  In this work, we complete our CT18qed study with the neutron's photon parton
distribution function (PDF), which is essential for the nucleus scattering
phenomenology. Two methods, CT18lux and CT18qed, based on the LUXqed formalism
and the DGLAP evolution, respectively, to determine the neutron's photon PDF
have been presented. Various low-$Q^2$ non-perturbative variations have been
carefully examined, which are treated as additional uncertainties on top of
those induced by quark and gluon PDFs. The impacts of the momentum sum rule as
well as isospin symmetry violation have been explored, and turn out to be
negligible. A detailed comparison with other neutron's photon PDF sets has been
performed, which shows a great improvement in the precision and a reasonable
uncertainty estimation in our results. Finally, two phenomenological
implications are demonstrated with photon-initiated processes: neutrino-nucleus
$W$-boson production, which is important for the near-future TeV--PeV neutrino
observations, and the axion-like particle production at a high-energy muon
beam-dump experiment.
