Within cosmology, there are two entirely independent pillars which can jointly drive this field towards precision: Astronomical observations of primordial element abundances and the detailed surveying of the cosmic microwave background. However, the comparatively large uncertainty stemming from the nuclear physics input is currently still hindering this effort, i.e. stemming from the 2H(p,γ)3He reaction. An accurate understanding of this reaction is required for precision data on primordial nucleosynthesis and an independent determination of the cosmological baryon density. Elsewhere, our Sun is an exceptional object to study stellar physics in general. While we are now able to measure solar neutrinos live on earth, there is a lack of knowledge regarding theoretical predictions of solar neutrino fluxes due to the limited precision (again) stemming from nuclear reactions, i.e. from the 3He(α,γ)7Be reaction. This thesis sheds light on these two nuclear reactions, which both limit our understanding of the universe. While the investigation of the 2H(p,γ)3He reaction will focus on the determination of its cross- section in the vicinity of the Gamow window for the Big Bang nucleosynthesis, the main aim for the 3He(α,γ)7Be reaction will be a measurement of its γ-ray angular distribution at astrophysically relevant energies. In addition, the installation of an ultra-low background counting setup will be reported which further enables the investigation of the physics of rare events. This is essential for modern nuclear astrophysics, but also relevant for double beta decay physics and the search for dark matter. The presented setup is now the most sensitive in Germany and among the most sensitive ones worldwide.

A possible mechanism to explain the origin of around 35 neutron–deficient stable isotopes with mass A≥75 between 74 Se and 196 Hg, known as the p–nuclei is the nucleosynthesis in the proton–rich neutrino–driven winds of core–collapse supernovae via the νp–process. However this production scenario is very sensitive to the underlying supernova dynamics and the nuclear physics input. As far as nuclear uncertainties are concerned, the breakout reaction from the pp-chains, 7Be(α, γ)11C, has been identified as an important link which can influence the nuclear flow and therefore the efficiency of the νp–process. However its reaction rate is not well known over the relevant energy range (T9 = 1.5–3). In this thesis we report on the direct first measurement of two resonances of the 7Be(α, γ)11 C reaction with previously unknown strengths using an intense radioactive 7Be beam from ISAC and the DRAGON recoil separator in inverse kinematics. Since resonance strength measurements with low mass beams using recoil separators depend strongly on the recoil angular distribution, which can exceed the acceptance of the separator, we first performed a proof–of–principle test by measuring a known resonance of the 6Li(α, γ)10B reaction, which also presents a similar challenge. Our results from the 6Li(α, γ)10B reaction are in agreement with literature, showing that DRAGON can measure resonance strengths of reactions for which the maximum momentum cone of the recoils exceeds its acceptance. From the newly measured 7Be(α, γ)11C resonance strengths we calculated the new reaction rate which is lower than the current recommended by 10–50% and constrained to 5–10% in the relevant temperature region. Using this new rate, we performed detailed nucleosynthesis calculations which suggest that there is no effect the production of light p–nuclei, but a production increase for CNO elements of up to an order of magnitude is observed. / Dissertation / Doctor of Philosophy (PhD)

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.

Dans les milieux astrophysiques explosifs tels que les novae ou les sursauts X, la densité d'hydrogène et la température sont suffisamment grandes pour que le temps entre deux réactions impliquant un proton soit plus court que le temps de vie de certains ions radioactifs. La connaissance de la section efficace des réactions de capture d’un proton par un ion radioactif est un des ingrédients important permettant la modélisation de tels milieux. Dans ce travail, un nouveau dispositif expérimental permettant d'étudier la force de résonance de réactions (p,gamma) en cinématique inverse est présenté. Ce dispositif, baptisé ARES (Astrophysical REcoil Separator), a été d’abord caractérisé à l'aide de l'étude de la réaction 19F(p,gamma)20Ne et plus particulièrement de la mesure de la force de la résonance bien connue à 635 keV au dessus du seuil 19F+p. De plus, la simulation de cette expérience est en accord avec les mesures effectuées. Une première mesure de force de résonance d'une réaction impliquant un faisceau d'ions radioactifs est ensuite présentée. Il s'agit de la réaction 19Ne(p,gamma)20Na et plus particulièrement de la résonance à 448 keV au dessus du seuil 19Ne+p. Une limite supérieure de 15.2 meV avec un niveau de confiance de 90% est obtenue. Cette limite supérieure améliore légèrement les résultats de mesures antérieures. / In explosive astrophysical environments such as novae or X-ray bursts, the temperature and the hydrogen density are so large that the time between two reactions involving protons is smaller than the live time of radioactive ions. The cross section of such reactions is an important ingredient of the modeling of such environments. In this work, a new experimental device, allowing the study of resonance strength of (p,gamma) reactions, is presented. This setup, called ARES (Astrophysical REcoil Separator), is first characterized using the study of the well known reaction, 19F(p,gamma)20Ne and more precisely the measurement of the resonance strength of the 635 keV level above the 19F+p threshold. The simulation of this experiment is found in good agreement with the measurement. Then the first resonance strength measurement of a reaction involving radioactive ions beams is presented, i.e. the resonance strength of the 448 keV level above the 19Ne+p threshold in the 19Ne(p,gamma)20Na reaction. An upper limit of 15.2 meV with a confidence level of 90% is obtained. This upper limit improves slightly the results of previous measurements.

For many centuries, the study of the Sun has been an important testbed for understanding stars that are further away. One of the first astronomical observations Galileo Galilei made in 1612 with the newly invented telescope concerned the sunspots, and in 1814, Joseph von Fraunhofer employed his new spectroscope to discover the absorption lines in the solar spectrum that are now named after him. Even though more refined and new modes of observation are now available than in the days of Galileo and Fraunhofer, the study of the Sun is still high on the agenda of contemporary science, due to three guiding interests. The first is connected to the ages-old human striving to understand the structure of the larger world surrounding us. Modern telescopes, some of them even based outside the Earth’s atmosphere in space, have succeeded in observing astronomical objects that are billions of lightyears away. However, for practical reasons precision data that are important for understanding stars can still only be gained from the Sun. In a sense, the observations of far-away astronomical objects thus call for a more precise study of the closeby, of the Sun, for their interpretation. The second interest stems from the human desire to understand the essence of the world, in particular the elementary particles of which it consists. Large accelerators have been constructed to produce and collide these particles. However, man-made machines can never be as luminous as the Sun when it comes to producing particles. Solar neutrinos have thus served not only as an astronomical tool to understand the Sun’s inner workings, but their behavior on the way from the Sun to the Earth is also being studied with the aim to understand their nature and interactions. The third interest is strictly connected to life on Earth. A multitude of research has shown that even relatively slight changes in the Earth’s climate may strongly affect the living conditions in a number of densely populated areas, mainly near the ocean shore and in arid regions. Thus, great effort is expended on the study of greenhouse gases in the Earth’s atmosphere. Also the Sun, via the solar irradiance and via the effects of the so-called solar wind of magnetic particles on the Earth’s atmosphere, may affect the climate. There is no proof linking solar effects to short-term changes in the Earth’s climate. However, such effects cannot be excluded, either, making it necessary to study the Sun. The experiments summarized in the present work contribute to the present-day study of our Sun by repeating, in the laboratory, some of the nuclear processes that take place in the core of the Sun. They aim to improve the precision of the nuclear cross section data that lay the foundation of the model of the nuclear reactions generating energy and producing neutrinos in the Sun. In order to reach this goal, low-energy nuclear physics experiments are performed. Wherever possible, the data are taken in a low-background, underground environment. There is only one underground accelerator facility in the world, the Laboratory Underground for Nuclear Astrophysics (LUNA) 0.4MV accelerator in the Gran Sasso laboratory in Italy. Much of the research described here is based on experiments at LUNA. Background and feasibility studies shown here lay the base for future, higher-energy underground accelerators. Finally, it is shown that such a device can even be placed in a shallow-underground facility such as the Dresden Felsenkeller without great loss of sensitivity.

<p>The astrophysical ³⁰P(<em>p</em>,<em>γ</em>)³¹S reaction rate is a key quantity used in both classical nova and type I X-ray burst models that predict isotopic abundances produced during nucleosynthesis in the outburst. Currently, uncertainties in ³¹S structure parameters lead to a variation in the reaction rate by a factor of 20 at nova temperatures causing predicted isotopic abundance ratios in the Si-Ar mass region to vary by factors of up to 4. The ³⁰P(<em>p</em>,<em>γ</em>)³¹S reaction rate can be determined indirectly by measuring transfer reactions populating excited states in ³¹S. Nuclear structure information for ³¹S resonant states above the proton threshold of 6131 keV and within the Gamow window that contribute most significantly to the reaction rate can be used to re-evaluate the rate for nova and type I X-ray burst temperatures and reduce current uncertainties. We have performed an experiment in order to study the level structure of ³¹S via the ³²S(<em>d</em>,<em>t</em>)³¹S single-nucleon transfer reaction using the MP tandem accelerator and Q3D magnetic spectrograph at MLL in Munich, Germany. Excited states of ³¹S in the 6-7 MeV region were observed and spin-parity constraints have been suggested. In this work we describe the experimental setup, data analysis and results for both experiments and provide recommendations for further investigation of the ³⁰P(<em>p</em>,<em>γ</em>)³¹S astrophysical reaction rate.</p> / Master of Science (MSc)

<p>This thesis describes a new experimental approach that has been designed to determine the alpha-branching ratio of the 4.03-MeV excited state of Ne-19. A precise measurement of this quantity is needed to reduce reaction rate uncertainties in Type I x-ray bursts models. The alpha-branching will be measured by the detection of the beta+ activity of the associated alpha-decay product. This activity method has been modelled using two separate simulations. The first, a Monte Carlo code to simulate the reaction process and energy distributions of the decay products. Secondly, a GEANT4 simulation was created to predict the detector response to the O-15 beta+ activity. Along with the simulations two NaI(Tl) detectors, which were customized to this experiment's geometric constraints, have been tested and their response and resolution have been determined. The results of this work will be used to refine the experimental setup such that the proposed test run and eventual alpha-branching ratio measurement of the 4.03-MeV state will be successful. With the results of the simulations and subsequent yield calculations, it has been found that reasonable statistical significance in the O-15 yield from the 4.03-MeV excited state in Ne-19 can be achieved within 10 days of beamtime.</p> / Master of Science (MSc)

L'astrophysique nucléaire est la discipline scientifique qui étudie la production d'énergie et la synthèse des éléments (nucléosynthèse) dans les étoiles. Les réactions nucléaires entre des noyaux légers (A < 20-30) et des protons ou des particules alpha jouent un rôle fondamental. Elles produisent les éléments lourds à partir des éléments plus légers et permettent à l'étoile de dégager de l'énergie. Cette production d'énergie et des éléments explique à son tour la structure et l'évolution des étoiles et, donc de l'Univers. Quels que soient les processus considérés (nucléosynthèse primordiale, stellaire ou explosive) le calcul des abondances des éléments dans les étoiles requiert la connaissance de nombreuses sections efficaces de réactions nucléaires [B2FH57]. Les réactions nucléaires sous-coulombiennes entre noyaux légers interviennent au cœur des étoiles lors des phases de combustion de l'hydrogène et de l'hélium en équilibre hydrostatique. Dans des processus à température plus élevée (novae, supernovae, sursauts X) les éléments radioactifs sont largement impliqués dans le réseau de réactions, ce qui, d'autre part, amène des problèmes spécifiques aux expérimentateurs [Bla06]. Ma dissertation est une récapitulation des travaux expérimentaux et théoriques en astrophysique nucléaire que j'ai réalisés de janvier 1993 à janvier 2006 (cette thèse ne présente pas mes autres travaux en physique nucléaire, principalement sur la structure et les interactions des noyaux exotiques, voir par exemple [Ang03a, Raa04, Cas06]). Je présente quelques méthodes expérimentales en astrophysique nucléaire et je discute brièvement certains problèmes techniques liés aux mesures des sections efficaces très petites, et en particulier, au caractère spécifique de certains appareils (accélérateurs, spectromètres, etc.). Un premier exemple se rapporte à la mise au point de l'accélérateur de protons de 250 kV construit au CSNSM à Orsay (France) dont j'ai été responsable de la calibration [Bog94]. Ensuite, je présente les tests du spectromètre de recul ARES [Cou03] construit au Centre de Recherches du Cyclotron et l'étude d'une réaction d'intérêt astrophysique avec le premier faisceau radioactif produit par le cyclotron CYCLONE44 [Cou04]. Je discute également d'une manière générale les mesures de sections efficaces de réactions nucléaires par des méthodes directes et indirectes. Pour les méthodes directes, je me suis intéressée plus particulièrement aux mesures avec des faisceaux de protons très intenses (accélérateur de 100 kV au DTL-Bochum), ainsi qu'aux expériences avec des éléments radioactifs -cibles ou faisceaux- (CSNSM, Orsay et cyclotrons de Louvain-la-Neuve). Dans ce contexte, je présente plusieurs cas de réactions d'intérêt en astrophysique comme, par exemple, 9Be(p,gamma)10B [Zah95] (nucléosynthèse stellaire), 7Be(p,gamma)8B [Ham98, Ham01] (liée au problème du neutrino solaire), 7Be(p,p)7Be [Ang03b] (liée à la réaction 7Be(p,?)8B), 18F(p,alpha)15O [Ser03] (nucléosynthèse explosive dans les novae), 7Be(d,p)2alpha [Ang05a] (nucléosynthèse du Big Bang). Quand une mesure directe est très difficile à réaliser, il existe des méthodes indirectes comme, par exemple, les réactions de transfert. Elles sont utiles, entre autres, dans l'étude des largeurs alpha des états d'importance astrophysique de certains noyaux. Je discute les cas du 19F (nucléosynthèse du fluor) étudié par la réaction 15N(7Li,t)19F [Oli95] et du 19Ne (nucléosynthèse dans les novae) étudié avec 18F(d,p)19F [Ser03]. Un traitement théorique rigoureux de la dépendance en énergie des sections efficaces est nécessaire pour extrapoler ces dernières aux énergies caractéristiques des processus astrophysiques, à partir des sections efficaces expérimentales (prises à des énergies plus élevées). Un des effets dont il faut tenir compte pour ces extrapolations est l'effet d'écrantage électronique en laboratoire [Ass87] qui devient important à des énergies très basses (< 10-20 keV). Parmi les différents modèles théoriques généralement appliqués aux réactions d'intérêt astrophysique, je me suis spécialement intéressée à la méthode de la matrice R [Lan58]. Cette approche contient des paramètres ajustables sur les sections efficaces disponibles, et les valeurs des sections efficaces aux énergies stellaires sont obtenues par extrapolation. Je présente l'étude de l'écrantage électronique de réactions non-résonnantes en appliquant le modèle de la matrice R [Ang98]. Je montre également son application à deux réactions de capture radiative importantes en astrophysique, 12C(alpha,gamma)16O [Ang00] et 14N(p,gamma)15O [Ang01]. L'analyse de la réaction 14N(p,gamma)15O est discutée en détails [For04, Run04, Ang05b]. Elle joue un rôle important dans la détermination de l'âge des amas globulaires, donc dans la détermination de l'âge de l'Univers [Deg04], et dans la nucléosynthèse dans des étoiles géantes rouges [Her06]. Finalement, je discute l'importance des bases de données pour l'astrophysique nucléaire, et en particulier, des compilations de taux de réactions. Je présente la compilation de taux de réactions la plus récente [Ang99], que j'ai coordonnée pendant mon séjour à l'Université Libre de Bruxelles (1995-1998) dans le cadre d'une collaboration entre 10 laboratoires européens (cette publication a actuellement plus de 350 citations à la date de mai 2006). Je présente aussi une compilation des réactions les plus importantes impliquées dans la nucléosynthèse du Big Bang [Des04] et une évaluation de l'effet des nouveaux taux de réactions sur la valeur de la densité baryonique de l'univers [Coc04]. Les conclusions situent ces travaux dans leur contexte actuel.

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.

The Bethe-Weizsäcker cycle consists of a set of nuclear reactions that convert hydrogen into helium and release energy in the stars. It determines the luminosity of low-metal stars at their turn-off from the main-sequence in the Hertzsprung-Russel diagram, so its rate enters the calculation of the globular clusters’ age, an independent lower limit on the age of the universe. The cycle contributes less than 1% to our Sun’s luminosity, but it produces neutrinos that can in principle be measured on Earth in underground experiments and bring direct information of the physical conditions in the solar core, provided that the nuclear reaction rate is known with sufficient precision. The 14N(p,γ)15O reaction is the slowest reaction of the Bethe-Weizs¨acker cycle and establishes its rate. Its cross section is the sum of the contributions by capture to different excited levels and to the ground state in 15O. Recent experiments studied the region of the resonance at Ep = 278 keV. Only one modern data set from an experiment performed in 1987 is available for the high-energy domain. Both energy ranges are needed to constrain the fit of the excitation function in the R-matrix framework and to obtain a reliable extrapolated S-factor at the very low astrophysical energies. The present research work studied the 14N(p,γ)15O reaction in the LUNA (Laboratory for Underground Nuclear Astrophysics) underground facility at three proton energies 0.36, 0.38, 0.40MeV, and in Dresden in the energy range Ep = 0.6 - 2MeV. In both cases, an intense proton beam was sent on solid titanium nitride sputtered targets, and the prompt photons emitted from the reaction were detected with germanium detectors. At LUNA, a composite germanium detector was used. This enabled a measurement with dramatically reduced summing corrections with respect to previous studies. The cross sections for capture to the ground state and to the excited states at 5181, 6172, and 6792 keV in 15O have been determined. An R-matrix fit was performed for capture to the ground state, that resolved the literature discrepancy of a factor two on the extrapolated S-factor. New precise branching ratios for the decay of the Ep = 278 keV resonance were measured. In Dresden, the strength of the Ep = 1058 keV resonance was measured relative to the well-known resonance at Ep = 278 keV, after checking the angular distribution. Its uncertainty is now half of the error quoted in literature. The branching ratios were also measured, showing that their recommended values should be updated. Preliminary data for the two most intense transitions off resonance are provided. The presence in the targets of the other stable nitrogen isotope 15N with its well- known isotopic abundance, allowed to measure the strength of two resonances at Ep = 430 and 897 keV of the 15N(p,αγ)12 C reaction, improving the precision for hydrogen depth profiling.