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.

In neutron capture for magic-shell nuclei the direct reaction mechanism can be important and may even dominate. As an example we investigated the reaction 48 Ca(n,γ) 49 Ca for projectile energies below 250 keV in a direct capture model using the folding procedure for optical and bound state potentials. The obtained theoretical cross sections are in agreement with the experimental data showing the dominance of the direct reaction mechanism in this case. The above method was also used to calculate the cross section for 50 Ca(n,γ) 51 Ca.

No abstract

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.

We calculate neutron capture cross sections for r-process nucleosynthesis in the 48 Ca region, namely for the isotopes 40−44 S, 46−50 Ar, 56−66 Ti, 62−68 Cr, and 72−76 Fe. While previously only cross sections resulting from the compound nucleus reaction mechanism (Hauser-Feshbach) have been considered, we recalculate not only that contribution to the cross section but also include direct capture on even-even nuclei. The level schemes, which are of utmost importance in the direct capture calculations, are taken from quasi-particle states obtained with a folded-Yukawa potential and Lipkin-Nogami pairing. Most recent deformation values derived from experimental data on β-decay half lives are used where available. Due to the consideration of direct capture, the capture rates are enhanced and the "turning points" in the r-process path are shifted to slightly higher mass numbers. We also discuss the sensitivity of the direct capture cross sections on the assumed deformation.