  For an unsteady baryonic gamma-ray burst (GRB) outflow, the fast and slow
proton shells collide with each other and produce energetic soft gamma-ray
emission. If the outflow has a significant neutron component, the
ultra-relativistic neutrons initially expand freely until decaying at a larger
radius. The late time proton shells ejected from the GRB central engine, after
powering the regular internal shocks, will sweep these $\beta-$decay products
and give rise to very bright UV/optical emission. The naked-eye optical flash
from GRB 080319B, an energetic explosion in the distant universe, can be well
explained in this way.
