Radiation effects in organic materials

Basic distinctions between the irradiation of organic and aqueous systems arise not only from the nature of the chemical bonds involved, but also from the dielectric constant and polarisability of the molecule. In organic molecules, absorbed energy can break homopolar bonds giving rise to radicals, or may give ionic intermediates. A number of mechanisms can intervene before or after these processes and serve to modify the final reaction products. In certain organics, notably monomers, a chain reaction may occur, leading to large chemical changes at relatively low doses. Such polymerisation reactions may also take place in the solid state depending on morphology and defects. In other, high molecular weight materials, radiation can promote chemical changes which though small, have a large effect on the subsequent physical properties. These reactions may be modified by additives in low concentration, capable of enhancing or reducing the radiation-induced charges. In several instances these high molecular weight molecules of simple chemical structure may serve as models for radiobiological research. At a much earlier stage than the chemical reaction, the electrons resulting from ionisation give rise to physical changes such as radiation-induced conductivity and thermoluminescence. A number of complementary techniques are available to study the behaviour of electrons in organic liquids and solids. These are briefly outlined, and attention is drawn to aspects not yet adequately clarified. (Auth.)