PKA may refer to:
- Professionally Known As (referring to pen names, stage personas and similar)
- pK, the symbol for the acid dissociation constant at logarithmic scale
- Protein kinase A, a class of cAMP-dependent enzymes
- Pi Kappa Alpha, the North-American social fraternity
- Public key authentication, establishing key authenticity in public-key cryptography
- Professional Karate Association
- Primary knock-on atom
- Russian Federal Space Agency
A Primary Knock-on Atom or 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, or come to rest in the lattice at an interstitial site if it does not.
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 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.