Radiation In The Environment J. L. Hunt i This work is licensed under the Creative Commons Attribution- NonCommercial-NoDerivs 2.5 License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/2.5/ or send a letter to Creative Commons, 543 Howard Street, 5th Floor, San Francisco, California, 94105, USA. ii CONTENTS Introduction Chapter 1. Electromagnetic Radiation: The Classical Description. Chapter 2. Electromagnetic Radiation: The Quantum Description. Chapter 3. Radiometry and Photometry. Chapter 4. Visible and Ultraviolet Radiation. Chapter 5. Infrared and Radio Frequencies. Chapter 6. Lasers and Hazards to the Eye Chapter 7. The Atomic Nucleus and Radioactivity. Chapter 8. The Interaction of Ionizing Radiation with Matter and its Biological Effects. Chapter 9. Ionizing Radiation Detectors. Chapter 10. Sound Chapter 10. Environmental Noise. Appendix I. Numerical Constants. Appendix II Symbols. Appendix III Loudness Chart Bibliography Index iii Acknowledgment The author would like to acknowledge the important contribution to Chapters 1, 4, 5, 8 in the first edition of this textbook by Prof. W. G. Graham. This material has been very little altered in the later editions. iv INTRODUCTION Radiation in Our World. n the post-nuclear era the word ‘radiation’, in the popular lexicon, has taken I on a specialized and negative meaning. Largely because of media misinterpretation the word now suggests to the non-scientist the malignant effects associated with the misuse of nuclear energy. But ‘radiation’ is not all harmful, nor indeed is it all nuclear in origin. We live in an environment bathed in radiation of natural origin and essential to the maintenance of life on the planet. Sunlight and its secondary thermal emissions of heat radiation is the engine of life on the planet Earth. In addition to this ubiquitous low-energy radiation there are natural high-energy radiations in which life has developed and for which evolution and nature have provided effective defence mechanisms. High energy radiation damage to DNA molecules is repaired by appropriate enzymes, and our atmosphere protects us from most of the high-energy portion of the Sun’s radiation. To the overwhelming flux of natural radiations humankind has added a small amount of artificial ones for various specialized purposes. In general, the intensity of these is miniscule compared with the natural sources, but in certain cases they can be hazardous. Exposure to an unshielded nuclear reactor fuel element can harm or even kill you. Exposure to an intense laser beam can blind you (but so can the Sun). Uncontrolled exposure to intense microwaves can heat flesh and cook it. Human-made radiations are created with some purpose; usually the purpose is beneficent. X-rays are an invaluable tool in diagnosis and treatment in medicine and there is hardly anyone who would want to ban their use, but the potential for harm or misuse is always present. An example of such misuse was the widespread use of X-rays in the fitting of shoes in the 1940s and 50s. The accurate measurement of the exposure of both customers and salespersons led to the banning of this frivolous practice. It is essential, therefore, that we be able to understand radiation, be able to measure it in a reproducible way, and as a result of our understanding and measurements, be able to control it and protect ourselves and the public. Fitting shoes with X- rays. Photo provided by Not all of the radiation of environmental concern is Oak Ridge Associated electromagnetic (e.g., light, X-rays) or consists of Universities particles v (e.g., α, β-nuclear particles). Sound is also radiation and has an environmental impact, usually in the form of ‘noise’. Although there are certainly physical effects of intense sound, much of its impact depends on our psychological reaction, and so the subject of ‘ psychoacoustics ’ has been developed to measure human perception of sound radiation. Energy in Radiation. The reason that radiation interacts with us and our environment is because it carries energy. The energy of radiation interacts with the environment through various atomic, molecular and nuclear mechanisms which can sometimes be usefully characterized by the amount of the energy involved in the process. The energy of a typical chemical bond is of the order of a few electron-volts (eV) 1. Visible light and the near-ultraviolet (UV) also have energies of this magnitude, so it is not surprising that these radiations interact with the outer (or valence-bond-forming) electrons in atoms, but not at all with the inner, more tightly bound electrons. Thus visible and near UV radiation can influence or initiate chemical reactions as for instance in photosynthesis or in the complex process of skin tanning. With just a little more energy in the UV the chemical reactions can be violent enough to cause severe damage to sensitive biological molecules; e.g., sunburn. At the lower energies involved in the infrared (IR) only more subtle molecular processes, such as the denaturing (i.e. cooking) of proteins, are possible but this can be serious if it is caused by an intense IR laser beam ‘cooking’ the retinal cells in the eye. At even lower energies it becomes more and more difficult to couple electromagnetic radiation into biological systems as there are fewer and fewer available energy states in the molecules with which to interact. It is for this reason that most scientists are immediately sceptical about claims of the biological harm of radiation from 60 Hz power lines. At this frequency, the energy of the radiation is so low that any possible energy states are already activated by just the thermal energy of the biological system. What further can the EM radiation do? For higher energy EM waves, X- and gamma-rays for example, the energy range is from10 3 eV (keV) to 10 6 eV (MeV). These interactions can take place with the inner, more tightly bound, electrons in the atom detaching them ( photoelectric effect ) and producing a fast moving electric charge in the medium. There are also mechanisms by which high energy EM waves can interact with the outer weakly bound electrons ( Compton Effect ) and also produce a fast moving charge in the medium. High energy particle radiation, such as α and β, also involves fast moving charges in the medium. A fast moving charge can detach electrons from the molecules of the medium creating chemically active species that can 1 The quantitative definition of the electron-volt will be given in Chapter 1. vi go on to produce extensive damage in living systems. Indeed almost all high- energy radiation damage in living cells is of this type. The interaction of radiation with the nuclei of atoms is negligible in the environmental context. The energy regime of nuclei is of the order of 10 6 eV (MeV) and there are certainly EM radiations of this energy. However, because of the small size of the nucleus, such interactions are extremely rare in the radiation fluxes encountered in the natural environment. To make such processes important the fluxes found in the cores of nuclear reactors are required. Organization of the Text. Because so much of the natural radiation is electromagnetic, a review of electric and magnetic fields in Chapter 1 leads to a consideration of the properties of EM waves from a classical point of view in Chapter 1 and a quantum point of view in Chapter 2. Chapter 3 covers the measurement of visible light, a subject known as ‘photometry ’. Here the human perception (seeing) of the physical phenomenon (EM waves) introduces the reader to one aspect of psychophysics and a plethora of new units and measures. Chapters 4 and 5 discuss the environmental effects of visible and UV radiation, and infrared and radio radiation respectively. Chapter 6 describes the construction of lasers and their classification by output power. This leads to an analysis of the hazard of laser radiation and the standards that have been established to minimize risk. Chapter 7 discusses the physics of nuclear radiations, and Chapter 8 their interactions with biological systems, and the associated hazards. Chapter 9 is a brief survey of some nuclear radiation detection and monitoring methods. Chapters 10 and 11 introduce sound radiation and the measurement and classification of a selection of examples of environmental noise. Of course, in an elementary survey, none of the topics are pursued to the detailed level that might be required of an environmental consultant or scientist in the field. Nevertheless, the physical principles are established and just with these, it is surprising how many realistic environmental situations can be understood and quantified. Throughout, there is an emphasis on quantitative methods with many numerical examples and problems. vii CHAPTER 1: ELECTROMAGNETIC RADIATION: THE CLASSICAL DESCRIPTION 1.1 Introduction. his chapter is a review of the classical or wave nature of electromagnetic (EM) T radiation; it is assumed that the reader is familiar with some of this material. More elementary and detailed treatments may be found in any introductory university physics book. Since visible light and radio are familiar forms of EM radiation, these are used extensively as examples of respectively, short-wavelength and long-wavelength radiations. It is well known that light transmits energy, can travel through the best laboratory vacuum, and exhibits diffraction and interference patterns under the proper conditions. This indicates that light (and other EM radiation) is some type of wave but not a mechanical wave (like sound) which requires matter for its transmission. Further, the familiar polarization effects with light (e.g., two ‘crossed’ polaroids; see Sec. 1.5) indicate that light is a transverse wave; there are no similar polarization phenomena with longitudinal waves such as sound.