The study of chemical changes resulting from the absorption of high-energy, ionizing radiation. Such radiation includes alpha particles, electrons, gamma rays, fission fragments, protons, deuterons, helium nuclei, and heavier charged projectiles. X-rays are distinguished from gamma rays only as being extranuclear in origin. In absorbing materials of low and intermediate atomic weight such as aqueous systems and most biological systems, these radiations deposit energy in a largely indiscriminate manner, leaving behind a complex mixture of short-lived ions, free radicals, and electronically excited molecules. This contrasts with the absorption of visible and ultraviolet radiation, in which one or a few specific electronically excited species are formed. Radiation chemical change results from the further reaction of these intermediates.