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The E2 component of the 12C(α,γ )16O cross section is investigated by a microscopic cluster model, and by R-matrix fits. The first approach provides S E2(300 keV)≈50 keV-b for ground-state transitions. In the R-matrix theory, we show that the background term plays a crucial role, and cannot be determined without ambiguity. Only an upper limit on the extrapolated S factor can be obtained [SE2(300 keV)<190 keV-b]. To constrain the R-matrix analysis, we use the GCM Asymptotic Normalization Constant (ANC) of the 21+ level. This procedure strongly reduces the uncertainties on the R-matrix fit, and we end up with a recommended value of SE2 (300 keV) =42± 2 keV-b. As ANC values derived from indirect methods are not consistent with the 12C(α,γ )16O cascade transitions to the 2 +1 state, we suggest a remeasurement of this cross section. © 2010 American Institute of Physics., SCOPUS: cp.p, info:eu-repo/semantics/published

The effect of variations of the fundamental constants on the thermonuclear rate of the triple alpha reaction, 4He(α α λ )12C, that bridges the gap between 4He and 12C is investigated. We have followed the evolution of 15 and 60 M ⊙ zero metallicity stellar models, up to the end of core helium burning. They are assumed to be representative of the first (Population IH) stars. The calculated oxygen and carbon abundances resulting from helium burning can then be used to constrain the variation of the fundamental constants. © 2010 American Institute of Physics., SCOPUS: cp.p, info:eu-repo/semantics/published

We measure the S-factor of the 13C(p,γ)14N reaction in reverse kinematics for energies ranging from 561 down to 225 keV with a low background experimental setup. The results are compared with previous measurements and an R-matrix treatment is applied to the data in order to obtain the properties of the 511 keV resonance that dominates the cross section at low energies. © 2010 IOP Publishing Ltd., SCOPUS: cp.j, info:eu-repo/semantics/published

The 12C(α,γ )16O reaction plays an important role in helium burning in massive stars and their evolution. However, despite many experimental studies, the low-energy cross section of 12C(α,γ )16O remains highly uncertain. The extrapolation of the measured cross sections to stellar energies (E=300 keV) is made difficult by the presence of the two sub-threshold states at 6.92 (2 +) and 7.12 (1-) MeV of 16O. In order to further investigate the contribution of these twosubthreshold resonances to the 12C(α,γ )16O cross section, we performed a new determination of the α-reduced widths of the 6.92 and 7.12 MeV of 16O via a measurement of the transfer reaction 12C(7Li,t) 16O at two incident energies, 34 and 28 MeV. The measured and calculated differential cross sections are presented as well as the obtained spectroscopic factors and the α-reduced widths for the 2+ and 1- sub-threshold states and their effect on the R-matrix calculations of 12C(α,γ )16O. Copyright owned by the author(s)., SCOPUS: cp.p, info:eu-repo/semantics/published

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The structure of 18Ne, and in particular of the 1- (6.18 MeV) level, is investigated in a microscopic cluster model involving 16O+p+p configurations. We use the three-cluster generator coordinate method associated with the hyperspherical formalism. The spectroscopy of 18Ne and 18O low-lying states is analyzed. Using the analytical continuation in the coupling constant (ACCC) method, we determine the width of the 1- (6.15 MeV) resonance in 18Ne and find Γ = 40 10 keV. This result is in fair agreement with experiment (50 5 keV). © 2010 IOP Publishing Ltd., SCOPUS: ar.j, info:eu-repo/semantics/published

Aims. A variation of the fundamental constants is expected to affect the thermonuclear rates important for stellar nucleosynthesis. In particular, because of the very low resonant energies of 8Be and 12C, the triple α process is extremely sensitive to any such variations. Methods. Using a microscopic model for these nuclei, we derive the sensitivity of the Hoyle state to the nucleon-nucleon potential, thereby allowing for a change in the magnitude of the nuclear interaction. We follow the evolution of 15 and 60 $Ṁ zero-metallicity stellar models, up to the end of core helium burning. These stars are assumed to be representative of the first, Population III stars. Results. We derive limits on the variation in the magnitude of the nuclear interaction and model dependent limits on the variation of the fine structure constant based on the calculated oxygen and carbon abundances resulting from helium burning. The requirement that some 12C and 16O be present at the end of the helium burning phase allows for permille limits on the change in the nuclear interaction and limits of the order of 10-5 on the fine structure constant relevant at a cosmological redshift of z ∼ 15-20. © 2010 ESO., SCOPUS: ar.j, info:eu-repo/semantics/published

We analyze the 12Be spectrum with two microscopic methods. Microscopic cluster calculations are performed within the Generator Coordinate Method (GCM) framework. An extended two-cluster model which includes the 8He (0+, 2+) + α, and 6He (0+, 2+) + 6He (0+, 2+) channels is developed. It represents an extension of a previous work by Descouvemont and Baye where only the 0+ ground states of 8He and 6He were taken into account. The GCM calculations lead to a good description of the molecular band suggested by Freer et al. and by Saito et al. The 02+ isomeric state and the 22+ state are also well described by the GCM. New bands of positive and negative parities are proposed. This approach is complemented by No-Core Shell Model (NCSM) calculations, in order to clarify the structure of the ground state and the hypothesis of an associated rotational band. The GCM and NCSM calculations do not support the existence of a band built on the ground state. © 2010 Elsevier B.V. All rights reserved., SCOPUS: ar.j, info:eu-repo/semantics/published

We investigate three-α continuum states in the hyperspherical formalism for J = 0+ and J = 2+. Two different types of α + α interactions are used: the shallow Ali-Bodmer potential and the deep potential of Buck et al. We determine the 3α phase shifts up to E = 6 MeV, in parallel with an analysis of resonances in the framework of the complex scaling method. We show that shallow potentials provide additional narrow resonances, in contrast with experimental data. Deep potentials, however, only give rise to broad resonances, and are more consistent with the data. © 2010 IOP Publishing Ltd., SCOPUS: ar.j, info:eu-repo/semantics/published

Elastic scattering of light weakly bound nuclei is studied within the Feshbach generalized optical model. The model explicitly takes into account the contribution of the projectile break-up continuum treated within a microscopic few-cluster model. The model is tested on deuteron and 6Li elastic scattering by different targets at intermediate energies. The generalized optical potentials are then calculated for 6He (230 MeV) + 12C reaction treating the projectile nucleus either within two-body (α + 2n) or three-body (α + n + n) approximations. The obtained differences are analyzed. © 2010 American Institute of Physics., SCOPUS: cp.p, info:eu-repo/semantics/published

The neutron-rich Li11 halo nucleus is unique among nuclei with known separation energies in its ability to emit a proton and a neutron in a β-decay process. The branching ratio toward this rare decay mode is evaluated within a three-body model for the initial bound state and with Coulomb three-body final scattering states. The branching ratio should be comprised between two extreme cases, i.e. a lower bound 6×10-12 obtained with a pure Coulomb wave and an upper bound 5×1010-10 obtained with a plane wave. A simple model with modified Coulomb waves provides plausible values between 0.8×10-10 and 2.2×10 -10, with most probable total energies of the proton and neutron between 0.15 and 0.3 MeV. © 2010 The American Physical Society., SCOPUS: ar.j, info:eu-repo/semantics/published

The RIBRAS facility (Radioactive Ion Beams in Brasil) is installed in connection with the 8MV Pelletron tandem of the University of São Paulo Physics Institute. It consists of two superconducting solenoids which focalize light radioactive secondary beams of low energy, produced by transfer reactions. Recent experimental results include the elastic scattering and transfer reactions of 6He halo nucleus on 9Be, 27Al, 51V and 120Sn targets. The elastic scattering and transfer of 8Li and 7Be on several targets is also being studied. The transfer reaction 8Li(p,α)5He of astrophysical interest was also studied in the Ec m = 0.2 - 2.5 MeV energy range. © 2010 Elsevier B.V. All rights reserved., SCOPUS: ar.j, info:eu-repo/semantics/published

We show that the E1 and E2,S factors recommended by Katsuma [Phys. Rev. C 78, 034606 (2008)] do not include well-known constraints and that the model is inappropriate for the C12(α,γ)O16 reaction. © 2010 The American Physical Society., SCOPUS: no.j, info:eu-repo/semantics/published

Relations between propagators and Greenfunctions of Hamiltonians which are SUSY partners have been obtained. New exact propagators for the family of multi-well, time-dependent and non-hermitian potentials have been calculated.Non-conservative SUSY transformation has been studied inthe case of multichannel Schrodinger equation with different thresholds. Spectrum (bound/virtual states and resonances) of thenon-conservative SUSY partner of zero potential has been founded. Exactly solvable model of the magnetic induced Feshbach resonancehas been constructed. This model was tested in the case of Rb$^{85}$.Conservative SUSY transformations of the first and the second order have been studied in the case of multichannel Schrodinger equation with equal thresholds. Transformations which introduce non-trivial coupling between scattering channels have been founded. The first order SUSY transformation which preserves $S$-matrix eigen-phase shifts andmodifies mixing parameter has been founded in the case of two channel scattering with partial waves of different parities. In the case of coinciding parities we have found the second order SUSY transformation which preserves $S$-matrix eigen-phase shifts and modifies mixing parameter. Phenomenological two channel $^3S_1-^3D_1$neutron-proton potential has been obtained by using single channel, phase equivalent and coupling SUSY transformations applied to zero potential., Doctorat en Sciences de l'ingénieur, info:eu-repo/semantics/nonPublished

Based on a microscopic cluster model that reproduces well the C17 spectrum below the neutron threshold, we study the spectrum of its mirror nucleus Na17. We find that the Na17 ground state should be located at 2.4MeV above the Ne16(01+)+p threshold, being a broad 1/2+ resonance with a width of 1.4MeV. However, there should exist at least four narrow excited states in Na17; three of them, 3/21+, 7/21+, and 9/21+, decay into the Ne16(21+)+p channel, and the other, 5/21+, decays mainly into the Ne16(01+)+p channel. Because the daughter nucleus Ne16 is a two-proton emitter, the narrow Na17 states must undergo a sequential three-proton decay via intermediate states in Ne16. © 2010 The American Physical Society., SCOPUS: ar.j, info:eu-repo/semantics/published

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We apply the Lagrange-mesh technique to the Continuum Discretized Coupled Channel (CDCC) theory. The CDCC equations are solved with the R-matrix method, using Lagrange functions as variational basis. The choice of Lagrange functions is shown to be efficient and accurate for elastic scattering as well as for breakup reactions. We describe the general formalism for two-body projectiles, and apply it to the d + 58 Ni collision at Ed = 80 MeV. Various numerical and physical aspects are discussed. Benchmark calculations on elastic scattering and breakup are presented., SCOPUS: ar.j, info:eu-repo/semantics/published

The R-matrix method is applied to the Continuum Discretized Coupled Channel (CDCC) approximation. The variational basis is chosen as Lagrange functions, which are shown to be efficient and accurate. We apply the general formalism to the d + 58Ni elastic scattering at Ed = 80 MeV. Future developments are briefly discussed. © 2010 World Scientific Publishing Company., SCOPUS: cp.j, info:eu-repo/semantics/published

In this thesis, we present a theoretical investigation of reactive processes involving the HeH$^+$ molecular ion, with applications in laboratory and astrophysical plasma physics. We consider in particular two processes, which are the charge transfer in H + He$^+$ collisions at low energy from a molecular approach and the photodissociation of HeH$^+$.At the molecular level, the cross section is the basic quantity that has to be determined in order to achieve an understanding of reactive processes. Its calculation will be based on the description of the reactions using an emph{ab initio}, quantum mechanical approach. In this work, we will rely on the Born-Oppenheimer approximation, which allows the molecular motion to be separated into an electronic and a nuclear motion. The evaluation of cross sections then follows two steps.The first is the determination of the electronic structure of the molecule. We will calculate the adiabatic potential energy curves of the excited electronic states as well as the dipole matrix elements between these states. The non-adiabatic radial and rotational couplings, which result from the breakdown of the Born-Oppenheimer approximation, are also estimated. The second step is to solve the nuclear motion, which we achieve using a time-dependent method based on the propagation of wave packets on the coupled electronic states. A particular emphasis will be put on the importance of the excited states and of the non-adiabatic couplings in the description of reactive processes. In the treatment of the charge transfer reaction between H and He$^+$ in excited states, it is well known that the non-adiabatic radial couplings cannot be neglected. However, we will show that the inclusion of the non-adiabatic rotational couplings is also necessary in order to obtain accurate state-to-state cross sections.In the description of the photodissociation of HeH$^+$ from its ground state, we will show the influence of the excited states o, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

The elastic breakup of a three-body projectile on a target is studied within the eikonal approximation with full account of final-state interactions. Bound and scattering states are calculated in hyperspherical coordinates on a Lagrange mesh. A correction is introduced to avoid the divergence of breakup cross sections due to the Coulomb interaction. The eikonal approximation allows the direct calculation of various cross sections, and in particular multidifferential cross sections can be obtained. The model is applied to the breakup of He6 on Pb208. The He6 halo nucleus is described within a three-body α+n+n model involving effective αn and nn interactions. The eikonal phase is obtained from optical potentials between α and n, and the target. Around 0.8 MeV, the total breakup cross sections exhibit a narrow 2+ resonant peak superimposed over a broad bump corresponding to a 1- resonance. These results suffer from a disagreement with experimental data at 240 MeV/nucleon, where cross sections are much smaller at low energies. The obtained E1 strength distribution resembles other theoretical results and reopens a long-standing problem about the existence of a 1- low-energy resonance in the He6 continuum. © 2009 The American Physical Society., info:eu-repo/semantics/published

De nombreux noyaux atomiques et hypernoyaux se modélisent comme des structures à trois corps. C'est le cas, par exemple, de noyaux à halo, comme 6He, ou de noyaux stables, comme 12C et 9Be. En effet, 6He se caractérise comme un système à trois corps, formé d'un coeur (une particule alpha) et de deux neutrons de valence faiblement liés. Le noyau de 12C peut s'étudier comme un système lié formé de trois particules alphas, tandis que 9Be peut être décrit comme la liaison de deux particules alphas et d'un neutron.Dans les exemples précédents, les particules alphas sont des amas de nucléons. Elles possèdent donc une structure interne dont il faut tenir compte en raison du principe de Pauli.Les modèles les plus réalistes pour décrire les structures à trois corps sont les modèles "microscopiques". Ces modèles prennent en compte explicitement tous les nucléons et respectent exactement le principe d'antisymétrisation de Pauli. Cependant, l'application de ces modèles est fortement limitée en pratique, car ils exigent de trop nombreux et trop longs calculs.Par conséquent, pour simplifier considérablement les calculs et permettre l'étude des structures à trois corps, des modèles moins détaillés, de type "semi-microscopiques", sont également développés. Dans ces modèles, on représente les amas de nucléons comme de simples particules ponctuelles. Dans ce cas, la modélisation consiste à construire les potentiels effectifs entre les amas, puis à les employer dans les modèles à trois corps.Dans ce travail, nous avons développé les modèles "semi-microscopiques à trois corps". Les potentiels effectifs entre amas sont directement déduits des forces entre nucléons (selon la RGM à 2 corps). Ces potentiels sont "non-locaux", et dépendent des énergies des amas qui interagissent. Ils permettent de simuler le principe de Pauli et les échanges de nucléons entre les amas. La dépendance en l'énergie se révèle être un inconvénient dans les modèles à trois corps. Les poten, Doctorat en Sciences de l'ingénieur, info:eu-repo/semantics/nonPublished

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The cross section of the nuclear reactions involved in stellar nucleosynthesis (e.g. AGB stars) are often very difficult to measure directly at stellar energies because of their very small value. Moreover, this situation can be complicated by the existence of very low energy resonances and/or subthreshold resonances. Indirect methods such as transfer reactions and the ANC method offer the possibility to overcome these difficulties. In this context, recent indirect measurement of the reaction 13C(α,n) 16O is presented. The 13C(α,n)16O reaction is considered as the main neutron source for s-process in low-mass asymptotic giant branch stars. At low energies of astrophysical interest, the contribution of the subthreshold state 6.356 MeV of 17O to the 13C(α,n)16O cross section should be taken into account. However, the results of previous studies of this contribution lead to different conclusions. Hence, we investigated the effect of this resonance on the astrophysical S-factor through a new precise measurement of the alpha spectroscopic factor, Sα, and the corresponding ANC of the 6.356 MeV state using the transfer reaction 13C(7Li,t) 17O at two different incident energies. The measured angular distributions and the obtained spectroscopic factor and the asymptotic normalization constant (ANC) are presented as well as their impact on 13C(α,n)16O cross section and reaction rate. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlikeLicence., SCOPUS: cp.p, info:eu-repo/semantics/published

We present an experimental study of 19Na states in the excitation energy range between 2 and 3 MeV. The presence of 19Na single-particle levels at these energies was first predicted by a microscopic cluster model and then experimentally confirmed by measuring the elastic and inelastic scattering of a 66 MeV 18Ne radioactive beam on a (CH2)n target. The H(18Ne, p)18Ne(g.s.) and H(18Ne, p′)18Ne*(2+, 1.887 MeV) cross sections have been obtained in the laboratory angular range θlab = 6.1 ° - 18.4 ° and analyzed by using the R-matrix method. Two new states in 19Na have been observed at centre-of-mass energies Ec .m .= 2.78 ± 0.01 MeV and 3.09 ± 0.05 MeV. Both resonances exhibit large widths in the 18Ne (2+) + p channel, and low branching ratios into the elastic channel. The reduced proton widths confirm the single-particle nature of these states, with a 18Ne (2+) + p structure. © 2007 Elsevier B.V. All rights reserved., info:eu-repo/semantics/published

We use the Generator Coordinate Method to investigate the 5H and 5He systems in the hyperspherical formalism. The microscopic wave functions are described by a 3H + n + n cluster structure for 5H, and by a mixing of 3H + p + n and 3H + n + n configurations for 5He. The resonance properties are analyzed with the Analytic Coupling in the Continuum Constant (ACCC) method. For 5H, we find results consistent with the literature, but the ground-state width is lower than in previous works. The calculation suggests J = 1 / 2+ and J = 3 / 2+ states in 5He which correspond to the isobaric analog states of low-lying levels in 5H. A J = 1 / 2+ ;T = 3 / 2 state is found near Ex = 21.3 MeV in 5He, with a width of Γ ≈ 1 MeV. © 2008 Elsevier B.V. All rights reserved., info:eu-repo/semantics/published

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info:eu-repo/semantics/published, Clustering aspects of nuclear structure and dynamics (CLUSTERS'07). 9th International conference, Stratford upon Avon, UK, 3-7 September 2008

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info:eu-repo/semantics/published, Nuclear physics in astrophysics III, Dresde, Germany, 26-31 March 2006

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info:eu-repo/semantics/published, Clustering aspects of nuclear structure and dynamics (CLUSTERS'07). 9th International conference, Stratford upon Avon, UK, 3-7 September 2009

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info:eu-repo/semantics/published, Clustering aspects of nuclear structure and dynamics (CLUSTERS'07). 9th International conference, Stratford upon Avon, UK, 3-7 September 2007