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The largest nuclear physics uncertainty in studies of gamma-ray emission from novae arises from the F-18(p,alpha)O-15 reaction rate that affects the abundance of F-18. Direct measurements have been made of the F-18(p,p)F-18 and F-18(p,alpha)O-15 reaction differential cross sections in the energy range 0.6 <= E-c.m.<= 1.6 MeV. Several resonances have been observed in both measurements, with simultaneous R-matrix fits used to determine the properties of corresponding states in Ne-19. A recently reported state at 7.420 MeV is not seen, while new states are seen including one of J(pi)=3/2(+). A candidate for a recently predicted broad J(pi)=1/2(+) state is seen, which would have significant impact on the low energy F-18(p,alpha)O-15 reaction rate, but its alpha width is significantly smaller than expected.

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.

The low-energy levels of /sup 19/Na were studied in the center-of-mass energy (E/sub CM/) range from 2.6 to 3.4 MeV, by using the elastic and inelastic scattering methods in inverse kinematics. The /sup 18/Ne+p scattering reactions, leading to the ground state and the first excited state of /sup 18/Ne, were investigated by using a 66 MeV radioactive /sup 18/Ne beam and a polyethylene (CH/sub 2/)/sub n/ target. The experimental methods and the set-up are described and preliminary results are presented., Anglais

We have investigated the low-energy states of N-11 by the resonant elastic-scattering method in inverse kinematics using a C-10 beam and a (CH2)(n) target at the CYCLONE facility at Louvain-la-Neuve. Recoil protons were detected at laboratory angles theta(lab)=5.2 degrees-18.3 degrees in a Delta E-E detector system. The absolute C-10+p elastic cross-section data were analyzed in the R-matrix framework. We found N-11 to be unbound with respect to proton emission by 1.54 +/- 0.02 MeV, with a decay width of 0.83 +/- 0.03 MeV. These results are used to calculate the two-proton decay width of the O-12 ground state.

We have investigated the low-energy states of /sup 11/N by the elastic scattering method in inverse kinematics using a /sup 10/C beam and a (CD/sub 2/)/sub n/ target. Recoil protons were detected at 16 laboratory angles thetas /sub lab/ = 4.6 degrees - 18.9 degrees in a Delta E - E detector system. Using the R-matrix method to analyse the /sup 10/C + p elastic cross sections, we found /sup 11/N to be unbound with respect to proton emission by 1.54+/-0.02 MeV with a decay width of 0.83+/-0.03. These results are used to calculate the two-proton decay width of the /sup 12/O ground state., Anglais

It has been realised recently that charge symmetry of the nucleon-nucleon interaction leads to a certain relation between Asymptotic Normalization Coefficients (ANCs) in mirror-conjugated one-nucleon overlap integrals. This relation can be approximated by a simple analytical formula that involves mirror neutron and proton separation energies, the core charge and the range of the strong nucleon-core interaction. We perform detailed microscopic multi-channel cluster model calculations and compare their predictions to the simple analytical formula as well as to calculations within a single-particle model in which mirror symmetry in potential wells and spectroscopic factors are assumed. The validity of the latter assumptions is verified on the basis of microscopic cluster model calculations. For mirror pairs in which one of the states is above the proton decay threshold, a link exists between the proton partial width and the ANC of the mirror neutron. This link is given by an approximate analytical formula similar to that for a bound-bound mirror pair. We compare predictions of this formula to the results of microscopic cluster model calculations. Mirror symmetry in ANCs can be used to predict cross sections for proton capture at stellar energies using neutron ANCs measured with stable or “less radioactive” beams.

The astrophysical reaction rates of the 16O(a,g)20Ne capture reaction and its inverse 20Ne(g,a)16O photodisintegration reaction are given by the sum of several narrow resonances and a small direct capture contribution at low temperatures. Although the thermal population of low lying excited states in 16O and 20Ne is extremely small, the first excited state in 20Ne plays a non-negligible role for the photodisintegration rate. Consequences for experiments with so-called quasi-thermal photon energy distributions are discussed. Comment: 4 pages, 2 figures, Proceedings Nuclear Physics in Astrophysics-II, Debrecen, Hungary, 2005

We use the R-matrix theory to fit S-factor data on nuclear reactions involved in Big Bang nucleosynthesis. We derive the reaction rates with associated uncertainties, which are evaluated on statistical grounds. We provide S factors and reaction rates in tabular and graphical formats (available at http://pntpm3.u1b.ac.be/bigbang)

From the observations of the anisotropies of the Cosmic Microwave Background (CMB) radiation, the WMAP satellite has provided a determination of the baryonic density of the Universe, with an unprecedented precision: 4%. This imposes a careful reanalysis of the standard Big-Bang Nucleosynthesis (SBBN) calculations. In a recent paper, we used the R-matrix theory to fit S-factor data on nuclear reactions involved in Big Bang nucleosynthesis. We derived the reaction rates with associated uncertainties, which were evaluated on statistical grounds (available at http://pntpm3.ulb.ac.be/bigbang). Combining these BBN results with the Omega(b)h(2) value from WMAP, we deduced the light element (He-4, D, He-3 and Li-7) primordial abundances and compare them with spectroscopic observations. There is a very good agreement with deuterium observed in cosmological clouds, which strengthens the confidence on the estimated baryonic density of the Universe. However, there is a discrepancy between the deduced Li-7 abundance and the one observed in halo stars of our Galaxy, supposed, until now, to represent the primordial abundance of this isotope. The origin of this discrepancy, observational, nuclear or more fundamental remains to be clarified. The possible role of the up to now neglected Be-7(d,p)2 alpha and 'Be(d,a)5Li reactions is considered and we present here a dedicated experiment performed at Louvain-la-Neuve to measure these cross sections.

The He-6 + He-4 elastic scattering cross section was measured at E-c.m.=11.6 MeV using a He-6 radioactive beam and a He-4-implanted Al foil as target. The use of the thin implanted target allowed to enlarge the angular range in which the data were collected, with respect to a previous measurement at the same center-of-mass energy. The new and previous data are compared with several calculations. The inclusion of a description for the 2n-transfer process, either with a parity-dependent term, or with a distorted-wave Born approximation transfer amplitude, is crucial to reproduce the increase of the cross section at large angles, thus assessing the contribution of the 2n-transfer process to the reaction. At the present energy, the complicated nature of the transfer process and the possible role played by the coupling to other channels, in particular, the breakup, require sophisticated reaction models in order to extract the relevant information on the peculiar He-6 nucleus. The quality of the data is such that the differences between He-6 and the similar Li-6 system should be identified by such calculations.

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. (C) 2003 Elsevier Science B.V. All rights reserved.

The second excited state (J(pi)=1/2(+), l=0) of the proton-rich nucleus Na-19 has been observed for the first time using the elastic scattering reaction H-1(Ne-18,H-1)Ne-18. An intense (4x10(6) pps) Ne-18 radioactive beam was produced at the CRC-RIB facility at Louvain-la-Neuve to bombard a polyethylene target. The recoil protons were detected at 20 different angles in the LEDA segmented silicon detector. The resulting elastic scattering cross sections have been analyzed using a global R-matrix fit. We find that the second excited state in Na-19 lies at the center-of-mass energy E-c.m.=1.066+/-0.003 MeV with respect to the Ne-18+p threshold, and has a proton width Gamma(p)=101+/-3 keV. The Coulomb shift between the 1/2(+) mirror levels in O-19 and Na-19 is 0.73 MeV, among the largest values currently observed in light exotic nuclei.

We have studied the resonant elastic scattering reaction Ne-18+p in reverse kinematics, by using the intense (4 x 10(6) pps) Ne-18 radioactive beam at the Centre de Recherches du Cyclotron RIB facility at Louvain-la-Neuve. The recoil protons were detected at 20 different angles in the LEDA segmented silicon detector. The resulting elastic scattering cross sections have been analysed using the R-matrix method. We have observed for the first time the second excited state (J(pi) = 1/2(+), l = 0) of the proton-rich nucleus Na-19 at the center-of-mass energy E-c.m. = 1.066 +/- 0.003 MeV with a proton width p = 101 +/- 3 keV. The Coulomb shift between the 1/2(+) mirror levels in O-19 and Na-19 is 0.73 MeV, among the largest values currently observed in light exotic nuclei.