Radiation is energy given off in the form of waves or tiny particles of matter. Radiation is found throughout the universe. It comes in many forms. Most people have heard of X rays, gamma rays, and radiation from nuclear reactors. These kinds of radiation are often mentioned as possible health hazards. X rays and gamma rays also have valuable uses in scientific research. But there are many other forms of radiation as well. The most familiar is probably the light we see, for example from the sun or a flashlight. The sun’s ultraviolet rays, which cause suntan and sunburn, are another form of radiation. Other examples include heat from a fireplace, the signals that carry radio and television broadcasts, the intense light from a laser, and the microwaves used to cook food.
Radiation occurs 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 is called electromagnetic radiation. It consists only of energy. The other type is known as particle radiation or particulate radiation. It consists of tiny bits of matter.
There are many sources of electromagnetic radiation. All materials that have been heated give off 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 Earth and other planets.
Particle radiation comes from radioactive elements. 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. They do this 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. Doctors use X rays, for example, 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 warm Earth. But sunlight also causes sunburn and skin cancer. Doctors use gamma radiation to treat disease by killing cancer cells. But gamma rays are also thought to cause birth defects. Nuclear power plants produce electric energy. But the same facilities create radioactive waste that can kill living things if not properly handled.
Uses of radiation
In medicine,
radiation and radioactive substances are used for diagnosis, treatment, and research. X rays, for example, pass through muscles and other soft tissue. However, they are stopped by dense materials. This property enables doctors to use X rays to find broken bones and to locate cancers in the body. Doctors also look for certain diseases by injecting a radioactive substance. To find the disease, they monitor the radiation given off as the substance moves through the body.
In communication.
Communication systems use forms of electromagnetic radiation to carry information. Variations in the radiation represent changes in the sound, pictures, or other information being sent. 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 thing. The age of such materials can be estimated by measuring the amount of radioactive carbon they contain. This process is 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 use doses of radiation to kill bacteria on certain foods, thus preserving the food. Radiation is used to make plastics because it can cause molecules to link together and harden. Industry also uses radiation to look for flaws in manufactured materials, a process called industrial radiography.
Nuclear power plants obtain energy from nuclear fission. Nuclear fission is the splitting of the nucleus of an atom into the nuclei of two lighter elements. Fission releases large amounts of radiation, producing heat. The heat is used to turn water into steam. This steam powers a turbine that produces electric energy.
The opposite process is called nuclear fusion. It 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. It also creates the explosive force of the hydrogen bomb. Scientists are working to harness fusion to produce electric energy.
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—that is, the heat—given off by living bodies.
Radiation and radioactivity
Scientists distinguish radiation from radioactivity, a property of some types of matter. Radioactivity results from changes in the nuclei of the atoms that make up such matter. These changes cause the matter to release certain forms of radiation.
To understand radiation and radioactivity, it helps to know the structure of an atomic nucleus. The nucleus of the most common isotope (form) of hydrogen consists of a single proton. A proton is a particle with a positive electric charge. All other nuclei consist of at least one proton and one neutron. A neutron is a particle with no charge. The most common form of helium, for example, has two protons and two neutrons in the nucleus.
In certain circumstances, an atom can change the number of protons and neutrons in its nucleus. This change is accomplished by giving off or taking in atomic particles or bursts of energy—that is, by giving off or taking in radiation. 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 isotopes 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 and isotopes. The rate of decay is measured by the half-life. The half-life is the length of time taken for half the atoms in a sample to decay. For example, the half-life of cesium 137, a radioactive isotope of cesium, is about 30 years. This fact means that after about 30 years, only half the atoms in a sample of cesium 137 will remain undecayed. After about 60 years, about 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
Electromagnetic radiation consists of electric and magnetic energy. Every electrically charged body is surrounded by an electric field. This field is the region where the body’s electric force can be felt. Every magnetic body is surrounded by a similar region known as a magnetic field. An electric current or a changing electric field creates a magnetic field. Likewise, a changing magnetic field creates an electric field. Electric and magnetic fields act together to produce electromagnetic radiation.
Electromagnetic radiation moves through space as a wave. However, 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. A wave’s frequency is the number of times each second that the wave passes through one cycle. The wavelength is the distance the wave travels in the time it takes to pass through one cycle.
The energy of a photon 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 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.
Particle radiation
Particle radiation includes protons, neutrons, and negatively charged electrons. These three types of tiny particles serve as the building blocks of an atom. All types of particle radiation have both mass and energy. Most such radiation travels at high speeds but slower than the speed of light.
We usually think of protons, neutrons, and electrons as particles. But scientists have discovered that they also behave like waves. These waves, called matter waves, have wavelengths. The faster a particle is moving, the shorter its wavelength. Thus, 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. Another particle, the neutrino, is also common, but it is so tiny that it barely interacts with other forms of matter.
Alpha particles
consist of two protons and two neutrons. They are identical with the nuclei of helium atoms. Alpha particles have a positive electric charge. The mass of an alpha particle is about 7,300 times that of an electron. Alpha particles are given off by the nuclei of some radioactive atoms. Most alpha particles eventually gain two electrons to become atoms of helium gas.
Beta particles
are generally electrons. Most beta particles form when a radioactive nucleus creates and releases an electron. In the process, a neutron in the nucleus changes into a proton and a beta particle is released.
Most beta particles are negatively charged. But some are positively charged particles called positrons. Positrons are produced when an atom changes a proton into a neutron. Positrons are a form of antimatter. Antimatter is a type of matter that resembles ordinary matter but with electric charge or certain other properties reversed. When a positron collides with a negatively charged electron, the two particles destroy each other. The collision produces two or three gamma ray photons. This process is called pair annihilation.
Two other small particles, neutrinos and antineutrinos, accompany beta radiation. When a nucleus produces a positron, it also releases a neutrino. When a nucleus releases a negatively charged beta particle, it also gives off an antineutrino, the antimatter equivalent of a neutrino.
Protons and neutrons
can also be released from some radioactive nuclei. Each of these particles has a mass about 1,850 times that 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 occurs naturally on Earth.
Sources of radiation
Natural sources of radiation include the sun, other stars, and naturally radioactive elements. There are also many artificial sources of radiation.
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, releasing a large amount of energy. This process produces electromagnetic radiation across the entire spectrum. Besides visible light, a star gives off everything from radio waves to high-energy gamma radiation. The gamma radiation, however, is produced when new elements form deep in the core of the star. It 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. The sun puts on brief displays called solar flares, bathing Earth in cosmic rays. These rays are strong enough to interfere with communications and electrical service lines.
Naturally radioactive substances.
Many elements have isotopes that are naturally radioactive. Most naturally radioactive substances belong to one of three sequences of change called radioactive decay series. They are: (1) the uranium series, (2) the thorium series, and (3) the actinium series. In each of these series, heavy isotopes decay into various lighter isotopes by giving off radiation until they eventually become stable. Different isotopes of an element have the same number of protons in their nuclei but a different number of neutrons.
The uranium series begins with uranium 238, the heaviest isotope of uranium. It 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.
The thorium series begins with thorium 232, an isotope of thorium. The actinium series begins with uranium 235, another isotope of uranium. These two series also end with lead.
A fourth group of naturally radioactive substances includes a variety of materials that do not belong to a radioactive series. Cosmic radiation striking Earth’s atmosphere makes many of these elements. Such elements include carbon 14, potassium 40, and samarium 146. 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 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, non-radioactive atoms with beams of these high-speed particles. The resulting collisions produce new radioactive atoms. Such experiments help scientists learn more about the structure and properties of atoms.
Causes of radiation
Within an atom, electrons are confined to regions called electron shells. The shells lie at various distances from the nucleus, according to how much energy the electrons have. Electrons with less energy travel in inner shells. Electrons with more energy are in outer shells. Protons and neutrons in the nucleus are also arranged according to their energy levels in layers known as nuclear shells. All the protons, neutrons, or electrons in a shell have almost the same amount of energy.
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. This packet, a photon, escapes from the atom. The energy of the photon equals the difference in energy between the electron’s original shell and the new one. If the energy difference is small, the atom will give off visible light, infrared radiation, or both. Light bulbs produce light in this way. If the energy 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. This happens 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 slow down. For example, electrons give off X rays when they collide with atoms of metal inside 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 << BREHM shtrah lung >> . Bremsstrahlung is a German word that means braking radiation.
Effects of radiation
Radiation produces two main effects in atoms or molecules: (1) excitation and (2) ionization. In excitation, an atom or molecule absorbs energy from radiation, moving its electrons to higher-energy shells. In most cases, the excited atom can hold the extra energy for only a fraction of a second. Then it releases the energy as a photon, falling back to a state of lower energy. In ionization, the radiation transfers enough energy to the electrons that they leave the atom. Atoms that have lost electrons become positively charged particles called positive ions. The electrons may then join other atoms.
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, chemical bonds may be broken and the shape of a molecule change. These changes can disrupt normal chemical processes in cells, causing the cells to become abnormal or die.
The hereditary material in living cells is in the form of a molecule called DNA (deoxyribonucleic acid). If radiation affects DNA, 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 damage cells by excitation. If the damage is severe, the cell may become cancerous or die while trying to divide. The damage 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 must first transfer energy to an atom. These types include X rays, gamma radiation, and neutron radiation. 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. The older system, still sometimes 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 and more commonly used system was introduced in 1975. It measures dosage in units called grays, named after the British scientist Louis H. Gray. One gray equals 100 rads or 1 joule per kilogram of material. A joule is a unit of energy. It equals 10 million ergs.
Different types of radiation produce different effects at the same dose. To account for this, scientists have developed a measure called the radiation weighting factor. This factor indicates how much the radiation damages living tissue compared with an equal dose of gamma rays or X rays. For example, a dose of alpha particles causes about 20 times as much damage as the same dose of X rays. Thus, alpha particles have a radiation weighting factor of 10. X rays, gamma radiation, and beta particles have a radiation weighting factor of 1. Neutrons range from 2 to 20, depending on their energy.
Multiplying the dose by the radiation weighting factor gives a measure of damage called the equivalent dose. If the dose is given in rads, the dose equivalent will be in rems. Rem stands for roentgen equivalent in man. One rem is the amount of radiation that causes the same effect on a human being as 1 rad of gamma rays or X rays. If the dose is in grays, the equivalent dose will be in sieverts. Sieverts are named for the Swedish scientist Rolf M. Sievert. One sievert equals 100 rems. Grays and sieverts are part of the metric system of measurement.
Large single doses
cause a combination of effects called radiation sickness. Doses above roughly 0.70 sievert damage the parts of the body that produce blood cells. Death can result from infections and hemorrhaging within a few months to a few weeks depending on the dose. At doses above about 6 sievert, the cells lining the digestive tract die and bacteria in the intestines invades the bloodstream. Death often occurs within a few weeks. At doses of tens of sieverts, the circulatory system collapses and the brain is severely injured leading to death in as little as a few days.
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 nuclear bombings of Hiroshima and Nagasaki, Japan. The worst reactor accident in history was a 1986 explosion and fire at the Chernobyl (now Chornobyl) nuclear power plant in Ukraine, then part of the Soviet Union. Thirty-one workers died in the accident.
Small single doses.
The doses typically encountered in daily life are much smaller. Many scientists believe that the average person is exposed to about 0.003 to 0.004 sievert of radiation 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 0.0004 to 0.0008 sievert per year on average. Other sources, such as nuclear power plants and waste disposal sites, typically account for less than 0.0001 sievert 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. However, it does not increase the severity of any resulting condition. The chief risks of repeated small doses of radiation are cancer and birth defects. Extremely small doses do not always produce clear evidence of harmful effects. Some researchers believe that continual exposure to low doses of ionizing radiation are beneficial. They think that such exposure may stimulate repair mechanisms within the cell, protecting against disease. This idea is called radiation hormesis.
The International Commission on Radiological Protection (ICRP), a panel of experts from many countries, sets guidelines to protect people from the effects of radiation. It recommends a maximum permissible dose (MPD) for radiation workers and the general public. The MPD states the maximum amount of radiation a person should be exposed to over a one-year and a five-year period. Other agencies set similar guidelines, including the National Council on Radiation Protection and Measurements in the United States and the Canadian Nuclear Safety Commission in Canada.
History
Scientists have studied radiation since ancient times. In the 300’s and 200’s B.C., the Greek philosopher Epicurus wrote of particles “streaming off” the surface of bodies. Euclid, a Greek mathematician of the same time, thought the eye sent out radiation to enable an object to be seen.
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.
Electromagnetic waves.
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. Huygens, on the other hand, 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 Scottish physicist 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 New Zealand-born 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 another substance that produced radiation. They named it polonium. Later that year, with the French chemist Gustave Bémont, they discovered an additional substance that gave off radiation. They named it 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 (give off) and absorb this radiation in packets of energy called quanta. The name quanta was 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. They found that 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 the Italian-born physicist Enrico Fermi and 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.
Refining the uses of radiation
has benefited many scientific fields. For example, researchers have refined the use of radiation in cancer therapy. Early radiation therapies exposed patients to radiation that was less controlled, sometimes damaging healthy parts of the body. Newer methods of radiation therapy, including intensity modulated radiation therapy (IMRT), can shape the beam of radiation to target the area of the body that shows cancer. In contrast, doctors increasingly use total body irradiation, which exposes the entire body to a low dose of radiation, to prepare patients for bone marrow or stem cell transplants. Medical research has also increased its use of radioactive tracers. Such tracers are radioactive atoms that enable researchers to track where molecules travel in the body. Radiation from across the electromagnetic spectrum has also been developed and refined for use in communication, industry, and research.
