During big bang nucleosynthesis , any injection of extra neutrons around the time of the ^ { 7 } Be formation , i.e .
at a temperature of order T \simeq 50 keV , can reduce the predicted freeze-out amount of ^ { 7 } Be + ^ { 7 } Li that otherwise remains in sharp contradiction with the Spite plateau value inferred from the observations of Pop II stars .
However , the growing confidence in the primordial D/H determinations puts a strong constraint on any such scenario .
We adress this issue in detail , analyzing different temporal patterns of neutron injection , such as decay , annihilation , resonant annihilation , and oscillation between mirror and standard model world neutrons .
For this latter case , we derive the realistic injection pattern taking into account thermal effects ( damping and refraction ) in the primordial plasma .
If the extra neutron supply is the sole non-standard mechanism operating during the BBN , the suppression of lithium abundance below Li/H \leq 1.9 \times 10 ^ { -10 } always leads to the overproduction of deuterium , D/H \geq 3.6 \times 10 ^ { -5 } , well outside the error bars suggested by recent observations .