AccessScience from McGraw-Hill Education Page 1 of 4 www.accessscience.com Radiation Contributed by: McAllister H. Hull, Jr. Publication year: 2019 Key Concepts • Radiation is the emission and propagation of energy, and may also refer to the emitted energy itself. • The major types of radiation are electromagnetic, particle, acoustic, and gravitational. • Electromagnetic radiation consists only of photons, whereas particle radiation includes high-energy photons as well as other rapidly moving subatomic particles and atomic nuclei. • Ionizing radiation can be harmful to living tissue, although it is widely used in medicine. • Acoustic, or sound, radiation, is classified by frequency. Sonic radiation is audible to the human ear. Infrasonic sound is below the range of human hearing, and ultrasonic is above that range. • Gravitational radiation consists of waves that propagate through spacetime at the speed of light and are generated by the acceleration of mass. The emission and propagation of energy; also, the emitted energy itself. Radiation is a ubiquitous phenomenon that transfers energy and matter from one place to another. The major types of radiation are electromagnetic (Fig. 1), particle, acoustic, and gravitational. Within these major divisions there are many subdivisions. See also: ENERGY ; MATTER (PHYSICS) . Electromagnetic radiation Massless, elementary particles called photons carry the electromagnetic force, one of nature’s four fundamental interactions along with gravitation and the strong and weak nuclear interactions. Electromagnetic radiation is most familiar to us as visible light, which is a small range of possible energies (and associated wavelengths) that photons may possess. The energy of a photon is inversely proportional to the wavelength. In order of decreasing wavelength (and thus increasing energy) along the electromagnetic spectrum, electromagnetic radiation is subdivided into radio, microwave, visible, ultraviolet, x-ray, and gamma ray. In the last three subdivisions, and frequently in the visible, the behavior of the radiation is more particlelike than wavelike. See also: ELECTROMAGNETIC RADIATION ; FUNDAMENTAL INTERACTIONS ; GAMMA RAYS ; GRAVITATION ; LIGHT ; PHOTON ; STRONG NUCLEAR INTERACTIONS ; WEAK NUCLEAR INTERACTIONS ; X-RAYS . AccessScience from McGraw-Hill Education Page 2 of 4 www.accessscience.com CrepuscularFig. 1 Crepuscular rays from rays, the which Sun are shafts of air illuminated by electromagnetic radiation (light) radiating from the Sun. [Credit: Danny Chapman ∕ Flickr ∕ Attribution-ShareAlike 2.0 Generic ( CC BY-SA 2.0 )] Particle radiation Particle radiation consists of rapidly moving subatomic particles as well as atomic nuclei. High-energy photons, and in particular gamma rays, are by convention categorized as particle radiation. A classic example of particle radiation is the alpha particle, which consists of two protons and two neutrons bound as a doubly ionized helium nucleus (meaning the helium atom is stripped of its two electrons). Another classic example of particle radiation is the beta particle, which consists of either an electron or its antimatter counterpart, a positron. Another kind of particle radiation is the cosmic ray, generated in a range of energies by astrophysical phenomena. Most cosmic rays are a lone proton (hydrogen nucleus), but others are heavier ionized nuclei. See also: ALPHA PARTICLES ; ANTIMATTER ; ATOMIC NUCLEUS ; BETA PARTICLES ; COSMIC RAY ; ELECTRON ; ELEMENTARY PARTICLE ; HELIUM ; HYDROGEN ; PROTON . Ionizing versus non-ionizing radiation In terms of damage to living tissue, an important categorization across electromagnetic and particle radiation is ionizing versus non-ionizing. Ionizing radiation includes highly energetic electromagnetic radiation (light), such as gamma rays, and particles that remove electrons from atoms, potentially breaking chemical bonds. Non-ionizing radiation involves less energetic radiation, for instance infrared radiation. Non-ionizing radiation exposure can cause heating in biological tissue, which can also of course be harmful, but in general, non-ionizing radiation is of considerably less concern than ionizing radiation. See also: INTERACTION OF PHOTONS WITH IONIZED MATTER ; IONIZATION ; RADIATION INJURY (BIOLOGY) . AccessScience from McGraw-Hill Education Page 3 of 4 www.accessscience.com gravitationalFig. 2 Illustration waves of gravitationalmoving through radiation space generated by the impending merger of two neutron stars. (Credit: R. Hurt ∕ Caltech-JPL) Both categories of electromagnetic and particle radiation are widely used in medicine. Radiology uses the tissue-penetrating power of radio waves, for instance, as well as x-rays, which are absorbed by bones, for diagnostic and imaging purposes. Nuclear medicine more broadly utilizes ionizing radiation generated by materials ingested or implanted in the body for diagnostic, research, and therapeutic purposes. Radiation therapy uses targeted doses of ionizing radiation to kill cancer cells. See also: MAMMOGRAPHY ; MEDICAL IMAGING ; NUCLEAR MEDICINE ; NUCLEAR RADIATION (BIOLOGY) ; RADIATION THERAPY ; RADIOGRAPHY ; RADIOISOTOPE (BIOLOGY) ; RADIOLOGY . Acoustic radiation Acoustic or sound radiation may be classified by frequency as infrasonic, sonic, or ultrasonic in order of increasing frequency. Infrasonic is "below" and ultrasonic is "above" the range of human hearing because of its frequency band being lower and higher, respectively, than the human auditory system can detect. Sonic radiation—in the range of hearing—spans between about 16 and 20,000 Hz. Infrasonic sound can result from explosions or other sources so loud that exceptional waves are set up because the large amplitudes of the source vibrations exceed the elastic limit of the transmitting medium. Ultrasonic sound can be produced by means of crystals which vibrate rapidly in response to alternating electric voltages applied to them. Ultrasound is widely used in medical imaging, with some examples of its applications including imaging a gestating fetus or a beating adult heart. See also: BIOMEDICAL ULTRASONICS ; ECHOCARDIOGRAPHY ; HEART (VERTEBRATE) ; INFRASOUND ; SOUND . Gravitational radiation Gravitational radiation consists of waves that travel through spacetime at the speed of light and are generated by the acceleration of mass (Fig. 2). German-born U.S. theoretical physicist Albert Einstein first proposed the existence of gravitational waves in his general theory of relativity in 1916. The waves proved too small to directly AccessScience from McGraw-Hill Education Page 4 of 4 www.accessscience.com detect until 2015, when an experiment called the Laser Interferometer Gravitational-wave Observatory (LIGO) 22 registered waves passing through Earth. LIGO is sensitive to a strain of one part in 10, , or on the order of one ten-thousandth the diameter of a proton. The gravitational waves were theoretically generated by the collision of black holes more than a billion light-years distant. LIGO has since made numerous detections from similar events, as well as neutron star mergers. See also: BLACK HOLE ; GRAVITATIONAL RADIATION ; LIGO (LASER INTERFEROMETER GRAVITATIONAL-WAVE OBSERVATORY) ; MASS ; NEUTRON STAR ; RELATIVITY ; SPACETIME . McAllister H. Hull, Jr. Keywords Radiation; electromagnetic radiation; acoustic radiation; sound; particle radiation; gravitational radiation; gravitational waves; light; medical imaging; radiation injury; nuclear reactions; nuclear radiation; gamma radiation Test Your Understanding 1. Define radiation. 2. What are the four major types of radiation? 3. Critical Thinking: Describe at least three factors that should be considered when using radiation in medicine. Support your conclusions with reasoning and relevant examples. 4. Critical Thinking: Identify three natural sources of radiation experienced on Earth and explain some ways in which those sources may affect humans. 5. Critical Thinking: What category of radiation could have more energy than microwave radiation and a longer wavelength than ultraviolet radiation? Explain your answer. Additional Readings P. Andreo, D. T. Burns, A. E. Nahum, and J. Seuntjens, Fundamentals of Ionizing Radiation Dosimetry: Textbook and Solutions , Wiley, 2017 A. Bettini, A Course in Classical Physics 3 — Electromagnetism , Springer, 2016 F. Fahy and D. Thompson, Fundamentals of Sound and Vibration , 2nd ed. CRC Press, 2015 D. Reitze, P. Saulson, and H. Grote, Advanced Interferometric Gravitational-Wave Detectors (In 2 Volumes) , World Scientific, 2019 .