Slow 7Be and the primordial7Li abundance

Abstract The cross sections of neutron-induced nuclear reactions are commonly large at low energies. Given this feature, the 7 Be( n , p ) 7 Li process with slow 7 Be is studied in connection with the cosmological lithium problem. An idea is tested whether slow (non-Maxwellian) 7 Be born in the radiative capture 3 He( α , γ ) 7 Be reaction can boost the 7 Be( n , p ) 7 Li( p , α ) α mechanism of 7 Li burn-up in the early universe, reducing the amount of primordial 7 Li. The rate of this sequential process is examined and its effect on the 7 Li abundance is clarified. In the absence of 7 Be thermalization in the primordial plasma, the above mechanism can decrease 7 Li/H by several percent, leaving unchanged the abundances of D and 4 He which are consistent with observations. However, taking into account the 7 Be thermalization due to scattering off electrons, positrons, nuclei, and photons radically suppresses the effect. It is found that the typical thermalization time of 7 Be is less than 10 −5 s, while the fraction of non-Maxwellian particles in the 7 Be component is at most 2 × 10 −7 . Under this condition, the corresponding impact on 7 Li/H is estimated to be no more than 10 −5 %.