2 00 2 Astrophysical reaction rate for α ( αn , γ ) 9 Be by photodisintegration

We study the astrophysical reaction rate for the formation of Be through the three body reaction α(αn, γ). This reaction is one of the key reactions which could bridge the mass gap at A = 8 nuclear systems to produce intermediate-to-heavy mass elements in alphaand neutron-rich environments such as r-process nucleosynthesis in supernova explosions, s-process nucleosynthesis in asymptotic giant branch (AGB) stars, and primordial nucleosynthesis in baryon inhomogeneous cosmological models. To calculate the thermonuclear reaction rate in a wide range of temperatures, we numerically integrate the thermal average of cross sections assuming a two-steps formation through a metastable Be, α+ α ⇀↽ Be(n,γ)Be. Off-resonant and on-resonant contributions from the ground state in Be are taken into account. As input cross section, we adopt the latest experimental data by photodisintegration of Be with laser-electron photon beams, which covers all relevant resonances in Be. Experimental data near the neutron threshold are added with γ-ray flux corrections and a new least-squares analysis is made to deduce resonance parameters in the Breit-Wigner formulation. Based on the photodisintegration cross section, we provide the reaction rate for α(αn, γ)Be in the temperature range from T9=10 −3 to T9=10 1 (T9 is the temperature in units of 10 9 K) both in the tabular form and in the analytical form for potential usage in nuclear reaction network calculations. The calculated reaction rate is compared with the reaction rates of the CF88 and the NACRE compilations. The CF88 rate, which is based on the photoneutron cross section for the 1/2 state in Be by Berman et al., is valid at T9 > 0.028 due to lack of the off-resonant contribution. The CF88 rate differs from the present rate by a factor of two in a temperature range T9 ≥ 0.1. The NACRE rate, which adopted different sources of experimental information on resonance states in Be, is 4–12 times larger than the present rate at T9 ≤ 0.028, but is consistent with the present rate to within ±20% at T9 ≥ 0.1.