  The neutron-proton effective $k$-mass splitting in asymmetric nucleonic
matter of isospin asymmetry $\delta$ and normal density is found to be
$m^{*}_{n-p}\equiv(m^{*}_{n}-m^{*}_{p})/m=(0.41 \pm0.15)\delta$ from analyzing
globally 1088 sets of reaction and angular differential cross sections of
proton elastic scattering on 130 targets with beam energies from 0.783 MeV to
200 MeV, and 1161 sets of data of neutron elastic scattering on 104 targets
with beam energies from 0.05 MeV to 200 MeV within an isospin dependent
non-relativistic optical potential model. It sets a useful reference for
testing model predictions on the momentum dependence of the nucleon isovector
potential necessary for understanding novel structures and reactions of rare
isotopes.
