Background

Nuclei are held together by the spin pairing of their nucleons; neutron-neutron. proton-proton and neutron-proton. Such spin pairings are exclusive in the sense that one neutron can pair with one proton and one other neutron. The same applies for a proton.

With respect to the increment in binding energy due to spin pairing there should be no difference between neutrons and protons. Here are the graphs of the increments for an additional neutron and for an additional proton.

Remarkably this shows a range over which the two functions essentially coincide. This is very significant. It suggests that the difference in incremental binding due to the formation of neutron-neutron and proton-proton spin pairs is a function of (p−n).

There is a near-coincidence for the case of p=n=27.

This too is consistent with the difference being a function of (p−n).

However this degree of coincidence prevails over a much shorter interval even for the close-by case of p=n=28.