Radiation chemistry

(nucleonics) The branch of chemistry that is concerned with the chemical effects, including decomposition, of energetic radiation or particles on matter.

The study of chemical changes resulting from the absorption of high-energy, ionizing radiation, including alpha particles, electrons, gamma rays, fission fragments, protons, deuterons, helium nuclei, and heavier charged projectiles. In absorbing materials of low and intermediate atomic weight such as aqueous systems and most biological systems, such radiation deposits energy in a largely indiscriminate manner, leaving behind a complex mixture of short-lived ions, free radicals, and electronically excited molecules. Radiation-induced chemical changes result from reaction with these intermediates. See also Photochemistry.

Sources of high-energy radiation include radioactive nuclides [for example, cobalt-60 (⁶⁰Co), strontium-90 (⁹⁰Sr), and hydrogen-3 (³H)] and instruments such as x-ray tubes, Van de Graaff generators, the betatron, the cyclotron, and the synchrotron. An electron accelerator known as the Linac (linear electron accelerator) has proved particularly valuable for the study of transient species that have lifetimes as short as 16 picoseconds; and another electron accelerator, known as the Febetron, has been used for the study of the effects of single pulses of electrons with widths of several nanoseconds at very high currents.

The primary absorption processes for high-energy radiation are ionization and molecular excitation. The distribution of the absorbed energy, however, depends significantly upon the nature of the radiation and absorbing medium.

Evaluation of the yields of radiation-induced reactions requires knowledge of the energy imparted to the reacting system. The energy deposited in the system is termed the dose, and the measurement process is called dosimetry. Absorbed energy from ionizing radiation is described in terms of grays (Gy; joule/kg), in rads (100 ergs/g), or in electronvolts per gram or per cubic centimeter.

Because of its importance in both chemical and biological systems, the radiation chemistry of water has been extensively studied and serves as an example of radiation-induced chemical change. A primary radiation interaction process may be represented by the reaction below, where H₂O* represents an

electronically excited water molecule. The secondary electron (e⁻), if formed with sufficient energy, will form its own trail of ionization and excitation. Within 10⁻¹⁰ to 10⁻⁸ s, reactions within spurs form hydrogen (H) atoms, hydroxyl (OH) radicals, hydrated electrons and molecular products, molecular hydrogen (H₂), and hydrogen peroxide (H₂O₂). In pure water, radicals escaping the spurs undergo further radical-radical reactions and reactions with molecular products. Upon continuous irradiation, steady-state concentrations of H₂, H₂O₂, and smaller amounts of dioxygen (O₂) result and no further decomposition occurs.

In addition to basic kinetics and mechanistic studies, the principles of radiation chemistry find application in any process in which ionizing radiation is used to study, treat, or modify a biological or chemical system.

In radiation therapy, tumors are destroyed by the application of ionizing radiation from external or internally administered sources. Gamma rays are used for treatment of internal tumors; electron or charged-particle beams are applied to external or invasively accessible lesions.

The physiological concentration of iodine in the thyroid is the basis for the treatment of hyperthyroidism with ¹³¹I. The beta radiation from this isotope is effective in localized tissue destruction.

A goal of any radiation therapy is maximum tumor cell destruction with minimum damage to healthy cells.

Radiation chemistry is used in food preservation by using ionizing radiation in doses that are lethal to microorganisms. The use of ionizing radiation for pathogen control has been approved by most governments for a wide range of foods. In general, limitations on dose have been specified for all products. Radiation is used to control pathogens in meat and meat products.

Processing of commercial quantities of food supplies requires a source of stable intensity and a radiation of sufficient penetrating power to deposit energy throughout the product in an economic, brief time period.