Pierre Descouvemont

A compilation of charged-particle induced thermonuclear reaction rates

Low-energy cross section data for 86 charged-particle induced reactions involving light (1 less than or equal to Z less than or equal to 14), mostly stable, nuclei are compiled. The corresponding Maxwellian-averaged thermonuclear reaction rates of relevance in astrophysical plasmas at temperatures in the range from 10(6) K to 10(10) K are calculated. These evaluations assume either that the target nuclei are in their ground state, or that the target states are thermally populated following a Maxwell-Boltzmann distribution, except in some cases involving isomeric states. Adopted values complemented with lower and upper limits of the rates are presented in tabular form. Analytical approximations to the adopted rates, as well as to the inverse/direct rate ratios, are provided

Updated Big Bang nucleosynthesis confronted to WMAP observations and to the abundance of light elements

We improve Standard Big Bang Nucleosynthesis (SBBN) calculations taking into account new nuclear physics analyses (Descouvemont et al. 2003). Using a Monte-Carlo technique, we calculate the abundances of light nuclei versus the baryon to photon ratio.The results concerning omegab are compared to relevant astrophysical and cosmological observations. Consistency between WMAP, SBBN results and D/H data strengthens the deduced baryon density and has interesting consequences on cosmic chemical evolution. A significant discrepancy between the calculated Li-7 deduced from WMAP and the Spite plateau is clearly revealed. To explain this discrepancy three possibilities are invoked : uncertainties on the Li abundance, surface alteration of Li in the course of stellar evolution or poor knowledge of the reaction rates related to Be-7 destruction. In particular, the possible role of the up to now neglected Be-7(d,p)2He-4 and Be-7(d,alpha)Li5 reactions is considered. The impressive advances in CMB observations provide a strong motivation for more efforts in experimental nuclear physics and high quality spectroscopy to keep BBN in pace. Comment: accepted in ApJ, 22 pages, 5 figures

Compilation and R-matrix analysis of Big Bang nuclear reaction rates

We use the R-matrix theory to fit low-energy data on nuclear reactions involved in Big Bang nucleosynthesis. A special attention is paid to the rate uncertainties which are evaluated on statistical grounds. We provide S factors and reaction rates in tabular and graphical formats. Comment: 40 pages, accepted for publication at ADNDT, web site at http://pntpm3.ulb.ac.be/bigbang

Microscopic study of the 7Li(n,γ)8Li and 7Be(p,γ)8B reactions in a multiconfiguration three-cluster model

Abstract The three-cluster generator coordinate method is applied to the 7Li(n, γ)8Li and 7Be(p, γ)8B capture reactions. The 8B (or 8Li) nucleus is defined by a mixing of (α+3He)+p and (α + p) + 3He (or mirror) configurations. We investigate the sensitivity of different observables with respect to the nucleon-nucleon interaction by considering four different Volkov forces. For all of them, the model fairly reproduces most of the experimentally known spectroscopic properties of the 8B and 8Li mirror nuclei. Recent measurements of the quadrupole moments are well explained without a halo structure. The 7Li(n, γ)8Li cross section supports the data of Imhof et al. rather than those of Wiescher et al. At zero energy, the 7Be(p, γ)8B astrophysical S-factors slightly depend on the nucleon-nucleon interaction. They are consistent with the currently accepted value, but contradict lower estimates.

S-factor of 14N(p,γ)15O at astrophysical energies⋆

Abstract.The astrophysical S(E) factor of 14N(p,γ)15O has been measured for effective center-of-mass energies between Eeff = 119 and 367 keV at the LUNA facility using TiN solid targets and Ge detectors. The data are in good agreement with previous and recent work at overlapping energies. R-matrix analysis reveals that due to the complex level structure of 15O the extrapolated S(0) value is model dependent and calls for additional experimental efforts to reduce the present uncertainty in S(0) to a level of a few percent as required by astrophysical calculations.

Electromagnetic transitions and radiative capture in the generator-coordinate method

Abstract A microscopic study of the electromagnetic properties of nuclei and of radiative capture reactions is presented in the framework of the generator-coordinate method. Fully antisymmetrized wave functions, with a correct asymptotic behaviour, are used to calculate matrix elements of the multipole operators in the long-wavelength approximation. Centre-of-mass corrections are shown to vanish exactly in this approximation. Approximate forms are derived when antisymmetrization is negligible. The E2, E3 and M2 transitions in 20 Ne and the 16 (α, γ) 20 Ne radiative capture reaction are treated as illustrative examples.

The 7Be(p, γ)8B reaction in a microscopic three-cluster model

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 32− and 12− 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.

Experimental determination of the 7Be+p scattering lengths

The Be-7 + p elastic cross section has been measured at the Centre de Recherches du Cyclotron RIB facility at Louvain-la-Neuve in the c.m. energy region from 0.3 to 0.75 MeV by bombarding a proton-rich target with a radioactive 7Be beam. The recoil protons have been detected in the angular range theta(c.m.) = 120.2degrees-131.1degrees and theta(c.m.) = 156.6degrees-170.2degrees using the LEDA system. From a R-matrix analysis of the cross section data. we obtain the energy and the width of the I resonance (E-X = 0.77 MeV). The iota = 0 scattering lengths a(01) = 25 +/- 9 fm (channel spin I = 1) and a(02) = -7 +/- 3 fm (channel spin I = 2) have been deduced. They are compared to values expected from charge-symmetry properties. Implications on the low energy S-factor of the Be-7(p,gamma)B-8 reaction are discussed

Antisymmetrization effects in radiative capture reactions

The role played by the Pauli principle in low-energy radiative capture reactions is investigated through a comparison between a microscopic model-the generator coordinate method-and two of its approximations based on an orthogonality condition. The electromagnetic transition matrix element is shown to contain a dominant local term and a smaller residual-antisymmetrization term. The validity of the approximations is studied for the 16O(α, γ) 20Ne reaction under conditions of calculation leading to spectroscopic and elasticscattering properties as close as possible to those of the microscopic model. Nonnegligible differences are found between the three calculations. The local approximation is not significantly less accurate than the more complicated nonlocal one. A simple criterion is proposed to determine the validity of both approximations for other radiative capture reactions. At very low energies, the variation of the astrophysical factor can be well reproduced by the extranuclear capture model. The slope of the S-factor at zero energy is explained qualitatively for different reactions by a simple analytical formula.

New reaction rate for O-16(p, gamma)F-17 and its influence on the oxygen isotopic ratios in massive AGB stars

The O-16(p, gamma)F-17 reaction rate is revisited with special emphasis on the stellar temperature range of T=60-100 MK, important for hot bottom burning in asymptotic giant branch (AGB) stars. We evaluate existing cross-section data that were obtained since 1958 and, if appropriate, correct published data for systematic errors that were not noticed previously, including the effects of coincidence summing and updated effective stopping powers. The data are interpreted by using two different models of nuclear reactions, that is, a potential model and R-matrix theory. A new astrophysical S factor and recommended thermonuclear reaction rates are presented. As a result of our work, the O-16(p, gamma)F-17 reaction has now the most precisely known rate involving any target nucleus in the mass A >= 12 range, with reaction rate errors of about 7% over the entire temperature region of astrophysical interest (T=0.01-2.5 GK). The impact of the present improved reaction rate with its significantly reduced uncertainties on the hot bottom burning in AGB stars is discussed. In contrast to earlier results we find now that there is not clear evidence to date for any stellar grain origin from massive AGB stars.