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The connection between phase shifts and the ANC has been explored in the frame of the effective range theory. The main result is that, in practice and under rather simple requirements, scattering states (phases shifts) can be correctly described and connected with bound states via the effective range function, and therefore, ANCs can be accurately determined thanks to the analytic properties of this function. This result has an important impact in stellar evolution due to the ANC and phases shifts are directly connected with capture cross sections which, for instance, determine partially the stage and evolution of stars.As a first step, the effective range function is approximated via the effective range expansion which shows that a successful phase-shift description depends on how precise the effective range parameters are determined. Thus, a technique to compute accurately these parameters is developed here. Its construction is based on a set of recurrence relations at low energy, that allows a compact and general description of the truncatedeffective range expansion. Several potential models are used to illustrate the effectivenessof this technique and to discuss its numerical limitations. The results shows that a very good precision of the effective-range parameters can be achieved; nevertheless, to describe experimental phase shifts several effective-range parameters can be needed, which shows a limitation for practical applications.As a second step, the effective range function is analyzed theoretically in an arbitrary energy range. This analysis shows that this function can be decomposed in such a way that contributions of bound states, resonances and background can be separated in a similar way as in the phenomenological R-matrix. In this new form experimental data can be better fitted because the free parameter space is reduced considerably,and therefore, extrapolations are better handled. By construction, the method agrees with the sca, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

Microscopic approaches enable one to study nuclear bound states as well as nuclear collisions in a unified framework.At non-relativistic energies, all physical quantities are determined by the solutions of the many-body Schrödinger equation based on an interaction potential between nucleons.The difficulty of solving this equation for collisions and taking the antisymmetrization principle into account restricts these approaches to light nuclei and requires the development of nuclear models based on some simplifying assumptions.One of these assumptions, which is done in this work, is to consider that the nucleons are aggregated in clusters in the nuclear systems. Another major problem of the microscopic description is the difficulty of determining a reliable interaction potential between nucleons.In spite of many years of efforts to establish such potentials, none has yet been proved to accurately describe both the spectroscopic properties of nuclei and the reactions between light nuclei.For this reason, many effective NN interactions, adapted to the model space and to the studied collision, have been built and used in microscopic models.In parallel, for a few years, some efforts have been done to use in the microscopic models more realistic NN interactions, adjusted to reproduce the two-nucleon properties.However, this requires solving much more accurately the Schrödinger equation by relaxing, for instance, the cluster assumption.These approaches therefore need large computational times, which limits the size of the systems that can be studied.In this work, a two-body realistic interaction has been adapted to the simple microscopic cluster model by using the Unitary Correlation Operator Method. This new realistic effective interaction has been adjusted so that the α+α elastic phase shifts obtained with the microscopic cluster model agree rather well with the experimental data.This interaction has been used to study α+N and α+3He, Doctorat en Sciences de l'ingénieur, info:eu-repo/semantics/nonPublished

The 2H(d, γ)4He capture reaction and the 2H(d, p)3H and 2H(d, n)3He transfer reactions at very low energies are studied in an extended microscopic cluster model with a realistic nucleon-nucleon force. Our results show that the tensor force in realistic interactions plays an essential and indispensable role to reproduce the very low-energy astrophysical S factor of these reactions. © 2013 Springer-Verlag Wien., SCOPUS: ar.j, info:eu-repo/semantics/published

We propose an extension of the continuum discretized coupled channels (CDCC) method, where the projectile is described by a microscopic cluster model. This microscopic generalization (MCDCC) relies only on nucleon-target interactions, and therefore presents an important predictive power. Core excitations can be included without any further parameter. As an example, we investigate the Li7+Pb208 elastic scattering at Elab=27 and 35 MeV. The Li7 nucleus is known to present an α+t cluster structure, and is well described by the resonating group method. An excellent agreement is obtained for the Li7+Pb208 elastic cross sections, provided that breakup channels are properly included. We also present an application to inelastic scattering and discuss future applications of the MCDCC. © 2013 American Physical Society., SCOPUS: ar.j, info:eu-repo/semantics/published

The α + α bremsstrahlung is studied in a microscopic cluster approach with the Generator Coordinate Method associated with the Microscopic R-matrix Method. Two effective nucleon-nucleon interactions are considered: the Minnesota potential and a potential derived from the realistic Argonne AV18 potential by the Unitary Correlation Operator Method. The influence of the nucleon-nucleon interactions are discussed and the results are compared with experimental data., SCOPUS: cp.j, info:eu-repo/semantics/published

12C and 16O nuclei are investigated within nonmicroscopic and microscopic theoretical frameworks, respectively. For the 12C nucleus viewed as a 3α system, 3-body Faddeev equations are solved in configuration space. Positions of the resonant states are obtained through the complex scaling method. We show that the nonlocal potential developed by Z. Papp and S. Moszkowski appears to be well-adapted to study 3α system. In particular, we show evidence for 12C states of positive-parity which share common features with the well-known 0 +2 Hoyle state, currently interpreted as a condensate state. For the 16O nucleus, a 12C+α multicluster generator coordinate method is used to solve the 16-nucleon problem. The 16O nucleus is described by four α cluster. We comment the difficulty to interpret broad resonant states. The phase-shift analysis of the 12C(02+) + α channel reveals the existence of two 0+ states located above the 4α threshold which can be interpreted as condensate states., SCOPUS: cp.j, info:eu-repo/semantics/published

The 2H(d,γ)4He capture reaction and the 2H(d,p)3H and 2H(d,n)3He transfer reactions at very low energies are studied in an extended microscopic cluster model, in which the 3H, 3He, and d clusters are given by the precise few-body wave functions with a realistic nucleon-nucleon force and the cluster relative motions are solved with the same realistic nucleon-nucleon force. Our results show that the tensor force in realistic interactions plays an essential and indispensable role to reproduce the very low energy astrophysical S factor of these reactions., info:eu-repo/semantics/published

The 12C+12C fusion reaction is investigated in a multichannel folding model, using the density-dependent DDM3Y nucleon-nucleon interaction. The 12C(01+,2+,02+,3-) states are included, and their densities are taken from a microscopic cluster calculation. Absorption to fusion channels is simulated by a short-range imaginary potential, and the model does not contain any fitting parameter. We compute elastic and fusion cross sections simultaneously. The role of 12C+12C inelastic channels, and in particular of the 12C(01+)+12C(02+) channel involving the Hoyle state, is important even at low energies. In the Gamow region, the energy range relevant in astrophysics, inelastic channels increase the S factor by a factor of three. © 2013 Elsevier B.V., SCOPUS: ar.j, info:eu-repo/semantics/published

The role of core excitations in exotic nuclei is discussed in the framework of a microscopic cluster model. This cluster approach is complemented by the R-matrix theory to take account of the long-range part of the wave functions. We briefly describe the model, and present two recent examples: the neutron-rich nucleus 16B, described by a 15B+n structure, and the proton-rich nucleus 17Na, described by a 16Ne+p structure. In both cases core excitations are shown to play an important role., SCOPUS: cp.j, info:eu-repo/semantics/published

The 11Li breakup on a 208Pb target is studied in the Coulomb corrected eikonal approximation by using a 9Li+n+n three-body description of the projectile. The 11Li wave functions are defined in the hyperspherical formalism for bound and scattering states, and are obtained from effective 9Li+n and n+n interactions. The 9Li+n+n three-body phase shifts suggest the existence of a narrow 1- resonance near 0.5 MeV above threshold. This resonance shows up as a peak in the breakup cross section, and is supported by experimental data., SCOPUS: cp.j, info:eu-repo/semantics/published

The structure of exotic nuclei is one of the main interests in current nuclear physics. Exotic nuclei present unusual properties, such as a low breakup energy, a short lifetime and/or a halo structure. Because of their short lifetimes, they can not be studied by usual spectroscopic techniques. Indeed, targets of such nuclei are impossible to build. But since the availability of radioactive beams, nuclear reactions have provided possibilities of exploring nuclei far from stability.The investigation of exotic nuclei has been recently reactivated by the development of intense radioactive nuclear beams. As firstly observed for the deuteron, and then for other exotic projectiles such as $^6$He and $^{11}$Be, the internal structures of the interacting nuclei can have a significant effect on the elastic cross sections. Due to their low binding energy, the projectile dissociation process, leaving the target in its ground state, highly affects elastic cross sections but also other measurements such as transfer and fusion reactions. Accurate reaction theories are therefore needed. The coupled discretized-continuum channel (CDCC) method is one of those theories and assumes a projectile made of N clusters (usually N=2 or 3) impinging on a target which is structureless. The N+1-body Schrödinger equation is approximately solved by expanding the total wave function over the bound and continuum states of the projectile. These latter take into account the dissociation events and are approximately described by a truncated set of square-integrable wave functions. There are two available methods for discretizing the continuum, the pseudostate method where the projectile Hamiltonian is diagonalized within a finite basis of square-integrable functions, or the bin method where exact scattering wave functions of the projectile are averaged over bins in a finite region of space. In both cases, the N+1-body Schrödinger equation is replaced by a set of coupled-channel differential equat, Doctorat en Sciences de l'ingénieur, info:eu-repo/semantics/nonPublished

We present the results of the 8Li(p, α) 5He reaction of astrophysical interest, measured at the RIBRAS system. It was realized in inverse kinematics and using a thick CH 2 polyethylene target. Using the thick target method the complete excitation function could be measured between E cm=0.2-2.5 MeV, which includes the Gamow peak energy region. The contribution of contaminating 12C( 8Li, 4He) 16N and 12C( 7Li, 4He) 15N reactions is still under analysis. However, if the cross section is expected to be somewhat reduced, the subtraction of the contamination will not change the general feature of the excitation function. © 2012 American Institute of Physics., SCOPUS: cp.p, info:eu-repo/semantics/published

We investigate the effect of a variation of fundamental constants on primordial element production in Big Bang nucleosynthesis (BBN). We focus on the effect of a possible change in the nucleon-nucleon interaction on nuclear reaction rates involving the A = 5 (5Li and 5He) and A = 8 (8Be) unstable nuclei. The reaction rates for 3He(d,p) 4He and 3H(d,n)4He are dominated by the properties of broad analog resonances in 5Li and 5He compound nuclei respectively. While the triple-alpha process 4He(αα,γ)12C is normally not effective in BBN, its rate is very sensitive to the position of the "Hoyle state" and could in principle be drastically affected if 8Be were stable during BBN. We found that the effect of the variation of constants on the 3He(d,p)4He, 3H(d,n)4He and 4He(αα,γ)12C reaction rates is not sufficient to induce a significant effect on BBN, even with a stable 8Be. The main influences come from the weak rates and the A = 2, n(p,γ)d, bottleneck reaction. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence., SCOPUS: cp.p, info:eu-repo/semantics/published

The radiative capture reaction 12C(α, γ) 16O plays a key role in stellar nucleosynthesis and evolution. However the cross section of this reaction at stellar energies remains highly uncertain despite various experimental studies. The extrapolation down to stellar energy (Ecm∼300 keV) of the measured cross sections at higher energies is made difficult by the overlap of various contributions of which some are badly known such as that of the 2+ (Ex=6.92 MeV) and 1- (Ex=7.12 MeV) sub-threshold states of 16O. Indeed, the α-reduced widths and so the α-spectroscopic-factors of these two sub-threshold states are spread over too-large a range of values. Accordingly, a new determination of these quantities through 12C(7Li,t)16O transfer reaction measurements at two incident energies and a detailed DWBA analysis of the data was performed recently at Orsay. The measured and calculated differential cross sections are presented as well as the obtained spec-troscopic factors and the α-reduced widths for the 2+ and 1- sub-threshold states. The R-matrix calculations of the 12C(α, γ)16O cross section using our obtained α-reduced widths for the two sub-threshold resonances are presented and discussed. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence., SCOPUS: cp.p, info:eu-repo/semantics/published

We investigate the effect of a variation of fundamental constants on primordial element production in big bang nucleosynthesis (BBN). We focus on the effect of a possible change in the nucleon-nucleon interaction on nuclear reaction rates involving the A=5 (Li5 and He5) and A=8 (Be8) unstable nuclei. The reaction rates for He3(d,p)He4 and H3(d,n)He4 are dominated by the properties of broad analog resonances in He5 and Li5 compound nuclei, respectively. While the triple alpha process He4(αα,γ)C12 is normally not effective in BBN, its rate is very sensitive to the position of the "Hoyle state" and could in principle be drastically affected if Be8 were stable during BBN. The nuclear properties (resonance energies in He5 and Li5 nuclei, and the binding energies of Be8 and D) are all computed in a consistent way using a microscopic cluster model. The n(p,γ)d, He3(d,p)He4, H3(d,n)He4, and He4(αα,γ)C12 reaction rates are subsequently calculated as a function of the nucleon-nucleon interaction that can be related to the fundamental constants. We found that the effect of the variation of constants on the He3(d,p)He4, H3(d,n)He4, and He4(αα, γ)C12 reaction rates is not sufficient to induce a significant effect on BBN, even if Be8 was stable. In particular, no significant production of carbon by the triple alpha reaction is found when compared to standard BBN. Finally, we complement our earlier work on the effect of coupled variation of fundamental constants inducing variations of the binding energy of deuterium, the Fermi constant, and the neutron-proton mass difference, that affect the weak rates and the n(p,γ)d, A=2 bottleneck reaction. © 2012 American Physical Society., SCOPUS: ar.j, info:eu-repo/semantics/published

The 12C(α,γ)16O reaction plays a crucial role in stellar evolution. The rate of this reaction determines directly the 12C-to-16O abundance ratio at the end of the helium burning phase of stars and consequently has a big effect on the subsequent nucleosynthesis and even on the evolution of massive stars. However, despite many experimental studies, the low-energy cross section of 12C(α,γ)16O remains uncertain. The extrapolation of the measured cross sections to stellar energies (E∼300 keV) is made particularly difficult by the presence of the 2 + (E x=6.92 MeV) and 1 - (E x=7.12 MeV) subthreshold states of 16O. To further investigate the contribution of these two subthreshold resonances to the 12C(α,γ)16O cross section, we determine their α-reduced widths via a measurement of the transfer reaction 12C(7Li,t)16O at two incident energies, 28 and 34 MeV. The uncertainties on the determined α-spectroscopic factors and the α-reduced widths were reduced thanks to a detailed distorted-wave Born approximation analysis of the transfer angular distributions measured at the two incident energies. The R-matrix calculations of 12C(α,γ)16O cross section using our obtained α-reduced widths for the 2 + and 1 - subthreshold resonances lead to an E1 S factor at 300 keV of 100±28 keVb, which is consistent with values obtained in most of the direct and indirect measurements as well as the NACRE collaboration compilation while the result for the E2 component S E2(300keV)=50±19 keVb disagrees with the NACRE adopted value. © 2012 American Physical Society., SCOPUS: ar.j, info:eu-repo/semantics/published

The most intense γ-ray line observable from novae is likely to be from positron annihilation associated with the decay of 18F. The uncertainty in the destruction rate of this nucleus through the 18F(p,α)15O reaction presents a limit to interpretation of any future observed γ-ray flux. Direct measurements of the cross section of both this reaction and the 18F(p,p)18F reaction have been performed between center of mass energies of 0.5 and 1.9 MeV. Simultaneous fits to both data sets with the R-matrix formalism reveal several resonances, with the inferred parameters of populated states in 19Ne in general agreement with previous measurements. Of particular interest, extra strength has been observed above E c.m.∼1.3 MeV in the 18F(p,p)18F reaction and between 1.3-1.7 MeV in the 18F(p,α)15O reaction. This is well described by a broad 1/2 + state, consistent with both a recent theoretical prediction and an inelastic scattering measurement. The astrophysical implications of a broad subthreshold partner to this state are discussed. © 2012 American Physical Society., SCOPUS: ar.j, info:eu-repo/semantics/published

The 2H(d, γ)4He capture reaction and the 2H(d, p) 3H and 2H(d, n) 3He transfer reactions at very low energies are studied in an extended microscopic cluster model with a realistic nucleon-nucleon force. Our results show that the tensor force in realistic interactions plays an essential and indispensable role to reproduce the very low energy astrophysical S factor of these reactions., SCOPUS: cp.j, info:eu-repo/semantics/published

The 2H(d,p)3H, 2H(d,n)3He, and 2H(d,γ)4He reactions at low energies are studied with realistic nucleon-nucleon interactions in an ab initio approach. The obtained astrophysical S-factors are all in very good agreement with experiment. The most important channels for both transfer and radiative capture are all found to dominate thanks to the tensor force. © 2012 American Institute of Physics., SCOPUS: cp.p, info:eu-repo/semantics/published

We present a direct measurement of the low-energy 8Li(p ,α)5He cross section, using a radioactive 8Li beam impinging on a thick target. With four beam energies, we cover the energy range between Ec.m.=0.2 and 2.1 MeV. An R-matrix analysis of the data is performed and suggests the existence of two broad overlapping resonances (5/2+ at Ec.m.=1.69 MeV and 7/2+ at E c.m.=1.76 MeV). At low energies our data are sensitive to the properties of a subthreshold state (Ex=16.67 MeV) and of two resonances above threshold. These resonances were observed in previous experiments. The R-matrix fit confirms spin assignments, and provides partial widths. We propose a new 8Li(p,α)5He reaction rate and briefly discuss its influence in nuclear astrophysics. © 2012 American Physical Society., SCOPUS: ar.j, info:eu-repo/semantics/published

We present an overview of microscopic cluster models, by focusing on the Resonating Group Method (RGM) and on the Generator Coordinate Method (GCM). The wave functions of a nuclear system are defined from cluster wave functions, with an exact account of antisymmetrization between all nucleons. For the sake of pedagogy, the formalism is mostly presented in simple conditions, i.e. we essentially assume spinless clusters, and single-channel calculations. Generalizations going beyond these limitations are outlined. We present the GCM in more detail, and show how to compute matrix elements between Slater determinants. Specific examples dealing with +nucleus systems are presented. We also discuss some approximations of the RGM, and in particular, the renormalized RGM which has been recently developed. We show that the GCM can be complemented by the microscopic variant of the R-matrix method, which provides a microscopic description of unbound states. Finally, extensions of the GCM to multicluster and multichannel calculations are discussed, and illustrated by typical examples. In particular we compare different three- descriptions of © 2012 Springer-Verlag Berlin Heidelberg., SCOPUS: cp.k, info:eu-repo/semantics/published

We discuss continuum effects in the scattering of exotic nuclei, and more specifically on the 11Be + 64Zn scattering. 11Be is a typical example of an exotic nucleus, with a low binding energy. Elastic, inelastic and breakup cross-sections of the 11Be + 64Zn system are computed in the Continuum Discretized Coupled Channel formalism, at energies near the Coulomb barrier. We show that converged cross-sections need high angular momenta as well as as large excitation energies in the wave functions of the projectile. Extensions to other systems are simulated by different collision energies, and by varying the binding energy of 11Be. © Società Italiana di Fisica / Springer-Verlag 2012., SCOPUS: ar.j, info:eu-repo/semantics/published

Nuclear astrophysics is a relatively young science; it is about half a century old. It is a multidisciplinary subject, since it combines nuclear physics with astrophysics and observations in astronomy. It also addresses fundamental issues in astrobiology through the formation of elements, in particular those required for a carbon-based life. In this paper, a rapid overview of nucleosynthesis is given, mainly from the point of view of nuclear physics. A short historical introduction is followed by the definition of the relevant nuclear parameters, such as nuclear reaction cross sections, astrophysical S-factors, the energy range defined by the Gamow peak and reaction rates. The different astrophysical scenarios that are the sites of nucleosynthesis, and different processes, cycles and chains that are responsible for the building of complex nuclei from the elementary hydrogen nuclei are then briefly described. Copyright © Cambridge University Press 2012., SCOPUS: ar.j, info:eu-repo/semantics/published

We present the first predictions for the widths of resonance states in a so-far unobserved exotic isotope beyond the proton dripline, 21Al. For this purpose we employ mirror symmetry between the widths of low-lying resonance states and the asymptotic normalization coefficients (ANC) of their mirror analogs. The latter are extracted from a recently measured peripheral reaction 20O(d,p)21O while the positions of resonances are estimated within a microscopic cluster model and a two-body potential model. We have found that 21Al should have two low-lying states decaying into the 20Mg+p channel and three states at higher energy, built on the 2 + core excited state and decaying into the 20Mg(2 +)+p channel. The widths of all these states are much smaller than their energies, which makes experimental spectroscopic studies of this isotope beyond the proton dripline possible. © 2012 American Physical Society., SCOPUS: ar.j, info:eu-repo/semantics/published

The RIBRAS facility (Radiactive Ion Beams in Brasil) is installed next to the 8 MV Pelletron Tandem of the Nuclear Physics Laboratory of the Institute of Physics of the University of São Paulo. It consists of two superconducting solenoids with maximum magnetic field of B=6.5T. Light radioactive ion beams are produced through transfer reactions, using solid or gaseous production targets of Be, LiF, 3He etc. The solenoids make a magnetic rigidity selection and the use of the two solenoids with a degrader between them allows the production of quite pure secondary beams. Beams of 6He, 8Li, 7Be, 10Be, 8B, 12B are currently produced and used to study elastic, inelastic, and transfer reactions on a variety of light, medium mass and heavy secondary targets. Some examples of reactions recently studied are presented below. © 2012 American Institute of Physics., SCOPUS: cp.p, info:eu-repo/semantics/published

We discuss properties of three-body continuum states in the hyperspherical formalism. The associated coupled-channel system is solved on a Lagrange basis, and is complemented by the R-matrix theory for the treatment of scattering states. Three-body ?+ n + n phase shifts are determined under various conditions. Two approximations of the continuum are considered: the complex scaling and the pseudostate methods. Both are briefly outlined, and compared with the R-matrix approach for the dipole-strength distribution of 6He. A fair agreement is obtained between the three methods. We also discuss the influence of the ?+ n and n + n interactions, as well as the effect of the Pauli principle. Some differences are obtained, but all calculations predict a broad 1 ?resonance near 1 MeV., SCOPUS: cp.j, info:eu-repo/semantics/published

The 11Li breakup on a 208Pb target is studied in the Coulomb-corrected eikonal approximation by using a 9Li+n+n three-body description of the projectile. The 11Li wave functions are defined in the hyperspherical formalism for bound and scattering states and are obtained from effective 9Li+n and n+n interactions. We first determine 0 +, 1 -, and 2 + 9Li+n+n phase shifts, which suggest the existence of a narrow 1 - resonance near 0.5 MeV above threshold. The calculated breakup cross sections show a peak at low energies, in agreement with the data. The influence of monopole and quadrupole components is analyzed and shown to be non-negligible. We discuss the derivation of dipole strengths from experimental breakup cross sections and suggest that the simple Coulomb dipole approximation, traditionally used in the literature, should be replaced by more elaborate models. We also show that 11Li+208Pb elastic scattering measurements would provide an indirect test of the model. © 2012 American Physical Society., info:eu-repo/semantics/published

Elastic-scattering phase shifts for four-nucleon systems are studied in an ab-initio type cluster model in order to clarify the role of the tensor force and to investigate cluster distortions in low energy d+d and t+p scattering. In the present method, the description of the cluster wave function is extended from (0s) harmonic-oscillator shell model to a few-body model with a realistic interaction, in which the wave functions of the subsystems are determined with the Stochastic Variational Method. In order to calculate the matrix elements of the four-body system, we have developed a Triple Global Vector Representation method for the correlated Gaussian basis functions. To compare effects of the cluster distortion with realistic and effective interactions, we employ the AV8′ potential + a three nucleon force as a realistic interaction and the Minnesota potential as an effective interaction. Especially for 1S 0, the calculated phase shifts show that the t+p and h+n channels are strongly coupled to the d+d channel for the case of the realistic interaction. On the contrary, the coupling of these channels plays a relatively minor role for the case of the effective interaction. This difference between both potentials originates from the tensor term in the realistic interaction. Furthermore, the tensor interaction makes the energy splitting of the negative parity states of 4He consistent with experiments. No such splitting is however reproduced with the effective interaction. © 2011 Springer-Verlag., info:eu-repo/semantics/published

The Continuum Discretized Coupled Channel (CDCC) method is extended to a fully microscopic treatment of the projectile, which is described by the Resonating Group Method. We present a first application to 7Li+ 208Pb elastic and inelastic scattering around the Coulomb barrier. Experimental cross sections are well reproduced by the model. © 2012 American Institute of Physics., SCOPUS: cp.p, info:eu-repo/semantics/published

Le but principal de ma thèse de doctorat est d’étudier la dissociation de noyaux exotiques. Ces noyaux sont peu liés et présentent une structure en halo dominante. Ceci conduit à des propriétés très particulières, telles que des rayons beaucoup plus grands que ceux des noyaux voisins. Les noyaux 6He et 11Li sont des exemples typiques, et sont décrits par des structures alpha+n+n et 9Li+n+n, respectivement. La principale technique expérimentale pour étudier ces noyaux consiste à développer des faisceaux d’ions radioactifs, et à mesurer la section efficace de dissociation sur des cibles lourdes. Les données expérimentales nécessitent donc le développement de modèles théoriques précis pour leur interprétation.Mon travail a consisté en le développement de la méthode eikonale pour décrire les processus de diffusion élastique et de dissociation. Cette méthode nécessite les fonctions d’onde du projectile qui, dans le cas présent, est décrit par une structure à trois corps. Cette situation rend les calculs particulièrement difficiles, en particulier pour la réaction de dissociation où les fonctions d’onde du continu à trois corps sont nécessaires. Les fonctions d’onde à trois corps sont décrites dans le cadre des coordonnées hypersphériques, à la fois pour les états liés et pour les états de diffusion.J'ai d'abord calculé les sections efficaces de dissociation de 6He en utilisant des fonctions d'onde &, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

Our thesis tackles the many-electronic problem considering a non-relativistic and a relativistic orbital approach. Using the suites of programs ATSP and GRASP, we are able to approximate many-electron wave functions beyond the independent particle model by considering a superposition of CSFs. The optimization process, based on the variational principle, provides the best possible mixing coefficients fixing the linear combination of CSFs and spin-orbital basis on which we impose the orthonormality condition between functions of the same l or kappa subspace. Using this conventional approach within the relativistic framework, we estimate different properties of the triply ionized antimony atom (Sb IV), namely transition energies, transition probabilities, isotope shifts and a hyperfine-induced transition.In the aim of partially relaxing the orthogonality constraints between correlation orbitals, we use the variational principle for targeting specific correlation effects by tailoring the configuration space. Independent sets of correlation orbitals, embedded in PCF, are produced from MCHF calculations. These non-orthogonal functions span CSF spaces that are coupled to each other by solving the associated generalized eigenvalue problem. The Hamiltonian and overlap matrix elements are evaluated using the biorthonormal orbital transformations and the efficient counter-transformations of the configuration interaction eigenvectors. This original method is successfully applied for describing different light atomic systems such as Li I, Be I, B I, C II and Ne I. An unwanted effect, called the ``constraint effect', is described and studied for these particular atomic systems. Even if this constraint can be completely relaxed through the DPCFI method, the computational resources required by such an approach lead us to study some simple strategies relaxing partially this constraint. This study takes it place in the context of neutral beryllium for which we test two particul, Doctorat en Sciences de l'ingénieur, info:eu-repo/semantics/nonPublished

The main source of gamma ray emission from novae below 511 keV is likely to be associated with the β+ decay of 18F. The main uncertainty in the abundance of this nucleus comes from the 18F(p,α)15O reaction. In 2006, through microscopic techniques, two previously unseen energy levels in the compound 19Ne nucleus, at 6 and 7.9 MeV were proposed by Dufour and Descouvemont. In light of this there have been two published attempts in the search for the higher of these states, by Murphy et al. and Dalouzy et al. The aim of this work is to address the contradiction between these works and come to a conclusion as to the possible existence of this state. An experiment has taken place that utilised a 4 MeV/u 18F beam, degraded to ∼1.9 MeV/u, incident upon a thick CH2 target. The data has been analysed within the R-matrix formalism and preliminary results are presented here., SCOPUS: cp.p, info:eu-repo/semantics/published

Durant le milieu des années quatre-vingt, le développement des faisceaux radioactifs a permis l’étude de noyaux exotiques situés à la limite de la stabilité nucléaire. Parmi ceux-ci, l’6He possède une structure particulière constituée d’un cœur α et de deux neutrons de valence faiblement liés. Il fait partie des noyaux à halo. Il s’agit également d’un système possédant au moins un état lié à trois amas (α+n+n) alors qu’aucun des sous-systèmes à deux amas (α+n ou n+n) n’est lié. Un tel système est dit borroméen. Deux autres exemples de systèmes borroméens sont le 9Be=n+α+α et le 12C=α+α+α. Les noyaux borroméens possèdent un ou deux états liés dont les propriétés sont maintenant bien connues. En revanche, les propriétés des états du continu le sont moins. Il existe aussi des systèmes à trois amas non liés mais caractérisés par une ou plusieurs résonances étroites à basse énergie. Par exemple, nous pouvons citer le 6Be=α+p+p et le 9B=p+α+α qui sont les noyaux miroirs de l’6He et du 9Be. L’étude des états du continu à trois corps est un sujet important de la recherche en physique nucléaire actuelle.D’un point de vue théorique, les modèles en amas permettent de décrire les noyaux possédant une structure à plusieurs corps. En particulier, les modèles en amas microscopiques tiennent compte de tous les nucléons et le principe de Pauli est traité de manière exacte à l’aide de l’opérateur d’antisymmétrisation. Les modèles microscopiques, plus réalistes, requièrent de longs temps de calculs et sont difficiles à implémenter.Dans le présent travail, nous avons développé une méthode de calcul semi-analytique plus rapide et plus précise qu’une méthode purement numérique. Notre modèle est basé sur la méthode des coordonnées génératrices dans le formalisme hypersphérique. Les états du continu sont étudiés à l’aide de la méthode des déphasages. Ces derniers sont obtenus à partir de la méthode de la matrice R microscopique.Avec ce modèle, nous avons étudié les effets, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

We investigate continuum states of the α+n+n and α+p+p systems using a microscopic three-cluster model in the hyperspherical formalism. The A-nucleon wave functions are defined in the generator coordinate method (GCM). The R-matrix method is used to connect the wave functions to their exact asymptotic behaviors and to derive the collision matrix. We present the three-body phase shifts of the 0+ and 1- continuum of the α+n+n and α+p+p systems. © 2011 World Scientific Publishing Company., SCOPUS: cp.j, info:eu-repo/semantics/published

We present the results of the 8Li(p,α) 5He transfer reaction, measured at the RIBRAS facility. It was performed in inverse kinematics using a thick CH 2 polyethylene target. The complete excitation function was measured between E cm=0.4-2.7 2MeV, which includes the Gamow peak energy region. The excitation function was reproduced with the R-matrix theory. © 2011 American Institute of Physics., SCOPUS: cp.p, info:eu-repo/semantics/published

We investigate three-a continuum states in the hyperspherical formalism for J = 0+ and J = 2+. We use the shallow Ali-Bodmer potential and the deep potential of Buck et al. We determine the 3a 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. © Published under licence by IOP Publishing Ltd., SCOPUS: cp.j, info:eu-repo/semantics/published

We present different models used in nuclear astrophysics. In particular the role of microscopic cluster theories is emphasized. Recent applications on the triple-α process, and on the 12C(α, γ)16O and 3He(α, γ)7Be reactions are discussed. © 2010 Springer-Verlag., SCOPUS: ar.j, info:eu-repo/semantics/published

The β-decay process of the 11Li halo nucleus into 9Li and d is reanalyzed within a three-body model. The 11Li nucleus is described as a 9Li+n+n system in hyperspherical coordinates. The transition probability per time and energy units measured in a recent experiment can be reproduced with a broad resonance located around 0.8 MeV and a weak absorption from the 9Li + d final channel. © 2011 World Scientific Publishing Company., SCOPUS: cp.j, info:eu-repo/semantics/published

The 2H(d,p)3H, 2H(d,n)3He, and 2H(d,γ)4He reactions are studied at low energies in a multi-channel ab initio model that takes into account the distortions of the nuclei. The internal wave functions of these nuclei are given by the stochastic variational method with the AV8' realistic interaction and a phenomenological three-body force included to reproduce the two-body thresholds. The obtained astrophysical S-factors are all in very good agreement with experiment. The most important channels for both transfer and radiative capture are identified by comparing to calculations with an effective central force. They are all found to dominate thanks to the tensor force., SCOPUS: ar.j, info:eu-repo/semantics/published

We review different models used in nuclear astrophysics. Definitions of the relevant cross sections and reaction rates are presented. In particular the role of the R-matrix theory and of microscopic cluster models is emphasized. We discuss three applications: R-matrix fits of the12C(α,γ)16O E2 S factor, microscopic cluster calculations of the7Be(p,γ)8B reactions, and recent results o,2H(d,γ)4He,2H(d, p)3H and2H(d,n)3He obtained with realistic nucleon-nucleon interactions. These d + d cross sections show evidence for an important role of the tensor force in A = 4 systems., SCOPUS: cp.p, info:eu-repo/semantics/published

d+d (S-wave) elastic scattering phase shifts are studied in an extended microscopic cluster model in order to investigate cluster-breaking effects appearing in the low-energy scattering. The description of the cluster wave function is extended from a simple (0s) harmonic-oscillator shell model to a few-body model, in which the wave function of the sub-system is determined with the Stochastic Variational Method (SVM). To compare the cluster breaking effects with realistic and effective interactions, we employed the G3RS potential as a realistic interaction and the Minnesota (MN) potential as an effective interaction. The calculated phase shifts show that t+p and 3He+n channels are strongly coupled to the d+d channel for the case of the realistic interaction (G3RS). On the contrary, the coupling of these channels plays a relatively minor role for the case of the effective interaction (MN). © 2011 World Scientific Publishing Company., SCOPUS: cp.j, info:eu-repo/semantics/published

We investigate the 16B spectrum within a microscopic 15B + n model, involving several channels. The wave functions are defined in the Generator Coordinate Method, associated with the microscopic R-matrix formalism. We suggest that the narrow peak observed in recent experiments above the 15B + n threshold can be assigned to a 0- resonance which decays by d-wave neutron emission. The GCM calculation shows that this state cannot be reproduced in a single-channel B15(3/2-)+n model, and that excited configurations are necessary. Other 1- and 2- s-wave resonances are found at low excitation energies. In particular, we find a 1- state which represents a possible candidate for the 16B ground state. In general, our results show the importance of a multichannel framework. Comparisons with shell-model calculations are briefly discussed. © 2010 Elsevier B.V., SCOPUS: ar.j, info:eu-repo/semantics/published

General aspects of microscopic cluster models based on the combination of the Generator-Coordinate-Method and of the R-matrix method are presented. The adequacy of such methods to describe the physics of exotic light nuclei is illustrated with the unbound 16B nucleus. © Published under licence by IOP Publishing Ltd., SCOPUS: cp.j, info:eu-repo/semantics/published

This work is devoted to a Generator-Coordinate-Method investigation of the 16B spectrum with an Extended Two-Cluster Model which includes many 15B+n channels. We find that the narrow peak above the 15B+n threshold seen in the experiments of Kalpachieva et al. and of Lecouey et al. can be assigned to a 01- resonance. Several resonances are obtained near the 15B+n threshold, in particular a 11- state which could be a possible candidate for the 16B ground state. Comparison with Shell Model calculations is performed. © 2011 World Scientific Publishing Company., SCOPUS: cp.j, info:eu-repo/semantics/published

General aspect of microscopic cluster models based on the combination of the Generator-Coordinate-Method and of the R-matrix method are presented. The adequacy of such a method to describe cluster states and reactions of astrophysical interest is illustrated by two typical examples. © 2011 World Scientific Publishing Company., SCOPUS: cp.j, info:eu-repo/semantics/published

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We investigate the 6He dipole distribution in a three-body ́+n+n model. Two approaches are used to describe the three-body 1́ continuum: the discretized-continuum method, where the scattering wave functions are approximated by square-integrable functions, and the R-matrix formalism, where their asymptotic behaviour is taken into account. We show that some ambiguity exists in the pseudostate method, owing to the smoothing technique, necessary to derive continuous distributions. We show evidence for the important role of the halo structure in the E1 dipole strength. We also address the treatment of Pauli forbidden states in the three-body wave functions. © 2011 Elsevier B.V., SCOPUS: ar.j, info:eu-repo/semantics/published