Transfer reaction measurements and the stellar nucleosynthesis of 26A1 and 44Ti

Progress in the description of stellar evolution is driven by the collaborative effort of nuclear physics, astrophysics and astronomy. Using those developments, the theory of the origin of elements in the Universe is challenged. This thesis addresses the problem behind the abundance of 44Ti and the origin of 26Al. The mismatch between the predicted abundance of 44Ti as produced by the only sites known to be able to create 44Ti, core collapse supernovae (CCSNe), and the observations, highlight the current uncertainty that exists in the physics of these stars. Several satellite based γ-ray observations of the isotope 44Ti have been reported in recent times and confirm the disagreement. As the amount of this isotope in stellar ejecta is thought to critically depend on the explosion mechanism, the ability to accurately model the observed abundance would be a pivotal step towards validating that theory. The most influential reaction to the amount of 44Ti in supernovae is 44Ti(α, p)47V. Here we report on a direct study of this reaction conducted at the REX-ISOLDE facility, CERN. The experiment was performed at a centre of mass energy 4.15±0.23 MeV, which is, for the first time, well within the Gamow window for core collapse supernovae. The experiment employed a beam of 44Ti extracted from highly irradiated components of the SINQ spallation neutron source of the Paul Scherrer Institute. No yield above background was observed, enabling an upper limit for the rate of this reaction to be determined. This result is below expectation, suggesting that the 44Ti(α, p)47V reaction proceeds more slowly than previously thought. Implications for astrophysical events, and remnant age, are discussed. In Wolf-Rayet and asymptotic giant branch (AGB) stars, the 26gAl(p,γ)27Si reaction is expected to govern the destruction of the cosmic γ-ray emitting nucleus 26Al. The rate of this reaction, however, is highly uncertain due to the unknown properties of several resonances in the temperature regime of hydrogen burning. We present a high-resolution inverse kinematic study of the 26gAl(d, p)27Al reaction as a method for constraining the strengths of key astrophysical resonances in the 26gAl(p,γ)27Si reaction. In particular, the results indicate that the resonance at Er = 127 keV in 27Si determines the entire 26gAl(p, γ)27Si reaction rate over almost the complete temperature range of Wolf-Rayet stars and AGB stars. The measurements of spectroscopic factors for many states in 27Al and a shell model calculation of nuclear properties of rp-resonant states in 27Si also allow for testing the structure model

Locating a weak change using diffuse waves (LOCADIFF) : theoretical approach and inversion procedure

We describe a time-resolved monitoring technique for heterogeneous media. Our approach is based on the spatial variations of the cross-coherence of coda waveforms acquired at fixed positions but at different dates. To locate and characterize a weak change that occurred between successive acquisitions, we use a maximum likelihood approach combined with a diffusive propagation model. We illustrate this technique, called LOCADIFF, with numerical simulations. In several illustrative examples, we show that the change can be located with a precision of a few wavelengths and its effective scattering cross-section can be retrieved. The precision of the method depending on the number of source receiver pairs, time window in the coda, and errors in the propagation model is investigated. Limits of applications of the technique to real-world experiments are discussed.Comment: 11 pages, 14 figures, 1 tabl

Levels in 210Fr and the decay of a high-spin, multi-particle isomer

The structure of 210Fr has been studied through the 197Au(18O, 5n)210Fr reaction. A high-spin iso-meric state has been identified at ∼4.4 MeV. It has a lifetime of 686(17) ns and decays by two γ-rays that are very likely to be either M2 or E3 multipol

Imagerie de chargements locaux en régime de diffusion multiple

Dans les milieux fortement hétérogènes, les ondes ne se propagent pas de façon balistique. Elles peuvent interagir de nombreuses fois avec les hétérogénéités du milieu et entrer ainsi dans le régime de diffusion multiple. Dans ce régime, les méthodes classiques d'imagerie basées sur les trajets des ondes directes ou simplement diffusées sont inefficaces. Les formes d'ondes multiplement diffusées (coda) sont trop complexes pour être modélisées exactement mais elles sont parfaitement reproductibles et très sensibles aux variations du milieu de propagation. Des travaux récents ont démontré la possibilité de mesurer de faibles changements de vitesse d'un milieu grâce aux ondes diffuses, à l'échelle de la croûte terrestre (coda sismique) comme à l'échelle des matériaux (coda ultrasonore). Ces travaux s'intéressent majoritairement à des changements globaux ou régionaux des différents milieux. La problématique de cette thèse concerne la possibilité d'utiliser la coda pour étudier des changements locaux du milieu. Deux études complémentaires sont développées : Le problème direct consiste à modéliser les variations de la coda engendrées par un changement local. Nous distinguons le cas d'un changement local de structure (fort contraste d'impédance) du cas d'un changement local de vitesse (faible contraste d'impédance). Le problème inverse consiste à utiliser les mesures de variations de la coda pour tenter de localiser et de caractériser les changements survenus. Les applications potentielles de ces travaux concernent entre autres le suivi temporel de structures géologiques ainsi que le contrôle non destructif de matériaux hétérogènes. Avec cet objectif, les différentes méthodes développées sont illustrées par des simulations numériques d'ondes acoustiques et sismiques ainsi que par des expériences en ultrasons dans des éléments en béton.In highly heterogeneous media, waves don't propagate ballistically. They can interact several times with the heterogeneities of the medium and enter the multiple scattering regime. In this regime, classical imaging techniques, based on direct or singly scattered waves fail. Multiply scattered waveforms (coda) are too complex for being exactly modeled but are perfectly reproducible and very sensitive to small variations of the medium. Recent works demonstrated the possibility of measuring small velocity variations with diffuse waves, either at the geophysical scale (seismic coda) or at the material scale (ultrasonic coda). These works are mainly focused in monitoring global or regional changes of the medium. The present thesis deals with the possibility of using coda waves to study local changes of the medium. Two complementary studies are developed: The forward problem addresses the modeling of the coda variations generated by a local change. We distinguish the case of a structural change (strong impedance contrast) from the case of a velocity change (small impedance contrast). The inverse problem consists in using the coda variations measurements to locate and characterize the changes that occurred. Potential applications may concern, among others, monitoring of geological structures and non-destructive testing of heterogeneous materials. This in mind, we illustrate the different studies with numerical simulations of acoustic and seismic waves and with ultrasound experiments in concrete blocks.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Pair conversion spectroscopy of the Hoyle state

The triple-alpha reaction leading to the formation of stable carbon in the Universe is one of the most important nuclear astrophysical processes. The radiative width of the so-called Hoyle state, involving the 7.654 MeV E0 and the 3.2148 MeV E2 transitio

Transfer reaction measurements and the stellar nucleosynthesis of 26Al and 44Ti

Progress in the description of stellar evolution is driven by the collaborative effort of nuclear physics, astrophysics and astronomy. Using those developments, the theory of the origin of elements in the Universe is challenged. This thesis addresses the problem behind the abundance of 44Ti and the origin of 26Al. The mismatch between the predicted abundance of 44Ti as produced by the only sites known to be able to create 44Ti, core collapse supernovae (CCSNe), and the observations, highlight the current uncertainty that exists in the physics of these stars. Several satellite based gamma-ray observations of the isotope 44Ti have been reported in recent times and conrm the disagreement. As the amount of this isotope in stellar ejecta is thought to critically depend on the explosion mechanism, the ability to accurately model the observed abundance would be a pivotal step towards validating that theory. The most in influential reaction to the amount of 44Ti in supernovae is 44Ti(alpha, p)47V. Here we report on a direct study of this reaction conducted at the REX-ISOLDE facility, CERN. The experiment was performed at a centre of mass energy 4.15 (+/-0.23) MeV, which is, for the first time, well within the Gamow window for core collapse supernovae. The experiment employed a beam of 44Ti extracted from highly irradiated components of the SINQ spallation neutron source of the Paul Scherrer Institute. No yield above background was observed, enabling an upper limit for the rate of this reaction to be determined. This result is below expectation, suggesting that the 44Ti(alpha, p)47V reaction proceeds more slowly than previously thought. Implications for astrophysical events, and remnant age, are discussed. In Wolf-Rayet and asymptotic giant branch (AGB) stars, the 26gAl(p, gamma)27Si reaction is expected to govern the destruction of the cosmic gamma-ray emitting nucleus 26Al. The rate of this reaction, however, is highly uncertain due to the unknown properties of several resonances in the temperature regime of hydrogen burning. We present a high-resolution inverse kinematic study of the 26gAl(d, p)27Al reaction as a method for constraining the strengths of key astrophysical resonances in the 26gAl(p, gamma)27Si reaction. In particular, the results indicate that the resonance at Er = 127 keV in 27Si determines the entire 26gAl(p, gamma)27Si reaction rate over almost the complete temperature range of Wolf-Rayet stars and AGB stars. The measurements of spectroscopic factors for many states in 27Al and a shell model calculation of nuclear properties of rp-resonant states in 27Si also allow for testing the structure model

Time Correlated gamma-ray Spectroscopy and High-Spin Structure of the Odd-Odd Nucleus ²¹⁰Fr

The structure of ²¹⁰Fr has been studied with the ¹⁹⁷Au(¹⁸O, 5n)²¹⁰Fr reaction. Experiments were performed using the 14UD Pelletron accelerator at the Heavy- Ion Accelerator Facility at the ANU to provide an ¹⁸O beam at an energy of 97 MeV. Several structural features have been discovered including the presence of four isomers. A high-spin isomeric state, with spin more likely being 23⁺, has been identified at ∼4.4 MeV with a lifetime of 686(17) ns that decays by two γ-rays at 573.5 and 663.6 keV that are very likely to have either M2 or E3 multi-polarity. The measured strengths in the case of E3 assignments are approximatively 8 and 21 W.u., respectively. These would be less enhanced than is observed in the decay from the high spin isomers in ²⁰⁹Fr and ²¹¹Fr that are believed to be from a related configuration. Another isomer at ∼1 MeV with Jπ=10⁻ and lifetime of ∼30 ns is found to decay via a hindered E1 transition that is j-forbidden. Possible configurations of excited states in ²¹⁰Fr are discussed within the semi-empirical shell model. Comparisons are made with calculations performed using the code Sesame developed at the ANU

Localisation d'un petit changement en milieu multiplement diffusant (LOCADIFF): Application à la localisation d'un défaut millimétrique dans le béton

National audienceNous présentons une technique d'imagerie qui permet de localiser une petite perturbation apparue dans un environnement multiplement diffusant. Cette technique se base sur une formulation directe de la décorrélation spatio-temporelle de la coda diffuse induite par l'apparition d'un diffuseur supplémentaire (le défaut à imager). La localisation du défaut, qui constitue le problème inverse à résoudre, est obtenue par une approche de maximum de vraisemblance. La technique LOCADIFF est appliquée à la localisation expérimentale d'un défaut millimétrique dans le béton par des ondes ultrasonores. Dans le domaine de fréquence 100-500 kHz, la précision obtenue est centimétrique. Nous démontrons la robustesse de la technique vis-à-vis des incertitudes de mesure

Tracking a defect in the multiple scattering regime

Program abstracts of the 162nd meeting of the Acoustical Society of America.International audienceWe describe a time-resolved monitoring technique for heterogeneous media, especially multiply scattering media. Our approach is based on the spatial variations of the cross-coherence of diffuse waves acquired at fixed positions but at different dates. The technique applies to all kind of waves, but a particular attention will be paid to ultrasound propagating in concrete. To locate and characterize a defect occurring between successive acquisitions, we use a maximum likelihood approach combined with a diffusive propagation model. We quantify the performance of this technique called LOCADIFF with numerical simulations. In several illustrative examples, we show that the change can be located with a precision of a few wavelengths and that its effective scattering cross-section can be retrieved. We investigate how the accuracy and precision of the method depends on the number of source-receiver pairs, on the time window used to compute the cross-correlation and on the errors in the propagation model. Applications can be found in nondestructive testing (civil engineering), seismology, radar, and sonar location

Measuring the scattering mean free path of Rayleigh waves on a volcano from spatial phase decoherence

International audienceWe analyse the statistics of phase fluctuations of seismic signals obtained from a temporary small aperture array deployed on a volcano in the French Auvergne. We demonstrate that the phase field satisfies Circular Gaussian statistics. We then determine the scattering mean free path of Rayleigh waves from the spatial phase decoherence. This phenomenon, observed for diffuse wavefields, is found to yield a good approximation of the scattering mean free path. Contrary to the amplitude, spatial phase decoherence is free from absorption effects and provides direct access to the scattering mean free path