Direct measurement of the Be-7(n, alpha)He-4 reaction cross sections for the cosmological Li problem

The cross sections of the 7Be(n, alpha)He-4 reaction for p-wave neutrons were experimentally determined at = 0.20-0.81 MeV close to the Big Bang nucleosynthesis (BBN) energy window for the first time on the basis of the detailed balance principle by measuring the time-reverse reaction. The obtained cross sections are much larger than the cross sections for s-wave neutrons inferred from the recent measurement at the n_TOF facility in CERN, but significantly smaller than the theoretical estimation widely used in the BBN calculations. The present results suggest the 7Be(n,alpha)He-4 reaction rate is not large enough to solve the cosmological lithium problem.