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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. Keywords: Stellar hydrogen burning, Hydrostatic stellar nucleosynthesis

We test the sensitivity of transfer reactions to the bound state wave functions within a distorted wave Born approximation formalism. Using supersymmetric transformations, we remove the Pauli-forbidden states from the two-body potentials and generate an equivalent supersymmetric partner. Wave functions from these potentials have the same asymptotics, but they differ in the nuclear interior. This allows us to study the influence of the nuclear interior on transfer cross sections. We apply the calculations to the O16(d,p)17O and C12(7Li, t)16O reactions, which are typical examples of nucleon and α transfer, respectively. The spectroscopic factors for 17O are decreased by about 30% when using supersymmetric potentials. For 16O, the differences are smaller. However, we show that ambiguities exist in the determination of the spectroscopic factors, due to the choice of the angular range where the fit is performed.

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.

We present applications of microscopic models to nuclear reactions of astrophysical interest, and we essentially focus on few-body systems. The calculation of radiative-capture and transfer cross sections is outlined, and we discuss the corresponding reaction rates. Microscopic theories are briefly presented, and we emphasize on the matrix elements of four-body systems. The microscopic extension of the R-matrix theory to nuclear reactions is described. Applications to the 2H(d, γ)4He, 2H(d, p)3H and 2H(d, n)3He reactions are presented. We show the importance of the tensor force to reproduce the low-energy behaviour of the cross sections.

Coulomb breakup excitation functions of 22C are calculated as a function of its ground state energy. We consider three ground states that fall into the recent experimental limit S2n < 0.3 MeV. The ground states are constructed by varying the scattering length value of a 1s1/2 virtual state in 21C. We find enhanced breakup cross sections sensitive to the weakly bound character of the ground state.

We present different reaction models commonly used in nuclear astrophysics, in particular for the nucleosynthesis of light elements. Pioneering works were performed within the potential model, where the internal structure of the colliding nuclei is completely ignored. Significant advances in microscopic cluster models provided the first microscopic description of the 3He(α,γ )7 Be reaction more than thirty years ago. In this approach, the calculations are based on an effective nucleon-nucleon interaction, but the cluster approximation should be made to simplify the calculations. Nowadays, modern microscopic calculations are able to go beyond the cluster approximation, and aim at finding exact solutions of the Schrödinger equation with realistic nucleon-nucleon interactions. We discuss recent examples on the d+d reactions at low energies.

We investigate the 9Be+208Pb elastic scattering, breakup and fusion at energies around the Coulomb barrier. The three processes are described simultaneously, with identical conditions of calculations. The 9Be nucleus is defined in an α + α + n three-body model, using the hyperspherical coordinate method. We first analyze spectroscopic properties of 9Be, and show that the model provides a fairly good description of the low-lying states. The scattering with 208Pb is then studied with the Continuum Discretized Coupled Channel (CDCC) method, where the α + α + n continuum is approximated by a discrete number of pseudostates. Optical potentials for the α+208Pb and n+208Pb systems are taken from the literature. We present elasticscattering and fusion cross sections at different energies.

We propose an alternative for Coupled-Channels calculations with looselybound exotic nuclei(CDCC), based on the the Random Matrix Model of the statistical theory of nuclear reactions. The coupled channels equations are divided into two sets. The first set, described by the CDCC, and the other set treated with RMT. The resulting theory is a Statistical CDCC (CDCCs), able in principle to take into account many pseudo channels.

We present the results of the 8Li(p,α)5He and 8Li(p,p)8Li reactions measured at the RIBRAS (Radioactive Ion Beams in Brazil) system. The experiment was realized in inverse kinematics using a thick [CH2]n polyethylene target and an incident 8Li beam, produced by RIBRAS. Using the thick target method, the complete excitation function could be measured between Ecm = 0.2 − 2.1 MeV, which includes the Gamow peak energy region. The excitation function of the 8Li(p,α)5He reaction, populating resonances between 16.888 and 19.0 MeV in 9Be, was obtained[1] and the resonances were fitted using R-matrix calculations. This study shed light on spins, parities, partial widths and isospin values of high lying resonances in 9Be. The measurement of the resonant elastic scattering 8Li(p,p)8Li populating resonances in the same energy region can constrain the resonance parameters. Preliminary results of the elastic scattering are also presented.

The 12C+12C fusion reaction is investigated in a multichannel folding model, using the density-dependent DDM3Y nucleon-nucleon interaction. The 12C(01+, 2+, 02+, 3−) states are included, and their densities are taken from a microscopic cluster calculation. Absorption to fusion channels is simulated by a short-range imaginary potential and the model does not contain any fitting parameter. We compute elastic and fusion cross sections simultaneously. The role of 12C+12C inelastic channels and, in particular, of the 12C(01+)+12C(02+) channel involving the Hoyle state is important even at low energies.

A generalized optical potential for elastic scattering induced by light weakly bound nuclei is calculated within the Feshbach projection operator method. The model explicitly takes into account the contribution of the projectile break-up continuum treated within a microscopic cluster model. The model is tested on deuteron and 6Lielastic scattering by different targets at intermediate energies. The optical potentials are then calculated for the 6He(230 MeV)+12Creaction treating the projectile nuclei within two (α + 2n) and three (α + n + n) cluster models. The differences are analyzed. [ABSTRACT FROM AUTHOR]

Abstract: 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 and states, corresponding to s waves, and expected to represent the main contribution in the 18F(p, α) 15O cross section at stellar energies. The 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 contribution. The relevant states have been observed in 19F, but have not been searched for in 19Ne. An elastic-scattering experiment above 1 MeV is suggested. [Copyright &y& Elsevier]