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 9Be because, in explosive environments rich in neutrons, α(αn, γ)9Be is the primary mechanism for bridging the stability gaps at A = 5 and A = 8. The α(αn, γ)9Be reaction represents a two-step process, consisting of α +α → 8Be followed by 8Be(n, γ)9Be. We report here on a new absolute cross-section measurement for the 9Be(γ, rc)8Be reaction conducted using a highly efficient, 3He-based neutron detector and nearly monoenergetic photon beams, covering energies from Eγ = 1.5 MeV to Eγ = 5.2 MeV, produced by the High Intensity γ-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 α(αn, γ)9Be reaction rate could have implications for the r process in explosive environments such as type II supernovae. [ABSTRACT FROM AUTHOR]