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Massive stars are crucial to galactic chemical evolution for elements heavier than iron. Their contribution at early times in the evolution of the Universe, however, is unclear due to poorly constrained nuclear reaction rates. The competing 17O(α, γ)21Ne and 17O(α, n)20Ne reactions strongly impact weak s-process yields from rotating massive stars at low metallicities. Abundant 16O absorbs neutrons, removing flux from the s-process, and producing 17O. The 17O(α, n)20Ne reaction releases neutrons, allowing continued s-process nucleosynthesis, if the 17O(α, γ)21Ne reaction is sufficiently weak. While published rates are available, they are based on limited indirect experimental data for the relevant temperatures and, more importantly, no uncertainties are provided. The available nuclear physics has been evaluated, and combined with data from a new study of astrophysically relevant 21Ne states using the 20Ne(d ,  p)21Ne reaction. Constraints are placed on the ratio of the (α, n)/(α, γ) reaction rates with uncertainties on the rates provided for the first time. The new rates favour the (α, n) reaction and suggest that the weak s-process in rotating low-metallicity stars is likely to continue up to barium and, within the computed uncertainties, even to lead. [ABSTRACT FROM AUTHOR]

The level structure of 30P up to 8.25 MeV was investigated via in-beam &#947-ray spectroscopy using the 28Si(³He,p&#947)30P reaction at 9 MeV at the University of Tsukuba Tandem Accelerator Complex in Japan. An energy level scheme was deduced from &#947-&#947 coincidence measurements. 47 new transitions have been observed from the previously known states (mostly resonances), thereby reducing the uncertainties in the excitation energies of 17 states from 3 to 10 keV to values of < 1 keV. Furthermore, spin assignments based on measurements of &#947-ray angular distributions and &#947-&#947 directional correlation of oriented nuclei (DCO ratios) were made for several observed levels of 30P. [ABSTRACT FROM AUTHOR]

Abstract: The 29P(p,γ)30S reaction plays an important role in understanding the Si isotopic abundances in presolar grains of nova origin, which in turn provide us with information on the nature of the probable white dwarf progenitor''s core, as well as the peak temperatures achieved during nova outbursts. This rate at nova temperatures is determined by two low-lying and resonances above the proton threshold at 4399 keV in 30S. We have studied the 30S nuclear structure via the 32S(p,t)30S reaction at 5 laboratory angles between to . We have observed 14 states including two levels at and that are candidates for the and the previously “missing” state, respectively. [Copyright &y& Elsevier]

We present the preparation and characterization of a large sample of implanted noble gas targets for use in precision nuclear astrophysics measurements with intense proton beams. Tantalum and titanium backings were prepared using wet-acid etching, outgassed via resistive heating, and implanted with 20Ne + beams from differing ion sources. These experimental targets were investigated using both nuclear reaction analysis techniques on the 1169-keV resonance in 20Ne(p, γ)21Na and Rutherford backscattering spectrometry analysis with 2-MeV α -particle beams. Results from these analyses reveal small target-to-target variations in stoichiometry, while exhibiting excellent agreement independent of ion-beam analysis method. We also present a self-consistent validation of the nuclear reaction analysis results using ion transport simulations in TRIM-2013 that rely on input parameters from SIMNRA scattering-yield fits to Rutherford backscattering spectra. In addition to a complete description of the implantation profile, this analysis method provides an alternate solution for characterizing a large sample of implanted targets when no suitable resonances are available for nuclear reaction analysis. [ABSTRACT FROM AUTHOR]

The 17O(p,γ)18F reaction plays an important role in hydrogen-burning nucleosynthesis. Conflicting values for the low-energy behavior of its cross section exist in the literature. We present direct measurements of the astrophysical S factor of the 17O(p,γ)18F reaction at center-of-mass energies between 250 and 500 keV. These measurements were conducted in inverse kinematics at the DRAGON recoil separator. [ABSTRACT FROM AUTHOR]

Model predictions of the amount of the radioisotope 26A1 produced in hydrogen-burning environments require reliable estimates of the thermonuclear rates for the 26gAl(p,γ) 27Si and 26mAl(p,γ)27Si reactions. These rates depend upon the spectroscopic properties of states in 27Si within about 1 MeV of the 26sAl + p threshold (Sp = 7463 keV). We have studied the 28Si(3He,α)27Si reaction at 25 MeV using a high-resolution quadrupole-dipole-dipole-dipole magnetic spectrograph. For the first time with a transfer reaction, we have constrained Jπ values for states in 27Si over Ex = 7.0-8.1 MeV through angular distribution measurements. Aside from a few important cases, we generally confirm the energies and spin-parity assignments reported in a recent γ-ray spectroscopy study. The magnitudes of neutron spectroscopic factors determined from shell-model calculations are in reasonable agreement with our experimental values extracted using this reaction. [ABSTRACT FROM AUTHOR]