un estudio sobre detectores basados en xenón líquido. En esta tesis se ha realizado un trabajo de investigación y desarrollo con tecnologías vanguardistas propuestas para la próxima generación de telescopios de rayos gamma. Esta instrumentación debe enfrentarse al reto de alcanzar la sensibilidad requerida para dar respuesta a las cuestiones aun no resultas por la astrofísica de rayos gamma en el rango de energía de las transiciones nucleares.
few MeV) has an extraordinary potential for understanding the evolving and violent Universe. In spite of the strong efforts accomplished by past and current instruments, in order to perform observations in this energy range, an improvement in sensitivity over present technologies is needed to take full advantage of the scientific potential contained in this energy range. In order to achieve higher sensitivities, γ-ray astronomy has been looking over the last decades for new ways to increase the efficiency of its instruments while reducing the background noise. With the objective of reducing or avoiding as much background as possible (through shielding mechanisms and data analysis techniques), a strong effort in innovation and design (build-up of prototypes and numerical simulations studies) is being conducted by a community facing the challenge of preparing the next generation of γ-ray telescopes. In particular, the progress achieved during the last decade on focusing optics based on Laue lenses is especially remarkable. Conceptually, a focusing telescope will reduce drastically the background noise by concentrating γ-rays onto a small size detector. Focusing γ-rays with a Laue lens is not just a theoretical concept, but a reality, mainly thanks to the development of a first prototype of Laue lens for nuclear astrophysics accomplished as part of the CLAIRE project. Moreover, the development of focusing optics during these years has also encouraged the development of new detector technologies. The focal plane detector of a focusing telescope should provide imaging capabilities, perform high-resolution spectroscopy and measure the polarization of the incident photons in order to achieve the ambitious scientific goals. The research presented in this thesis covers both main areas of a γ-rays telescope: focusing optics and focal plane detector. As far as the optics is concerned, a test of the lens CLAIRE was performed in order to confirm the principles of a Laue diffraction lens. Concerning the focal plane detector, theoretical and experimental studies with new detector technologies have been carried out. Our main research has evolved in the framework of two mission concept studies -GRI (2007) and DUAL (2010)- submitted to the ESA Calls for a Medium-size mission opportunity within the Cosmic Vision 2015-2025 program. As far as the GRI mission is concerned, a focal plane detector configuration based on Cd(Zn)Te pixelated detectors is proposed, whilst development and testing of a detector prototype is accomplished. It is noteworthy that the detector configuration was successfully registered under a European Patent and is being considered for applications in the field of nuclear medicine. Regarding the DUAL mission, simulations of the expected space radiation environment and the resulting detector activation were carried out in order to estimate the performances of the all-sky Compton telescope of DUAL (based on Germanium-strip detectors). The results show that DUAL could achieve, after two years of operation, a continuum sensitivity one order of magnitude better than any past and current observatory in the MeV energy range and up to a factor 30 of improvement with its Laue lens. Beyond the detector technology proposed for GRI and DUAL, a wide variety of technologies could be explored for the focal plane of a γ-ray lens mission as well as for a stand-alone detector. In this thesis a focal plane detector based on liquid xenon is also considered. This work faces the challenges of the next generation of γ-ray telescopes, where high performance γ-ray detectors are necessary to achieve the required sensitivity in order to answer several hot scientific topics of Gamma-ray astrophysics in the energy range of nuclear transitions.

The 22Ne(p,gamma)23Na reaction takes part in the neon-sodium cycle of hydrogen burning. This cycle plays a crucial role for the synthesis of the elements with mass A = 20-25 in asymptotic giant branch stars, classical novae explosions and type Ia supernovae, where hydrogen burning occurs at high temperatures. The 22Ne(p,gamma)23Na thermonuclear reaction rate is highly uncertain because of a large number of poorly known resonances lying at astrophysical energies. This thesis reports on a new experimental study of the 22Ne(p,gamma)23Na reaction. In particular, two experiments have been performed to pin down the cross section of the proton capture on 22Ne: a measurement of the resonances at proton energies below 400 keV has been performed at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Gran Sasso (Italy), while a high-precision study of the resonances between 400 and 660 keV has been performed at the Helmoltz-Zentrum Dresden-Rossendorf (Germany). For the LUNA measurement, a windowless gas target filled with enriched 22Ne was used and the gamma rays emitted in the 23Na decay were detected by two high-purity germanium detectors. The experiment performed at LUNA led to the first detection of three previously unobserved resonances. Moreover, the decay schemes of the corresponding excited states of 23Na have been extended with the observation of new transitions. The LUNA measurement also allowed to reduce the upper limits on three unobserved resonances that represent the main source of uncertainty for the reaction rate. The HZDR experiment was performed with a 22Ne solid target and two high-purity germanium detectors surrounded by BGO anti-Compton shields. The target were prepared implanting 22Ne on a tantalum backing. The implantation was performed at the 200 kV high-current implanter of Legnaro National Laboratories (Italy). To improve the precision on the strengths of the resonances between 400 and 660 keV, the well known 22Ne(p,gamma)23Na resonances at 1279 keV and 478 keV were used for normalization. This measurement allowed to reduce the uncertainty on the strengths of the 436 keV and 638.5 keV resonances up to a factor of three. Moreover, the strength of the 661 keV resonance has been revised downward by one order of magnitude. Taking into account the new results, an updated thermonuclear reaction rate has been calculated. At the temperatures of classical novae explosions, the uncertainty on the reaction rate has been reduced by about one order of magnitude compared to the literature.
La reazione 22Ne(p,gamma)23Na fa parte del ciclo neon-sodio per il bruciamento dell' idrogeno. Il ciclo neon-sodio gioca un ruolo fondamentale per la sintesi degli elementi con massa A = 20-25 nelle stelle in fase di asymptotic giant branch, nelle esplosioni di novae di tipo classico e nelle esplosioni di supernovae di tipo Ia, dove il bruciamento di idrogeno avviene ad alte temperature. In particolare, la reazione 22Ne(p,gamma)23Na è la più incerta del ciclo neon-sodio. L'incertezza sulla sezione d'urto è dovuta al contributo, alle energie di interesse astrofisico, di un gran numero di risonanze. Alcune di queste risonanze non sono mai state osservate, per altre, invece, l'intensità è conosciuta con grande incertezza. Per misurare la sezione d'urto della 22Ne(p,gamma)23Na alle energie di interesse astrofisico, due esperimenti sono stati condotti nell'ambito di questa tesi: il primo, svolto con l'apparato sperimentale di LUNA, ha permesso di esplorare le risonanze di energia inferiore a 400 keV. Il secondo, invece, è stato svolto all'Helmoltz-Zentrum Dresden-Rossendorf (HZDR), in Germania, ed ha permesso di miglirare la precisione sulle intensità delle risonanze tra 400 e 660 keV. Per la misura svolta al Gran Sasso è stato usato un bersaglio di tipo gassoso senza finestre di ingresso e i fotoni emessi nel decadimento del 23Na sono stati osservati con due rivelatori al germanio. L'esperimento svolto a LUNA ha permesso di rivelare per la prima volta tre risonanze. Per queste risonanze sono stati osservati anche nuovi modi di decadimento gamma. Questo ha permesso di ampliare gli schemi di decadimento di letteratura. Questa misura ha permesso, inoltre, di ridurre di due ordini di grandezza i limiti superiori sulle intensità di tre risonanze la cui esistenza è tuttora dubbia. Per l'esperimento svolto all'HZDR è stato utilizzato un bersaglio solido di 22Ne e due rivelatori al germanio circondati da schermi anti Compton. I target sono stati realizzati all'impiantatore da 200 kV dei Laboratori Nazionali di Legnaro impiantando il 22Ne su una targhetta di tantalio. L'intensità delle risonanze tra 400 e 660 keV è stata misurata usando come riferimento le risonanze a 1279 keV e 478 keV, che sono intense e ben note. Questo esperimento ha permesso di ridurre l'incertezza sull'intensità della risonanza a 436 keV di un fattore tre, mentre, per la risonanza a 661 keV, è stata determinata un'intensità un ordine di grandezza inferiore rispetto alla letteratura. Il rate di reazione astrofisico è stato aggiornato tenendo conto dei nuovi risultati descritti sopra. Alle temperature caratteristiche delle esplosioni di novae di tipo classico, l'incertezza sul nuovo rate è un ordine di grandezza inferiore rispetto alla letteratura.

>Magister Scientiae - MSc
This project aimed to investigate proton unbound states in 32Cl using the 32S(3He; t) charge-exchange reaction. This research is relevant both in the context of nuclear structure and astrophysics. Excited states in 32Cl up to Ex 6 MeV were produced using a 50 MeV 3He++ beam from the K200 separated sector cyclotron at iThemba LABS. The triton ejectiles were mass analysed and detected at the focal plane of the K600 magnetic spectrometer. An additional segmented silicon detector array called CAKE was used to detect the unbound protons from states in 32Cl in conjunction with the tritons. In this work we looked for potential sources of isospin admixture that could explain the apparent violation of the Isobaric Multiplet Mass Equation (IMME) for the A = 32, T = 2 quintet. We also investigated the possibility of determining the 31S(p; ) reaction rate indirectly, via measurements of the partial proton widths of unbound states in 32Cl.

>Magister Scientiae - MSc
Measurements of nuclear level lifetimes are an important aspect of experimental nuclear physics. Such measurements determine transition matrix elements for nuclear structure research and also provide the widths of relevant excited states in nuclei that are of astrophysical interest. In the latter, the measured widths are used to obtain reaction rates in main sequence stars such as the Sun and in binary-star systems where the accretion of material from one star to another provides an opportunity to study extreme stellar environments such as novae and x-ray bursts. This thesis work describes the design and simulation of a new experimental set up at iThemba LABS that will allow for highprecision femtosecond-level lifetime measurements of nuclear states using the Doppler Shift Attenuation Method (DSAM). We use the Solid Edge computer-aided design (CAD) software to design a new scattering chamber with a cooled target ladder specifically for such measurements using inverse-kinematic transfer reactions with ion implanted targets. The light charged ejectiles from the reaction will be detected with a ΔE - E silicon telescope, while Doppler shifted rays will be registered using a high-purity and 100% efficient germanium (HPGe) detector. We also describe preliminary Monte Carlo simulation codes that are being developed in a relativistically invariant framework to optimize the experimental set up and to obtain predicted lineshapes of γ rays from several astrophysically relevant states in nuclei using this experimental set up.
National Research Foundation (NRF)

In dieser Diplomarbeit wurde das astrophysikalisch interessante Resonanztriplett der Reaktion 40Ca(α,γ)44Ti bei 4,5MeV untersucht. Am 3-MV-Tandetron des Helmholtz-Zentrums Dresden-Rossendorf wurden dafür die Energien von Protonen- und -Strahlen kalibriert, Anregungsfunktionen im Energiebereich der drei Resonanzen aufgenommen, vier CaOTargets aktiviert und deren Struktur mittels der Reaktion 40Ca(p,γ)41Sc überprüft. Im Felsenkeller-Niederniveaumesslabor wurde anschließend die Aktivität der Proben gemessen. Schließlich konnte die Summe der Resonanzstärken bei 4497 und 4510 keV -Energie im Laborsystem zu (12;8 2;3) eV und die Summe der Resonanzstärken des gesamten Tripletts, d.h. zusätzlich bei 4523 keV, zu (12;0 2;0) eV bestimmt werden. Bei der ersten Resonanzstärke konnte die Unsicherheit im Vergleich zur Literatur von 19% auf 18% verbessert werden. Außerdem bieten die Daten der vorliegenden Arbeit die Grundlage, zukünftig die Unsicherheiten noch erheblich weiter zu reduzieren
In this thesis the astrophysically interesting resonance triplet of the 40Ca(α ,γ)44Ti reaction at 4.5MeV has been studied. For this purpose energies of proton and beams provided by 3MVTandetron at Helmholtz-Zentrum Dresden-Rossendorf have been calibrated. Excitation functions of energy regions near the resonances and in-beam spectra of four different targets have been measured. The 40Ca(p,γ)41Sc reaction has been used to scan the structure of the activated targets. Afterwards their activity has been measured in the underground laboratory Felsenkeller Dresden. Hence the sum of resonance strengths at laboratory energies of 4497 and 4510 keV of (12:8 2:3) eV has been determined as well as the sum of the total triplet strength, including 4523 keV, of (12:0 2:0) eV. In the case of the first resonance, the uncertainty was decreased from 19% to 18 %. Furthermore the results of this work establish a basis for reaching much lower uncertainties in the future

Plusieurs observations astronomiques indiquent que la densité de matière dans notre Univers est dominée par sa composante invisible. Parmi les particules pouvant constituer cette matière, le neutralino est un candidat supersymétrique favorisé. Cette thèse s'est faite dans le cadre de la collaboration EDELWEISS. Cette expérience tente de mettre en évidence l'interaction d'un neutralino avec un noyau cible. Les bolomètres utilisés par EDELWEISS associent à la détection des phonons celle des charges créées par le dépôt d'énergie. Cette double détection permet de rejeter une partie importante du bruit de fond associé aux interactions électroniques. Actuellement, les expériences utilisant des bolomètres double composante permettant une discrimination du bruit de fond sont les plus sensibles. Les sensibilités diminuent en augmentant la masse de détecteurs et bientôt ces expériences commenceront à être limitées par le bruit de fond neutron induit par les muons cosmiques résiduels. Ce travail de thèse présente une étude détaillée des interactions inélastiques des muons (d'énergie moyenne 300 GeV au Laboratoire Souterrain de Modane) et de la production de neutrons engendrés par les muons dans différents matériaux à l'aide de simulation GEANT. Cette étude s'inscrit dans le développement des simulations de l'expérience EDELWEISS-II qui prévoit à terme l'installation de 120 détecteurs germanium. Pour rendre ces simulations les plus réalistes possibles, nous intégrons les distributions caractéristiques des muons au LSM obtenues à l'aide des données du détecteur Fréjus. Les flux de neutrons produits par les muons atteignent un tel niveau que la collaboration a décidé de mettre en place un veto muon afin de signer leur passage à l'intérieur de l'expérience EDELWEISS-II. Nous développons aussi un étude expérimentale d'un scintillateur plastique qui sera le constituant principal du veto muon. L'expérience M3 (Mesure Muon Modane) permet l'identification des muons au LSM et donne accès au spectre expérimental des pertes d'énergie des muons par ionisation. Enfin, les résultats obtenus par simulation sur l'efficacité de détection du veto muon seront présentés.Plusieurs observations astronomiques indiquent que la densité de matière dans notre Univers est dominée par sa composante invisible. Parmi les particules pouvant constituer cette matière, le neutralino est un candidat supersymétrique favorisé. Cette thèse s'est faite dans le cadre de la collaboration EDELWEISS. Cette expérience tente de mettre en évidence l'interaction d'un neutralino avec un noyau cible. Les bolomètres utilisés par EDELWEISS associent à la détection des phonons celle des charges créées par le dépôt d'énergie. Cette double détection permet de rejeter une partie importante du bruit de fond associé aux interactions électroniques. Actuellement, les expériences utilisant des bolomètres double composante permettant une discrimination du bruit de fond sont les plus sensibles. Les sensibilités diminuent en augmentant la masse de détecteurs et bientôt ces expériences commenceront à être limitées par le bruit de fond neutron induit par les muons cosmiques résiduels. Ce travail de thèse présente une étude détaillée des interactions inélastiques des muons (d'énergie moyenne 300 GeV au Laboratoire Souterrain de Modane) et de la production de neutrons engendrés par les muons dans différents matériaux à l'aide de simulation GEANT. Cette étude s'inscrit dans le développement des simulations de l'expérience EDELWEISS-II qui prévoit à terme l'installation de 120 détecteurs germanium. Pour rendre ces simulations les plus réalistes possibles, nous intégrons les distributions caractéristiques des muons au LSM obtenues à l'aide des données du détecteur Fréjus. Les flux de neutrons produits par les muons atteignent un tel niveau que la collaboration a décidé de mettre en place un veto muon afin de signer leur passage à l'intérieur de l'expérience EDELWEISS-II. Nous développons aussi un étude expérimentale d'un scintillateur plastique qui sera le constituant principal du veto muon. L'expérience M3 (Mesure Muon Modane) permet l'identification des muons au LSM et donne accès au spectre expérimental des pertes d'énergie des muons par ionisation. Enfin, les résultats obtenus par simulation sur l'efficacité de détection du veto muon seront présentés.

For about 20 years now, observations of 6Li in several old metal-poor stars inside the halo of our galaxy have been reported, which are largely independent of the stars’ metallicity, and which point to a possible primordial origin. The observations exceed the predictions of the Standard Big-Bang Nucleosynthesis model by a factor of 500. In the relevant energy range, no directly measured S-factors were available yet for the main production reaction 2H(α,γ)6Li, while different theoretical estimations have an uncertainty of up to two orders of magnitude. The very small cross section in the picobarn range has been measured with a deuterium gas target at the LUNA acceler- ator (Laboratory for Underground Nuclear Astrophysics), located deep underground inside Laboratori Nazionali del Gran Sasso in Italy. A beam-induced, neutron-caused background in the γ-detector occurred which had to be analyzed carefully and sub- tracted in an appropriate way, to finally infer the weak signal of the reaction. For this purpose, a method to parameterize the Compton background has been developed. The results are a contribution to the discussion about the accuracy of the recent 6Li observations, and to the question if it is necessary to include new physics into the Standard Big-Bang Nucleosynthesis model.

A hipótese de que a matéria estranha de quarks (um plasma composto por quarks up, down e strange em quantidades aproximadamente iguais) é estável `a pressão nula vem sendo considerada há mais de vinte anos, tanto teoricamente quanto em experimentos específicos para sua eventual detecção. Se a matéria estranha é de fato estável, então poderia haver importantes implicações para a Astrofísica. Entre as mais estimulantes destas implicações está a possibilidade de conversão da matéria nuclear ordinária em matéria estranha no interior das estrelas de nêutrons devido ás altíssimas densidades ali encontradas. Processos tais como coalescência em binárias de estrelas de nêutrons e as próprias supernovas, eventos responsáveis pelo nascimento destas estrelas, poderiam ejetar estruturas finitas de matéria estranha, chamadas strangelets (equivalentes aos núcleos), no meio interestelar. Desta forma, strangelets estariam presentes entre os primários de raios cósmicos e seriam sujeitas a processos elementares análogos aos núcleos ordinários. Nesta Tese, as strangelets são estudadas desde os prováveis sítios para sua produção astrofísica, passando por interações com a matéria do meio interestelar, ate a chegada ás imediações terrestres. Estima-se o fluxo de strangelets de baixa energia que poderia ser mantido aprisionado na magnetosfera da Terra, bem como são estudados os processos de suas interações com componentes da atmosfera, com vistas às assinaturas observacionais então resultantes. Desta forma, são determinadas as características relevantes para a identificação destes exóticos por experimentos que testam o fluxo de raios cósmicos, ajudando na melhor compreensão das propriedades da matéria nuclear em alta densidade bariônica e baixa temperatura.
The strange quark matter hypothesis, which states that a plasma composed of quarks up, down and strange in roughly equal amounts is absolutely stable at zero pressure, has been studied for more than twenty years, both theoretically and during searches for its detection in specific experiments. If strange quark matter is indeed stable, then there could be important implications for the field of Astrophysics. Among the most stimulating ones is the possibility of conversion of ordinary nuclear matter in strange quark matter in the interior of neutron stars due to the extremely high densities reached in the core of these compact objects. Processes such as the merger in neutron star binaries systems and supernovae themselves, responsible for the birth of these stars, may eject lumps of strange quark matter, termed strangelets, in the interstellar medium. In this way, strangelets may be present among the cosmic ray flux and be subjected to elementary processes much in the same way as ordinary nuclei. In this Thesis, strangelets are studied from their likely astrophysical production sites, passing through the interstellar medium until they reach the Earth neighborhood. Estimates of the low energy flux of strangelets that could be trapped in the terrestrial magnetosphere are given. Also, the interaction of these particles with components in the Earth atmosphere are studied with the aim of providing better understanding of the resulting observational signatures. It allows the determination of the relevant characteristics for the identification of these exotics by experiments testing the cosmic ray flux, helping to better understand the properties of nuclear matter at high densities and low temperatures.

Cette thèse se compose de deux parties différentes qui s'inscrivent dans une thématique générale astrophysique. Les titres de ces parties sont : "La capture alpha en cinématique inverse liée au processus p ; mesure de la réaction 78Kr( $/alpha$ , $/gamma$ )82Sr "et "Mesures de la décroissance de 19O et 19Ne implantés dans le niobium ". L'objet de la première partie est directement liée à l'astrophysique nucléaire. Il s'agit d'établir une technique expérimentale pour la mesure directe à basse énergie de sections efficaces de réactions de capture radiative alpha en cinématique inverse. Ces réactions sont importantes en astrophysique, elles vont permettre d'améliorer les potentiels du modèle optique pour les particules alpha utilisés dans des modèles nucléaires pour prédire les sections efficaces des réactions ayant lieu dans les supernovae. Ici, nous avons insisté surtout sur la faisabilité technique de ce type d'expériences. En seconde partie de la thèse, on a examiné l'influence de l'environnement sur les probabilités de décroissance bêta d'un noyau, et en particulier de l'influence de l'écrantage électronique de la barrière de Coulombienne par les paires de Cooper d'un matériau supraconducteur. Un effet extrêmement faible, dans la limite des erreurs de mesure, aurait été observé.

For about 20 years now, observations of 6Li in several old metal-poor stars inside the halo of our galaxy have been reported, which are largely independent of the stars’ metallicity, and which point to a possible primordial origin. The observations exceed the predictions of the Standard Big-Bang Nucleosynthesis model by a factor of 500. In the relevant energy range, no directly measured S-factors were available yet for the main production reaction 2H(α,γ)6Li, while different theoretical estimations have an uncertainty of up to two orders of magnitude. The very small cross section in the picobarn range has been measured with a deuterium gas target at the LUNA acceler- ator (Laboratory for Underground Nuclear Astrophysics), located deep underground inside Laboratori Nazionali del Gran Sasso in Italy. A beam-induced, neutron-caused background in the γ-detector occurred which had to be analyzed carefully and sub- tracted in an appropriate way, to finally infer the weak signal of the reaction. For this purpose, a method to parameterize the Compton background has been developed. The results are a contribution to the discussion about the accuracy of the recent 6Li observations, and to the question if it is necessary to include new physics into the Standard Big-Bang Nucleosynthesis model.

The proper understanding of the equation of state (EoS) of highly asymmetric nuclear matter is essential when studying systems such as neutron stars (NSs). Using zero-range Skyrme interactions and finite-range interactions such as Gogny forces, momentum-dependent interactions (MDI) and simple effective interactions (SEI), we analyze the properties of the EoS and the influence they may have on the calculations for NSs. We start by studying the convergence properties of the Taylor series expansion of EoS in powers of the isospin asymmetry. Next, we analyze the accuracy of the results for β-stable nuclear matter, which is found in the interior of NSs, when it is computed using the Taylor expansion of the EoS. The agreement with the results obtained using the full expression of the EoS is better for interactions with small-to-moderate values of the slope of the symmetry energy L. The mass and radius relation for a NS is obtained by integrating the so-called Tolman-Oppenheimer-Volkoff (TOV) equations, where the input is the EoS of the system. We have studied the mass-radius relation for Skyrme and Gogny interactions, and we see that that very soft forces are not able to give stable solutions of the TOV equations and only the stiff enough parametrizations can provide 2M0 NSs. We also notice that none of the existing parametrizations of the standard Gogny D1 interaction is able to provide a NS inside the observational constraints. Because of that, we propose a new parametrization, which we name D1M∗, that is able to provide NSs of 2M0 while still providing the same good description of finite nuclei as D1M. A parametrization D1M∗∗ is also presented, which is fitted in the same way as D1M∗ and provides NSs up to 1.91M0. Moreover, we estimate the core-crust transition in NSs by finding where the nuclear matter in the core is unstable against fluctuations of the density. To do that, we employ two methods, the thermodynamical method and the dynamical method. In the case of finite-range interactions, such as the Gogny ones, to use the dynamical method we have had to derive the explicit expression of the energy curvature matrix in momentum space for this type of interactions. We observe a decreasing trend of the transition density with the slope L of the symmetry energy, while the correlation between the transition pressure and L is much lower. Finally different NS properties are studied. The crustal properties, such as the crustal mass, crustal thickness and crustal fraction of the moment of inertial have lower values if one computes them using the core-crust transition density obtained with the dynamical method instead of the one obtained with the thermodynamical method, pointing out the importance of the accurate evaluation of the transition density when studying observational phenomena. We have also studied the moment of inertia of NSs, which is compared to constraints proposed in the literature. Finally, the tidal deformability for NSs is also calculated and compared with the constraints coming from the GW170817 event detected by the LIGO and Virgo observatories and which accounts for the merger of two NSs in a binary system.
El coneixement de l’equació d’estat (EoS) de matèria altament densa i assimètrica és essencial per tal d’estudiar les estrelles de neutrons (NSs). En aquesta tesi s’analitzen, utilitzant interaccions de camp mig no relativistes, les propietats de l’EoS i la seva influència en càlculs de NSs. Primerament, s’estudia la convergència del desenvolupament en sèrie de Taylor de l’EoS en potències de l'assimetria d’isospí. Seguidament, s’analitza l’exactitud dels resultats per matèria β-estable, la qual es troba a l’interior de les NSs, quan es calcula utilitzant el desenvolupament de Taylor de l’EoS. La relació entre la massa i el radi obtinguda integrant les equacions Tolman-Oppenheimer-Volkoff (TOV) també és estudiada. A causa de que les interaccions de Gogny de la família D1 no aconsegueixen donar NSs compatibles amb observacions astrofísiques, en aquesta tesi proposem dues noves forces de Gogny, anomenades D1M∗ i D1M∗∗, les quals poden donar, respectivament, NSs de 2 i 1.91 masses solars. Una altra part de la tesi es dedica a l’estudi de la transició entre l’escorça i el nucli, buscant la densitat a la qual la matèria uniforme al nucli és inestable contra fluctuacions de densitat. Ho estudiem amb dos mètodes, el mètode termodinàmic i el mètode dinàmic. Finalment, s’analitzen diverses propietats de les NSs, com són la relació entre la massa i el radi de l’estrella, les propietats de l’escorça, el moment d’inèrcia, així com la deformació deguda als corrents de marea (tidal deformability) que està relacionada amb l’emissió d’ones gravitacionals en sistemes binaris d’estrelles de neutrons.

L'émission gamma des novae à, et en dessous, de 511 keV provient essentiellement de l'annihilation des positrons venant de la décroissance beta+ du 18F. L'interprétation de cette émission, à l'aide d'observations par le satellite INTEGRAL par exemple, nécessite une bonne connaissance de la nucléosynthèse du 18F. Dans ce contexte, le taux de la réaction 18F(p,alpha)15O est le moins bien connu à cause de deux résonances correspondant aux niveaux excités Ex = 6.419 et 6.449 MeV dans le 19Ne dont les largeurs protons sont totalement inconnues. Nous avons déterminé ces largeurs protons par le biais d'une réaction de transfert d'un nucléon D(18F,p alpha)15N peuplant les niveaux analogues, dans le 19F, des niveaux d'intérêt astrophysique. Nous avons utilisé un faisceau radioactif de 18F accéléré à 14 MeV au Centre de Recherche du Cyclotron de Louvain--la--Neuve sur une cible de CD2 en cinématique inverse ainsi que le détecteur multi--piste au silicium LEDA. Une analyse en DWBA a permi de déterminer la largeur proton de ces deux résonances et a montré qu'elles ne pouvaient pas être négligées dans le calcul du taux de réaction. Une étude détaillée des incertitudes restantes sur le taux de réaction a été entreprise et particulièrement en ce qui concerne les interférences entre ces résonances et une autre résonance à plus haute énergie dans le 19Ne. Le taux de réaction ainsi établi diffère peu de l'ancien taux nominal mais repose maintenant sur des bases plus solides permettant une meilleure interprétation des futures observations gamma des novae et donc une meilleure contrainte des modèles astrophysiques.

Ce manuscrit de thèse présente l’étude des processus d’émission de deux types de noyaux actifs de galaxie détectés aux rayons gamma : des blazars vues au TeV et des Narrow Line Seyfert 1s (NLS1s) détectés au GeV. La distribution spectrale d’énergie des blazars peut être décrite en général par des modèles 'one-zone synchrotron self-Compton'. Ce modèle a été appliqué au blazar 1ES2322-409 qui fut premièrement détecté au TeV par l’expérience HESS. Des composantes externes comme le tore, le disque d’accrétion, la couronne X ou la 'Broad Line Region' sont nécessaires pour expliquer la radiation observée dans des NLS1 qui émettent des rayons gamma. Un modèle numérique qui considère ces champs des photons externes a été développé. Ce modèle explique l’émission observée et la transition entre des états bas et des états d’émission augmentée pour trois NLS1s vues au régime gamma : 1H0323+342, B20954+25A et PMN J0948+0022
This thesis manuscript presents the study of the emission processes of two types of gamma-emitting active galactic nuclei: TeV-detected blazars and GeV-detected Narrow Line Seyfert 1s (NLS1s).The Spectral Energy Distribution (SED) of TeV blazars can in general be well described by simple one-zone synchrotron self-Compton models. Such model has been applied to the blazar 1ES2322-409 that was first detected at TeV by the HESS collaboration.Additional external photon fields such as the obscuring torus, the accretion disc, the X-ray corona or the broad line region are necessary to describe the observed radiation and broad-band SED of gamma-emitting NLS1s. A numerical model that takes into account emission from these external fields has been developed. The model explains the observed emission and the transition from quiescent to gamma-ray flaring states of three gamma-emitting NLS1s: 1H0323+342, B20954+25A and PMN J0948+0022

Thesis (Ph. D.)--Georgia State University, 2008.
Title from file title page. Xiaochun He, committee chair; William Nelson, Steven Manson, Brian Thoms, Douglas Gies, committee members. Description based on contents viewed Aug. 21, 2009. Includes bibliographical references (p. 104-108).

Le spectromètre magnétique AMS sera installé sur la station spatiale internationale ISS en 2003. Il sera en particulier équipé d'un imageur d'anneaux Tcherenkov à focalisation de proximité (RICH). Après une présentation des objectifs de recherche de la collaboration internationale AMS et de son détecteur, une étude détaillée du fonctionnement et des performances des RICH à focalisation de proximité est proposée. Le travail se poursuit par une étude par simulation du rayonnement cosmique nucléaire attendu dans AMS grâce au RICH. La thèse rapporte ensuite le travail mené sur le prototype d'imageur conçu, développé et testé à l'Institut des Sciences Nucléaires de Grenoble, de 1997 à 1999. Le fonctionnement du détecteur et son étalonnage est expliqué. Le travail se poursuit par l'analyse des données prises pendant les campagnes de tests au moyen des rayons cosmiques au sol et sur faisceau d'ions, auprès de l'accélérateur de GSI-Darmstadt (Allemagne). Les données expérimentales sont comparées aux résultats du programme de simulation. Enfin, les résultats d'une campagne de test simulant la détection de particules " Albedo " sont présentés et interprétés.

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.

Tese de mestrado em Engenharia Física, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2012
A dissertação ”Experimental advances on alpha nuclear potentials for the astrophysical pprocess” reflecte a actividade experimental realizada durante o período de estágio no ATOMKI, em Debrecen na Hungria, realizado sobre alçada do programa ERASMUS. Nesta dissertação será descrito o processo de preparação para a experiência de dispersão de partículas α em alvos de 64Zn, 113In e 115In. As ligações eléctricas, detectores e respectiva calibração, pré-amplificadores, amplificadores, assim como a resultante montagem experimental serão descritas em detalhe. Os resultados da experiência de dispersão de partículas α irão permitir calcular a secção eficaz dos elementos estudados, efectuar a comparação dos dados experimentais com as previsões teóricas dos modelos globais de potenciais α-nucleares. Permitirão também um estudo detalhado de ”famílias” de potenciais locais com vista a melhorar os modelos de previsão teóricos e assim aprofundar o conhecimento na área da astrofísica.
The subject of the present Master Thesis ”Experimental advances on alpha nuclear potentials for the astrophysical p-process” is the experimental activity performed during my training period at ATOMKI in Debrecen, Hungary, under the ERASMUS program tutelage. In this dissertation the process will be described for preparation of the experiment of elastic α scattering from 64Zn, 113In and 115In targets. The electric connections, detectors and their calibration, pre-amplifiers, amplifiers, and the overall experimental setup is described in detail. The results of this experiment will allow the calculation of the cross section of the studied elements, perform the comparison of the experimental data with theoretical predictions of the most accepted models for global α–nucleus potentials . The results will also allow for a detailed study about the ”families” of local α–nucleus potentials in order to improve the predictive power of the existent theoretical models and the overall knowledge in the area of astrophysics.