Cross-section measurement of 9Be(γ,n)8Be and implications for α+α+n→9Be in the r process

Models of the r process are sensitive to the production rate of Be-9 because, in explosive environments rich in neutrons, alpha(alpha n, gamma)Be-9 is the primary mechanism for bridging the stability gaps at A = 5 and A = 8. The alpha(alpha n, gamma)Be-9 reaction represents a two-step process, consisting of alpha + alpha -> Be-8 followed by Be-8(n, gamma)Be-9. We report here on a new absolute cross-section measurement for the Be-9(gamma, n)Be-8 reaction conducted using a highly efficient, He-3-based neutron detector and nearly monoenergetic photon beams, covering energies from E-gamma = 1.5MeV to E-gamma = 5.2 MeV, produced by the High Intensity gamma-ray Source of Triangle Universities Nuclear Laboratory. In the astrophysically important threshold energy region, the present cross sections are 40% larger than those found in most previous measurements and are accurate to +/- 10% (95% confidence). The revised thermonuclear alpha(alpha n, gamma)Be-9 reaction rate could have implications for the r process in explosive environments such as type II supernovae.