Two-neutron capture reactions in supernovae neutrino bubbles

Recent calculations suggest that the neutrino heated bubbles in the postcollapse phase of type II supernovae may be the site of the r process. The nucleosynthesis process depends on the expansion and cooling rates as well as on the rates of the α-particle and neutron recombination processes that bridge the mass gaps at A = 5 and 8. We have reexamined the rate for the important reaction ${}_{}{}^4{}_{}{}^{}\mathrm{He}$(αn,γ${)}^9$Be, and estimated the rates for the two neutron capture reactions on ${}_{}{}^6{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^8{}_{}{}^{}\mathrm{He}$ which could provide an alternative reaction path. We find that the most important reaction for initiating the α process is the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$(αn,γ${)}^9$Be reaction, as suggested by earlier work.