  Models of the r-process are sensitive to the production rate of 9Be because,
in explosive environments rich in neutrons, alpha(alpha n,gamma)9Be is the
primary mechanism for bridging the stability gaps at A=5 and A=8. The
alpha(alpha n,gamma)9Be reaction represents a two-step process, consisting of
alpha+alpha -> 8Be followed by 8Be(n,gamma)9Be. We report here on a new
absolute cross section measurement for the 9Be(gamma,n)8Be reaction conducted
using a highly-efficient, 3He-based neutron detector and nearly-monoenergetic
photon beams, covering energies from E_gamma = 1.5 MeV to 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)9Be reaction rate could have implications for
the r-process in explosive environments such as Type II supernovae.
