213 research outputs found
Coulomb dissociation of B and Li
No full textInternational audienceThe analysis of high-energy Coulomb dissociation experiments with 8B and 6Li beams in order to extract low-energy S factors of the 7Be(p,γ)8B and 4He(d,γ)6Li radiative capture reactions relies on a precise knowledge of the reaction mechanism. A comparison of various experimental cross sections with predictions from theory allows us to identify the relevant ingredients that are necessary for a successful theoretical description. The cases of 8B and 6Li Coulomb breakup show substantial differences related to the structure of the nuclei
Electromagnetic dissociation of B and the astrophysical S factor for Be( p , γ ) B
No full textWe present S factor data obtained from the Coulomb dissociation of 83 MeV/nucleon B-8, and analyze Be-7 longitudinal momentum distributions measured at 44 and 81 MeV/nucleon using a potential model, first-order perturbation theory, and dynamical solution of the time-dependent Schrodinger equation. Comparing our results with independent continuum-discretized coupled channels calculations, we study the reaction model and beam energy dependence of the E2 contribution to the dissociation cross section. By fitting radiative capture and Coulomb breakup data taken below relative energies of 400 keV with potential models constrained by Li-7+n and Be-7+p elastic scattering data, we examine the mutual consistency of recent S-17 measurements and obtain a recommended value for S-17(0) of 18.6+/-0.4 (experimental) +/-1.1 (extrapolation) eV b (1sigma). This result is in good agreement with recent experimental determinations of the asymptotic normalization coefficient of the valence proton wave function in B-8
The Trojan-Horse method for nuclear astrophysics
No full textThe Trojan-Horse method is an indirect approach to determine the low-energy astrophysical S-factor of direct nuclear reactions by studying closely related transfer reactions with three particles in the final state under quasi-free scattering conditions. The theoretical foundation and basic features of this approach are presented. General considerations for the application of method and two examples are discussed
The Trojan-Horse Method applied to the6Li(p,α)3He reaction down to astrophysical energies
No full textThe Trojan-Horse Method has been applied to the three-body d(6Li,alpha 3He)n breack-up reaction in order to extract the bare nucleus S(E) factor for the 6Li(p,alpha)3He reaction down to astrophysical energies
Pseudospin, supersymmetry and the shell structure of atomic nuclei
No full textThe evolution of single-particle energies with varying isospin asymmetry in the shell model is an important issue when predicting changes in the shell structure for exotic nuclei. In many cases pseudospin partner levels, that are almost degenerate in energy for stable nuclei, are relevant in extracting the size of the shell gaps. A breaking of the pseudospin symmetry can affect the size of these gaps and change the magic numbers accordingly. The strength of the pseudospin splitting is expected to depend in particular on isovector-dependent and tensor contributions to the effective nuclear interaction. A description employing supersymmetric quantum mechanics allows to derive a pseudospin symmetry breaking potential that is regular in contrast to the pseudospin-orbit potential in the conventional relativistic treatment. The derived perturbation potential provides a measure to quantify the symmetry breaking and it can be employed to improve mean-field calculations in order to better reproduce the experimentally observed shell evolution. General potentials with exact pseudospin symmetry are obtained that can be used in relativistic mean-field Hamiltonians
Indirect study of the astrophysically relevant reaction 6Li(p,alpha)3He by means of the Trojan Horse Method
No full textThe Trojan Horse Method was applied to the 2H(6Li,α3He)n three-body reaction in order to extract the bare nucleus S(E) factor for the 6Li(p,α)3He reaction. The three-body reaction was performed in two kinematically complete experiments at beam energies of 25 and 14 MeV. The selected quasi-free coincidence yield was compared with the result of a Monte Carlo calculation where the entering two-body cross section was the result of a R-matrix parameterization of the direct two-body cross section. The quite good agreement throughout the investigated region above and below the p-6Li Coulomb barrier, allowed for the extraction of the bare S(E) factor in the astrophysical energy region. The S(0) value together with an independent estimate of the screening potential Ue were derived and compared with those obtained from direct measurements
Variations on the excluded-volume mechanism
No full textThe conventional excluded-volume mechanism in the theoretical description of matter properties is generalized by introducing more general functional dependencies of the available volume fraction. The requirement of thermodynamic consistency governs the appearance of rearrangement contributions to thermodynamic quantities and to particle potentials. The main features of the method are studied in three examples: the dissolution of deuterons in warm and dense nuclear matter, the stiffening or softening of the nuclear matter equation of state in a relativistic mean-field model, and the effects of medium-dependent effective degeneracy factors in a Fermi gas model for quark matter
Bare astrophysical S(E)-factor for the Li-6(d, alpha)He-4 and Li-7(p,alpha)He-4 reactions at astrophysical energies
No full textClustering in dilute matter with medium effects
No full textThe formation and dissolution of light and heavy clusters in warm and dilute strongly interacting matter is studied using a generalized relativistic density functional. This phenomenological theoretical description is an extension of a relativistic mean-field model with density dependent nucleons-meson couplings. It considers nuclei as explicit degrees of freedom in addition to nucleons. Nuclei and nucleons are treated as quasiparticles with medium dependent masses. For clusters, one contribution to the mass shift represents an effective way to take into account the Pauli exclusion principle for nucleons, whether free or bound in nuclei. This mechanism causes the dissolution of clusters at high densities. The consequences are explored for compact star matter as an example for the application of the model in astrophysics. General features and open questions of the approach are discussed
Electromagnetic strength of neutron and proton single-particle nuclei
No full textElectromagnetic strength functions of halo nuclei exhibit universal features that can be described in terms of characteristic scale parameters. For a nucleus with nucleon + core structure the reduced transition probability, as determined, e.g., by Coulomb dissociation experiments, shows a typical shape that depends on the nucleon separation energy and the orbital angular momenta in the initial and final states. The sensitivity to the final-state interaction (FSI) between the nucleon and the core can be studied systematically by varying the strength of the interaction in the continuum. In the case of neutron + core nuclei analytical results for the reduced transition probabilities are obtained by introducing the effective-range expansion. The scaling with the relevant parameters is found explicitly. General trends are observed by studying several examples of neutron +core and proton +core nuclei in a single-particle model assuming Woods-Saxon potentials. Many important features of the neutron halo case can be obtained from a square-well model. Rather simple analytical formulas are found. The nucleon-core interaction in the continuum affects the determination of astrophysical S factors at zero energy in the method of asymptotic normalisation coefficients (ANC). It is also relevant for the extrapolation of radiative capture cross sections to low energies. (c) 2005 Elsevier B.V. All rights reserved
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