Recent investigations suggest that the neutrino-heated hot bubble between the nascent neutron star and the overlying stellar mantle of a type-II supernova may be the site of the r-process. In the preceding α-process building up the elements to A ≈ 100, the 4 He(2n,γ) 6 Heand 6 He(α,n) 9 Be-reactions bridging the instability gap at A = 5 and A = 8 could be of relevance. We suggest a mechanism for 4 He(2n,γ) 6 He and calculate the reaction rate within the α+n+n approach. The value obtained is about a factor 1.6 smaller than the one obtained recently in the simpler direct-capture model, but is at least three order of magnitude enhanced compared to the previously adopted value. Our calculation confirms the result of the direct-capture calculation that under representative conditions in the α-process the reaction path proceeding through 6 He is negligible compared to 4 He(αn,γ) 9 Be.

At the Karlsruhe pulsed 3.75 MV Van de Graaff accelerator the thermonuclear 48 Ca(n,γ) 49 Ca(8.72 min) cross section was measured by the fast cyclic activation technique via the 3084.5 keV γ-ray line of the 49 Ca-decay. Samples of CaCO3 enriched in 48 Ca by 77.87 % were irradiated between two gold foils which served as capture standards. The capture cross-section was measured at the neutron energies 25, 151, 176, and 218 keV, respectively. Additionally, the thermal capture cross-section was measured at the reactor BR1 in Mol, Belgium, via the prompt and decay γ-ray lines using the same target material. The 48 Ca(n,γ) 49 Ca cross-section in the thermonuclear and thermal energy range has been calculated using the direct-capture model combined with folding potentials. The potential strengths are adjusted to the scattering length and the binding energies of the final states in 49 Ca. The small coherent elastic cross section of 48 Ca+n is explained through the nuclear Ramsauer effect. Spectroscopic factors of 49 Ca have been extracted from the thermal capture cross-section with better accuracy than from a recent (d,p) experiment. Within the uncertainties both results are in agreement. The non-resonant thermal and thermonuclear experimental data for this reaction can be reproduced using the direct-capture model. A possible interference with a resonant contribution is discussed. The neutron spectroscopic factors of 49 Ca determined from shell-model calculations are compared with the values extracted from the experimental cross sections for 48 Ca(d,p) 49 Ca and 48 Ca(n,γ) 49 Ca.

The reaction rates of neutron capture reactions on light nuclei are important for reliably simulating nucleosynthesis in a variety of stellar scenarios.Neutron capture reaction rates on neutron-rich C-, N-, and O-isotopes are calculated in the framework of a hybrid compound and direct capture model.

Thermal neutron capture cross sections for 34 S(n,γ) 35 S and 36 S(n,γ) 37 S have been measured and spectroscopic factors of the final states have been extracted. The calculated direct-capture cross sections reproduce the experimental data.

An introduction to nucleosynthesis, the creation of the elements in the big bang, in interstellar matter and in stars is given. The two-step process 4 He(2n,γ) 6 He and the reverse photodisintegration 6 He(γ,2n) 4 He involving the halo nucleus 6 He could be of importance in the α-process in type-II supernovae. The reaction rates for the above processes are calculated using three-body methods and show an enhancement of more than three orders of magnitude compared to the previous adopted value. Direct-capture calculations give similar values for the above reaction rates. Therefore, this method was also used to calculate the reaction rates of the two-step processes 6 He(2n,γ) 8 He and 9 Li(2n,γ) 11 Li and the reverse photodisintegration of 8 He and 11 Li that could be also of importance in the α-process.