After interaction with a proton, generation of free radical occurs in:

In direct effects, biologic molecules absorb energy from ionizing radiation and form unstable free radicals  (atoms or molecules having an unpaired electron in the valence orbital).  Generation of free radicals occurs in less than 10-10 seconds after interaction with a photon. Free radicals are extremely reactive and have very short lives, quickly reforming into stable configurations by dissociation (breaking apart) or cross-linking (joining of two molecules). Free radicals play a dominant role in producing molecular changes in biologic molecules. Ref:Oral Radiology, Principles and Interpretation / Stuart C. White, Michael J. Pharoah - 7th ed - pg - 16