Introduction to Radiation
Radiation, the emission and transmission of energy and particles of matter from atoms. Electromagnetic radiation consists entirely of energy; nuclear radiation or particle radiation consists of (1) energy and (2) various particles of matter. Visible light is an example of electromagnetic radiation. The radiation produced by radium and other radioactive elements is an example of nuclear radiation.
Radiation is found all over the universe, though in various forms such as x-rays, gamma rays, and radiation from nuclear reactors. Radiation happens whenever energy moves from one place to another. Atoms and molecules radiate to give off excess energy.
The energy of radiation is called radiant energy. (Other forms of energy are chemical, mechanical, and nuclear.) Some forms of radiant energy, such as the heat produced by infrared radiation, are evident to the senses. Special instruments are needed to detect other forms, such as that of radio waves.
Energy radiated from the sun warms Earth and provides the energy that supports life on Earth. When plants make food by photosynthesis, they convert light energy into chemical energy, which is essential to almost every kind of living thing. The same energy is locked in wood, coal, and petroleum, from which it is released by combustion to do work.
Causes of Radiation
When a proton or neutron shifts between cells within the nucleus, it releases gamma radiation. Atoms release particle radiation during radioactive decay; most emit gamma radiation as well, as their protons and neutrons shift between shells. During nuclear reactions, radiation is emitted as protons, neutrons, and electrons shift between shells; for example, in nuclear fission, when a nucleus splits into two, particles move to the shells of the new nuclei.
Electromagnetic radiation is released when an electrically charged particle changes direction, or speed, or both. A particle that enters an electric or magnetic field, for example, slows down and shifts direction and, therefore, emits radiation. Whenever electrons lose speed suddenly, such as when in an x-ray machine they collide with metal atoms, x-rays form. X-rays form when electrons pass a large nucleus as well. The nucleus, which has a positive charge, draws electrons, which have a negative charge. As the electrons change direction, they produce x-rays called bremsstrahlung, a German word that means braking radiation.
Depending on the energy electrons have, they may be found at varying distances from the nucleus, in regions called electron shells. Electrons with little energy are found in inner shells, and those with high energy are found in outer shells. In the nucleus, protons and neutrons are arranged in layers, called nuclear shells according to their energy levels. All the particles in a shellprotons, neutrons, and electronshave almost identical quantities of energy.
Electrons seek the state of lowest energy. When an electron shifts from an outer shell to one closer to the nucleus, the electron discharges energy in the form of a photon, which moves away from the atom. The difference in energy of the original shell of the electron and its new shell is the photons energy. If the difference is small the atom will emit visible light, infrared radiation, or both. If the difference is large, the atom will emit x-rays.
Types of Radiation
There are two chief types of radiation. One is electromagnetic radiation consisting of energy. The other type is known as particle radiation or particulate radiation consisting of tiny particles of matter.
Electromagnetic radiation travels in waves called electromagnetic waves. These waves carry energy in the form of oscillating (pulsating) electric and magnetic fields. (In physics, a field is the region through which a given force is effective.) Every electrically charged body is surrounded by an electric field. An electric field is a region where the bodys electric force can be found. In the same way, every magnetic body is surrounded by its magnetic field. The lines of force of the magnetic and electric fields are at right angles to each other. The two fields together form an electromagnetic field.
Electromagnetic waves are transverse waves, the oscillations of the electromagnetic field being at right angles to the direction of travel of the waves. Each complete oscillation of the electromagnetic field is called a cycle. The number of cycles that occur in a given time is called the frequency of the electromagnetic wave. The wavelength can be determined by dividing the speed of the wave by the wave's frequency.
Though electromagnetic radiation travels as a wave through waves, it also has the properties of particles. Atoms release electromagnet radiation in the form of photons. A photon is a tiny packet of energy, which, like particles, occupies a fixed amount of space. However, like waves, photons too have a definite frequency and wavelength. The energy of a photon depends on its frequency and wavelength. When 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. There are many sources of electromagnetic radiation. Materials that are being heated act as sources of electromagnetic radiation. The sun produces electromagnetic radiation from nuclear reactions in its core.
Unlike other waves, electromagnetic waves can travel through a vacuum. In a vacuum, the waves move at a speed of about 186,300 miles (299,800 km) per second. In passing through matter, the speed of the waves is reduced. For example, in water the speed of light (a kind of electromagnetic radiation) is only about three-fourths as great as its speed in a vacuum. Different kinds of radiation differ in their frequency and wavelength. The electromagnetic spectrum can be seen when white light passes through a prism and splits into all the visible colors, ranging from red to violet. The colors are formed by different wavelengths of light. They make up the visible spectrum, the part of the electromagnetic spectrum that can be seen. The electromagnetic spectrum is made up of wavelengths of all sizes, including those too long and too short to be seen.
Each kind of electromagnetic radiation occupies a band, or range of wavelengths, of the electromagnetic spectrum. There are no sharp divisions between the bands. The wavelengths of each band blend into those of adjacent bands. The approximate range of wavelengths of each band, in centimeters (a centimeter is 0.3937 inch), is as follows:
|Radio Waves||3,000,000 to .03|
|Infrared Waves||.03 to .000076|
|Visible Light||.000076 to .00004|
|Ultraviolet Waves||.00004 to .0000004|
|X Rays||.0000004 to .000000001|
|Gamma Rays||.000000001 to .000000000056|
Particle radiation consists of protons, neutrons, and electrons, which are the building blocks of an atom. It has both mass and energy. Most of the types of particle radiation travel at high speeds but they are slower than the speed of light. However, there is a type of particle called neutrino, which travels almost at the speed of light. Its mass is also undetermined. According to scientists, protons, neutrons, and electrons also behave like waves. These waves are called matter waves. Like electromagnet radiation, particle radiation too has characteristics of both particles and waves. There are four types of particle radiation—i) alpha particles, ii) beta particles, iii) protons, and iv) neutrons.
Alpha particles consist of two protons and two neutrons and have positive electric charge. These particles are identical with the nuclei of helium atoms. The mass of an alpha particle is about 7,300 times as large as the mass of an electron. Some radioactive atoms give off alpha particles.
Beta particles are electrons, which 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. However, some beta particles are positively charged. These are called positrons. Positrons are a form of antimatter, matter that resembles ordinary matter except that its electric charge is reversed. Positrons are produced when an atom changes a proton into a neutron. When a positron collides with a negatively charged electron, both of them destroy each other. As a result, two or three gamma ray photons are produced. This type of collision is known as pair annihilation.
Neutrinos and antineutrinos, two other particles, accompany beta radiation. A neutrino is a particle with no charge and an undetermined mass, which is released when a nucleus produces a positron. When a nucleus produces a negatively charged beta particle, it also releases an antineutrino, which is the antimatter form of a neutrino.
Protons and neutrons can also be discharged from some radioactive nuclei. Each has a mass about 1,850 times greater than the mass of an electron. The mass of a neutron is slightly greater than the mass of a proton. Neutron radiation is more common than proton radiation, which rarely happens naturally on Earth.
Sources of Radiation
There are two types of sources of radiationnatural and artificial.
Natural sources of radiation are the sun and other stars and naturally radioactive elements. The sun and other stars release both electromagnetic and particle radiation.
Radiation from the stars results from the fusion of hydrogen nuclei in the stars. The hydrogen changes into helium and gives off a large amount of energy. This produces electromagnetic radiation across the entire spectrum. Besides visible light, a star releases everything from radio waves to high-energy gamma radiation.
Stars also produce alpha and beta particles, protons, neutrons, and other forms of particle radiation. They release high-energy particles called cosmic rays. Solar flare is a phenomenon where the sun releases cosmic rays strong enough to disrupt communications on Earth.
The sources of particle radiation are naturally radioactive substances such as radium, uranium, and many other heavy elements that are found in rocks and soil. However, scientists are capable of creating radioactive forms of any element in a laboratory.
Most naturally radioactive substances belong to one of three sequences of change, which are called radioactive decay seriesi) the uranium series, ii) the thorium series, and iii) the actinium series. Heavy isotopes which are forms of the same element but have different numbers of neutrons decay into various lighter isotopes in each of these series by giving off radiation till they become stable.
The uranium series starts with uranium 238. It is the heaviest isotope of uranium. It has 92 protons and 146 neutrons. After losing an alpha particle, the nucleus has 90 protons and 144 neutrons. It no longer remains uranium, but becomes a radioactive isotope of thorium. This process of changing from one into another element is called transmutation. The thorium breaks down in several steps to radium 226. The radium 226 again decays into radon. Radon is a naturally occurring radioactive gas. Radon might turn hazardous if it accumulates in certain buildings, especially where ventilation is poor. The series continues till the isotope becomes a stable form of lead.
The thorium series starts with thorium 232, an isotope of thorium. It also ends with lead.
The actinium series begins with uranium 235, which is another isotope of uranium. It is also called U-235. Like the other two series, the actinium series continues until the isotope becomes a stable form of lead.
There is another group of naturally radioactive substances, which includes a wide variety of materials that do not belong to a radioactive series. Many of these elements, such as carbon 14, potassium 40, and samarium 146, are produced when cosmic radiation strikes Earth's atmosphere.
Artificial radioactive substances are formed by human activities, such as the fission that takes place in nuclear weapons and nuclear reactors, or in laboratories. When fission breaks up a nucleus, it releases several types of radiation, including neutrons, gamma radiation, and beta particles. The process also creates new radioactive atoms called fission products. In the 1950s and 1960s, atomic bomb tests produced a new fission product called cesium 137, which was a radioactive isotope of cesium. Nuclear plants also create new radioactive elements. These elements are known as activation products. The pipes and other materials in a nuclear reactor absorb neutrons and other types of radiation and become radioactive. These are then called activation products. Used fuel from nuclear plants also contains fission products like plutonium 239, strontium 90, and barium 140. This used fuel, called nuclear waste, remains radioactive and potentially harmful for many years.
Sometimes physicists use powerful devices to speed up the movement of electrically charged particles that include the entire nuclei. Then physicists shoot stable, non-radioactive atoms with beams of these high-speed particles. This results in collisions producing new radioactive atoms.
Uses of Radiation
All types of modern communication systems use different forms of electromagnet radiation. Variations in the intensity of the radiation depict changes in the sound, pictures, or other elements in the information being transmitted.
In medicines, radiation and radioactive substances are used in diagnosis, treatment, and also research. Doctors use x-rays to locate broken bones and cancer in a body. Sometimes to detect certain diseases, doctors inject a radioactive substance into the body and monitor the radiation given off as the substance moves through the body.
Researchers use radioactive atoms to determine the age of materials that were once part of a living organism. They measure the amount of radioactive carbon the materials contain to identify the age of the materials. This process is called radiocarbon dating.
Scientists also use radiation to determine the composition of materials. The process is known as neutron activation analysis. Here scientists take a sample of a substance. They then bombard the sample with neutrons. Some of the atoms in the sample absorb neutrons and turn radioactive. This helps the scientists to identify the elements in the sample by studying the radiation given off.
Environmental scientists use radioactive atoms to identify the paths different pollutants take through the environment. These atoms are known as tracer atoms.
Radio waves come to use in military operations. Radar systems use radio waves to locate aircraft and ships. Microwaves and the light from lasers are used for communication and also to guide missiles to their targets. During night detection heat-sensing devices depend on the infrared radiation that is given off by living bodies.
Radiation is used in food processing plants to kill bacteria on certain foods. It is also used in plastics creation. In industry, radiation is used to identify flaws in manufactured materials. This process is called industrial radiography.
Nuclear fission releases infrared radiation, which is used to turn water into steam. This steam, in turn, runs a turbine that produces electric energy.
Nuclear fusion, which occurs when the nuclei of two lighter elements join to form the nucleus of a heavier one, releases vast amounts of radiation. Fusion creates the explosive force of the hydrogen bomb. Scientists are trying to utilize fusion to produce electric energy.
Effects of Radiation
Radiation affects atoms and molecules, and living tissues as well. It has two main effects on atoms and moleculesexcitation and ionization. Excitation is a process in which radiation energizes an atom or molecule so that its electrons move to higher-energy shells. Usually, the excited atom retains this energy for only a fraction of a second, releases it in the form of a photon, and reverts to its initial level of energy. Ionization is a process in which radiation energizes an atom or molecule so that its electrons leave the atom and move through space. Atoms that lose electrons become positive ionsparticles that have a positive charge. The electrons may then join other atoms.
Radiation may be ionizing or non-ionizing. Ionizing radiation is radiation of two typesthe type that is powerful enough to take away electrons directly from atoms on and around their path, and the type that has to first transfer energy to an atom. The first type includes alpha and beta particles and protons, and is more dangerous than the second type, which includes x-rays, gamma radiation, and neutron radiation. Non-ionizing radiation has photons too weak to ionize particles. It includes radio waves, microwaves, infrared radiation, and visible light. Each of these causes excitation only.
Excitation and ionization also affect living tissues. Electrons bind many of the molecules in the body's cells. When these molecules are excited or ionized by radiation, chemical bonds may break and molecules may change shape, upsetting intracellular chemical processes and consequently destroying or distorting cells. Mutation, which is a permanent change in physical characteristics, may occur if radiation affects molecules of deoxyribonucleic acid (DNA), the transmissible material in living cells. If radiation causes mutation, disagreeable characteristics may be passed on to progeny in rare cases. Excitation caused by low-energy photons, especially ultraviolet light from the sun, may cause damage. If the damage is critical, the cell develops cancer or dies while trying to divide. The degree of damage depends on the ionizing ability of the radiation, the dose, and the type of tissue. Birth defects, cancer, and death are the chief effects of radiation.
The quantity of radiation taken in by a substance is referred to as the dose of radiation. There are two systems used to measure dosage. The older system uses a unit called rad (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, uses a unit called gray. It is 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.
The effect of a dose of radiation depends on its type. This is measured by the quality factor, which indicates how much the radiation damages living tissue compared with an equal dose of x-rays. The quality factor of alpha particles is 10; that means a dose of alpha particles damages living tissue around 10 times as much as x-rays. The quality factor of x-rays, gamma radiation, and beta particles is 1, and that of neutrons varies between 2 and 11.
The damage caused is calculated by multiplying the dose of radiation by its quality factor; the measure is called dose equivalent. If the unit of the dose is a rad, the unit of the dose equivalent is rem (roentgen equivalent in man). It 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 unit of the dose equivalent is sievert. Grays and sieverts are metric measurements.
Radiation sickness may occur from large doses of radiation. Doses above 100 rems damage red and white blood cells (the hematopoietic effect). Doses above 300 rems may cause death in several weeks. When subjected to doses above 1,000 rems, the cells lining the digestive tract die, bacteria from the intestines invade the bloodstream (the gastrointestinal effect), and infection causes death within a week. At doses of several thousand rems, the brain is injured, and death may occur within hours. Deaths from radiation sickness happen only very rarely. People have suffered large doses in reactor accidents, in a few cases where radioactive material was mishandled, and in the 1945 bombings of Hiroshima and Nagasaki, during World War II. The worst reactor accident in history was a 1986 explosion and fire at the Chernobyl nuclear power plant in Ukraine, which killed 31 workers.
The dosage of radiation received in daily life is much smaller, and is sometimes called background dose. Sources are radon, a gas released by radioactive rocks and soil; medical and dental x-rays; nuclear power plants; waste disposal sites; and radioactive isotopes in tobacco smoke. Repeated exposure to small doses of radiation increases the risk for cancer and congenital defects.
Experts from many countries formed the International Commission on Radiological Protection to set exposure protocols and to protect people from the effects of radiation. The commission has set the annual maximum permissible dose (MPD) for nuclear workers at 5 rems per year, and that for the public at 0.5 rem per 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.
Study of Radiation
Radiation has been studied since ancient times. In the 3rd and 4th centuries BC, Epicurus, a Greek philosopher, wrote of particles "streaming off" the surface of objects. Euclid, a Greek mathematician, thought around the same time that an object could be seen because the eye sent out radiation.
Robert Grosseteste, an English bishop and scholar of the 13th century, regarded light as the root of all knowledge. He thought that understanding the laws governing light would unravel the laws of nature.
In the 17th century, Sir Isaac Newton, the English scientist, considered light to be composed of tiny particles, and Christiaan Huygens, the Dutch physicist, thought that light was made up of waves. Their followers and scientists argued each view for over a century until Thomas Young, a British physicist, showed early in the 19th century that light had properties similar to those of sound and water waves. Augustin Fresnel, a French physicist, provided more evidence for Youngs view a few years later. By 1850, most scientists had accepted that light consisted of waves.
In 1864, James Clerk Maxwell, a British scientist, suggested that light consisted of electromagnetic waves, and predicted the discovery of other, invisible forms of electromagnetic radiation. Heinrich R. Hertz and Wilhelm C. Roentgen, two German physicists, proved Maxwell's predictions correct. Hertz discovered radio waves in the late 1880s, and Roentgen discovered x-rays in 1895.
The discovery of radioactivity was a landmark in the study of radiation. In 1896, Antoine Henri Becquerel, a French physicist, discovered that crystals of a uranium compound darken photographic plates even when they were not exposed to light, and conjectured that uranium gave off energy in the form of radiation. Experiments conducted later by Ernest Rutherford, a New Zealand-born physicist, showed that this radiation was made up of two types of particles, which he named alpha and beta.
In 1898, Marie and Pierre Curie, both French physicists, found another material that produced radiation, and called it polonium. Also that year, along with Gustave Bemont, a French chemist, they discovered yet another material that gave off radiation, and called it radium. Rutherford showed a few years later that the process of transmutation could convert radioactive substances into new elements.
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 was another landmark in the study of radiation. Max Planck, a German physicist, analyzed radiation from hot objects and suggested in 1900 that objects could give off and take in this radiation only in packets of energy. These packets are called photons; initially, they were called quanta.
Albert Einstein, another German physicist, used Planck's theory to explain in 1905 the photoelectric effect. This is a process in which metal gives off electrons when struck by a bright beam of light. Einstein proposed that an electron could be freed from an atom in the metal by the energy supplied by a single photon. The localized manner in which photons act to produce the photoelectric effect resembles that of particles rather than of waves. Einstein's proposition led scientists again to consider the particle theory of light; now they know that radiation has features of both particles and waves.
Niels Bohr, a Danish physicist, based his explanation of the structure of the hydrogen atom in 1913 on the quantum theory. Bohr theorized that electrons can have only certain specific values of energy, and proved that atoms release photons of radiation when their electrons lose energy. In 1924, Louis de Broglie, a French physicist, hypothesized that electrons act as waves, called matter waves.
The first artificial nuclear chain reaction began the nuclear age in 1942. Enrico Fermi, a physicist born in Italy, and his co-workers at the University of Chicago produced that reaction. Many scientists have since then tried to discover applications of radioactivity and radiation instead of finding out what causes them. Nuclear weapons based on fission (the atomic bomb) and fusion (the hydrogen bomb) were developed. The first full-scale nuclear power plant began operating in 1956. Radiation from across the entire electromagnetic spectrum has been put to use in industry, research, communication, and medicine.