17 research outputs found
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 Îł-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,Îł)27Si
reaction is expected to govern the destruction of the cosmic Îł-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,Îł)27Si reaction. In particular, the results indicate that the resonance
at Er = 127 keV in 27Si determines the entire 26gAl(p, Îł)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 âŒ4.4 MeV. It has a lifetime of 686(17) ns and decays by two Îł-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 hĂ©tĂ©rogĂšnes, les ondes ne se propagent pas de façon balistique. Elles peuvent interagir de nombreuses fois avec les hĂ©tĂ©rogĂ©nĂ©itĂ©s du milieu et entrer ainsi dans le rĂ©gime de diffusion multiple. Dans ce rĂ©gime, les mĂ©thodes classiques d'imagerie basĂ©es sur les trajets des ondes directes ou simplement diffusĂ©es sont inefficaces. Les formes d'ondes multiplement diffusĂ©es (coda) sont trop complexes pour ĂȘtre modĂ©lisĂ©es exactement mais elles sont parfaitement reproductibles et trĂšs sensibles aux variations du milieu de propagation. Des travaux rĂ©cents ont dĂ©montrĂ© la possibilitĂ© de mesurer de faibles changements de vitesse d'un milieu grĂące aux ondes diffuses, Ă l'Ă©chelle de la croĂ»te terrestre (coda sismique) comme Ă l'Ă©chelle des matĂ©riaux (coda ultrasonore). Ces travaux s'intĂ©ressent majoritairement Ă des changements globaux ou rĂ©gionaux des diffĂ©rents milieux. La problĂ©matique de cette thĂšse concerne la possibilitĂ© d'utiliser la coda pour Ă©tudier des changements locaux du milieu. Deux Ă©tudes complĂ©mentaires sont dĂ©veloppĂ©es : Le problĂšme direct consiste Ă modĂ©liser les variations de la coda engendrĂ©es par un changement local. Nous distinguons le cas d'un changement local de structure (fort contraste d'impĂ©dance) du cas d'un changement local de vitesse (faible contraste d'impĂ©dance). Le problĂšme inverse consiste Ă utiliser les mesures de variations de la coda pour tenter de localiser et de caractĂ©riser les changements survenus. Les applications potentielles de ces travaux concernent entre autres le suivi temporel de structures gĂ©ologiques ainsi que le contrĂŽle non destructif de matĂ©riaux hĂ©tĂ©rogĂšnes. Avec cet objectif, les diffĂ©rentes mĂ©thodes dĂ©veloppĂ©es sont illustrĂ©es par des simulations numĂ©riques d'ondes acoustiques et sismiques ainsi que par des expĂ©riences en ultrasons dans des Ă©lĂ©ments en bĂ©ton.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.Ă©lectronique (730659901) / SudocGRENOBLE1/INP-Bib.Ă©lectronique (384210012) / SudocGRENOBLE2/3-Bib.Ă©lectronique (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 ÂČÂčâ°Fr
The structure of ÂČÂčâ°Fr has been studied with the Âčâčâ·Au(ÂčâžO, 5n)ÂČÂčâ°Fr reaction.
Experiments were performed using the 14UD Pelletron accelerator at the Heavy-
Ion Accelerator Facility at the ANU to provide an ÂčâžO 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 23âș,
has been identified at âŒ4.4 MeV with a lifetime of 686(17) ns that decays by
two Îł-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 ÂČâ°âčFr and ÂČÂčÂčFr that are
believed to be from a related configuration. Another isomer at âŒ1 MeV with
JÏ=10â» and lifetime of âŒ30 ns is found to decay via a hindered E1 transition
that is j-forbidden. Possible configurations of excited states in ÂČÂčâ°Fr 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 problÚme 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