  We probe the momentum dependence of the isovector mean-field potential by
comparing the energy spectra of neutrons and protons emitted in
$^{112}$Sn+$^{112}$Sn and $^{124}$Sn+$^{124}$Sn collisions at incident energies
of E/A=50 and 120 MeV. We achieve experimental precision that discriminates
between different momentum dependencies for the symmetry mean-field potential.
Comparisons of the experimental results to Improved Quantum Molecular Dynamics
model calculations with Skyrme Interactions indicate small differences between
the neutron and proton effective masses.
