7Li(n,γ)8Li reaction and the S17 factor at Ec.m.>500 keV

The partial cross sections from the neutron capture state to the ground and first excited states in 8Li have been separately determined for the first time at stellar neutron energy. The direct and weak cascade γ rays from the capture and first excited states to the ground state were measured by means of anti-Compton NaI(Tl) and anti-Compton HPGe spectrometers, respectively. The γ-ray branching ratio and the cross sections thus determined agree with that for thermal neutrons assuming a 1/v neutron velocity dependence. By comparing the cross sections with calculations based on the nonresonant direct capture mechanism it is shown that the cross sections are sensitive to the interaction potential of the incident neutron with the 7Li target nucleus. This analysis confirms the possibility of deriving the parameters necessary for the calculation of the astrophysical S factor S17(E) for the 7Be(p,γ)8B reaction in the upper energy range above 500 keV