Fascinating, do you know if protons can exist within neutron stars? That would make it each one an element, right? Or if not, they would all be isotopes of Neutronium?
At the core, a neutron star is likely very nearly pure neutronium, as other particles simply won't exist long under the conditions present.
As depth decreases, other particles can exist, and at the surface there are likely free nuclei and atoms of elements, though my probably with low nuclear potential. Fission and fusion would be accelerated by the energies involved.
That's a lay reading / bits of stray collected memories, though quick checking seems to confirm most of this. See:
https://heasarc.gsfc.nasa.gov/docs/objects/binaries/neutron_...