In condensed-matter physics, a primary knock-on atom (PKA) is an atom that is displaced from its lattice site by irradiation; it is, by definition, the first atom that an incident particle encounters in the target. After it is displaced from its initial lattice site, the PKA can induce the subsequent lattice site displacements of other atoms if it possesses sufficient energy (threshold displacement energy), or come to rest in the lattice at an interstitial site if it does not (interstitial defect).
Most of the displaced atoms resulting from electron irradiation and some other types of irradiation are PKAs, since these are usually below the threshold displacement energy and therefore do not have sufficient energy to displace more atoms. In other cases like fast neutron irradiation, most of the displacements result from higher-energy PKAs colliding with other atoms as they slow down to rest.[1]