REACTION-RATE FOR 2-NEUTRON CAPTURE BY HE-4

Recent investigations suggest that the neutrino-heated hot bubble betw een the nascent neutron star and the overlying stellar mantle of a typ e-II supernova may be the site of the r-process. In the preceding alph a-process building up the elements to A approximate to 100, the He-4(2 n,gamma)He-6- and He-6(alpha,n)Be-9-reactions bridging the instability gap at A = 5 and A = 8 could be of relevance. We suggest a mechanism for He-4(2n,gamma)He-6 and calculate the reaction rate within the alph a+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-capt ure calculation that under representative conditions in the alpha-proc ess the reaction path proceeding through He-6 is negligible compared t o He-4(alpha n,gamma)Be-9.