I am discussing a question related to elastic coherent scattering of neutron from a nucleus. I am referring to the terms (optical and nuclear potential) which are used on page 272 of the review article 'Neutron optics' by AG Klein and SA Werner [Rep. Prog. Phys. 46 259 (1983)], where the author introduces the optical potential as the nuclear potential, and on page 273, where he later calls this optical/nuclear potential as Fermi potential.
I didn't understand that what is the fundamental difference between optical and nuclear potential. Also, since nuclear potential is negative, why is Fermi potential not carrying a negative sign?