  We have investigated the influence of different non-linear relativistic mean
field models ($NL$, $NL\rho$ and $NL\rho\delta$) on spallation neutrons for
p+Fe and Pb reactions at 0.8, 1.2 and 1.6 GeV by means of a relativistic
Boltzmann Uehling Uhlenbeck (RBUU) approach plus a statistical
multifragmentation (SM) decay model. We find that the "evaporation shoulder",
i.e. the neutron energy spectrum from 3 to 30 MeV, almost for any emission
angle is quite sensitive to the isospin part of the mean field. For the more
neutron-rich Pb-target the evaporation component can be directly related to the
low density behavior on the symmetry energy in the thermal expansion phase of
the excited compound system. It turns out that the spallation data for the
reactions under study are shown to be more consistent with RBUU+SM employing
the $NL\rho$ effective lagrangian.
