Difference between optical potential and nuclear potential (used in neutron optics)

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?

• Please don't link to pdfs, particularly when paywalled. Link instead to the abstract, or any suitable landing page unless there's no such thing, and use DOI link whenever they're available. – Emilio Pisanty Oct 24 '17 at 20:21
• @EmilioPisanty Sure! I'll see if I can get the same relevant material somewhere else. But the pdf link I sent can be assessed even by someone associated with his institution (you do not need to pay for the article) by using the institutional login ID (like we do for moodle) – kbg Oct 24 '17 at 20:26
• No, you misunderstand me. I'm not asking you to change what papers you cite, I'm suggesting that you change how you link to those papers: pdf links are annoying to people on mobile devices, and straight journal links are susceptible to link rot, whereas DOIs are not. That said, if the papers are paywalled and there are no versions freely available, and you can spare the time to locate alternative references that do have accessible copies, then all the better. (For example, it is a mistake to assume that everybody in your audience has institutional access to the papers you do.) – Emilio Pisanty Oct 24 '17 at 21:03
• The nuclear potential models the interaction of the neutron with one nucleus. It is only discussed qualitatively because its specific shape does not matter, only the rough orders of magnitude from which to justify the argument at the top of p. 272, and in more details in the last paragraph of p. 275. The optical potential deals with the average wave in the material, that is the sum of all the wave diffracted by each nuclei. – user154997 Oct 24 '17 at 21:49
• @LucJ.Bourhis And what about the sign? We say that nuclear potential is $-V$. But when the Fermi potential is defined, it is defined as $V$ with further argument that for most of the material, the scattering length is positive thus rendering a positive nuclear potential. How do we explain the sign confusion? – kg__ Oct 25 '17 at 15:34