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I investigate the Li11(p,t)Li9 two-neutron transfer reaction at Elab=33 MeV within the distorted wave Born approximation (DWBA). The Li11 and H3 nuclei are described in the hyperspherical formalism, as Li9+n+n and p+n+n configurations, respectively. The calculation of the cross section is shown to be much more complicated than in one-neutron transfer reactions, owing to the three-body structure of Li11. The Li11+p and Li9+t scattering wave functions are determined with the continuum discretized coupled channel (CDCC) method. Approximations derived from equivalent local potentials are used to compute the transfer cross section at the DWBA. The model reproduces experimental data reasonably well, considering that there is no adjustable parameter. I show that the cross section is fairly sensitive to the long-range part of the Li11 wave function, and therefore to its halo structure., SCOPUS: ar.j, info:eu-repo/semantics/published

We develop a four-body CDCC (continuum discretized coupled channel) model, aimed at describing reactions where both nuclei present a low breakup threshold. The only inputs are the interactions describing the colliding nuclei, and the four optical potentials between the fragments. The method is applied to the (formula presented) and (formula presented) systems. We show that, in general, breakup channels are crucial to reproduce the elastic cross sections. We suggest that the present wave functions could be used in future DWBA calculations., SCOPUS: cp.p, info:eu-repo/semantics/published

Background: Given the established 2α structure of Be8, a realistic model of four interacting α clusters must be used to obtain a Be8+Be8 interaction potential. Such a four-body problem poses a challenge for the determination of the Be8+Be8 optical potential (OP) that is still unknown due to the lack of the elastic Be8+Be8 scattering data. Purpose: To probe the complex Be8+Be8 optical potential in the coupled-reaction-channel (CRC) study of the α transfer C12(α,Be8) reaction measured at Eα=65 MeV and to obtain the spectroscopic information on the α+Be8 cluster configuration of C12. Method: The three- and four-body continuum-discretized coupled-channel (CDCC) methods are used to calculate the elastic α+Be8 and Be8+Be8 scattering at the energy around 16 MeV/nucleon, with the breakup effect taken into account explicitly. Based on the elastic cross section predicted by the CDCC calculation, the local equivalent OP's for these systems are deduced for the CRC study of the C12(α,Be8) reaction. Results: Using the CDCC-based OP's and α spectroscopic factors given by the cluster model calculation, a good CRC description of the α transfer data for both the Be8+Be8 and Be8+Be2+∗8 exit channels is obtained without any adjustment of the (complex) potential strength. Conclusion: The α+Be8 and Be8+Be8 interaction potential can be described by the three- and four-body CDCC methods, respectively, starting from a realistic α+α interaction. The α transfer C12(α,Be8) reaction should be further investigated not only to probe of the 4α interaction but also the cluster structure of C12., SCOPUS: ar.j, info:eu-repo/semantics/published

The Li11+p and Li11+d reactions are investigated in the continuum discretized coupled channel (CDCC) method with a three-body description (Li9+n+n) of Li11. I first discuss the properties of Li11, and focus on E1 transition probabilities to the continuum. The existence of a 1- resonance at low excitation energies is confirmed, but the associated E1 transition from the ground state does not have an isoscalar character, as suggested in a recent experiment. In a second step, I study the Li11+p elastic cross section at Elab=66 MeV in the CDCC framework. I obtain a fair agreement with experiment, and show that breakup effects are maximal at large angles. The breakup cross section is shown to be dominated by the 1- dipole state in Li11, but the role of this resonance is minor in elastic scattering. From CDCC equivalent Li11+p and Li11+n potentials, I explore the Li11+d cross section within a standard three-body Li11+(p+n) model. At small angles, the experimental cross section is close to the Rutherford scattering cross section, which is not supported by the CDCC. A five-body (Li9+n+n)+(p+n) calculation is then performed. Including breakup states in Li11 and in the deuteron represents a numerical challenge for theory, owing to the large number of channels. Although a full convergence could not be reached, the CDCC model tends to overestimate the data at small angles. I suggest that measurements of the Li9+p elastic scattering would be helpful to determine more accurate optical potentials. The current disagreement between experiment and theory on Li11+d scattering also deserves new experiments at other energies., SCOPUS: ar.j, info:eu-repo/semantics/published

Given the established 2 alpha structure of 8Be, a realistic model of 4 interacting alpha clusters must be used to obtain a 8Be+8Be interaction potential. Such a four-body problem poses a challenge for the determination of the 8Be+8Be optical potential (OP) that is still unknown due to the lack of the elastic 8Be+8Be scattering data. The main goal of the present study is to probe the complex 8Be+8Be optical potential in the coupled reaction channel (CRC) study of the alpha transfer 12C(alpha,8Be) reaction measured at 65 MeV, and to obtain the spectroscopic information on the alpha+8Be cluster configuration of 12C. The 3- and 4-body Continuum-Discretized Coupled Channel (CDCC) methods are used to calculate the elastic alpha+8Be and 8Be+8Be scattering at the energy around 16 MeV/nucleon, with the breakup effect taken into account explicitly. Using the CDCC-based OP and alpha spectroscopic factors given by the cluster model calculation, a good CRC description of the alpha transfer data without any adjustment of the (complex) potential strength., Comment: Phys. Rev. C {\bf 102}, 024622 (2020)

The 11Li + p and 11Li + d reactions are investigated in the Continuum Discretized Coupled Channel (CDCC) method with a three-body description (9Li + n + n) of 11Li. I first discuss the properties of 11Li, and focus on E1 transition probabilities to the continuum. The existence of a 1- resonance at low excitation energies is confirmed, but the associated E1 transition from the ground state does not have an isoscalar character, as suggested in a recent experiment. In a second step, I study the 11Li + p elastic cross section at Elab = 66 MeV in the CDCC framework. I obtain a fair agreement with experiment, and show that breakup effects are maximal at large angles. The breakup cross section is shown to be dominated by the 1- dipole state in 11Li, but the role of this resonance is minor in elastic scattering. From CDCC equivalent 11Li + p and 11Li + n potentials, I explore the 11Li + d cross section within a standard three-body 11Li +(p + n) model. At small angles, the experimental cross section is close to the Rutherford scattering cross section, which is not supported by the CDCC. A five-body (9Li+n+n)+(p+n) is then performed. Including breakup states in 11Li and in the deuteron represents a numerical challenge for theory, owing to the large number of channels. Although a full convergence could not be reached, the CDCC model tends to overestimate the data at small angles. I suggest that measurements of the 9Li + p elastic scattering would be helpful to determine more accurate optical potentials. The current disagreement between experiment and theory on 11Li + d scattering also deserves new experiments at other energies., Comment: 12 pages, 10 figures, accepted at Phys. Rev. C

We use a microscopic multicluster model to investigate the structure of $^{10}$Be and of $^{11}$Be. These nuclei are described by $\alpha+\alpha+n+n$ and $\alpha+\alpha+n+n+n$ configurations, respectively, within the Generator Coordinate Method (GCM). The 4- and 5-body models raise the problem of a large number of generator coordinates (6 for $^{10}$Be and 9 for $^{11}$Be), which requires specific treatment. We address this issue by using the Stochastic Variational Method (SVM), which is based on an optimal choice of the basis functions, generated randomly. The model provides good energy spectra for low-lying states of both nuclei. We also compute rms radii and densities, as well as electromagnetic transition probabilities. We analyze the structure of $^{10}$Be and of $^{11}$Be by considering energy curves, where one of the generator coordinates is fixed during the minimization procedure.

We apply a microscopic version of the Continuum Discretized Coupled Channel (CDCC) method, referred to as MCDCC, to 8Li and 8B scattering on different targets. The 8Li and 8B nuclei are described in a microscopic three-cluster model (α+ t+ n and α+ 3He + p), using the hyperspherical coordinates. We first present spectroscopic properties of these nuclei. Then, we determine 8Li + nucleus and 8B + nucleus potentials by using proton+ target and neutron+ target interactions. We compute various elastic-scattering cross sections and confirm that breakup effects are important, in particular at low energies. In general, we find a fair agreement with experiment, except for 8B+58Ni where we suggest that the data might be overestimated., SCOPUS: ar.j, info:eu-repo/semantics/published

This white paper reports on the discussions of the 2018 Facility for Rare Isotope Beams Theory Alliance (FRIB-TA) topical program "From bound states to the continuum: Connecting bound state calculations with scattering and reaction theory". One of the biggest and most important frontiers in nuclear theory today is to construct better and stronger bridges between bound state calculations and calculations in the continuum, especially scattering and reaction theory, as well as teasing out the influence of the continuum on states near threshold. This is particularly challenging as many-body structure calculations typically use a bound state basis, while reaction calculations more commonly utilize few-body continuum approaches. The many-body bound state and few-body continuum methods use different language and emphasize different properties. To build better foundations for these bridges, we present an overview of several bound state and continuum methods and, where possible, point to current and possible future connections., Comment: 48 pages, 23 figures; paper from the FRIB Theory Alliance Workshop on "From Bound States to the Continuum", June 11-22, 2018

The β-decay process of the He6 halo nucleus into the α+d continuum is studied in an updated three-body model. The He6 nucleus is described as an α+n+n system in hyperspherical coordinates on a Lagrange mesh. The shape and absolute values of the transition probability per time and energy units of new experiments are reproduced with a modified α+d potential. The obtained total transition probabilities are (2.48±0.16)×10-6s-1 for the full energy region and (2.40±0.16)×10-6s-1 for the cut-off E>150 keV. The strong cancellation between the internal and halo parts of the β decay matrix element is a challenge for future ab initio calculations., SCOPUS: ar.j, info:eu-repo/semantics/published

Primordial or big bang nucleosynthesis (BBN) is one of the three historical strong evidences for the big bang model. Standard BBN is now a parameter free theory, since the baryonic density of the Universe has been deduced with an unprecedented precision from observations of the anisotropies of the cosmic microwave background (CMB) radiation. There is a good agreement between the primordial abundances of 4He, D, 3He and 7Li deduced from observations and from primordial nucleosynthesis calculations. However, the 7Li calculated abundance is significantly higher than the one deduced from spectroscopic observations and remains an open problem. In addition, recent deuterium observations have drastically reduced the uncertainty on D/H, to reach a value of 1.6%. It needs to be matched by BBN predictions whose precision is now limited by thermonuclear reaction rate uncertainties. This is especially important as many attempts to reconcile Li observations with models lead to an increased D prediction. Here, we re-evaluates the D(p,g)3He, D(d,n)3He and D(d,p)3H reaction rates that govern deuterium destruction, incorporating new experimental data and carefully accounting for systematic uncertainties. Contrary to previous evaluations, we use theoretical ab initio models for the energy dependence of the S-factors. As a result, these rates increase at BBN temperatures, leading to a reduced value of D/H = (2.45$\pm0.10)\times10^{-5}$ (2$\sigma$), in agreement with observations., Comment: Submitted to Phys. Rev. D. (without the non-essential Tables IV, IX, X and XI provided here)

Primordial or big bang nucleosynthesis (BBN) is one of the three historical strong evidences for the big bang model. Standard BBN is now a parameter free theory, since the baryonic density of the Universe has been deduced with an unprecedented precision from observations of the anisotropies of the cosmic microwave background (CMB) radiation. There is a good agreement between the primordial abundances of 4He, D, 3He and 7Li deduced from observations and from primordial nucleosynthesis calculations. However, the 7Li calculated abundance is significantly higher than the one deduced from spectroscopic observations and remains an open problem. In addition, recent deuterium observations have drastically reduced the uncertainty on D/H, to reach a value of 1.6%. It needs to be matched by BBN predictions whose precision is now limited by thermonuclear reaction rate uncertainties. This is especially important as many attempts to reconcile Li observations with models lead to an increased D prediction. Here, we re-evaluates the D(p,g)3He, D(d,n)3He and D(d,p)3H reaction rates that govern deuterium destruction, incorporating new experimental data and carefully accounting for systematic uncertainties. Contrary to previous evaluations, we use theoretical ab initio models for the energy dependence of the S-factors. As a result, these rates increase at BBN temperatures, leading to a reduced value of D/H = (2.45$\pm0.10)\times10^{-5}$ (2$\sigma$), in agreement with observations.

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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 (Li-5 and He-5) and A=8 (Be-8) unstable nuclei and complement earlier work on its effect on the binding energy of deuterium. The reaction rates for He3(d,p)He4 and H3(d,n)He4 are dominated by the properties of broad analog resonances in He-5 and Li-5 compound nuclei respectively. While the triple alpha process 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 Be-8 were stable during BBN. The nuclear properties (resonance energies in He-5 and Li-5 nuclei, and the binding energies of Be-8 and D) are all computed in a consistent way using a microscopic cluster model. The n(p,gamma)d, He3(d,p)He4 and H3(d,n)He4 and triple-alpha 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 and H3(d,n)He4 and triple-alpha reaction rates is not sufficient to induce a significant effect on BBN, even if Be-8 was stable. In particular, no significant production of carbon by the triple alpha reaction is found when compared to standard BBN. We also update our previous analysis on the effect of a variation of constants on the n(p,gamma)d reaction rate., 14 pages, 12 figures

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Dissociation has become an essential tool in several domains of nuclear physics. It provides useful information about the structure of halo nuclei, and Coulomb breakup can be used as an indirect method to measure radiative-capture cross sections at stellar energies. Though simple it may seem, this indirect technique relies on peculiar assumptions. Recent theoretical analyses of the Coulomb breakup of 8B have shown that these assumptions are not all satisfied. Whereas many experimental investigations on such a phenomenon have been conducted on 8B, the case of 17F has been poorly addressed up to now. An exclusive study of 17F breakup reactions has thus been performed at the FRIBs facility of the Laboratori Nazionali del Sud, Catania (Italy). The experimental setup and the detector systems allowed the measurement, event-by-event, of the X-Y coordinates of the interaction point on the target as well as the momenta and angles of all outgoing decay particles with a geometrical efficiency of 72% and a resolution of approximately 300 keV. The first results and preliminary model comparison are reported., info:eu-repo/semantics/published

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Antisymmetric + 14Cg.s. and + 14C(21+) wave functions are used in the generator coordinate formalism to investigate molecular properties of O18 states. Quadrupole moments, rms radii, and reduced widths of these states are calculated as well as reduced E1 and E2 transition probabilities between them. A number of molecular bands are found and analyzed. The assignment of experimental states to these bands is discussed. The alternating-parity band proposed by Gai et al. is interpreted as a fortuitous alignment of molecular states belonging to different bands. © 1985 The American Physical Society., info:eu-repo/semantics/published

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Doctorat en Sciences, Thèse d'agrégation, info:eu-repo/semantics/nonPublished

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The generator-coordinate method is applied to the study of the three-+-particle system. The choice of the basis functions allows us to calculate the ++ 8Be phase shifts. We perform a comparison between the full three-cluster approach and two of its approximations. Distortion effects are shown to be important in the spectroscopic properties of C12. We present the Be8(+,) 12C S factor derived in the three-+-particle model. At low temperatures, the reaction rate is found to be slightly different from earlier studies. © 1987 The American Physical Society., info:eu-repo/semantics/published

The E1 and E2 components of the C12() 16O S factor are investigated microscopically with a generator-coordinate calculation involving + 12Cg.s. + 12C(21+), p+ 15Ng.s. and n+ 15Og.s. configurations. The theoretical capture cross sections towards the ground state and excited states are in semiquantitative agreement with experiment. Simple corrections to the microscopic expressions lead to good agreement with experimental data. The corrected model provides an S factor of 0.24 MeV b at the astrophysically important energy 0.3 MeV. © 1987 The American Physical Society., info:eu-repo/semantics/published

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The 16O(α,γ)20Ne cross section is calculated with antisymmetric wave functions by the generator coordinate method. E1 and E2 gamma widths in the 01 + and 0- bands of 20Ne agree fairly withd experiment. At low energies, the non-resonant S-factor is found larger than usually assumed. The S-factor is also enhanced by broad resonances of the 04 + band. © 1983., SCOPUS: ar.j, info:eu-repo/semantics/published

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The 7Be(p, γ)8B reaction is investigated through a microscopic three-cluster model, using the generator coordinate method. The 7Be nucleus is described by an α +3He structure, projected on the 3 2- and 1 2- states of 7Be. The model is tested by the calculation of M1 transition probabilities in 8Li and 8B and of the 7Li(n, γ)8Li cross section. At zero energy, we find an S-factor S(0) = 0.030 keV·b, slightly larger than the currently accepted value. © 1988., SCOPUS: ar.j, info:eu-repo/semantics/published

The Be9 and B9 spectra are investigated in a microscopic three-cluster model, using the generator-coordinate method. Both nuclei are described by a Be8 core surrounded by a nucleon. We present the n+8Be and p+8Be phase shifts, and the electromagnetic transition probabilities in Be9. A good overall agreement is found with the available experimental data. We suggest spin assignments for different poorly known states. The energy of the (1/2+ first excited state in B9 is in excellent agreement with a recent experimental investigation. © 1989 The American Physical Society., info:eu-repo/semantics/published

The C13(α,n)16O and C13(α,α)13C reactions are investigated through the generator-coordinate method. Two bands with marked α+ 13C structure are found in the O17 spectrum. Experimental candidates are proposed. The microscopic C13(α,α)13C cross sections are in fair agreement with experimental data. We show that the C13(α,n)16O cross section at stellar energies is strongly affected by the 1/2 2+ (6.36 MeV) weakly bound state. The astrophysical S factor below Ec.m.=0.3 MeV is therefore significantly enhanced with respect to the extrapolations used for astrophysical purposes. © 1987 The American Physical Society., info:eu-repo/semantics/published

The recent results of Hahn et al. on the 16O(α,γ)20Ne capture reaction are discussed in the light of a microscopic calculation. We suggest that the S factor at 300 keV proposed by Hahn et al. should be substantially increased. © 1988 The American Physical Society., info:eu-repo/semantics/published

The Ne22 spectrum is investigated with microscopic wave functions defined in the generator-coordinate formalism. This nucleus is described in a three-cluster model, involving an O16 core surrounded by a dineutron and an particle. The O18 cluster is projected out on the 0+ and 2+ states. We propose a band structure of Ne22 with experimental candidates. The model suggests the existence of a 0+ band presenting a marked +18O cluster structure. We present the +18O(0+,2+) collision matrices, and discuss the resonance properties in the (18)18O scattering. Electromagnetic transition probabilities in the Ne22 nucleus are calculated. From the theoretical Coulomb shifts between the mirror Ne22 and Mg22 nuclei, we suggest spin assignments for different Mg22 states. The (18,)22Ne capture cross section is calculated, and is used for determining microscopically the nonresonant reaction rate. © 1988 The American Physical Society., info:eu-repo/semantics/published

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Doctorat en Sciences, info:eu-repo/semantics/nonPublished

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Specular reflectivity (SR) and off-specular neutron scattering (OSS) are non-destructive scattering techniques, which, through deuteration (exchange of hydrogen atoms with deuterium), give a high contrast even among very similar chemical species and are therefore highly suitable for investigations of soft thin films. While readily available software exists for analysis of SR, there are no such existing tools for OSS. Through a combination of both, the former yielding a density profile in the direction normal to the sample surface and the latter yielding an in-plane lateral structure at each of the buried interfaces, one can obtain detailed information of buried morphology on length scales ranging from angstroms to 10 micrometres. OSS analysis was developed to analyze buried interfaces in absolute scale, with only structural parameters being free. Moreover, the SR and OSS are intricately linked and have to be fitted to the data simultaneously, constraining the model to a high degree. The OSS was applied to investigation of soft interfaces as model systems.The stability and instability of thin polymer layers have been studied extensively in the last decades, both as their applications as coatings have become part of our daily life, and in fundamental soft matter research. The mechanism of immiscible polymer film decomposition was investigated by preparing bilayers of deuterated polystyrene (d-PS) and protonated poly(methyl methacrylate) (h-PMMA) of different thicknesses on Si substrates. The in-plane lateral cutoff length was measured at the PS/PMMA interface as a function of d-PS thickness. For preannealed films with thickness of dPS below 250 angstroms, the result follows the prediction of the capillary waves cutoff length. For thicker films, spontaneous nucleation of the top layer, with a clear OSS signature, was observed. This was also the case for the non-preannealed samples, showing that preannealing causes irreversible adsorption on the substrate and thus stabilizes the layer.The bilayers were then measured also as a function of annealing time, measuring the nucleation of the top layer from the beginning. The measurements show that the breakup starts immediately after the start of annealing. For a sufficiently thick PMMA layer, the expected PS/PMMA interfacial width was measured. Different cutoff lengths were measured at different regimes of nucleation. Furthermore, a thick/thin/thick PMMA/PS/PMMA trilayer was prepared on an Si substrate, isolating the influence of the substrate to study the hypothesized spinodal dewetting of the thin layer. The roughnesses of the interfaces of the thin layer grow with the same rate and their peaks and valleys are supposed to oscillate in anti-phase. As the simulation subsequently showed, the difference between the in-phase and anti-phase oscillations is too small to be measured with the current instruments. The minimum detectable amount of material would be 3 stacked repeating units of PMMA/PS/PMMA, where the results of the simulations show a noticeable difference. Such a system was measured and is qualitatively compared to the simulations., Doctorat en Sciences, info:eu-repo/semantics/nonPublished

The first aim of this thesis is to perform relativistic calculation of atomic and ionic polarizabilities and two-photon decay rates. Hydrogenic systems are treated by the Lagrange-mesh method. The extension to alkali-like systems is realized by means of a semiempirical-core-potential approach combined with the Lagrange-mesh method. The studied systems are partitioned into frozen-core electrons and an active valence electron. The core orbitals are defined by a Dirac-Hartree-Fock (DHF) calculation using the GRASP2K package. The valence electron is described by a Dirac-like Hamiltonian involving a core-polarization potential to simulate the core-valence electron correlation. Polarizabilities appear in a large number of fields and applications, namely in cold atoms physics, metrology and chemical physics. Two-photon transitions are part of a priori highly unlikely processes and are therefore called forbidden radiative processes. Experimental situations report decays from metastable excited states through these channels. Long lifetimes were measured for highly charged Be-like ions in recent storage-ring experiments, but their interpretation is problematic. The study of the competition between forbidden (one-photon beyond the dipole approximation, or multi-photon) and unexpected (hyperfine-induced or induced by external magnetic fields) radiative processes is all obviously relevant. The second aim of this thesis is to perform relativistic ab initio calculations of electronic isotope-shift (IS) factors by using the multiconfiguration DHF (MCDHF) method implemented in the RIS3/GRASP2K and RATIP program packages. Using the MCDHF method, two different approaches are adopted for the computation of electronic IS factors for a set of transitions between low-lying levels of neutral systems. The first one is based on the estimate of the expectation values of the one- and two-body nuclear recoil Hamiltonian for a given isotope, including relativistic corrections derived by Shabaev, combined with the calculation of the total electron densities at the origin. In the second approach, the relevant electronic factors are extracted from the calculated transition shifts for given triads of isotopes. These electronic quantities together with observed ISs between different pairs of isotopes provide the changes in mean-square charge radii of the atomic nuclei. Within this computational approach for the estimation of the mass- and field-shift factors, different models for electron correlation are explored in a systematic way to determine a reliable computational strategy, and to estimate theoretical error bars of the IS factors., Doctorat en Sciences de l'ingénieur et technologie, info:eu-repo/semantics/nonPublished

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 scattering matrix properties which allows a simple calculation of resonances (locations and widths) and asymptotic normalization constants (ANCs). Several tests are successfully performed via potential models. Phase shifts for the 2 + partial wave of the 12C+α are analyzed with this method. They are correctly described including bothresonances at Ec.m. = 2.7 and 4.4 MeV. For the 6.92 MeV (2+) exited state of 16O, the ANC estimation 112(8) × 10 3 fm^−1/2 is obtained taking into account statistical errors., Doctorat en Sciences, info:eu-repo/semantics/nonPublished

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 equations, which provides the physical quantities such as the collision matrix. In principle, the CDCC method can be very close to the exact N+1-body wave function and is adapted to low as well as to high energy reactions. However, its main interest consists in the low-energy domain.In the present work, we propose a new approach to solve the CDCC equations. This method is based on the R-matrix theory associated with a Lagrange mesh basis. We will show that the combination of both approaches provides a fast and accurate technique to solve the CDCC equations, even for large systems, where traditional methods meet convergence problems. Before investigating collisions with exotic projectiles, we restrict ourselves to the simplest nucleus, the deuteron. Then we make a step towards a more complicated system, the $^6$Li which is a well known stable nucleus. We apply the CDCC method to the d + $^{58}$Ni and $^6$Li + $^{40}$Ca elastic scattering and breakup. These systems are considered in the literature as test cases. They have been investigated by several authors who showed the importance of the breakup channels in the elastic cross sections.After having validated the present version of the CDCC method, we focus on $^{11}$Be, a typical example of a halo nucleus, with low binding energy and large quadrupole moment. Elastic, inelastic and breakup cross sections are computed in the CDCC formalism, at energies near the Coulomb barrier, where continuum effects in the scattering of exotic nuclei, and more specifically on the $^{11}$Be + $^{64}$Zn scattering, are observed. We show that converged cross sections need high angular momenta as well as large excitation energies in the wave functions of the projectile.A Borromean nucleus is made of three constituents which are weakly linked together, but where each pair of those three constituents does not form a bound system. The name "Borromean" comes from the Borromean rings where, if any one of three rings is removed, the remaining two become unbound. Collisions with $^6$He and $^9$Be Borromean projectiles are studied in the present work. Again we compare our method with the $^6$He + $^{208}$Pb and $^6$He + $^{12}$C benchmark calculations. Afterwards, the convergence against the parameters of the description of the $^9$Be projectile is tested for the elastic cross section. The sensitivity to the technique employed to remove the forbidden states and also the sensitivity to the collision energy are investigated., Doctorat en Sciences de l'ingénieur, 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 scattering. The calculated phase shifts give a rather good agreement with experimental data without additional adjustment, without three-body interactions and with simple basis functions. Besides this study of elastic scattering between light nuclei, this work deals with the nucleus-nucleus bremsstrahlung. Previous microscopic models of nucleus-nucleus bremsstrahlung were based on a photon-emission operator fully neglecting the meson-exchange currents. In this work, a microscopic cluster model of bremsstrahlung is developed, which implicitly takes them partially into account by using an extension of the Siegert theorem. Then, the photon-emission operator can be deduced from the charge density rather than from the current density. Although this extension of the Siegert theorem does not fully remove the nuclear-current dependence, the effects of the meson-exchange currents should be largely reduced, especially at low photon energy. The microscopic cluster model of nucleus-nucleus bremsstrahlung developed in this work has been applied to the α+ α and α+N systems. This model is based on an effective NN interaction, which enables a good reproduction of the elastic phase shifts for the α+ α and α+N systems. The agreement with experimental bremsstrahlung cross sections is rather good but the comparison between theory and experiment requires more numerous and more accurate data to be conclusive. With an extension to the p shell, the present model could also describe heavier cluster systems such as 12C+p and 16O+p for which experimental data exist at low energies., Doctorat en Sciences de l'ingénieur, info:eu-repo/semantics/published

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 scattering.The calculated phase shifts give a rather good agreement with experimental data without additional adjustment, without three-body interactions and with simple basis functions. Besides this study of elastic scattering between light nuclei, this work deals with the nucleus-nucleus bremsstrahlung.Previous microscopic models of nucleus-nucleus bremsstrahlung were based on a photon-emission operator fully neglecting the meson-exchange currents. In this work, a microscopic cluster model of bremsstrahlung is developed, which implicitly takes them partially into account by using an extension of the Siegert theorem. Then, the photon-emission operator can be deduced from the charge density rather than from the current density.Although this extension of the Siegert theorem does not fully remove the nuclear-current dependence, the effects of the meson-exchange currents should be largely reduced, especially at low photon energy.The microscopic cluster model of nucleus-nucleus bremsstrahlung developed in this work has been applied to the α+ α and α+N systems. This model is based on an effective NN interaction, which enables a good reproduction of the elastic phase shifts for the α+ α and α+N systems.The agreement with experimental bremsstrahlung cross sections is rather good but the comparison between theory and experiment requires more numerous and more accurate data to be conclusive. With an extension to the p shell, the present model could also describe heavier cluster systems such as 12C+p and 16O+p for which experimental data exist at low energies., Doctorat en Sciences de l'ingénieur, info:eu-repo/semantics/nonPublished

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 +n+n décrites par la méthode de la matrice R. Cette technique rigoureuse conduit à de longs temps de calcul, et j'ai donc développé des méthodes d'approximations telles que la méthode des pseudo-états et la méthode de la rotation complexe. J'ai ensuite étudié la dissociation de 11Li en 9Li+n+n. J’ai calculé les déphasages à trois corps du système 9Li+n+n. Le calcul prédit une résonnance dipolaire à basse énergie, en bon accord avec l'expérience. J'ai proposé que l'analyse expérimentale des données de transition E1 soit reconsidérée pour aller au-delà de l’approximation colombienne. J'ai également entrepris des calculs utilisant une fonction d’onde microscopique pour le projectile. Un exemple simple est la particule alpha décrite par deux neutrons et deux protons. J'ai appliqué ce modèle à la diffusion élastique alpha+58Ni et j'ai montré que la section efficace est sensible au rayon de la particule alpha. J'ai proposé, sans les appliquer, des extensions plus générales de ce modèle., Doctorat en Sciences, 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 particular strategies: one based on a weight criterion and one based on the type of excitations. Before concluding, we expose some developments combining the SCF process and the biorthonormal condition to relax the orthogonality constraints that are presently applied to the optimization process of the spin-orbital basis. / Dans notre thèse, nous abordons le problème polyélectronique dans un contexte non-relativiste et relativiste en adoptant une approche orbitalaire. En utilisant les suites de programmes reconnues ATSP et GRASP, nous sommes aptes à approcher des fonctions d'ondes polyélectroniques au-delà du modèle des particules indépendantes en utilisant une superposition de CSFs. Le processus d'optimisation, basé sur le principe des variations, fournit la meilleure estimation possible des coefficients de mélange, fixant la combinaison linéaire de CSFs et la meilleure base de spin-orbitales sur laquelle on impose la condition d'orthonormalité entre les fonctions appartenant au même sous-espace l ou kappa. En adoptant cette approche dans un cadre relativiste, nous évaluons des énergies de transition, des probabilités de transition, des déplacements isotopiques ainsi qu'une transition induite par mélange hyperfin pour l'atome d'antimoine trois fois ionisé (Sb IV). Dans le but de relâcher partiellement les contraintes d'orthogonalité entre les orbitales de corrélation, nous utilisons le principe des variations afin de cibler des effets précis de la corrélation en taillant l'espace des configurations. Les ensembles indépendants d'orbitales de corrélation sont obtenus via la méthode MCHF. Les espaces de CSFs, exprimés sur ces fonctions mono-électronique non-orthogonales, sont couplés en résolvant le problème aux valeurs propres généralisé associé. Les matrices Hamiltonienne et de recouvrement sont déterminées au moyen de la technique des transformations biorthonormales et de la contre-transformation des vecteurs propres associés. Cette méthode originale est utilisée avec succès pour décrire des systèmes atomiques légers comme Li I, Be I, B I, C II et Ne I. Un effet indésirable, appelé ``effet de contrainte', est déecrit et étudié pour ces derniers systèmes atomiques. Même si ces contraintes peuvent-être en principe totalement levées au travers de la méthode DPCFI, les ressources nécessaires à l'application de cette dernière approche nous ont conduit à la recherche de stratégies simples et efficaces autorisant leur levée partielle. Pour ce faire, dans le cadre de nos calculs réalisés sur l'atome de béryllium, nous avons envisagé deux stratégies particulières: l'une basée sur les coefficients de mélange et l'autre basée sur le type d'excitation. Avant de conclure, nous proposons quelques développements combinant le processus auto-cohérent et la condition de biorthonormalité dans le but de relâcher les contraintes d'orthogonalité appliquées lors du processus d'optimisation de la base de spin-orbitales., Doctorat en Sciences de l'ingénieur, info:eu-repo/semantics/published

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 particular strategies: one based on a weight criterion and one based on the type of excitations. Before concluding, we expose some developments combining the SCF process and the biorthonormal condition to relax the orthogonality constraints that are presently applied to the optimization process of the spin-orbital basis./Dans notre thèse, nous abordons le problème polyélectronique dans un contexte non-relativiste et relativiste en adoptant une approche orbitalaire. En utilisant les suites de programmes reconnues ATSP et GRASP, nous sommes aptes à approcher des fonctions d'ondes polyélectroniques au-delà du modèle des particules indépendantes en utilisant une superposition de CSFs. Le processus d'optimisation, basé sur le principe des variations, fournit la meilleure estimation possible des coefficients de mélange, fixant la combinaison linéaire de CSFs et la meilleure base de spin-orbitales sur laquelle on impose la condition d'orthonormalité entre les fonctions appartenant au même sous-espace l ou kappa. En adoptant cette approche dans un cadre relativiste, nous évaluons des énergies de transition, des probabilités de transition, des déplacements isotopiques ainsi qu'une transition induite par mélange hyperfin pour l'atome d'antimoine trois fois ionisé (Sb IV).Dans le but de relâcher partiellement les contraintes d'orthogonalité entre les orbitales de corrélation, nous utilisons le principe des variations afin de cibler des effets précis de la corrélation en taillant l'espace des configurations. Les ensembles indépendants d'orbitales de corrélation sont obtenus via la méthode MCHF. Les espaces de CSFs, exprimés sur ces fonctions mono-électronique non-orthogonales, sont couplés en résolvant le problème aux valeurs propres généralisé associé. Les matrices Hamiltonienne et de recouvrement sont déterminées au moyen de la technique des transformations biorthonormales et de la contre-transformation des vecteurs propres associés. Cette méthode originale est utilisée avec succès pour décrire des systèmes atomiques légers comme Li I, Be I, B I, C II et Ne I. Un effet indésirable, appelé ``effet de contrainte', est déecrit et étudié pour ces derniers systèmes atomiques. Même si ces contraintes peuvent-être en principe totalement levées au travers de la méthode DPCFI, les ressources nécessaires à l'application de cette dernière approche nous ont conduit à la recherche de stratégies simples et efficaces autorisant leur levée partielle. Pour ce faire, dans le cadre de nos calculs réalisés sur l'atome de béryllium, nous avons envisagé deux stratégies particulières: l'une basée sur les coefficients de mélange et l'autre basée sur le type d'excitation. Avant de conclure, nous proposons quelques développements combinant le processus auto-cohérent et la condition de biorthonormalité dans le but de relâcher les contraintes d'orthogonalité appliquées lors du processus d'optimisation de la base de spin-orbitales., Doctorat en Sciences de l'ingénieur, info:eu-repo/semantics/nonPublished

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 d’antisymmétrisation dans les noyaux d’6He, 6Be, 9Be, 9B et 12C. Nous avons montré que ces effets restent non négligeables mêmes à grandes distances dans le cas des noyaux 9Be, 9B et du 12C. Nous avons ensuite étudié les états du continu du 6He et du 6Be. Nous avons déterminé des valeurs théoriques pour les propriétés des résonances connues expérimentalement. Nos résultats sont en bon accord avec l’expérience. Nous avons également observé des structures larges dans les déphases associés aux états 0+ et 1- de l’6He et 1- du 6Be. Finalement, nous avons étudié les états du 12C à travers la collision 8Be+α. Dans notre calcul, le 8Be est considéré comme un système comportant deux amas α. Nous avons montré que les états 2+ et 4+ du 8Be devaient être pris en compte pour une étude appropriée des états 0+, 2+ et 4+ du 12C., Doctorat en Sciences, 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 d’antisymmétrisation dans les noyaux d’6He, 6Be, 9Be, 9B et 12C. Nous avons montré que ces effets restent non négligeables mêmes à grandes distances dans le cas des noyaux 9Be, 9B et du 12C. Nous avons ensuite étudié les états du continu du 6He et du 6Be. Nous avons déterminé des valeurs théoriques pour les propriétés des résonances connues expérimentalement. Nos résultats sont en bon accord avec l’expérience. Nous avons également observé des structures larges dans les déphases associés aux états 0+ et 1- de l’6He et 1- du 6Be.Finalement, nous avons étudié les états du 12C à travers la collision 8Be+α. Dans notre calcul, le 8Be est considéré comme un système comportant deux amas α. Nous avons montré que les états 2+ et 4+ du 8Be devaient être pris en compte pour une étude appropriée des états 0+, 2+ et 4+ du 12C., Doctorat en Sciences, info:eu-repo/semantics/nonPublished