  Materials subjected to neutron irradiation will suffer from a build-up of
damage caused by the displacement cascades initiated by nuclear reactions.
Previously, the main "measure" of this damage accumulation has been through the
displacements per atom (dpa) index. There are known limitations associated with
the dpa quantity and its domain of application and therefore this paper
describes a more rigorous methodology to calculate the primary atomic recoil
events (often called the primary knock-on atoms or PKAs) that lead to cascade
damage events as a function of energy and recoiling species for any simulated
or measured neutron irradiation scenario. Via examples of fusion relevant
materials, it is shown that the PKA spectra can be complex, involving many
different recoiling species, potentially differing in both proton and neutron
number from the original target nuclei, including high energy recoils of light
emitted particles such as alpha-particles and protons. The variations in PKA
spectra as a function of time, neutron field, and material are explored.
Example PKA spectra are applied to radiation damage quantification using the
binary collision approximation and stochastic cluster dynamics, and the results
from these different approaches are discussed and compared.
