Properties of the first 1/2+ state in 9Be from electron scattering and astrophysical implications

The low-energy level structure of the 9Be nucleus has long been a matter of interest, in particular with respect to the strength of three-body α+ α+ n cluster configurations. This nucleus has the lowest neutron threshold (Sn = 1.6654 MeV) of all stable nuclei. Already the first excited Jπ = 1/2+ state lies at an excitation energy of several tens of keV above the 8Be+n threshold. Parameters of this resonance are of great astrophysical importance since it is believed to provide an important route for the production of carbon and subsequently heavier nuclei triggering the r-process in core-collapse supernovae. Due to its closeness to the neutron threshold the resonance has a strongly asymmetric line shape but despite a large number of different experiments there still exist discrepancies between the various deduced resonance parameters. We present high-resolution inelastic electron scattering experiments on 9Be performed at the S-DALINAC. The resonance parameters of the first excited 1/2+ state in 9Be are derived in a one-level R-matrix approximation from the present and older (e,e') data. The astrophysically relevant α(αn,γ)9Be reaction rate is extracted and discussed.