Cross Section Measurement of 9 Be( ,n) 8 Be and Implications for α+α+n-> 9 Be in the r-Process C.W. Arnold, T.B. Clegg, C. Iliadis, H.J. Karwowski, G.C. Rich, J.R. Tompkins. Dept. of Physics and Astronomy, UNC-CH, and TUNL C.R. Howell. Dept. of Physics, Duke University, and TUNL This work was supported in part by the USDOE Office of Nuclear Physics Grants DE-FG02-97ER41041 and DE-FG02-97ER Below T = 10 GK, the rate is dominated by the 1/2 + threshold state. Realistic treatments for energy dependence of neutron partial widths leads to a rate that is 3x lower than NACRE at the lowest temperatures. In the temperature range relevant to the astrophysical r- process, 1 GK < T < 5 GK, the newly calculated reaction rate is 20% to 40% larger than NACRE. Reaction Rate Comparisons Experiment at HI  S The Los Alamos-designed inventory sample counter model IV was extensively characterized by experiments at TUNL. Simulations were designed and carried out using both MCNPX and GEANT4. Peak efficiency over 60%. Models of the r-process are sensitive to the production rate of 9 Be because, in explosive environments rich in neutrons, α(αn,  ) 9 Be is the primary mechanism for bridging the stability gaps at A = 5 and A = 8. The α(αn,  ) 9 Be reaction represents a two-step process, consisting of α+α-> 8 Be followed by 8 Be(n,  ) 9 Be. We report here on a new absolute cross section measurement for the 9 Be( ,n) 8 Be reaction conducted using a highly-efficient neutron detector and nearly-monoenergetic photon beams produced by HI  S. 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,  ) 9 Be reaction rate could have implications for the r-process in explosive environments such as Type II supernovae. New Cross Section Values New resonance parameters measured for important 1/2 + threshold state at 1731 keV. Narrow 5/2 - state at 2431 keV is better resolved and is 2x stronger than seen in previous works. The HI  S facility was used to create photon beams covering the energy range between 1.5 MeV and 5.2 MeV with energy spreads of ≤ 1% and fluxes between 10 5 and 10 6  /s. The photon beams were incident on targets located in the central cavity of the neutron detector (a). Upstream of the targets, the photon beam passed through thin scintillating paddles for relative flux monitoring. Downstream of the targets, the photons were incident on either a NaI(Tl) detector (c) for flux determination, or a HPGe detector (d) for energy measurement. As necessary, the beam was attenuated by lead of a selectable thickness (b). Neutron Detector Characterization Deconvolution of Finite-Energy Beam Beam energy profile was deconvolved to determine cross section. Effect especially significant in threshold energy region. In this plot, red squares represent deconvolved data points while black dots are points where a monoenergetic beam was assumed.