Time of primordial 7Be conversion into 7Li, energy release and doublet of narrow cosmological neutrino lines

One of the light elements created during the big bang nucleosynthesis is 7Be which then decays to 7Li by electron capture when recombination becomes effective but well before the Saha equilibrium recombination is reached. This means that 7Be should wait until its recombination epoch even though the half-life of the hydrogenic beryllium atom is only 106.4 days. We calculate when the conversion from primordial 7Be to 7Li occurs taking into account the population of the hyperfine structure sublevels and solving the kinetic equations for recombination, photoionization and conversion rate. We also calculate the energies and the spectrum of two narrow neutrino lines from 7Be decay.