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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

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The 16O nucleus is investigated within a C12+α four α multicluster model. The generator coordinate method combined with the microscopic R-matrix method is used to determine bound and resonant states. The 12C nucleus is described by 3 α clusters located at the apexes of isosceles triangles. We optimize the generator coordinates to get a reasonable description of the 02+-01+ energy gap, where the 02+ is the so-called Hoyle state, considered as a 3 α state. Our calculations mainly focus on resonances located near the C12(02+)+α threshold. We emphasize the difficulty to interpret broad resonant states and we show that bound state approximation methods are not well adapted in the context of our study. An R-matrix analysis suggests the existence of 0 +, 2 + and 4 + molecular states which present a significant C12(02+)+α structure, and which may form a rotational band. © 2014 Elsevier B.V., SCOPUS: ar.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 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

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

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

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

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The 18F(p, α) 15O S-factor is determined in a microscopic cluster model. The wave functions are defined in the Generator Coordinate Method, involving various 18F + p, 15O+α, and 18Ne + n configurations. We analyze the 19Ne level scheme, and test the model with the 19F spectrum, better known experimentally. We focus on 1 / 2+ and 3 / 2+ states, corresponding to s waves, and expected to represent the main contribution in the 18F(p, α) 15O cross section at stellar energies. The 1 / 2+ partial wave is shown to play an important role in a wide energy range. This property reduces the uncertainties associated with interference patterns in the 3 / 2+ contribution. The relevant 1 / 2+ states have been observed in 19F, but have not been searched for in 19Ne. An elastic-scattering experiment above 1 MeV is suggested. © 2007 Elsevier B.V. All rights reserved., info:eu-repo/semantics/published

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Different theoretical models adapted to low-energy reactions are briefly described. We emphasize on the microscopic approach, where all information is obtained from a nucleon-nucleon interaction. A first application deals with the 14C(n,γ )15C reaction, where we use the Asymptotic Normalization Constant (ANC) method. By using recent data on 14O+p elastic scattering combined with a microscopic analysis, we show evidence that existing direct and some indirect data on 14C(n,γ ) 15C are inconsistent with charge symmetry. We also present new results on the 18F(p,α)15O reaction, obtained in a microscopic cluster model. We point out that 1/2+ resonances, generally disregarded, may play a role. This property reduces the uncertainties associated with the 3/2+ contribution. The relevant 1/2+ states have been observed in 19F, but have not been searched for in 19Ne. An elastic-scattering experiment near Ecm ≈ 1.5 MeV is suggested. © Copyright owned by the author(s)., SCOPUS: cp.p, info:eu-repo/semantics/published

An extended form of the Volkov interaction, taking into account the Bartlett and Heisenberg operators is investigated. It contains two free parameters which are determined by adjusting the threshold energy of typical transfer reactions involving p and sd-shell nuclei. Low-lying states of each nucleus are systematically studied. One can conclude at the existence of parameters which reproduce simultaneously at least the ground-state energy of the nuclei involved in a given reaction. Furthermore, compared with the standard Volkov force, the new interaction gives other interesting results such as a better description of the 16O radius and 31- excitation energy. The force is tested on two-cluster systems with a new microscopic study of 13C + α. © 2004 Elsevier B.V. All rights reserved., info:eu-repo/semantics/published

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The 7Be(p,γ)8B and 17F(p,Γ 18Ne reactions are analyzed in a microscopic cluster model. The 7Be wave functions are defined by an α+3He cluster structure, including distortion effects. We find slight differences with respect, to a previous three-cluster calculation. For 17F(p,y)18Ne, the non-resonant (E1) as well as the resonant contributions (M1, E2) of the astrophysical S-factor are determined. We pay a special attention to the properties of the 31+ resonance, crucial for astrophysics. © 2004 Elsevier B.V. All rights reserved., info:eu-repo/semantics/published, Proceedings of the 8th International Conference on Clustering Aspects of Nuclear Structure and Dynamics, Nara, Japan, Edited by K. Ikeda, I. Tanihata and H. Horiuchi

α-cluster states of 22Ne are investigated with a α+18O two-cluster model, using the Generator Coordinate Method. The l8O internal wave functions are defined in the shell model involving the s, p and sd shells. As a consequence, the model allows the description of many α+18O channels. In this paper, we focus only on two results, particularly significant in the context of this work :the possible existence of a positive-parity molecular band and the confirmation of the existence of negative-parity doublets locatcd above the α+l8O threshold. © 2004 Elsevier B.V. All rights reserved., info:eu-repo/semantics/published, Proceedings of the 8th International Conference on Clustering Aspects of Nuclear Structure and Dynamics, Nara, Japan, Edited by K. Ikeda, I. Tanihata and H. Horiuchi

The 17F( p,γ)18Ne radiative capture reaction is investigated with a two-cluster generator-coordinate method. A multi-channel description of the 18Ne wave functions as a superposition of the 17F+p and 14O+α channels is used. The 17F and 14O wave functions are defined in the shell model involving s, p and sd orbitals. The model provides a good overall description of the 18Ne spectroscopy at energies typical of nova and x-ray burst processes. In particular, the 3+1 state, an s-wave resonance dominating the reaction rate at temperatures above 5×108 K, is well described. We find a proton width Γp=21.1 keV, in good agreement with the experimental value Γp=18.0±2stat ±1sys keV. Its gamma width, unknown experimentally, is Γγ=33 meV, similar to values deduced from the shell model, and used in astrophysics (Γγ= 30±20 meV). The non-resonant (E1) as well as the resonant contributions (M1,E2) of the astrophysical S-factor are determined. At zero energy we find S(0)=3.5 keV-b. The corresponding 17F( p,γ)18Ne reaction rate is calculated, and compared with previous approaches. © 2003 Elsevier B.V. All rights reserved., info:eu-repo/semantics/published

α-cluster states of 22Ne are investigated with an α+18O two-cluster model, using the generator coordinate method. The 18O internal wave functions are defined in the shell model involving the s, p and sd shells. As a consequence, the model allows the description of many α+18O channels. Our results suggest the existence of a positive-parity molecular band. Furthermore they confirm the existence of negative-parity doublets located above the α+18O threshold. Electromagnetic transition probabilities and reduced widths associated with these states are given. A similar study is performed in the 22Mg mirror nucleus. © 2003 Elsevier B.V. All rights reserved., info:eu-repo/semantics/published

info:eu-repo/semantics/published, Nuclei in the cosmos. Proceedings, 6th International Conference, Cosmos 2000, Aarhus, Denmark, 27 June-1 July 2000

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We investigate the 15N(α,γ)19F and 15O(α,γ)19Ne reactions by using a multicluster description of 15N and 15O. The 15N(α,γ)19F and 15O(α,γ)19Ne wave functions are defined in the Generator Coordinate Method, using α and triton (or 3He) wave functions. The band structure and the E2 transition probabilities are well reproduced by the model, but some resonances, important for astrophysics, are missing. The non-resonant part of the 15N(α,γ)19F and 15O(α,γ)19Ne capture cross sections is discussed. © 2000 Elsevier Science B.V., info:eu-repo/semantics/published

We use the Generator Coordinate Method to investigate some nuclei between A=12 and A=16, in a multicluster model. The microscopic wave functions are defined from cluster wave functions, involving three α particles and any s-shell nucleus. The α clusters are located at the apexes of an equilateral triangle, and form the basis of a tetrahedral structure, with the additional s cluster at the top of its height. Optimal configurations are determined for the 12C, 13C, 14C, 15N and 16O nuclei. We show that, with respect to the more usual shell-model approach, the multicluster description significantly improves the binding energy and the spectroscopic properties. Future applications to reactions of astrophysical interest are discussed., SCOPUS: ar.j, info:eu-repo/semantics/published