  The Fermi transition (\Delta L=\Delta S=0 and \Delta T=1) between the nuclear
isobaric analog states (IAS), induced by the charge-exchange (p,n) or (3He,t)
reaction, can be considered as "elastic" scattering of proton or 3He by the
isovector term of the optical potential (OP) that flips the projectile isospin.
The accurately measured (p,n) or (3He,t) scattering cross-section to the IAS
can be used, therefore, to probe the isospin dependence of the proton or 3He
optical potential. Within the folding model, the isovector part of the OP is
determined exclusively by the neutron-proton difference in the nuclear
densities and the isospin dependence of the effective nucleon-nucleon (NN)
interaction. Because the isovector coupling explicitly links the isovector part
of the proton or 3He optical potential to the cross section of the
charge-exchange (p,n) or (3He,t) scattering to the IAS, the isospin dependence
of the effective (in-medium) NN interaction can be well tested in the folding
model analysis of these charge-exchange reactions. On the other hand, the same
isospin- and density dependent NN interaction can also be used in a
Hartree-Fock calculation of asymmetric nuclear matter, to estimate the nuclear
matter energy and its asymmetry part (the nuclear symmetry energy). As a
result, the fine-tuning of the isospin dependence of the effective NN
interaction against the measured (p,n) or (3He,t) cross sections should allow
us to make some realistic prediction of the nuclear symmetry energy and its
density dependence.
