Radiation

Radiation is present whenever energy moves from one place to another. Atoms and molecules give off radiation to dispose of excess energy. When the radiation strikes a substance, it may transfer some or all of its energy to the substance. Often, the energy takes the form of heat, raising the temperature of the material. Except for light, most kinds of radiation are invisible.

There are two chief types of radiation. One type, called electromagnetic radiation, consists only of energy. The other type, known as particle radiation or particulate radiation, consists of tiny bits of matter.

There are many sources of electromagnetic radiation. All materials that have been heated act as sources of such radiation. The sun produces electromagnetic radiation from nuclear reactions in its core. This energy heats the sun's outer layer until the hot gases glow, giving off light and other radiation. This solar radiation travels through space to the earth and other planets.

Particle radiation comes from radioactive substances. Radium, uranium, and many other heavy elements found in rocks and soil are naturally radioactive. In addition, scientists can create radioactive forms of any element in a laboratory by bombarding the element with atomic particles, the tiny bits of matter that make up atoms.

All life on earth depends on radiation, but some forms of radiation can be dangerous if not handled properly. X rays, for example, allow doctors to locate and diagnose hidden diseases. But X rays also can damage cells, causing them to become cancerous or die. Light from the sun enables plants to grow and warms the earth, but it also causes sunburn and skin cancer. Gamma radiation is used to treat disease by killing cancer cells, but it also can cause birth defects. Nuclear power plants produce electric energy, but the same facilities create radioactive waste that can kill living things.

In communication. All modern communication systems use forms of electromagnetic radiation. Variations in the intensity of the radiation represent changes in the sound, pictures, or other information being transmitted. For example, a human voice can be sent as a radio wave or microwave by making the wave vary to correspond to variations in the voice.

In science, researchers use radioactive atoms to determine the age of materials that were once part of a living organism. The age of such materials can be estimated by measuring the amount of radioactive carbon they contain in a process called radiocarbon dating. Environmental scientists use radioactive atoms known as tracer atoms to identify the pathways taken by pollutants through the environment.

Radiation is used to determine the composition of materials in a process called neutron activation analysis. In this process, scientists bombard a sample of a substance with particles called neutrons. Some of the atoms in the sample absorb neutrons and become radioactive. The scientists can identify the elements in the sample by studying the radiation given off.

In industry, radiation has many uses. Food processing plants employ low doses of radiation to kill bacteria on certain foods, thus preserving the food. Radiation is used to make plastics because it causes molecules to link together and harden. Industry also uses radiation to look for flaws in manufactured materials in a process called industrial radiography.

Nuclear power plants obtain energy from nuclear fission, the splitting of the nucleus of an atom into the nuclei of two lighter elements. Fission releases large amounts of radiation, including infrared radiation that is used to turn water into steam. This steam then runs a turbine that produces electric energy.

Nuclear fission releases several types of radiation, including neutrons, alpha and beta particles, gamma rays, and X rays. Fission involves using a neutron to split a nucleus of a heavy element, such as uranium, into two fission fragments. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book illustration by Mark Swindle.

The opposite process, nuclear fusion, occurs when the nuclei of two lighter elements join to form the nucleus of a heavier one. Fusion, like fission, releases vast amounts of radiation. Fusion creates the heat and light of the sun and other stars, and the explosive force of the hydrogen bomb. Scientists are learning how to harness fusion to produce electric energy.

Nuclear fusion releases large amounts of radiation. Fusion occurs when the nuclei of two lightweight elements join to form the nucleus of a heavier one. In the example shown here, nuclei of deuterium and tritium unite and form a helium nucleus. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book illustration by Mark Swindle.

In military operations, radio waves are used in radar systems to locate aircraft and ships. Microwaves and the light from lasers have been used both for communication and to guide "smart" missiles to their targets. Heat-sensing devices for night detection rely on the infrared radiation given off by living bodies.

To understand radiation and radioactivity, it is necessary to understand how an atom is constructed and how it can change. An atom consists of tiny particles of negative electric charge called electrons surrounding a heavy, positively charged nucleus. Opposite electric charges attract each other, and like charges repel (push away) each other. The positively charged nucleus therefore attracts the negatively charged electrons and so keeps them within the atom.

The nucleus of every element except the most common form of hydrogen consists of particles called protons and neutrons. (A normal hydrogen nucleus is made up of a single proton and no neutrons.) Protons carry a positive charge, and neutrons have no charge. The most common form of helium, for example, has two protons and two neutrons in the nucleus and two electrons outside the nucleus. Protons and neutrons consist of even smaller particles called quarks.

Within the nucleus, the positively charged protons repel one another because they have like charges. The protons and neutrons remain together in the nucleus only because an extremely powerful force holds them together. This force is called the strong nuclear force or the strong interaction. See Atom.

An atom can change the number of protons and neutrons in its nucleus by giving off or taking in atomic particles or bursts of energy--that is, by giving off or taking in radiation. But any change in the number of protons in the nucleus produces an atom of a different element. Radioactive atoms spontaneously release radiation to take on a more stable form. The process of giving off atomic particles is called radioactive decay. As radioactive elements decay, they change into different forms of the same element or into other elements until they finally become stable and nonradioactive.

Radioactive decay takes place at different rates in different elements or different forms of the same element. The rate of decay is measured by the half-life, the length of time needed for half the atoms in a sample to decay. For example, the half-life of cesium 137, a radioactive form of the metal cesium, is about 30 years. After about 60 years, approximately a fourth of the original cesium 137 remains. After another 30 years, only an eighth remains, and so on. The half-life of radon 222 is about 3.8 days. Half-lives vary from fractions of a second to billions of years.

Electromagnetic radiation moves through space as a wave, but it also has properties of particles. Atoms release electromagnetic radiation in the form of a tiny packet of energy called a photon. Like a particle, a photon occupies a fixed amount of space. Like waves, however, photons have a definite frequency and wavelength, which can be measured. The number of times each second that a wave passes through one cycle is called its frequency. The distance a wave travels in the time it takes to pass through one cycle is called the wavelength. The energy of a photon of electromagnetic radiation varies according to the frequency and wavelength. If the radiation has a high frequency and a short wavelength, its photons have high energy. If the radiation has a low frequency and a long wavelength, its photons have low energy.

In a vacuum, all electromagnetic radiation moves at the speed of light--186,282 miles (299,792 kilometers) per second. The various kinds of radiation differ, however, in their frequency and wavelength. They are classified according to an arrangement called the electromagnetic spectrum. In order of increasing wavelength, the kinds of electromagnetic radiation are gamma rays, X rays, ultraviolet rays, visible light, infrared (pronounced ihn fruh REHD) rays, microwaves, and radio waves. Gamma rays and X rays are high-energy forms of radiation. Radio waves, on the other end of the spectrum, have relatively low energy.

Scientists have discovered that protons, neutrons, and electrons, which we usually think of as particles, also behave like waves. These waves, called matter waves, have wavelengths. The faster a particle is moving, the shorter its wavelength. This means that particle radiation, like electromagnetic radiation, has characteristics of both particles and waves. There are four common types of particle radiation: (1) alpha particles, (2) beta particles, (3) protons, and (4) neutrons.

Alpha particles consist of two protons and two neutrons that act as one particle. When the nucleus of a radioactive atom emits an alpha particle, it thus loses two protons and two neutrons. Beta particles are high-speed electrons emitted from the nuclei of certain radioactive elements. Beta particles can be either negative or positive. When a nucleus emits a negatively charged beta particle, it also gives off an antineutrino. When a nucleus emits a positively charged beta particle, called a positron, it also gives off a neutrino. Gamma rays are particles of electromagnetic energy called photons. Gamma rays are released when a nucleus, after radioactive decay, is in a high-energy state. The rays travel at the speed of light. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book illustrations by Sarah Woodward.

Alpha particles consist of two protons and two neutrons and are identical with the nuclei of helium atoms. Alpha particles have a positive electric charge. The mass of an alpha is about 7,300 times larger than the mass of an electron. Alpha particles are given off by the nuclei of some radioactive atoms. Most alpha particles eventually gain two electrons and become atoms of helium gas.

Beta particles are electrons. Most beta particles are produced when a radioactive nucleus creates and releases an electron. In the process, a neutron in the nucleus changes into a proton and a beta is released.

Most beta particles are negatively charged, but some are positively charged particles called positrons produced when an atom changes a proton into a neutron. Positrons are a form of antimatter, matter which resembles ordinary matter except that its electric charge is reversed. When a positron collides with a negatively charged electron, the two particles destroy each other, and two or three gamma ray photons are produced. This collision is called pair annihilation (pronounced uh ny uh LAY shuhn).

Two other small particles, neutrinos and antineutrinos, accompany beta radiation. When a nucleus produces a positron, it also releases a neutrino, which has no charge and an undetermined mass. When a nucleus creates and releases a negatively charged beta particle, it also gives off an antineutrino, the antimatter form of a neutrino.

Protons and neutrons can also be released from some radioactive nuclei. Each has a mass about 1,850 times larger than the mass of an electron. The mass of a neutron is slightly larger than the mass of a proton. Neutron radiation is more common than proton radiation, which rarely is produced naturally on earth.

The sun and other stars give off both electromagnetic and particle radiation. This radiation results from the fusion of hydrogen nuclei in the star. The hydrogen changes into helium and releases a large amount of energy, producing electromagnetic radiation across the entire spectrum. Besides visible light, a star gives off everything from radio waves to high-energy gamma radiation. However, the gamma radiation, which is produced when new elements form deep in the core of the star, does not reach earth directly.

Stars also produce alpha and beta particles, protons, neutrons, and other forms of radiation. The high-energy particles released by stars are called cosmic rays. Even the sun puts on brief displays called solar flares, bathing the earth in cosmic rays strong enough to interfere with communications.

Naturally radioactive substances. Most naturally radioactive substances belong to one of three sequences of change called radioactive decay series: (1) the uranium series, (2) the thorium series, and (3) the actinium series. In each of these series, heavy isotopes (forms of the same element that have different numbers of neutrons) decay into various lighter isotopes by giving off radiation until they eventually become stable.

The uranium series begins with uranium 238, the heaviest isotope of uranium, which has 92 protons and 146 neutrons. After losing an alpha particle, which consists of 2 protons and 2 neutrons, the nucleus has 90 protons and 144 neutrons. It is no longer uranium but a radioactive isotope of thorium. Scientists call this process of changing into another element transmutation. The thorium, in turn, breaks down in several steps to radium 226. The radium 226 decays into radon, a naturally occurring radioactive gas. Radon may become a health hazard if it accumulates in certain buildings, especially poorly ventilated ones. The series continues until the isotope becomes a stable form of lead.

A radioactive decay series is the process by which a radioactive atom releases radiation and changes into different forms of the same element or into other elements. The uranium series shown here begins with uranium 238. Losing an alpha particle, the atom changes into radioactive thorium 234. The series continues through many more steps of decay until the atom becomes a stable form of lead. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book illustration by Mark Swindle.

The thorium series begins with thorium 232, an isotope of thorium. The actinium series begins with uranium 235, also called U-235, another isotope of uranium. These two series also end with lead.

A fourth group of naturally radioactive substances includes a wide variety of materials that do not belong to a radioactive series. Many of these elements, including carbon 14, potassium 40, and samarium 146, are produced by cosmic radiation striking the earth's atmosphere. Carbon 14 and potassium 40 are also present in the human body.

Artificial radioactive substances are made by human activities, such as the fission that takes place in nuclear weapons and nuclear reactors, or in laboratories. When fission splits a nucleus, it releases several types of radiation, including neutrons, gamma radiation, and beta particles. Fission also produces new radioactive atoms called fission products. For example, atomic bomb tests in the 1950's and 1960's covered the earth with a fission product called cesium 137, a radioactive isotope of cesium. Used fuel from nuclear power plants also contains many fission products, such as plutonium 239, strontium 90, and barium 140. This used fuel, called nuclear waste, remains radioactive and dangerous for thousands of years.

In addition, nuclear plants create new radioactive elements known as activation products. Activation products form when the pipes and other materials in a nuclear reactor absorb neutrons and other types of radiation, becoming radioactive.

Many other types of radiation are created by human activities. Physicists use powerful devices called particle accelerators to speed up the movement of electrically charged particles, including electrons, protons, and entire nuclei. The physicists then bombard stable, nonradioactive atoms with beams of these high-speed particles. The resulting collisions produce new radioactive atoms and help scientists learn more about the structure and properties of atoms.

Just as water always seeks its lowest possible level, electrons seek the state of lowest energy. When an electron shifts from an outer shell to one closer to the nucleus, the electron releases a packet of energy called a photon, which escapes from the atom. The energy of the photon equals the difference in energy between the original shell of the electron and the new one. If the energy difference is small, as in a light bulb, the atom will give off visible light, infrared radiation, or both. If the difference is large, the atom might produce X rays.

When a proton or neutron moves from one nuclear shell to another, the nucleus releases gamma radiation. Most atoms that release particle radiation in the course of radioactive decay also produce gamma radiation because their protons and neutrons are shifting into new shells. The radiation produced by nuclear reactions also results from protons, neutrons, and electrons moving to new shells. In nuclear fission, for example, the particles are moving to the shells of new nuclei created when a nucleus splits into two smaller nuclei.

Electromagnetic radiation also is produced if an electrically charged particle changes direction, speed, or both. A particle that enters an electric or magnetic field, for example, slows down and changes course. As a result, the particle releases radiation. X rays are produced whenever electrons suddenly decelerate, such as when they collide with atoms of metal to create the X rays in an X-ray machine. Electrons also produce X rays if they pass near a large nucleus. The negatively charged electrons are attracted by the positively charged nucleus. As the electrons change direction, they produce X rays called bremsstrahlung (pronounced BREHM shtrah lung), a German word that means braking radiation.

Excitation and ionization also affect living tissues. The body's cells contain molecules, many of which are held together by electrons. When radiation excites or ionizes the molecules in cells, chemical bonds may be broken and the shape of a molecule may be changed. These changes disrupt the normal chemical processes of the cells, causing the cells to become abnormal or die.

If radiation affects molecules of DNA (deoxyribonucleic acid), the hereditary material in living cells, it may cause a permanent change called a mutation. In rare cases, mutations caused by radiation may pass on undesirable traits to offspring. Even low-energy photons, particularly ultraviolet light from the sun, may produce damage by excitation. If the injury is severe, the cell becomes cancerous or dies while trying to divide. The effect produced depends on the radiation's ionizing ability, the dose received, and the type of tissue involved.

Ionizing ability. Radiation may be classified as ionizing or non-ionizing. Ionizing radiation is the most dangerous. Some types of ionizing radiation have enough energy to directly strip electrons from any atoms near their path. Such radiation includes alpha and beta particles and protons. Other types of ionizing radiation, including X rays, gamma radiation, and neutron radiation, must first transfer energy to an atom. The added energy then causes the atom to lose an electron.

Non-ionizing radiation consists of photons with too little energy to cause ionization. Radio waves, microwaves, infrared radiation, and visible light are all non-ionizing radiation. Each will cause only excitation.

Dose. Scientists use two systems for measuring the amount, or dose, of radiation absorbed by a substance. The older system, still commonly used, measures doses in units called rads. Rad stands for radiation absorbed dose. One rad is produced when 1 gram of material absorbs 100 ergs. (An erg is an extremely small unit of energy.) The newer system, introduced in 1975, measures dosage in units called grays, named after Louis H. Gray, a British radiation biologist. One gray is equal to 100 rads or 1 joule per kilogram of material. A joule is a unit of energy equal to 10 million ergs. A typical dental X ray, for example, exposes the patient to about 0.25 rad (0.0025 gray).

Different types of radiation produce different effects at the same dose. To account for this, scientists have developed the quality factor. The quality factor indicates how much the radiation damages living tissue compared with an equal dose of X rays. For example, a dose of alpha particles causes about 10 times as much damage as the same dose of X rays, so alpha particles have a quality factor of 10. X rays, gamma radiation, and beta particles have a factor of 1. Neutrons range from 2 to 11.

Multiplying the dose by the quality factor gives a measure of damage called the dose equivalent. If the dose is given in rads, the dose equivalent will be in rems. A rem, which stands for roentgen equivalent in man, is the amount of radiation necessary to cause the same effect on a human being as 1 rad of X rays. If the dose is reported in grays, the dose equivalent will be in sieverts, named for Swedish radiologist Rolf M. Sievert. Grays and sieverts are part of the metric system of measurement, officially called the Systeme International d'Unites (International System of Units).

Large doses cause a combination of effects called radiation sickness. Doses above 100 rems damage red and white blood cells. This damage is known as the hematopoietic effect. At doses above 300 rems, death may follow in several weeks. Above 1,000 rems, the cells lining the digestive tract die and bacteria from the intestines invade the bloodstream. This effect, known as the gastrointestinal effect, may lead to death from infection within a week. At doses of several thousand rems, the brain is injured and death can come within hours.

Deaths from radiation sickness are extremely rare. People have only suffered such large doses in reactor accidents, in a few cases where radioactive material was mishandled, and in the 1945 bombings of Hiroshima and Nagasaki, Japan, during World War II. The worst reactor accident in history was a 1986 explosion and fire at the Chernobyl nuclear power plant in Ukraine, then part of the Soviet Union. Thirty-one workers died.

Small doses. The doses received in daily life, sometimes called background doses, are much smaller. Some scientists believe that the average background dose is 0.3 to 0.4 rem per year. About half of this amount comes from breathing radon gas released by radioactive rocks and soil. Medical and dental X rays add another 0.04 rem per year. Other sources, such as nuclear power plants and waste disposal sites, typically account for less than 0.01 rem per year. Smokers take in much higher doses from radioactive isotopes in smoke.

An accumulation of small doses of radiation increases the risk of developing a condition, but not the severity of the condition. The chief effects of repeated small doses of radiation are cancer and birth defects.

To protect people from the effects of radiation, the International Commission on Radiological Protection, a panel of experts from many countries, sets guidelines for exposure. This group recommends that nuclear workers receive a maximum permissible dose (MPD) of no more than 5 rems per year. The commission also urges that the general public receive no more than 0.5 rem in any year. Other agencies, including the National Council on Radiation Protection and Measurements in the United States and the Atomic Energy Control Board in Canada, set similar guidelines.

Robert Grosseteste, an English bishop and scholar of the 1200's, thought of light as the root of all knowledge. He believed that understanding the laws controlling light would uncover all the laws of nature.

The composition of light was debated in the 1600's by the followers of the English scientist Sir Isaac Newton and the Dutch physicist Christiaan Huygens. Newton insisted that light consisted of tiny particles, while Huygens suggested it was composed of waves. Scientists argued about these two theories for more than 100 years. Then, in the early 1800's, the British physicist Thomas Young showed that light had properties similar to those of sound and water waves. A few years later, the French physicist Augustin Fresnel provided more evidence. By 1850, most scientists accepted Young's and Fresnel's findings as proof of the wave nature of light.

In 1864, the British scientist James Clerk Maxwell suggested that light consisted of electromagnetic waves. Maxwell also predicted that other, invisible forms of electromagnetic radiation would be discovered. Maxwell's predictions came true with the work of two German physicists, Heinrich R. Hertz and Wilhelm C. Roentgen. Hertz discovered radio waves in the late 1880's, and Roentgen discovered X rays in 1895.

Discovery of radioactivity. In 1896, the French physicist Antoine Henri Becquerel discovered that crystals of a uranium compound would darken photographic plates even if the plates were not exposed to light. He proposed that uranium gave off energy in the form of radiation. Later experiments by the British physicist Ernest Rutherford showed that this radiation consisted of particles he named alphas and betas.

In 1898, the French physicists Marie and Pierre Curie found other substances that produced radiation, naming them polonium and radium. A few years later, Rutherford showed that radioactive substances could change into new elements in the process of transmutation.

The work of Rutherford and the Curies led to great interest in the structure of the atom. Rutherford, his colleagues, and other scientists soon proved that the atom had a nucleus of high mass and positive electric charge surrounded by negatively charged electrons.

The quantum theory. In 1900, the German physicist Max Planck studied radiation from hot objects. He suggested that objects could only emit and absorb this radiation in packets of energy called quanta, a name later changed to photons. Another German physicist, Albert Einstein, used Planck's theory in 1905 to explain a phenomenon known as the photoelectric effect. Earlier scientists had discovered this effect, in which a bright beam of light striking a metal causes the metal to release electrons. Einstein proposed that the energy supplied by a single photon could free an electron from an atom in the metal. To produce the photoelectric effect, photons act in a localized manner characteristic of particles rather than waves. Thus, Einstein's ideas revived the particle theory of light. Scientists now know that radiation has features of both particles and waves.

The Danish physicist Niels Bohr used the quantum theory in 1913 to explain the structure of the hydrogen atom. Bohr proposed that electrons can have only certain values of energy. He showed that atoms release photons of radiation when their electrons drop from a high-energy level to a lower one. In 1924, the French physicist Louis de Broglie predicted that electrons themselves might act as waves, called matter waves.

The nuclear age began in 1942, when Italian-born physicist Enrico Fermi and his co-workers at the University of Chicago produced the first artificial nuclear chain reaction. Since then, many scientists have turned their attention from understanding what causes radioactivity and radiation to finding uses for them. Nuclear weapons based on fission--the atomic bomb--and fusion--the hydrogen bomb--were developed. The first full-scale nuclear power plant began operation in 1956. Radiation from across the entire electromagnetic spectrum was harnessed for communication, medicine, industry, and research.

Contributor: Douglas John Crawford-Brown, Ph.D., Prof., Department of Environmental Sciences and Engineering, Univ. of North Carolina at Chapel Hill.

Related articles include:

Atom; Electromagnetic Waves; Energy; Fluorescence; Infrared Rays; Ion; Light; Luminescence; Phosphorescence; Photon; Quantum Mechanics; Radio; Sun; Ultraviolet Rays.

How does ionizing radiation damage living cells?

What is the final product of uranium decay?

How are positive ions produced?

What are some natural sources of radiation?

How do physicists create radioactive forms of elements?

Who first suggested that radiation came in packets of energy called quanta or photons?

Why do atoms give off gamma radiation?

What are the chief health risks caused by repeated low doses of radiation?

What is the difference between electromagnetic radiation and particle radiation?

What are the chief types of electromagnetic radiation?

Lillie, David W. Our Radiant World. Ia. State Univ. Pr., 1986.

Murphy, Wendy and Jack. Nuclear Medicine. Chelsea Hse., 1994.

Pringle, Laurence. Radiation. Enslow, 1983.

Tucker, Wallace, and Giacconi, Riccardo. The X-Ray Universe. Harvard Univ. Pr., 1985.

Wolfson, Richard. Nuclear Choices: A Citizen's Guide to Nuclear Technology. Rev. ed. MIT Pr., 1993.

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