Chemical Evidence of Track Effects in the Radiolysis of Liquids

Abstract

It is now well established that in a system exposed to ionizing radiations ions are produced along the path of the ionizing particles. These ions are originally located very close to the tracks, either in cylindrical columns surrounding the tracks when densely ionizing particles are used (a-rays, protons, slow electrons), or in separate clusters if hard X-rays, y-rays, or high-energy electrons are considered. Direct evidence for this phenomenon in gases is given by cloud chamber photographs. After a short time interval, the ions diffuse away from their original positions, the tracks becoming less sharp. The problem of ion diffusion and recombination in tracks was considered by Jaffe (1) in 1913 and has since been developed in greater detail by Lea (2), Gray (3, 4), Magee (5), and others. From the chemical point of view, however, it has become more and more evident in recent years that free radicals and not ions are responsible for most chemical reactions. This was first emphasized in the case of the radiolysis of water by Weiss (6) and has more recently been extended to several organic systems. This is why the important point for the chemist is not so much to consider the spatial distribution of ions but the original distribution of free radicals. In a liquid exposed to ionizing radiations, free radicals are created by several processes: a. Primary dissociative ionizations, a molecule AB being disrupted so as to form a charged fragment and a free radical: