Nonhomogeneous processes in radiation research: Radical diffusion models

Radiation research has come to mean investigations of the nature of penetrating radiations and their effects on the materials they traverse. The radiations themselves are fast charged particles (including atomic nuclei and electrons) and short wave length electromagnetic photons (X and γ rays). These radiations have the common property that they deposit their energy nonhomogeneously (in tracks) as they are stopped in matter. Chemically reactive species (excited molecules, ions, radicals, etc.) are created in the tracks. Irradiated systems are the earliest examples of systems recognized to be nonhomogeneous. The tracks formed by radiations develop by expansion and reaction of the newly created reactive species among themselves and with any molecules of the stopping medium. The radiation chemical yields of products depend on the nature of the track, which in turn depends on the radiation. Radiation chemists have been successful in the construction of models that describe track expansion and the prediction of radiation chemical yields. The development of this field, over the years, is reviewed.