  In the neutron-rich internal shocks model for gamma-ray bursts (GRBs), the
Lorentz factors (LFs) of ion shells are variable, and so are the LFs of
accompanying neutron shells. For slow neutron shells with a typical LF of
approximate tens, the typical $\beta$-decay radius is $\sim 10^{14}-10^{15}$
cm. As GRBs last long enough [$T_{90}>14(1+z)$ s], one earlier but slower
ejected neutron shell will be swept successively by later ejected ion shells in
the range $\sim10^{13}-10^{15}$ cm, where slow neutrons have decayed
significantly. Part of the thermal energy released in the interaction will be
given to the electrons. These accelerated electrons will be mainly cooled by
the prompt soft $\gamma-$rays and give rise to GeV emission. This kind of GeV
emission is particularly important for some very long GRBs and is detectable
for the upcoming satellite {\it Gamma-Ray Large Area Space Telescope} (GLAST).
