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We use a microscopic cluster model to investigate the C15 nucleus. Two variants are considered: a two-cluster C14+n description and a three-cluster C13+n+n approach. We show that C15 bound-state energies converge to similar values, provided that the number of C14 states is sufficient. We also study negative-parity states, and show that the three-cluster model is more appropriate. With the same nucleon-nucleon interaction, we analyze the F15 spectrum, where all known states are unbound., SCOPUS: ar.j, info:eu-repo/semantics/published

I investigate three-body spectroscopic amplitudes in a microscopic three-cluster model. The total wave functions are described in the resonating group method (RGM) formalism based on cluster wave functions and on relative functions defined in the hyperspherical method. Core excitations are included. I develop a method aimed to determine three-body spectroscopic amplitudes and spectroscopic factors. This technique also provides three-body wave functions where the antisymmetrization is treated approximately. In this way, the various channels are orthogonal to each other and can be used without the antisymmetrization operator. This method is well known in two-cluster systems, and is based on the eigenvalues and the eigenvectors of the antisymmetrization operator. I illustrate the formalism with the He6, Li11, and Be14 nuclei where Li9 and Be12 excited states are taken into account., SCOPUS: ar.j, info:eu-repo/semantics/published

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Background: Nuclear transfer reactions are a useful tool to study the structure of a nucleus. For reactions involving weekly bound nuclei, breakup effects can play significant role and theoretical calculations can be computational expensive in such cases.Purpose: To utilize the Lagrange-mesh and R-matrix methods for nuclear transfer reactions.Methods: We use the adiabatic distorted wave approximation (ADWA) method which can approximately treats the breakup effects in a simpler manner. In our approach, we apply the R-matrix method combining it with the Lagrange-mesh method, which is known to provide the fast and accurate computations.Results: As a test case, we calculate the angular distribution of the cross sections for the 56Fe(d, p)57Fe reaction, where deuteron breakup effects play important role.Conclusions: We show that these methods work well in the ADWA framework, and we look forward to applying these methods in coupled channel calculations., info:eu-repo/semantics/published

We analyze the C16+p and C16+d reactions within the four- and five-body continuum discretized coupled channel method. The C16 nucleus is described by a C14+n+n configuration in hyperspherical coordinates. This description reproduces fairly well several C16 low-lying states. First we analyze the 2+→0+E2 transition amplitude, which confirms that an effective charge must be introduced to reproduce the experimental value. Then, proton and deuteron elastic and inelastic scattering are investigated by including C16 pseudostates, which simulate the C14+n+n continuum. In C16+d, the deuteron breakup is taken into account with p+n two-body pseudostates. A fair agreement with experiment is obtained without any fitting parameter. Breakup effects are in general small, but improve the agreement with experiment., SCOPUS: ar.j, info:eu-repo/semantics/published

We discuss resonance properties in three-body systems, with examples on 12C and 24Mg. We use a microscopic cluster model, where the generator coordinate is defined in the hyperspherical formalism. The 12C nucleus is described by an α+ α+ α structure, whereas 24Mg is considered as an 16O + α+ α system. We essentially pay attention to resonances. We review various techniques which may extend variational methods to resonances. We consider 0 + and 2 + states in 12C and 24Mg. We show that the r.m.s. radius of a resonance is strongly sensitive to the variational basis. This has consequences for the Hoyle state (02+ state in 12C) whose radius has been calculated or measured in several works. In 24Mg ,we identify two 0 + resonances slightly below the three-body threshold., SCOPUS: ar.j, info:eu-repo/semantics/published

We present a scattering model for nuclei with similar masses. In this three-body model, the projectile has a core+valence structure, whereas the target is identical to the core nucleus. The three-body wave functions must be symmetrized for the exchange of the cores. This property gives rise to nonlocal potentials, which are computed without approximation. The present model is an extension of the continuum discretized coupled channel formalism, with an additional treatment of core exchange. We solve the coupled-channel system, including nonlocal terms, by the R-matrix method using Lagrange functions. This model is applied to the C13+C12, N13+C12, and O16+C12 systems. Experimental scattering cross sections are fairly well reproduced without any parameter fitting. The backward-angle enhancement of the elastic cross sections is due to the nonlocal potential. We discuss in more detail the various nonlocal contributions and present effective local potentials., SCOPUS: ar.j, info:eu-repo/semantics/published

We use a microscopic multicluster model to investigate the structure of 10Be and of 11Be. These nuclei are described by α + α + n + n and α + α + 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 10Be and 9 for 11Be), 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 10Be and of 11Be by considering energy curves, where one of the generator coordinates is fixed during the minimization procedure., SCOPUS: ar.j, info:eu-repo/semantics/published

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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., SCOPUS: re.j, info:eu-repo/semantics/published

The Lagrange-mesh R-matrix method is generalized to inhomogeneous equations. This method is numerically stable and efficient. It can be directly used for transfer reactions with the formalism discussed by Ascuitto and Glendenning [Phys. Rev. 181, 1396 (1969)10.1103/PhysRev.181.1396] and for inclusive breakup reactions modeled by Ichimura, Austern, and Vincent [Phys. Rev. C 32, 431 (1985)10.1103/PhysRevC.32.431]. We first present a simple example to assess the method. Then the application to the Nb93(d,pX) nonelastic breakup is discussed., SCOPUS: ar.j, info:eu-repo/semantics/published

The 18O(p,α)15N reaction affects the synthesis of 15N, 18O and 19F isotopes, whose abundances can be used to probe the nucleosynthesis and mixing processes occurring deep inside asymptotic giant branch (AGB) stars. We performed a low-background direct measurement of the 18O(p,α)15N reaction cross-section at the Laboratory for Underground Nuclear Astrophysics (LUNA) from center of mass energy Ec.m.=340 keV down to Ec.m.=55 keV, the lowest energy measured to date corresponding to a cross-section of less than 1 picobarn/sr. The strength of a key resonance at center of mass energy Er=90 keV was found to be a factor of 10 higher than previously reported. A multi-channel R-matrix analysis of our and other data available in the literature was performed. Over a wide temperature range, T=0.01–1.00 GK, our new astrophysical rate is both more accurate and precise than recent evaluations. Stronger constraints can now be placed on the physical processes controlling nucleosynthesis in AGB stars with interesting consequences on the abundance of 18O in these stars and in stardust grains, specifically on the production sites of oxygen-rich Group II grains., SCOPUS: ar.j, info:eu-repo/semantics/published

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

I develop an extension of the continuum-discretized coupled-channels (CDCC) method to reactions where both nuclei present a low breakup threshold. This leads to a four-body model, where the only inputs are the interactions describing the colliding nuclei, and the four optical potentials between the fragments. Once these potentials are chosen, the model does not contain any additional parameter. First I briefly discuss the general formalism, and emphasize the need for dealing with large coupled-channel systems. The method is tested with existing benchmarks on 4α bound states with the Ali-Bodmer potential. Then I apply the four-body CDCC to the Be11+d system, where I consider the Be10(0+,2+)+n configuration for Be11. I show that breakup channels are crucial to reproduce the elastic cross section, but that core excitation plays a weak role. The Li7+d system is investigated with an α+t cluster model for Li7. I show that breakup channels significantly improve the agreement with the experimental cross section, but an additional imaginary term, simulating missing transfer channels, is necessary. The full CDCC results can be interpreted by equivalent potentials. For both systems, the real part is weakly affected by breakup channels, but the imaginary part is strongly modified. I suggest that the present wave functions could be used in future DWBA calculations., SCOPUS: ar.j, info:eu-repo/semantics/published

We use microscopic Be9 wave functions defined in a α+α+n multicluster model to compute Be9+target scattering cross sections. The parameter sets describing Be9 are generated in the spirit of the stochastic variational method, and the optimal solution is obtained by superposing Slater determinants and by diagonalizing the Hamiltonian. The Be9 three-body continuum is approximated by square-integral wave functions. The Be9 microscopic wave functions are then used in a continuum-discretized coupled-channel (CDCC) calculation of Be9+Pb208 and of Be9+Al27 elastic scattering. Without any parameter fitting, we obtain a fair agreement with experiment. For a heavy target, the influence of Be9 breakup is important, while it is weaker for light targets. This result confirms previous nonmicroscopic CDCC calculations. One of the main advantages of the microscopic CDCC is that it is based on nucleon-target interactions only; there is no adjustable parameter. The present work represents a first step towards more ambitious calculations involving heavier Be isotopes., SCOPUS: ar.j, info:eu-repo/semantics/published

We use a three-body continuum discretized coupled channel (CDCC) model to investigate Coulomb and nuclear effects in breakup and reaction cross sections. The breakup of the projectile is simulated by a finite number of square integrable wave functions. First we show that the scattering matrices can be split in a nuclear term and in a Coulomb term. This decomposition is based on the Lippmann-Schwinger equation and requires the scattering wave functions. We present two different methods to separate both effects. Then, we apply this separation to breakup and reaction cross sections of Li7+Pb208. For breakup, we investigate various aspects, such as the role of the α+t continuum, the angular-momentum distribution, and the balance between Coulomb and nuclear effects. We show that there is a large ambiguity in defining the Coulomb and nuclear breakup cross sections, since both techniques, although providing the same total breakup cross sections, strongly differ for the individual components. We suggest a third method which could be efficiently used to address convergence problems at large angular momentum. For reaction cross sections, interference effects are smaller, and the nuclear contribution is dominant above the Coulomb barrier. We also draw attention to different definitions of the reaction cross section which exist in the literature and which may induce small, but significant, differences in the numerical values., SCOPUS: ar.j, info:eu-repo/semantics/published

We investigate the O26 unbound nucleus in a O24+n+n three-body model. We use the hyperspherical formalism, which relies only on O24+n and n+n interactions. As O26 is unbound, even in its ground state, the long-range behavior of the wave functions is crucial. We adapt the R-matrix technique to derive the wave functions and the associated phase shifts. We start from a O24+n potential which nicely reproduces the known O25 properties. Using a weakly repulsive three-body potential, we find the O26 ground state at 0.08 MeV, i.e. just above the threshold, in agreement with the latest experimental value. An analysis of the wave function suggests a dominant dineutron configuration. We extend the model to J=1- and J=2+. For J=2+, we find a relatively narrow resonance near ER=1.3 MeV (Γ≈0.4 MeV), which is supported by recent experimental data. Broad structures are seen in the 0+ and 1- phase shifts but cannot be associated with physical resonances., SCOPUS: ar.j, info:eu-repo/semantics/published

We study the Coulomb breakup cross section of the22C nucleus and its relation with three-body resonances and ground state properties. Cross sections are computed with a Coulomb corrected four-body eikonal model and they are constructed by fixing the continuum state and varying the ground state of22C. These states are calculated by using20C-n potentials that provide different scattering lengths and resonance energies, of a presumed virtual state and of a d resonance in21C. We find that the breakup cross section is strongly dependent on the ground state properties and we predict the existence of a 2+ narrow resonance in22C., SCOPUS: cp.j, info:eu-repo/semantics/published

We present a new reaction model, which permits the description of reactions where both colliding nuclei present a low threshold to breakup. The method corresponds to a four-body extension of the Continuum Discretized Coupled Channel (CDCC) model. We first discuss the theoretical formalism, and then apply the method to 11Be + d scattering at Ec.m.=45.5 MeV. The 11Be nucleus and the deuteron are described by 10Be + n and p+n structures, respectively. The model involves very large bases, but we show that an accurate description of elastic-scattering data may be achieved only when continuum states of 11Be and of the deuteron are introduced simultaneously. We also discuss breakup calculations, and show that the cross section is larger for 11Be than for the deuteron. The present theory provides reliable wave functions that may be used in the analysis of (d,p) or (d,n) experiments involving radioactive beams., SCOPUS: ar.j, info:eu-repo/semantics/published

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, Doctorat en Sciences de l'ingénieur et technologie, info:eu-repo/semantics/nonPublished