  The energy-dependent cross section of the 7Be(n,alpha)4He reaction, of
interest for the so-called Cosmological Lithium Problem in Big Bang
Nucleosynthesis, has been measured for the first time from 10 meV to 10 keV
neutron energy. The challenges posed by the short half-life of 7Be and by the
low reaction cross section have been overcome at n_TOF thanks to an
unprecedented combination of the extremely high luminosity and good resolution
of the neutron beam in the new experimental area (EAR2) of the n_TOF facility
at CERN, the availability of a sufficient amount of chemically pure 7Be, and a
specifically designed experimental setup. Coincidences between the two
alpha-particles have been recorded in two Si-7Be-Si arrays placed directly in
the neutron beam. The present results are consistent, at thermal neutron
energy, with the only previous measurement performed in the 60's at a nuclear
reactor. The energy dependence here reported clearly indicates the inadequacy
of the cross section estimates currently used in BBN calculations. Although new
measurements at higher neutron energy may still be needed, the n_TOF results
hint to a minor role of this reaction in BBN, leaving the long-standing
Cosmological Lithium problem unsolved.
