In Wolf-Rayet and asymptotic giant branch (AGB) stars, the 26g Alðp; γÞ 27 Si reaction is expected to govern the destruction of the cosmic γ-ray emitting nucleus 26 Al. The rate of this reaction, however, is highly uncertain due to the unknown properties of key resonances in the temperature regime of hydrogen burning. We present a high-resolution inverse kinematic study of the 26g Alðd; pÞ 27 Al reaction as a method for constraining the strengths of key astrophysical resonances in the 26g Alðp; γÞ 27 Si reaction. In particular, the results indicate that the resonance at E r ¼ 127 keV in 27 Si determines the entire 26g Alðp; γÞ 27 Si reaction rate over almost the complete temperature range of Wolf-Rayet stars and AGB stars.

The 18 F(p,α) 15 O reaction rate is crucial for constraining model predictions of the γ-ray observable radioisotope 18 F produced in novae. The determination of this rate is challenging due to particular features of the level scheme of the compound nucleus, 19 Ne, which result in interference effects potentially playing a significant role. The dominant uncertainty in this rate arises from interference between J π =3/2 + states near the proton threshold (Sp = 6.411 MeV) and a broad J π =3/2 + state at 665 keV above threshold. This unknown interference term results in up to a factor of 40 uncertainty in the astrophysical S-factor at nova temperatures. Here we report a new measurement of states in this energy region using the 19 F( 3 He,t) 19 Ne reaction. In stark contrast with previous assumptions we find at least 3 resonances between the proton threshold and Ecm=50 keV, all with different angular distributions. None of these are consistent with J π = 3/2 + angular distributions. We find that the main uncertainty now arises from the unknown proton-width of the 48 keV resonance, not from possible interference effects. Hydrodynamic nova model calculations performed indicate that this unknown width affects 18 F production by at least a factor of two in the model considered.PACS numbers: 26.50.+x, 26.30.Ca, 25.55.Kr Novae occur in binary systems where hydrogen-rich material is accreted from a companion star onto a white dwarf, leading to thermonuclear runaway and subsequent ejection of material. Their ejecta is thought to be the main source of 13 C, 15 N and 17 O in the Galaxy [1,2]. The relevant unstable nuclei are accessible to experiments, and consequently, novae are the only explosive environment where the nuclear physics input is almost entirely based on experimental data [3].However, there are a number of outstanding challenges in our understanding of nova explosions [4], one of which is to reproduce the amount of ejected material inferred from infrared and radio observations, which is systematically underestimated by models. An independent way to constrain the ejected masses would be the detection of γ-rays, produced at the explosion stage. When the envelope becomes optically thin, novae are expected to emit γ-rays, dominated by a prominent 511 keV line. Predicted detectability distances of this prompt γ-ray emission (about 2 -3 kpc [2]) strongly depend on the overall amount of 18 F (T 1/2 (β + )=110 mins) left over after the explosion. This is critically influenced by the 18 F(p,α) 15 O reaction. Sensitivity studies of the impact of reaction rates on nova nucleosynthesis suggest that rates should be known to a precision of, at least, 30% [3]. However, this rate is currently poorly understood and considerable experimental and theoretical effort has been focused on determining this rate ([5, 6] and references therein).Until recently, this rate was thought to be dominated by (i) the 3/2 − resonance at E cm = 330 keV, and (ii) the interference of the 3/2 + states, at 8 and 38 keV E cm , with the known, broad 3...

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During the slow neutron capture process in massive stars, reactions on light elements can both produce and absorb neutrons thereby influencing the final heavy element abundances. At low metallicities, the high neutron capture rate of 16 O can inhibit s-process nucleosynthesis unless the neutrons are recycled via the 17 O(α,n) 20 Ne reaction. The efficiency of this neutron recycling is determined by competition between the 17 O(α,n) 20 Ne and 17 O(α, γ) 21 Ne reactions. While some experimental data are available on the former reaction, no data exist for the radiative capture channel at the relevant astrophysical energies.The 17 O(α, γ) 21 Ne reaction has been studied directly using the DRAGON recoil separator at the TRIUMF Laboratory. The reaction cross section has been determined at energies between 0.6 and 1.6 MeV E cm , reaching into the Gamow window for core helium burning for the first time. Resonance strengths for resonances at 0.63, 0.721, 0.81 and 1.122 MeV E cm have been extracted. The experimentally based reaction rate calculated represents a lower limit, but suggests that significant s-process nucleosynthesis occurs in low metallicity massive stars.PACS numbers 26.20. Kn, 25.40.Lw

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