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Radiation Exposure

Radiation Exposure

The discovery of radiation

Radiation comes in many forms

Measuring exposure to radiation

Sources of radiation

Effects of radiation exposure

Future developments

Resources

Radiation exposure occurs any time that energy in the form of electromagnetic rays or fast-moving particles interacts with living tissue. Ionizing radiation is particularly damaging to tissue; examples include x rays, gamma radiation, and fast-moving subatomic particles such as neutrons. Biological damage caused by exposure to ionizing ranges from mild tissue burns to cancer, genetic damage, and ultimately, death. However, there are potential benefits of controlled exposures to certain kinds of radiation, which can be used for the detection, diagnosis, and treatment of certain diseases.

Exposure to many types of radiation is routinely monitored using sensitive devices, such as film badges and dosimeters.

The discovery of radiation

In the mid-1880s, James Maxwell (18311879) published a mathematical description of the wave motion of heat and light, the only forms of radiation known at the time. As scientists discovered other forms of radiation (such as x rays, radio waves, microwaves, and gamma rays) they found that their physical behavior could also be described by Maxwells equations, and that they were all part of the same, continuous, electromagnetic spectrum.

In 1895, the French physicist Henri Becquerel (18521909) began experimenting with the rare metal, uranium. He eventually discovered that uranium emitted a previously unknown form of radiation. Soon after, Pierre (18591906) and Marie Curie (1867 1934) discovered radium and polonium, which are also radioactive. These discoveries led to better understanding of the structure of the atom, and it became clear that there was another kind of radiation: ionizing radiation produced by radioactive substances. This type of radiation consists of extremely high-energy particles, which are released from the nuclei of radioactive atoms as they spontaneously undergo fission(i.e., break into smaller nuclei, forming different atomic elements). (Gamma rays, a form of electromagnetic radiation, are also released by some radioactive elements.) Because there are many kinds of radiation, it is subject to different classifications. Radiation can be described as electromagnetic or particulate (i.e., radioactive). These are further classified as being either ionizing or non-ionizing, depending on their energy level.

Radiation comes in many forms

The word radiation refers to two closely related things. First, it refers to forms of radiant energy, particularly that represented by subatomic particles (for example, the type of radiation released during a nuclear explosion), and by electromagnetism (for example, the type of radiation emitted by a light bulb, and by the sun). Sound is also considered a type of radiation.

The word radiation can also refer to the release and propagation through space of the energy itself. For example, a block of uranium releases radiation in the form of radioactive particles. Both the release of the particles, and the particles themselves, are called radiation. However, not all radiation is radioactive. The particle radiation released from uranium is radioactive, but the electromagnetic radiation emitted by a light bulb is not. Radioactivity is a form of radiation which involves the release of alpha particles, neutrons, electrons, and gamma rays, emitted by radioactive elements and substances.

Most of the radiation on Earths surface is electromagnetic radiation, which travels in waves of different frequency. (Frequency is the number of waves passing a point each second; it is the inverse of wavelength.) From the lowest to highest frequency, the spectrum of electromagnetic radiation is divided into the following ranges: radio waves, microwaves, visible light, ultraviolet light, x rays, and gamma rays. Visible light can be detected by the human eye, and is divided into the following color ranges: red, orange, yellow, green, blue, violet (arranged from lowest to highest frequency).

Sound, or acoustic radiation is also classified according to its frequency. In increasing order of frequency, sound radiation is classified as infrasonic, sonic, and ultrasonic.

Measuring exposure to radiation

The first commonly used unit for measuring the biological effects of x-ray exposure was the roentgen. It was named after the German physicist Wilhelm Roentgen (18451923), who discovered x rays in 1895. A roentgen is the amount of radiation that produces a set number of charged ions in a certain amount of air under standard conditions. This unit is not, however, particularly useful for describing the potential effects of radiation on human or animal tissues. The rad unit is slightly better in this regard. It is a measure of the radiation dose absorbed by one gram of something. A rad is equal to a defined amount of energy (100 ergs) absorbed per gram.

The problem with rads as a unit of measurement for human radiation exposure is that a dose of one rad of radiation from plutonium produces a different effect on living tissue than one rad of a less harmful type of radiation. Consequently, scientists introduced the rem, which stands for roentgen equivalent man. A rem is the dose of any radiation that produces the same biological effect, or dose equivalent, in humans as one rad of x rays.

Scientists continue to use these units, which were introduced earlier in the century, even as they become used to newer units for certain applications. the roentgen will still be the unit used to measure exposure to ionizing radiation, but the rad is being replaced with the gray as a measure of absorbed dose. One gray equals 100 rads. The sievert is replacing the rem as a measure of dose equivalent. One sievert equals 100 rems.

Sources of radiation

Exposure to ionizing radiation can be divided into two categories: natural and anthropogenic (i.e., associated with human activity). Background radiation is mostly due to solar radiation in the form of cosmic rays, and also radioactivity from rocks. Exposure to background radiation is continuous, although its intensity varies. The sun is also the main source of ultraviolet radiation. Each person in the United States receives an average radiation dose per year of about one millisievert (one-thousandth sievert; this is the same as 0.1 rem). About one-half of this exposure is due to radon, a natural radioactive gas released from rocks.

Radon is a breakdown product of uranium. Radon itself breaks down rapidly; its half-life is less than four days (this is the time for one-half of an initial quantity to decay through radioactivity). Unfortunately, radon decays into polonium-218, polonium-214, and polonium-220, which emit alpha particles. Alpha particles are heavy, charged particles that have trouble penetrating matter but can be dangerous if taken into the body, where they are in close contact with tissues and biochemicals (such as DNA) that are sensitive to suffering damage by ionization. Radon may be responsible for one-tenth of all deaths by lung cancer.

The actual and potential sources of anthropogenic radiation include: x rays and other types of radiation used in medicine, radioactive waste generated by nuclear power stations and scientific research centers, and radioactive fallout from nuclear weapons testing. Fallout is radioactive contamination of air, water, and land following the explosion of nuclear weapons or accidents at nuclear power stations.

Electromagnetic radiation from television sets and microwave ovens has been lowered to insignificant levels in recent years, thanks to federal regulations and improved designs. Some people consider high-voltage transmission lines a radiation threat, but scientific studies have not demonstrated a significant threat from this source.

Effects of radiation exposure

How energy from radiation is transferred to the body depends on the type of radiation. Visible light and infrared radiation, for example, transfer their energy to entire molecules. The absorbed energy causes greater molecular vibration, which can be measured as heat (or thermal energy).

With many forms of ionizing radiation, energy is transferred to electrons that surround atomic nuclei. Atoms affected by x rays usually absorb enough energy to lose some of their electrons, and so become ionized. (An atom is ionized when it gains or loses electrons and acquires a net electric charge.) Ultraviolet radiation causes electrons to absorb energy and jump to a higher energy orbit around the atomic nucleus. The sun and sunlamps emit enough ultraviolet radiation to cause sunburn, premature aging of the skin, and skin cancers. Exposure of humans and animals to ultraviolet radiation also results in the production of vitamin D, a biochemical necessary for good health.

Radiation that consists of charged particles can knock electrons out of their orbit around atoms. This also creates ions. Such radiation can also cause atoms to enter an exited state, if the electrons are bumped into higher-energy orbits. These changes result in atoms and molecules (including biochemicals) that are chemically reactive. Seeking to become stable, they interact with unaffected atoms and molecules, which may be damaged (i.e., changed) in the process, and are then unable to perform their usual metabolic functions. For example, nuclear material such as DNA molecules may be damaged to the degree that they can no longer be accurately copied. This may lead to impaired cell function, cell death, or genetic abnormalities.

Neutral particles of radiation, such as neutrons, transfer their energy to nuclei rather than to electrons. Often, neutrons strike a single proton, like that in a hydrogen nucleus, causing it to recoil and in the process be separated from its electrons, leaving a single, positively charged proton (this is also an ionization reaction). The neutron is then less energetic, and is captured by another nucleus, which releases charged particles in turn.

The specific effects of radiation on living beings depends on the type of radiation, the dose, the length of exposure, and the type of tissue exposed to the radiation. Damage caused by exposure to high levels of radiation is divided into two categories: somatic and genetic. Somatic refers to effects on the physiological functioning of the body; genetic refers to damage caused to reproductive cells, including heritable effects that can affect offspring.

Genetic damage can include mutations or broken chromosomes, the structures in cell nuclei that house DNA, and all the genetic information of an organism. Many mutations, or changes in genes, are harmful. Mutations caused by radiation are fundamentally the same as mutations caused by any other influence.

Somatic damage from high doses of ionizing radiation is indicated by burns and radiation sickness, with symptoms of nausea, vomiting, and diarrhea. long-term effects can include cancers such as leukemia. Cells are killed outright if a high dose of ionizing radiation is delivered in a short amount of time. Symptoms may appear within hours or days. The same dose delivered over a long time will not produce the same symptoms, because the body has time to repair some of the damage caused during a long-term exposure. However, some cells may experience genetic damage that causes some forms of cancer to develop years later (this is called a latent effect).

Exposure to intense doses of high-energy electromagnetic radiation, of the kind occurring close to radar towers or large radio transmitters, is less common than exposure to radioactivity. However, when it does occur it can cause cataracts, organ damage, hearing loss, and other disorders to develop. The health consequences of exposure to low doses of electromagnetic radiation are the subject of much controversy. Significant health effects have so far been difficult to detect.

Future developments

The public is increasingly becoming aware of the dangers of radiation exposure. Less than a generation ago, many people considered a dark suntan to be a sign of health and vigor. Today, health experts are working hard to convince people that excessive exposure to solar ultraviolet radiation, and to similar ultraviolet emitted by lamps in tanning salons, increases the risk of skin cancers and premature aging of the skin. It is risky to expose skin to full sunlight, especially for a reason as trivial as the esthetics of a suntan. Education campaigns are also being mounted to make home owners aware of the risks posed by radon, which can accumulate in well-insulated homes with certain kinds of concrete-walled or rock-floored basements.

Technological improvements are resulting in much smaller exposures to radiation during medical diagnostic procedures. Efforts are also being made to reduce and better focus the radiation exposures used for therapeutic purposes (for example, to treat some

KEY TERMS

Cosmic rays Ionizing radiation from the sun or other sources in outer space, consisting of atomic particles and electrons.

Electromagnetic spectrum The range of electromagnetic radiation that includes radio waves, x rays, visible light, ultraviolet light, infrared radiation, gamma rays, and other forms of radiation.

Ionization The production of atoms or molecules that have lost or gained electrons, and therefore have gained a net electric charge.

Nuclear reactor A device that generates energy by controlling nuclear fission, or splitting of the atom. The heat produced is used to heat water, which drives an electrical generator. Radioactive byproducts of the fission process are used for medical, scientific, and military purposes, or are disposed as nuclear waste.

Nuclear weapon A bomb or other explosive that derives its explosive force from the release of nuclear energy, either from fission or fusion reactions.

Radiation Energy in the form of waves, or particles.

Radioactivity Spontaneous release of subatomic particles or gamma rays by unstable atoms as their nuclei decay.

Uranium A heavy element found in nature. More than 99% of natural uranium is in the isotopic form of U-238. Only the less-common U-235 readily undergoes fission.

kinds of cancers). Sophisticated developments, such as the three-dimensional x-ray images produced by CAT scanners, allow health care workers to obtain more information with less exposure to radiation.

Steps are also being taken to prevent exposure resulting from anthropogenic sources of radiation in the environment. In 1986, a catastrophic accident at a nuclear reactor at Chernobyl in the Ukraine resulted in a huge emission of radioactive contaminants into the atmosphere, affecting much of Europe. After this disaster, networks of monitors were erected in many countries to detect future radiation leaks and warn threatened populations. The largest monitoring system is in Germany, which has installed several thousand radiation sensors. These systems will be able to detect radiation leaks coming from domestic or foreign sources shortly after nuclear accidents occur, allowing residents to seek shelter if necessary.

Most nations that do not already possess nuclear weapons, have signed a pact to not develop them, and nations that already have them have agreed not to test them above ground (which leads to particularly intense emissions of radioactivity into the atmosphere).

Some evidence exists that very low exposures to radiation may actual benefit cells, a hypothetical effect known as hormesis. However, the 2006 report of the prestigious U.S. National Academies of Science on the biological effects of low-level ionizing radiation concluded that there was insufficient evidence for hormesis in human beings and that the best available model for judging radiation risk is to extrapolate a linear relationship down to zerothat is, halved radiation exposure results in halved risk, all the way down to zero radiation and zero risk.

See also Mutation; Radioactive pollution.

Resources

BOOKS

Hendee, William R., et al. Radiation Therapy Physics. New York: Wiley-Liss, 2004.

National Academies of Science. Health Risks from Exposure to Low Levels of Ionizing Radiation: Beir vII Phase 2. Washington, DC: National Academies Press, 2006.

Waltar, Alan E. and Helene Langevin-Joliot. Radiation and Modern Life: Fulfilling Marie Curies Dream. Amherst, NY: Prometheus Books, 2004.

PERIODICALS

Cosmic Rays: Are Air Crews At Risk? Occupational and Environmental Medicine 59, no. 7 (2002): 428-432. Kasner, Darcy L., and Michael E. Spieth. The Day of Contamination. Journal of Nuclear Medicine Technology 31 (2003): 21-24.

Radiation Risk During Long-Term Spaceflight. Advances in Space Research 30, no. 4 (2002): 989-994.

Dean Allen Haycock

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Radiation Exposure

Radiation exposure


Radiation is defined as the emission of energy from an atom in the form of a wave or particle. Such energy is released as electromagnetic radiation or as radioactivity . Electromagnetic radiation includes radio waves, infrared waves or heat, visible light, ultraviolet radiation , x rays, gamma rays, and cosmic rays. Radioactivity, emitted when an atomic nucleus undergoes decay, usually takes the form of a particle such as an alpha particle or beta particle , though atomic decay can also release electromagnetic gamma rays.

While radiation in the form of heat, visible light, and even ultraviolet light is essential to life, the word "radiation" is often used to refer only to those emissions which can damage or kill living things. Such harm is specifically attributed to radioactive particles as well as the electromagnetic rays with frequencies higher than visible light (ultraviolet, x rays, gamma rays). Harmful electromagnetic radiation is also known as ionizing radiation because it strips atoms of their electrons, leaving highly reactive ions called free radicals which can damage tissue or genetic material.

Effects of radiation

The effects of radiation depend upon the type of radiation absorbed, the amount or dose received, and the part of the body irradiated. While alpha and beta particles have only limited power to penetrate the body, gamma rays and x rays are far more potent. The damage potential of a radiation dose is expressed in rems, a quantity equal to the actual dose in rads (units per kg) multiplied by a quality factor, called Q, representing the potency of the radiation in living tissue. Over a lifetime, a person typically receives 714 rems from natural sources. Exposure to 575 rems causes few observable symptoms. Exposure to 75200 rems leads to vomiting, fatigue, and loss of appetite. Exposure to 300 rems or more leads to severe changes in blood cells accompanied by hemorrhage. Such a dosage delivered to the whole body is lethal 50% of the time. An exposure of more than 600 rems causes loss of hair, loss of the body's ability to fight infection, and results in death. A dose of 10,000 rem will kill quickly through damage to the central nervous system.

The symptoms that follow exposure to a sufficient dose of radiation are often termed "radiation sickness" or "radiation burn." Bone marrow and lymphoid tissue cells, testes and ovaries, and embryonic tissue are most sensitive to radiation exposure. Since the lymphatic tissue manufactures white blood cells (WBCs), radiation sickness is almost always accompanied by a reduction in WBC production within 72 hours, and recovery from a radiation dose is first indicated by an increase in WBC production.

Any exposure to radiation increases the risk of cancer , birth defects , and genetic damage, as well as accelerating the aging process, and causing other health problems including impaired immunity. Among the chronic diseases suffered by those exposed to radiation are cancer, stroke, diabetes, hypertension, and cardiovascular and renal disease.

Sources of radiation

Some 82% of the average American's radiation exposure comes from natural sources. These sources include radon gas emissions from underground, cosmic rays from space, naturally occurring radioactive elements within our own bodies, and radioactive particles emitted from soil and rocks. Man-made radiation, the other 18%, comes primarily from medical x rays and nuclear medicine, but is also emitted from some consumer products (such as smoke detectors and blue topaz jewelry), or originates in the production and testing of nuclear weapons and the manufacture of nuclear fuels.

Environmental scientists believe that radon, a radioactive gas, accounts for most of the radiation dose Americans receive. Released by the decay of uranium in the earth, radon can infiltrate a house through pores in block walls, cracks in basement walls or floors, or around pipes. The Environmental Protection Agency estimates that eight million homes in the United States have potentially dangerous levels of radon, and calls radon "the largest environmental radiation health problem affecting Americans." Inhaled radon may contribute to 20,000 lung cancer deaths each year in the United States. The EPA now recommends that homeowners test their houses for radon gas and install a specialized ventilation system if excessive levels of gas are detected.

Though artificial sources of radiation contribute only a small fraction to overall radiation exposure, they remain a strong concern for two reasons. First, they are preventable or avoidable, unlike cosmic radiation, for example. Second, while the average individual may not receive a significant dose of radiation from artificial sources, geographic and occupational factors may mean dramatically higher doses of radiation for large numbers of people. For instance, many Americans have been exposed to radiation from nearly 600 nuclear tests conducted at the Nevada Test Site . From the early 1950s to the early 1960s, atmospheric blasts caused a lingering increase in radiation-related sickness downwind of the site, and increased the overall dose of radiation received by Americans by as much as 7%. Once the tests were moved underground, that figure fell to less than 1%.

A February 1990 study of the Windscale plutonium processing plant in Britain clearly demonstrated the importance of the indirect effects of radiation exposure. The study correlated an abnormally high rate of leukemia among children in the area with male workers at the plant, who evidently passed a tendency to leukemia to their children even though they had been receiving radiation doses that were considered "acceptable."

Recently, the EPA published their National Radon Results listing studies from 1984 to 1999. This study showed that 18 million homes have been tested for radon in the United States. In 1999 alone, there were about 1.4 million homes tested for radon. The knowledge of radon's harmful effects is on the rise (88% of Americans were aware of the harmful effects in 1999 compared to 73% in 1996) which is apparently contributing to the increase in testing.

[Linda Rehkopf and Jeffrey Muhr ]


RESOURCES

BOOKS

Caufield, C. Multiple Exposures. Harper & Row, Publishers, New York, 1989.

Wagner Jr., H. N., and L. E. Ketchum. Living With Radiation: The Risk, the Promise. Baltimore: Johns Hopkins University Press, 1989.

PERIODICALS

Cobb Jr., C. E. "Living With Radiation." National Geographic 175 (April 1989): 403437.

OTHER

Environmental Protection Agency. National Radon Results. June 2002 [cited July 2002]. <http://www.epa.gov/air/oarnew3.html>.

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Radiation Exposure

Radiation exposure

The term radiation exposure refers to any occasion on which a human or other animal or a plant has been placed in the presence of radiation from a radioactive source. For example, scientists have learned that the radioactive element radon is present in the basements of some homes and office buildings. Radon gas gives off radiation that can cause damage to human cells. Anyone living in a home or working in an office where radon is present runs some risk of being exposed to the radiation from this element.

The term radiation itself refers both to high speed subatomic particles, such as streams of alpha particles or beta particles, and to electromagnetic radiation. Electromagnetic radiation is a type of energy that travels in waves and includes such forms as X rays, gamma rays, ultraviolet radiation, infrared radiation, and visible light. Concerns about radiation exposure are, however, limited almost exclusively to effects caused by radiation emitted by radioactive materials: alpha and beta particles and gamma rays.

Sources of radiation

Radiation comes from both natural and human sources. Many elements exist in one or more radioactive forms. The most common of these is an isotope known as potassium 40. Isotopes are forms of an element that differ from each other in the structure of their nuclei. Other radioactive isotopes found in nature include hydrogen 3, carbon 14, chlorine 39, lead 212, radium 226, and uranium 235 and 238.

Humans and other organisms cannot escape exposure to radiation from these radioactive sources. They constitute a normal radiation, called background radiation, that is simply part of existing on Earth. Although some harmful effects can be produced by exposure to natural background radiation, those effects are relatively minor and, in most cases, not even measurable.

Human activities have added to normal background radiation over the past half century. When nuclear weapons are exploded, for example, they release radioactive isotopes into the atmosphere. As these radioactive isotopes are spread around the world by prevailing winds, they come into contact with humans and other organisms.

Words to Know

Background radiation: The natural level of radiation present on Earth at all times.

Ionization: The process by which atoms or molecules lose electrons and become positively charged particles.

Radiation: Energy transmitted in the form of electromagnetic waves or subatomic particles.

Radiation detectors: Instruments that are able to sense and relay information about the presence of radiation.

Radiation sickness: A term used to describe a variety of symptoms that develop when a person is exposed to radiation.

Radioactivity: The property possessed by some elements of spontaneously emitting energy in the form of particles or waves by disintegration of their atomic nuclei.

Subatomic particle: Basic unit of matter and energy (proton, neutron, electron, neutrino, and positron) smaller than an atom.

Nuclear power plants are also a potential source of radiation. Such plants are normally constructed with very high levels of safety in mind, and there is little or no evidence that humans are at risk as the result of the normal operation of a nuclear power plant. On those rare occasions when damage occurs to a nuclear power plant, however, that situation can change dramatically. Accidents at the Three Mile Island Plant in Pennsylvania in 1979 and at the Chernobyl Plant outside Kiev in Ukraine in 1986, for example, caused the release of substantial amounts of radiation to the areas surrounding the plants. In both cases, people were either injured or killed as a result of the release of radiation.

Effects of radiation

The harmful effects of exposure to radiation are due largely to its ionizing effects. Atoms and molecules contain electrons that can be removed from their orbits rather easily. For example, if a beta particle passes through an atom, it has the ability to repel any electrons in its path, ejecting them from the atom.

The bonds that hold atoms together in molecules are made of electrons. A molecule of water, for example, consists of oxygen and hydrogen atoms held together by electrons. If radiation passes through or very near a molecule of water, it may cause electrons to be ejected from the molecule. When that happens, the water molecule may fall apart. The same process occurs in any kind of molecule, including proteins, lipids, nucleic acids, and carbohydrates, the molecules of which living organisms are constructed.

Damage to molecules of this kind can have two general effects. First, when essential molecules are destroyed, an organism is no longer able to carry on all the normal functions it needs in order to stay alive and to function properly. A person, for example, may become sick if essential enzymes (substances that speed up chemical reactions) in his or her body are destroyed.

Some of the symptoms of radiation sickness include actual burns to the skin, nausea, vomiting, and diarrhea. The specific effects observed depend on the kind of radiation to which the person was exposed and the length of exposure. For example, a person exposed to low doses of radiation may experience some of the least severe symptoms of radiation sickness and then get better. A person exposed to higher doses of radiation may become seriously ill and even die.

Exposure to radiation can have long-term effects as well. These effects include the development of various kinds of cancer, leukemia being one of the most common types. Damage to a person's deoxyribonucleic acid (DNA) can also cause reproductive defects, such as children who are born deformed, blind, mentally impaired, or with other physical or mental challenges. (DNA is a complex molecule in the nucleus of cells that stores and transmits genetic information.)

Radiation detectors

All forms of radiation are invisible. You could stand in an area being flooded with life-threatening levels of alpha and beta particles and gamma rays, and you would never be able to tell. It is for this reason that anyone who works in an area where radioactivity is to be expected must be provided with radiation detectors. A radiation detector is an instrument that is able to sense and report on the presence of radiation.

Many kinds of radiation detectors are now available. One of the most common of these is the film badge. A film badge consists of a piece of photographic film wrapped in black paper behind the badge. When radiation passes through the badge, it exposes the photographic film. The film is removed from the badge at regular intervals and developed. The amount of radiation received by the wearer of the badge can then easily be determined by the amount of fogging on the photographic film.

A Geiger counter, another type of radiation detector, is a cylindrical glass tube with a thin wire down the middle. When radiation passes through the tube, it ionizes the gas inside the tube. The ions formed travel to the central wire where they initiate an electrical current through the wire. The wire is connected with an audible signal or a visual display. The clicking sound made or the light produced in the Geiger counter is an indication of the amount of radiation passing through the tube.

Cloud chambers and bubble chambers provide a visual display of radiation. When radiation passes through a cloud chamber, it causes moisture in the chamber to condense in much the same way a contrail (cloud) forms when a jet airplane passes through the sky. In a bubble chamber, it is a string of bubbles rather than a string of water droplets that forms. In either case, the path of the radiation and even the form of the radiation can be traced by the water droplets or bubbles formed in one of these devices.

[See also Mutation; Radiation; Radioactivity ]

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