  The effects of neutron irradiation on materials are often interpreted in
terms of atomic recoils, initiated by neutron impacts and producing crystal
lattice defects. In addition, there is a remarkable two-step process, strongly
pronounced in the medium-weight and heavy elements. This process involves the
generation of energetic {\gamma} photons in nonelastic collisions of neutrons
with atomic nuclei, achieved via capture and inelastic reactions. Subsequently,
high-energy electrons are excited through the scattering of {\gamma} photons by
the atomic electrons. We derive and validate equations enabling a fast and
robust evaluation of photon and electron fluxes produced by the neutrons in the
bulk of materials. The two-step n-{\gamma}-e scattering creates a
nonequilibrium dynamically fluctuating steady-state population of high-energy
electrons, with the spectra of photon and electron energies extending well into
the mega-electron-volt range. This stimulates vacancy diffusion through
electron-triggered atomic recoils, primarily involving vacancy-impurity
dissociation, even if thermal activation is ineffective. Tungsten converts the
energy of fusion or fission neutrons into a flux of {\gamma} radiation at the
conversion efficiency approaching 99%, with implications for structural
materials, superconductors, and insulators, as well as phenomena like
corrosion, and helium and hydrogen isotope retention.
