  One of the important light elements created during the big bang
nucleosynthesis is Be-7 which then decays to Li-7 by electron capture when
recombination becomes effective but well before the Saha equilibrium
recombination is reached. This means that Be-7 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 Be-7 to
Li-7 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 narrow
neutrino doublet lines resulting from Be-7 decay.
