Ionizing radiation is energetic and penetrating. Many of its chemical effects in biological matter are due to the geometry of the initial physical energy deposition events, referred to as the track structure. Ionizing radiation exists in either particulate or electromagnetic types. The particulate radiation interacts with the biological tissue either by ionization or excitation. The ionizations and excitations that it produced tend to be localized, along the tracks of individual charged particles. Whereas the photon can penetrate matter without interacting, it can be completely absorbed by depositing its energy, or it can be scattered (deflected) from its original direction and deposit part of its energy as follows: