Reaction rate fro two-neutron capture by 4He

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 4He(2n,γ)6He- and 6He(α,n)9Be-reactions bridging the instability gap at A=5 and A=8 could be of relevance. We suggest a mechanism for 4He(2n,γ)6He 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 orders 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 6He is negligible compared to 4He(αn,γ)9Be. (orig.)