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Background:The ratio between the rates of the reactions 17 O(α, n) 20 Ne and 17 O(α, γ) 21 Ne determines whether 16 O is an efficient neutron poison for the s process in massive stars, or if most of the neutrons captured by 16 O(n, γ) are recycled into the stellar environment. This ratio is of particular relevance to constrain the s process yields of fast rotating massive stars at low metallicity.Purpose: Recent results on the (α, γ) channel have made it necessary to measure the (α, n) reaction more precisely and investigate the effect of the new data on s process nucleosynthesis in massive stars. Method:The 17 O(α, n (0+1) ) reaction has been measured with a moderating neutron detector. In addition, the (α, n1) channel has been measured independently by observation of the characteristic 1633 keV γ-transition in 20 Ne. The reaction cross section was determined with a simultaneous R-matrix fit to both channels. (α, n) and (α, γ) resonance strengths of states lying below the covered energy range were estimated using their known properties from the literature. Results: The reaction channels17 O(α, n0) 20 Ne and 17 O(α, n1γ) 20 Ne were measured in the energy range Eα = 800 keV to 2300 keV. A new 17 O(α, n) reaction rate was deduced for the temperature range 0.1 GK to 10 GK. At typical He burning temperatures, the combination of the new (α, n) rate with a previously measured (α, γ) rate gives approximately the same ratio as current compilations. The influence on the nucleosynthesis of the s process in massive stars at low metallicity is discussed.Conclusions: It was found that in He burning conditions the (α, γ) channel is strong enough to compete with the neutron channel. This leads to a less efficient neutron recycling compared to a previous suggestion of a very weak (α, γ) channel. S process calculations using our rates confirm that massive rotating stars do play a significant role in the production of elements up to Sr, but they strongly reduce the s process contribution to heavier elements.

We report the first observation of the 108 Xe → 104 Te → 100 Sn α-decay chain. The α emitters, 108 Xe [Eα = 4.4(2) MeV, T1 /2 = 58 +106 −23 µs] and 104 Te [Eα = 4.9(2) MeV, T1 /2 <18 ns], decaying into doubly magic 100 Sn were produced using a fusion-evaporation reaction 54 Fe(58 Ni,4n) 108 Xe, and identified with a recoil mass separator and an implantation-decay correlation technique. This is the first time α radioactivity has been observed to a heavy self-conjugate nucleus. A previous benchmark for study of this fundamental decay mode has been the decay of 212 Po into doubly magic 208 Pb. Enhanced proton-neutron interactions in the N = Z parent nuclei may result in superallowed α decays with reduced α-decay widths significantly greater than that for 212 Po. From the decay chain, we deduce that the α-reduced width for 108 Xe or 104 Te is more than a factor of 5 larger than that for 212 Po.

Neutrons produced by the carbon fusion reaction 12 C( 12 C,n) 23 Mg play an important role in stellar nucleosynthesis. However, past studies have shown large discrepancies between experimental data and theory, leading to an uncertain cross section extrapolation at astrophysical energies. We present the first direct measurement that extends deep into the astrophysical energy range along with a new and improved extrapolation technique based on experimental data from the mirror reaction 12 C( 12 C,p) 23 Na. The new reaction rate has been determined with a well-defined uncertainty that exceeds the precision required by astrophysics models. Using our constrained rate, we find that 12 C( 12 C,n) 23 Mg is crucial to the production of Na and Al in Pop-III Pair Instability Supernovae. It also plays a non-negligible role in the production of weak s-process elements as well as in the production of the important galactic γ-emitter 60 Fe. The first stars in the early Universe formed about 400 million years after the big bang. Verification of the existence of these stars is important for our understanding of the evolution of the Universe [1]. It has been predicted that for Population-III (metal-free stars [2]) stellar production yields, the abundances of odd-Z elements are remarkably deficient compared to their adjacent even-Z elements [3]. Astronomers are searching for long-lived, low mass stars with the unique nucleosynthetic pattern matching the predicted yields [4]. The relevance of 12 C( 12 C,n) 23 Mg in the first stars has been discussed by Woosley, Heger, and Weaver [5]. By the end of helium burning in Pop-III stars, the neutron to proton ratio in the ash is almost exactly 1. However, in the subsequent carbon burning phase, frequent β + decay of produced 23 Mg converts protons into neutrons, thus increasing the neutron to proton ratio. A slight excess of neutrons would significantly affect the abundances of the odd-Z isotopes with neutron to proton ratios higher than 1, e.g.23 Na and 27 Al.12 C( 12 C,n) 23 Mg is also a potentially important neutron source for the so-called weak s-process occurring in massive ) and ) stars. The weak s-process takes place during the core helium and shell carbon burning phases and is largely responsible for the s-process abundances up to A≈90 [6]. Pignatari et al. recently performed a study of the weak s-process during carbon shell burning for a 25 M stellar model using different 12 C( 12 C,n) 23 Mg rates [7]. They found that a factor of 2 precision or better would be desirable to limit its impact on the s-process predictions to within 10%.Stellar carbon burning has three main reaction channels:12 C + 12 C → 23 Mg + n − 2.60 MeV → 23 Na + p + 2.24 MeVWith Q < 0, the probability of decay through the neutron channel is weakest among the three at the low energies relevant for astrophysics. For a typical carbon shell burning temperature T 9 = 1.1, the important energy range for this channel is 2.7 < E cm < 3.6 MeV. The reaction was first studied in 1969 by Patterson et al. [8] who measured t...