# Why does $S_p$ increase for closed neutron shell?

I'm taking an introductory nuclear physics course in which we just covered the shell model. In that model, the so-called magic numbers arise as a consequence of there being an energy level gap due to closed shells. Now I was playing around with some datasets and I saw this:

There is a clear jump in proton separation energy ($$S_p$$), when the number of neutrons ($$N$$) reaches the magic number 28. The shell model assumes (as far as I know) that the proton and neutron energy levels are independent.

Can this jump be explained starting from the shell model? Or is this not covered by the shell model?