radiation chemistry

a branch of chemistry encompassing the chemical processes that arise from the effect of ionizing radiation on matter. Both electromagnetic radiation, in the form of X rays, gamma rays, and radiant energy within the optical wavelengths, and fast charged particles, such as electrons, protons, alpha particles, and fragments of heavy nuclei, possess an ionizing capacity when their energy exceeds the ionization potential of atoms or molecules (usually 10–15 electron volts [eV]). The initiation of chemical reactions through the action of ionizing radiation derives from the capacity of this radiation to ionize and excite molecules.

History. The capacity of ionizing radiation to initiate chemical reactions was found soon after the discovery of radioactivity. Experiments demonstrating the presence of chemical effects caused by the emanations of radioactive elements date from the beginning of the 20th century. Radiation chemistry came into its own as a separate field in the 1940’s with the construction of nuclear reactors and the industrial production of fissionable elements, such as plutonium. With the development of nuclear technology, the need arose to study the various accompanying chemical effects, for example, the radiolysis of water, the transmutation of radioactive elements in solution, the changes in the various materials used in nuclear technology, and the reactions of the gaseous components of air (nitrogen, oxygen, and carbon dioxide). In connection with the effect of ionizing radiation on organisms, it became necessary to conduct a detailed study of the radiation-chemical transformations in biopolymers.

It gradually became clear that ionizing radiation could be applied in carrying out useful chemical processes. Broad studies were undertaken to stimulate various radiation-chemical processes by ionizing radiation, and a detailed investigation was begun to uncover the laws governing these processes.

Principles. It was found that gamma rays and fast particles (alpha particles, electrons, and protons), upon passing through matter, knock electrons out of molecules; that is, the rays or particles cause the ionization or, if the energy imparted is less than the ionization energy, the excitation of the molecule. As a result, a large number of electrically charged (in the case of ions and ion-radicals) or neutral (in the case of atoms and radicals) molecular fragments arise along the path of a fast particle, thus forming a track. The electrons ejected from the molecules are of lower energy (secondary electrons); they fly off and, in turn, produce an analogous effect but over a shorter distance (corresponding to their lower energy). As a result, the track of the primary fast particle branches out through the formation of smaller regions of ionization and excitation. At a sufficiently high radiation density, the tracks overlap and any initial non-uniformity in the spatial distribution of activated and fragmented particles is eliminated. Uniformity is also facilitated by the diffusion of particles from the tracks into those regions of the medium not affected by radiation.

The processes occurring in the irradiated medium may be divided into three major steps. In the initial, physical step, collisions occur between the fast charged particle and the molecules of the medium, resulting in a transfer of the particle’s kinetic energy and thus in a change in the energy state of the molecules. In this step, the energy imparted to the medium is distributed over various molecular (atomic) levels; as a result, a large number of activated molecules arise in various states of excitation. The initial step occurs over a very short time interval, 10^–15 to 10^–12 sec. In their excited state, the molecules are unstable and either decompose or interact with surrounding molecules. This instability results in the formation of ions, atoms, and radicals, that is, intermediate particles in radiation-chemical reactions. This is the second step and lasts 10^–13 to 10^–11 sec. In the third step (the actual chemical step), the activated particles that have been formed react with the surrounding molecules or with each other and form the final products of the radiation-chemical reactions. The duration of the third step depends on the activity of the intermediate particles and the properties of the medium and may last 10^–11 to 10^–6 sec.

The secondary electrons lose their kinetic energy in the ionization (or excitation) of molecules and gradually slow to a speed corresponding to the thermal energy. In a liquid medium, the deceleration occurs in 10^–13 to 10^–12 sec, after which the electrons are captured either by a single molecule, thus forming a negatively charged ion, or by a group of molecules (are solvat-ed). Solvated electrons have a lifetime of 10^–8 to 10^–5 sec, depending on the properties of the medium and the conditions; at the end of this period, they recombine with positively charged particles. The regularities governing these elementary processes are an important part of the theory of radiation chemistry. Furthermore, a significant role in radiation-chemical processes is played by the reactions of the excited molecules. The transmission of excitation energy in the irradiated medium is also very important for the occurrence of the processes and leads to the deactivation of the excited molecules and the dispersion of energy. Such processes are also studied in photochemistry, which is thus closely related to radiation chemistry.

Radiation-chemical transformations. The reactions of activated particles with molecules differ from the reactions of unex-cited molecules with each other. In most cases, molecules are rather stable, and in order for reactions to occur upon collision, additional energy is required to overcome the energy barrier of the reaction. This energy is usually imparted to molecules by increasing the temperature of the medium. The energy barrier is very low for the reactions of activated particles both with each other and with other molecules. Reactions are very effective when involving the recombination of electrons with positive ions, when radicals and atoms react with each other, or when positive ions react with molecules (ion-molecule reactions). In some cases, the dissociative capture of electrons by a molecule is effective; here, the molecule dissociates into a radical and a negative ion. These elementary processes lead either to the dissociation of molecules and complex ions or to the formation of molecules of a new substance. Reactions of radicals with molecules require the surmounting of relatively low energy barriers, from 5 to 10 kilocalories/mole (from 21 to 42 kilojoules/mole). As a consequence, radiation-chemical reactions proceed rapidly even at very low temperatures (below -200°C); in contradistinction to ordinary reactions, the rates of radiation-chemical reactions are only weakly dependent on temperature.

The course of radiation-chemical reactions depends on the state of aggregation of matter. In the gaseous phase, these reactions usually proceed with a greater yield than in the condensed, that is, liquid and solid, phases because of a more rapid dispersion of energy in condensed media. If these reactions are reversible, that is, able to proceed in both the forward and reverse directions, then the reaction rates in both directions gradually become equal, and a steady state is established in which no apparent chemical changes occur in the irradiated medium. The chemical composition in such a state differs considerably from the composition established upon chemical equilibrium, and the steady-state product concentrations may greatly exceed the equilibrium concentrations corresponding to a given temperature. For example, the steady-state concentrations of the oxides of nitrogen formed through the irradiation of a mixture of nitrogen and oxygen (or air) at room temperature are thousands of times higher than the concentrations established under conditions of thermal chemical equilibrium at a given temperature. The radiation energy absorbed by matter is usually not used entirely for carrying out chemical processes; a significant portion is dispersed and gradually converted into heat.

The efficiency of the chemical effect of radiation is usually characterized by the value of the radiation-chemical yield G, which represents the number of molecules converted or produced upon the absorption of 100 eV of energy by the medium. For ordinary reactions, values for G range between 1 and 20, but in chain reactions, the values may reach tens of thousands. The amount of energy absorbed is called the absorbed radiation dose and is measured in roentgens or rads.

Radiation-chemical reactions vary in nature. The simplest occur in the air under the action of cosmic rays or radiation from radioactive elements. Various chemical processes occur in the air under the action of ionizing radiation. For example, ozone is formed from oxygen, nitrogen reacts with oxygen to form various nitrogen oxides, and carbon dioxide decomposes to form carbon monoxide. In other cases, the decomposition of chemical compounds into simpler substances occurs. For example, water decomposes into hydrogen and oxygen, ammonia into hydrogen and nitrogen, and hydrogen peroxide into oxygen and water. The capacity of ionizing radiation to initiate chemical reactions at relatively low temperatures makes possible a number of industrially important processes, for example, the oxidation of hydrocarbons by atmospheric oxygen leading to the formation of substances used in detergents and lubricating oils.

One of the most interesting processes initiated by ionizing radiation is the polymerization of organic monomers leading to the formation of various polymers. Many of these polymers have valuable properties, such as a higher molecular weight, which are not obtained by other methods of synthesis. The action of radiation on polymers may give rise to internal processes resulting in an improvement in the physicochemical properties, such as thermal stability, of the polymers.

Various sources of ionizing radiation are used in carrying out radiation-chemical processes. One of the most common sources is radioactive cobalt, which emits gamma rays with energy above 1 megaelectron volt. Electron accelerators are common and are well suited for practical use because of their manageability and high radiation intensity. Means have also been de veloped for the direct use of the radiation from atomic reactors for carrying out radiation-chemical processes.

The current development of radiation chemistry is closely related to a number of areas in science and technology, including atomic physics, atomic power engineering, and space research. Work is also being done on the application of radiation chemistry to the problems of biology and medicine. Soviet scientists have dealt with a number of basic theoretical questions and have developed many of the practical aspects of radiation chemistry.