G. Bogaert

Low-energy measurement of the Be-7(p,gamma) B-8 cross-section

We have measured the cross section of the 7Be(p,gamma)8B reaction for E_cm = 185.8 keV, 134.7 keV and 111.7 keV using a radioactive 7Be target (132 mCi). Single and coincidence spectra of beta^+ and alpha particles from 8B and 8Be^* decay, respectively, were measured using a large acceptance spectrometer. The zero energy S factor inferred from these data is 18.5 +/- 2.4 eV b and a weighted mean value of 18.8 +/- 1.7 eV b (theoretical uncertainty included) is deduced when combining this value with our previous results at higher energies. Comment: Accepted for publication in Phys. Rev. Lett

Determination of the 13N(p, γ)14O reaction rate through the Coulomb break-up of a 14O radioactive beam

Abstract In stellar-evolution models, the 13N(p, γ)14O reaction plays an important role in the hot CNO cycle. Its reaction rate depends directly on the magnitude of the radiative width of the 5.17 MeV level in 14O. That width has been measured using the Coulomb break-up technique. A 70 MeV u 14O beam was excited in the Coulomb field of a 208Pb target, the 13N and proton fragments being recorded using a magnetic spectrometer and CsI scintillators, respectively. The experimental value Γγ = 2.4 ± 0.9 eV is in overall agreement with other recent measurements.

Astrophysical rate of the 11C+p reaction from the Coulomb break-up of a 12N radioactive beam

Abstract The Coulomb break-up technique has been used to determine the radiative width of one excited level in 12 N and the direct capture contribution to 11 C(p,γ) 12 N. The 12 N radioactive beam was produced through transfer and fragmentation reactions of a primary 95.5 MeV/u 14 N beam at GANIL. From the extracted cross section we deduce the rate of the 11 C+p reaction. Consequences for the hot pp chain are analysed.

Comparison of low-energy resonances in15N(α,γ)19F and15O(α,γ)19Ne and related uncertainties

A disagreement between two determinations of Gamma_alpha of the astro- physically relevant level at E_x=4.378 MeV in 19F has been stated in two recent papers by Wilmes et al. and de Oliveira et al. In this work the uncertainties of both papers are discussed in detail, and we adopt the value Gamma_alpha=(1.5^{+1.5}_{-0.8})10^-9eV for the 4.378 MeV state. In addition, the validity and the uncertainties of the usual approximations for mirror nuclei Gamma_gamma(19F) approx Gamma_gamma(19Ne), theta^2_alpha(19F) approx theta^2_alpha(19Ne) are discussed, together with the resulting uncertainties on the resonance strengths in 19Ne and on the 15O(alpha,gamma)19Ne rate.A disagreement between two determinations of {Gamma}{sub {alpha}} of the astrophysically relevant level at E{sub x}=4.378 MeV in {sup 19}F has been stated in two recent papers by Wilmes {ital et al.} and de Oliveira {ital et al.} In this work the uncertainties of both papers are discussed in detail, and we adopt the value {Gamma}{sub {alpha}} = (1.5{sub {minus}0.8}{sup +1.5}){times}10{sup {minus}9} eV for the 4.378 MeV state. In addition, the validity and the uncertainties of the usual approximations for mirror nuclei {Gamma}{sub {gamma}}({sup 19}F){approx}{Gamma}{sub {gamma}}({sup 19}Ne), {theta}{sub {alpha}}{sup 2}({sup 19}F){approx}{theta}{sub {alpha}}{sup 2}({sup 19}Ne) are discussed, together with the resulting uncertainties on the resonance strengths in {sup 19}Ne and on the {sup 15}O({alpha},{gamma}){sup 19}Ne rate. {copyright} {ital 1997} {ital The American Physical Society}

Determination of α-widths in 19F relevant to fluorine nucleosynthesis

Abstract Nucleosynthesis of fluorine in the context of helium burning occurs through the 15 N(α,γ) 19 F reaction. At temperatures where fluorine formation takes place in most astrophysical models, the narrow resonance associated with the 4.378 MeV level of 19 F is expected to dominate the reaction rate, but its strength is not known. We used a 15 N confined gas target to study this level by means of the transfer reaction 15 N 7 Li,t) 19 F at 28 MeV. Reaction products products were analysed with a split pole magnetic spectrometer and the angular distributions for the first 16 levels of 19 F were extracted. These distributions are fairly well reproduced by FR-DWBA calculations in the framework of an α-cluster transfer model with a compound-nucleus contribution obtained by Hauser-Feshbach calculations. α-spectroscopic factors were deduced and, for unbound levels, the α-widths were determined and compared with the existing direct measurements. The α-width of the level of astrophysical interest ( E x = 4.378 MeV) was found to be Γ α = 1.5 × 10 −9 eV, a value 60 times smaller than the commonly used one. The astrophysical consequences for 19 F production in AGB stars are discussed.

31S(p,γ)32Cl reaction in explosive hydrogen burning

Abstract In the present work we attempted to determine excitation energies and widths of proton unbound states in 32 Cl. These states may contribute as resonances to the 31 S(p,γ) reaction and will determine the reaction rate. Results were used to evaluate the reaction flow in the Si to Ar region obtained by nova outbursts in the case of an ONeMg white dwarf of 1.35 M ⊙ .

The 12C + α reaction rate from the elastic 16O breakup

Abstract Evidence for direct elastic breakup of 16 O into the α- 12 C continuum with relative energies ranging from 900 to 1800 keV has been obtained in the scattering of 1527 MeV 16 O projectiles off 208 Pb. An interpretation of E2 breakup including nuclear and Coulomb contributions leads to reduced electromagnetic transitions probabilities and astrophysical S e 2 factors in reasonable agreement with direct measurements, showing that the method can be applied to extract the E2 part of the 12 C(α, γ) reaction rate.

Rate of 3H(7Li, n0)9Be and big-bang nucleosynthesis☆

Abstract The differential cross sections for the 3 H( 7 Li, n 0 ) 9 Be reaction were measured at 5 angles in the energy range E (c.m.)=0.2–0.9 MeV using a pulsed 7 Li beam and time-of-flight technique. Absolute values of the cross section were obtained by comparison with the well-known cross section of 3 H(d, n) 4 He at E d =1.0 MeV. The resulting reaction rates are obtained at temperatures relevant to big-bang nucleosynthesis, and consequences for primordial 9 Be abundances are discussed.

FRAGMENT ANGULAR CORRELATION IN THE BREAKUP OF 16O IONS AT 95 MEV/A

Abstract Fragment angular correlations in projectile breakup reactions are very sensitive to interference between different multipolarities entering in the excitation-dissociation process of the projectile. In particular, it was proposed to disentangle L = 1 and L = 2 contributions in direct breakup reactions of 16 O with low relative energy between the α and 12 C fragment, which are of astrophysical interest. We studied the experimental aspects of extracting those angular correlations in extreme kinematical conditions usually encountered in breakup experiments of astrophysical interest. The breakup of 95 MeV/ A 16 O projectiles induced by a 208 Pb target was measured using the high-resolution spectrometer SPEG at Ganil for the coincident detection of the fragments. Sequential breakup via the 12.53 MeV level of 16 O is analyzed in this framework and it favors an one-step M2 excitation of this level.

A 250 kV high current ion accelerator for applications in nuclear astrophysics

Abstract A 250 kV ion accelerator which provides high proton beam currents of the order of 1 mA has been built at the CSNSM, Orsay. Energy spread and energy stability were found to be better than ± 50 eV. This accelerator will be used for precise determination of proton induced nuclear reactions at low energies relevant in the field of nuclear astrophysics. Experimental studies of nuclear reactions involving radioactive targets will be performed. The target setup for measuring particle emitting reactions consists of a solenoidal superconducting spectrometer to achieve a large solid angle for the detection of the reaction products and an improved signal/noise ratio.