RADIATION

either the process by which energy is emitted from a source and propagated through the surrounding medium or the energy involved in this process. Familiar examples of radiant energy include light (a form of electromagnetic radiation) and sound (a form of acoustic radiation). Both electromagnetic and acoustic radiations are commonly described as waves that can vary over great ranges of either frequency or intensity. Electromagnetic radiation also is often treated as discrete packets of energy, called photons, or quanta. At very high frequencies, the energy of electromagnetic radiation becomes equivalent to appreciable quantities of mass, and the distinction between waves and particles becomes arbitrary. Much of the radiation emitted by radioactive elements takes the form of alpha rays, beta rays, and streams of other subatomic particles. Radiation is treated in several articles. For the origin, nature, and propagation of energy in the form of gamma rays, X rays, ultraviolet rays, visible light, heat, radio waves, and the like, see electromagnetic radiation; light; radioactivity. For corresponding treatment of acoustic radiations, see acoustics. For discussion of the interaction of electromagnetic waves and subatomic particles with matter, living and nonliving, see radiation. flow of atomic and subatomic particles and of waves, such as those that characterize heat rays, light rays, and X rays. All matter is constantly bombarded with radiation of both types from cosmic and terrestrial sources. This article delineates the properties and behaviour of radiation and the matter with which it interacts and describes how energy is transferred from radiation to its surroundings. Considerable attention is devoted to the consequences of such an energy transfer to living matter, including the normal effects on many life processes (e.g., photosynthesis in plants and vision in animals) and the abnormal or injurious effects that result from the exposure of organisms to unusual types of radiation or to increased amounts of the radiations commonly encountered in nature. The applications of various forms of radiation in medicine and technological fields are touched upon as well. Additional reading General Historical works include Max Planck, Introduction to Theoretical Physics, vol. 4, Theory of Light (1932, reprinted 1957; originally published in German, 1927), the classic work on the subject of light and quanta. Other forms of electromagnetic radiation are covered in Otto Glasser, Wilhelm Conrad Rntgen and the Early History of the Roentgen Rays (1933; originally published in German, 1931). See also R.W. Ditchburn, Light, 3rd ed., 2 vol. (1976), a well-presented text on physical optics that, though not too mathematical, does require understanding of the use of differential equations. Interaction of radiation with matter Gerhard K. Rollefson and Milton Burton, Photochemistry and the Mechanism of Chemical Reactions (1939, reprinted 1946); William Albert Noyes and Philip Albert Leighton, The Photochemistry of Gases (1941, reprinted 1966), are both classic works that include material on internal conversion and predissociation. The language of intersystem crossing is discussed in detail in a comprehensive text, Jack G. Calvert and James N. Pitts, Photochemistry (1966). The actual effects of radiation on solids are thoroughly summarized in Hans A. Bethe and Julius Ashkin, Passage of Radiations Through Matter, in Emilio Segr (ed.), Experimental Nuclear Physics, vol. 1 (1953), pp. 166357; G.J. Dienes and G.H. Vineyard, Radiation Effects in Solids (1957); and Douglas S. Billington and James H. Crawford, Jr., Radiation Damage in Solids (1961). For a survey of radiation effects on aqueous solutions and organic compounds, see J.W.T. Spinks and R.J. Woods, An Introduction to Radiation Chemistry, 2nd ed. (1976); Actions chimiques et biologiques des radiations (annual 195571), the first survey on a large variety of subjects in radiation chemistry written by scientists largely about their own worksome volumes have been translated into English with the title, The Chemical and Biological Action of Radiations; and Max S. Matheson and Leon M. Dorfman, Pulse Radiolysis (1969), an excellent book on techniques in radiation chemistry. Advances in Photochemistry (irregular) is concerned mainly with surveys of advances in the field. See also J.F. Ziegler (ed.), Ion Implantation: Science and Technology (1984), a treatment of ion implantation mechanisms, techniques, effects, and practical applications; and Orlando Auciello and Roger Kelly (eds.), Ion Bombardment Modification of Surfaces: Fundamentals and Applications (1984), covering surface alteration mechanisms with major emphasis on topographical effects. Milton Burton Asokendu Mozumder Myron Luntz Radiological units and measurements For descriptions, see Ralph E. Lapp and Howard L. Andrews, Nuclear Radiation Physics, 4th ed. (1972); and International Commission on Radiation Units and Measurements, Radiation Quantities and Units (1980). Biologic effects of radiation Ionizing General information is given in Charles Wesley Shilling (ed.), Atomic Energy Encyclopedia in the Life Sciences (1964). Introductory information on radiation biology is given in J.E. Coggle, Biological Effects of Radiation, 2nd ed. (1983); John W. Gofman, Radiation and Human Health (1981); Daniel S. Grosch and Larry E. Hopwood, Biological Effects of Radiations, 2nd ed. (1979); and Eric J. Hall, Radiation and Life, 2nd ed. (1984). More specialized topics are covered in Assembly on Life Sciences (U.S.) Committee on the Biological Effects of Ionizing Radiations, The Effects on Populations of Exposure to Low Levels of Ionizing Radiation, 1980 (1980); Merrill Eisenbud, Environmental Radioactivity: From Natural, Industrial, and Military Sources, 3rd ed. (1987); Donald J. Pizzarello and Richard L. Witcofski, Medical Radiation Biology, 2nd ed. (1982); United Nations Scientific Committee on the Effects of Atomic Radiation, Ionizing Radiation: Sources and Biological Effects (1982), and Genetic and Somatic Effects of Ionizing Radiation (1986); Arthur C. Upton, Radiation Injury: Effects, Principles, and Perspectives (1969); and Arthur C. Upton et al. (eds.), Radiation Carcinogenesis (1986). Non-ionizing Microwave radiation is treated in National Council of Radiation Protection and Measurements, Biological Effects of Ultrasound: Mechanisms and Clinical Implications (1983), and Biological Effects and Exposure Criteria for Radiofrequency and Electromagnetic Fields (1986); and R.C. Petersen, Bioeffects of Microwaves: A Review of Current Knowledge, Journal of Occupational Medicine, 25(2):103110 (February 1983). Visible and ultraviolet radiations are the subject of Walter Harm, Biological Effects of Ultraviolet Radiation (1980); A. Jarret (ed.), The Photobiology of the Skin: Lasers and the Skin (1984); Kendric C. Smith (ed.), Topics in Photomedicine (1984); and Richard J. Wurtman, Michael J. Baum, and John T. Potts, Jr. (eds.), The Medical and Biological Effects of Light (1985). Nuclear war Radiation effects of a nuclear war are discussed in Samuel Glasstone and Philip J. Dolan (eds.), The Effects of Nuclear Weapons, 3rd ed. (1977); Jean Petersen and Don Hinrichsen (eds.), Nuclear War: The Aftermath (1982); Julius London and Gilbert F. White (eds.), The Environmental Effects of Nuclear War (1984); and Fredric Solomon and Robert Q. Marston (eds.), The Medical Implications of Nuclear War (1986). Radiation protection and safety Procedures and recommendations for protection are analyzed in International Commission on Radiological Protection, Recommendations of the International Commission on Radiological Protection (1977, reprinted with supplements, 1987), and Nonstochastic Effects of Ionizing Radiation (1984); National Council on Radiation Protection and Measurements, Ionizing Radiation Exposure of the Population of the United States (1987); and Marilyn E. Noz and Gerald Q. Maguire, Jr., Radiation Protection in the Radiologic and Health Sciences, 2nd ed. (1985). Applications of radiation Medical Radiological imaging techniques are explored in R.P. Clark and M.P. Goff (eds.), Recent Developments in Medical and Physiological Imaging (1986); W.-D. Heiss and M.F. Phelps (eds.), Positron Emission Tomography of the Brain (1983); Alexander R. Magulis and Charles A. Gooding (eds.), Diagnostic Radiology, 1987 (1987); and Albert A. Moss, Ernest J. Ring, and Charles B. Higgins (eds.), NMR, CT, and Interventional Radiology (1984). Radiation therapy is addressed by Gilbert H. Fletcher, Textbook of Radiotherapy, 3rd ed. (1980); and Ernest J. Ring and Gordon K. McLean, Interventional Radiology: Principles and Techniques (1981). Specific uses of phototherapy are outlined in Audrey K. Brown and Jane Showacre (eds.), Phototherapy for Neonatal Hyperbilirubinemia: Long-Term Implications (1977); Wayne F. March (ed.), Ophthalmic Lasers: Current Clinical Uses (1984); and Warwick L. Morison, Phototherapy and Photochemotherapy of Skin Disease (1983). Arthur Canfield Upton Scientific and industrial A review of ionizing radiation processing in medicine and industrial manufacturing is found in Vitomir Markovic, Modern Tools of the Trade, International Atomic Energy Agency Bulletin, 27(1):3339 (Spring 1985). Industrial uses are explored in Joseph Silverman, Radiation Processing: The Industrial Applications of Radiation Chemistry, Journal of Chemical Education, 58(2):168173 (Feb. 1981). International Atomic Energy Agency, Industrial Application of Radioisotopes and Radiation Technology (1982), is a collection of conference papers. Joseph Silverman Applications of radiation Medical applications The uses of radiation in diagnosis and treatment have multiplied so rapidly in recent years that one or another form of radiation is now indispensable in virtually every branch of medicine. The many forms of radiation that are used include electromagnetic waves of widely differing wavelengths (e.g., radio waves, visible light, ultraviolet radiation, X rays, and gamma rays), as well as particulate radiations of various types (e.g., electrons, fast neutrons, protons, alpha particles, and pi-mesons). Imaging techniques Advances in techniques for obtaining images of the body's interior have greatly improved medical diagnosis. New imaging methods include various X-ray systems, positron emission tomography, and nuclear magnetic resonance imaging. Biologic effects of ionizing radiation The biomedical effects of ionizing radiation have been investigated more thoroughly than those of any other environmental agent. Evidence that harmful effects may result from small amounts of such radiation has prompted growing concern about the hazards that may be associated with low-level irradiation from the fallout of nuclear weapons, medical radiography, nuclear power plants, and other sources. Assessment of the health impact of ionizing radiation requires an understanding of the interactions of radiation with living cells and the subsequent reactions that lead to injury. These subjects are surveyed in the following sections, with particular reference to the principal sources and levels of radiation in the environment and the different types of biologic effects that may be associated with them. Historical background Within weeks after Rntgen revealed the first X-ray photographs in January 1896, news of the discovery spread throughout the world. Soon afterward, the penetrating properties of the rays began to be exploited for medical purposes, with no inkling that such radiation might have deleterious effects. The first reports of X-ray injury to human tissue came later in 1896. Elihu Thomson, an American electrical engineer, deliberately exposed one of his fingers to X rays and provided accurate observations on the burns produced. That same year, Thomas Alva Edison was engaged in developing a fluorescent X-ray lamp when he noticed that his assistant, Clarence Dally, was so poisonously affected by the new rays that his hair fell out and his scalp became inflamed and ulcerated. By 1904 Dally had developed severe ulcers on both hands and arms, which soon became cancerous and caused his early death. During the next few decades, many investigators and physicians developed radiation burns and cancer, and more than 100 of them died as a result of their exposure to X rays. These unfortunate early experiences eventually led to an awareness of radiation hazards for professional workers and stimulated the development of a new branch of sciencenamely, radiobiology. Radiations from radioactive materials were not immediately recognized as being related to X rays. In 1906 Henri Becquerel, the French physicist who discovered radioactivity, accidentally burned himself by carrying radioactive materials in his pocket. Noting that, Pierre Curie, the co-discoverer of radium, deliberately produced a similar burn on himself. Beginning about 1925, a number of women employed in applying luminescent paint that contained radium to clock and instrument dials became ill with anemia and lesions of the jawbones and mouth; some of them subsequently developed bone cancer. In 1933 Ernest O. Lawrence and his collaborators completed the first full-scale cyclotron at the University of California at Berkeley. This type of particle accelerator was a copious source of neutrons, which had recently been discovered by Sir James Chadwick in England. Lawrence and his associates exposed laboratory rats to fast neutrons produced with the cyclotron and found that such radiation was about two and a half times more effective in killing power for rats than were X rays. Considerably more knowledge about the biologic effects of neutrons had been acquired by the time the first nuclear reactor was built in 1942 in Chicago. The nuclear reactor, which has become a prime source of energy for the world, produces an enormous amount of neutrons as well as other forms of radiation. The widespread use of nuclear reactors and the development of high-energy particle accelerators, another prolific source of ionizing radiation, have given rise to health physics. This field of study deals with the hazards of radiation and protection against such hazards. Moreover, since the advent of spaceflight in the late 1950s, certain kinds of radiation from space and their effects on human health have attracted much attention. The protons in the Van Allen radiation belts (two doughnut-shaped zones of high-energy particles trapped in the Earth's magnetic field), the protons and heavier ions ejected in solar flares, and similar particles near the top of the atmosphere are particularly important.