Sam M. Austin

Solar fusion cross-sections

We review and analyze the available information on the nuclear-fusion cross sections that are most important for solar energy generation and solar neutrino production. We provide best values for the low-energy cross-section factors and, wherever possible, estimates of the uncertainties. We also describe the most important experiments and calculations that are required in order to improve our knowledge of solar fusion rates.

Energy Systematics of the Giant Gamow-Teller Resonance and a Charge-Exchange Dipole Spin-Flip Resonance

Energy systematics of the giant Gamow-Teller resonance and a charge-exchange resonance excited by a L = 1, S = 1 interaction are presented. Plots of the energy separation between each resonance and the IAS versus (N − Z)A can be represented approximately by linear functions.

PRODUCTION OF 26Al, 44Ti, AND 60Fe IN CORE-COLLAPSE SUPERNOVAE: SENSITIVITY TO THE RATES OF THE TRIPLE ALPHA AND 12C(α, γ)16O REACTIONS

We have studied the sensitivity to variations in the triple alpha and 12C(?, ?)16O reaction rates of the production of 26Al, 44Ti, and 60Fe in core-collapse supernovae (SNe). We used the KEPLER code to model the evolution of 15 M ?, 20 M ?, and 25 M ? stars to the onset of core collapse and simulated the ensuing SN explosion using a piston model for the explosion and an explosion energy of 1.2 ? 1051 erg. Calculations were performed for the Anders & Grevesse and Lodders abundances. Over a range of twice the experimental uncertainty, ?, for each helium-burning rate, the production of 26Al, 60Fe, and their ratio vary by factors of 5 or more. For some species, similar variations were observed for much smaller rate changes, 0.5? or less. The production of 44Ti was less sensitive to changes in the helium-burning rates. Production of all three isotopes depended on the solar abundance set used for the initial stellar composition.

S17(0) determined from the Coulomb breakup of 83 MeV/nucleon 8B

A kinematically complete measurement was made of the Coulomb dissociation of 8B nuclei on a Pb target at 83 MeV/nucleon. The cross section was measured at low relative energies in order to infer the astrophysical S factor for the 7Be(p,gamma)8B reaction. A first-order perturbation theory analysis including E1, E2, and M1 transitions was employed to extract the E1 strength relevant to neutrino-producing reactions in the solar interior. By fitting the measured cross section from E(rel) = 130 to 400 keV, we find S17(0) = 17.8(+1.4)(-1.2) eV b.

Nuclear reaction rates and carbon star formation

We have studied how the third dredge-up and the carbon star formation in low-mass asymptotic giant branch stars depend on certain key nuclear reaction rates. From a set of complete stellar evolution calculations of a 2 M☉ model with Z = 0.01 including mass loss, we find that varying either the 14N(p, γ)15O or the triple-α reaction rate within their uncertainties as given in the NACRE compilation results in dredge-up and yields that differ by a factor of 2. Model tracks with a higher rate for the triple-α rate and a lower rate for the 14N(p, γ)15O reaction both show more efficient third dredge-up. New experimental results for the 14N(p, γ)15O reaction rates are surveyed, yielding a rate that is about 40% lower than the tabulated NACRE rate and smaller than NACREs lower limit. We discuss the possible implications of the revised nuclear reaction rate for stellar evolution calculations that aim to reproduce the observed carbon star formation at low mass, which requires efficient third dredge-up.

S{sub 17}(0 ) Determined from the Coulomb Breakup of 83MeV/ Nucleon {sup 8}B

A kinematically complete measurement was made of the Coulomb dissociation of 8B nuclei on a Pb target at 83 MeV/nucleon. The cross section was measured at low relative energies in order to infer the astrophysical S factor for the 7Be(p,gamma)8B reaction. A first-order perturbation theory analysis including E1, E2, and M1 transitions was employed to extract the E1 strength relevant to neutrino-producing reactions in the solar interior. By fitting the measured cross section from E(rel) = 130 to 400 keV, we find S17(0) = 17.8(+1.4)(-1.2) eV b.

Gamow-Teller strengths from (t,3He) charge-exchange reactions on light nuclei

Abstract Zero-degree cross sections have been measured for (t, 3 He) reactions on the light target nuclei 9 Be, 10 B, 11 B, 12 C, and 13 C at a bombarding energy of 127 MeV/A. The triton beam was produced by ( 4 He,t+p) breakup on a Be production target using a 4 He beam of 155 MeV/A. Dispersion matching was applied to obtain good energy resolution ( Δ E/E=0.002, Δ E=780 keV). The zero-degree (t, 3 He) cross sections are used to discuss Gamow-Teller strengths in 10 Be and 11 Be.

ON THE SENSITIVITY OF MASSIVE STAR NUCLEOSYNTHESIS AND EVOLUTION TO SOLAR ABUNDANCES AND TO UNCERTAINTIES IN HELIUM-BURNING REACTION RATES

We explore the dependence of presupernova evolution and supernova nucleosynthesis yields on the uncertainties in helium-burning reaction rates. Using the revised solar abundances of Lodders for the initial stellar composition, instead of those of Anders and Grevesse, changes the supernova yields and limits the constraints that those yields place on the 12C(α,γ)16O reaction rate. The production factors of medium-weight elements (A = 16-40) were found to be in reasonable agreement with observed solar ratios within the current experimental uncertainties in the triple-α reaction rate. Simultaneous variations by the same amount in both reaction rates or in either of them separately, however, can induce significant changes in the central 12C abundance at core carbon ignition and in the mass of the supernova remnant. It therefore remains important to have experimental determinations of the helium-burning rates so that their ratio and absolute values are known with an accuracy of 10% or better.

The initial organic products of fixation of 13N-labeled nitrogen gas by the blue-green alga Anabaena cylindrica

Methods have been developed for the rapid isolation and characterization of the first organic products of fixation of 13N-labeled N2. In experiments with the blue-green alga, Anabaena cylindrica, glutamine is the first 13N-labeled organic product observed, and glutamate is the second. The results indicate that the glutamine synthetase/glutamate synthase pathway is operative in this blue-green alga.

Dependence of s-Process Nucleosynthesis in Massive Stars on Triple-Alpha and 12C(α, γ)16O Reaction Rate Uncertainties

We have studied the sensitivity of s-process nucleosynthesis in massive stars to ±2σ variations in the rates of the triple-α and 12C(α, γ)16O reactions. We simulated the evolution of massive stars from H burning through Fe-core collapse, followed by a supernova explosion. We found that the production factors of s-process nuclides between 58Fe and 96Zr change strongly with changes in the He burning reaction rates; using the Lodders solar abundances rather than those of Anders and Grevesse reduces s-process nucleosynthesis; later burning phases beyond core He burning and shell C burning have a significant effect on post-explosive production factors. We also discuss the implications of the uncertainties in the helium burning rates for evidence of a new primary neutron capture process (LEPP) in massive stars.