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1.high-energy radiation capable of producing ionization in substances through which it passes
1.(MeSH)Electromagnetic or corpuscular radiation capable of producing ions, directly or indirectly, in its passage through matter. The wavelengths are equal to or smaller than those of short (far) ultraviolet radiation and include gamma and X-rays and high-energy elementary particles.
Ionizing Radiation (n.) [MeSH]
rayon lumineux (fr)[termes liés]
(radiation; radioactivity)[termes liés]
ionizing radiation (n.)
Ionizing (or ionising) radiation is radiation composed of particles that individually can liberate an electron from an atom or molecule, producing ions usually in ion-pairs. These ions are especially chemically reactive, and the reactivity produces the high biological damage per unit of energy that is a characteristic of all ionizing radiation.
While some types of radiation are always considered ionizing, including cosmic rays, gamma rays, X-rays, neutrons, and in general all particles produced by radioactive decay and nuclear reactions, the boundary between ionizing and non-ionizing radiation becomes fuzzy and difficult to define, as the energy of the radiation decreases. The reason is that the energy of ionization differs for different atoms and molecules, and this transition region occurs at particle energies of about 10 to 33 electronvolts (eV), an amount of energy that lies in the far ultraviolet in the electromagnetic spectrum.
Thus, for radiation with far less than 10 to 33 eV of energy per photon (or per particle) the radiation is not formally “ionizing,” and such non-ionizing radiation includes radiowaves, microwaves, infrared light, visible light, and most of the ultraviolet spectrum. For the energy region between 10 to 33 eV, categorization of radiation as "ionizing" or "non-ionizing" is not clear in the various literatures in science and law.
A related operational difficulty near these energies comes from the fact that the idea of "ionizing radiation" was originally conceived in large part because of a need to differentiate radiation that had the property to alter chemical bonds (and thus in living organisms cause tissue damage, mutation, and cancer) at low powers, and without large temperature changes. However, non-ionizing radiation in the ultraviolet, and occasionally even in the visible light range, may exhibit such chemical alteration character. Thus, it may cause “ionization type” chemical damage, without producing ion-pairs, and without technically being ionizing radiation. These similar behavioral characteristics sometimes results in further confusion about these terms.
There is no sharply defined boundary between ionizing and non-ionizing radiation. In general, if the characteristic kinetic energy of the radiation particles is greater than the ionization energy of the target material, then each particle collision can be expected to ionize a target atom, no matter how low the power of the beam. This is an appealing bright line between ionizing and non-ionizing radiation, but it is subject to several caveats:
The boundary of greatest interest is for low intensity photon radiation striking organic material. Since the first ionization energy of hydrogen and oxygen are both 14 eV, the spectrum of ionizing radiation is commonly defined to start at approximately 10 eV (equivalent to a far ultraviolet wavelength of 124 nanometers). Some sources use the ionization energy of air to define the boundary at 33 eV. (38 nm) Since the energy of a carbon-carbon bond is 4.9 eV (250 nm), it might be just as reasonable to draw a conservative boundary there. All of these figures lie partway within the spectrum of ultraviolet light. X-rays and gamma rays are above all of these definitions and are always considered ionizing radiation.
When considering high-intensity long exposure scenarios, as in suntanning, the probability of multiphoton ionization increases. Indeed, any ultraviolet light can cause radiation burns similar to those produced by x-ray or gamma radiation. The major difference is that skin is largely opaque to ultraviolet, and therefore protects internal tissues from ultraviolet damage. Ultraviolet can cause damage to skin as a result of photoreactions in collagen. DNA molecules may be directly or indirectly damaged by UV radiation carrying enough energy to excite certain molecular bonds to form thymine dimers (pyrimidine dimer)s (this causes sunburn). Even microwave radiation, which has a kinetic energy well below that of visible light and is usually considered non-ionizing, can be considered ionizing if it is intense enough.
Truly non-ionizing radiation can still heat materials, cause ordinary burns, and even raise materials to their ionization temperature. Such heating does not produce free radicals until higher temperatures (for example, flame temperatures or "browning" temperatures, and above) are attained. In contrast, ionizing radiation produces free radicals, such as reactive oxygen species, even at room temperatures and below. Free radical production is a primary basis for the particular danger to biological systems of relatively small amounts of ionizing radiation that are far smaller than needed to produce significant heating. Free radicals easily damage DNA, and ionizing radiation may also directly damage DNA by ionizing or breaking DNA molecules.
Free neutrons are able to cause many nuclear reactions in a variety of substances no matter their energy, because in many substances they give rise to high-energy nuclear reactions, and these (or their products) liberate enough energy to cause ionization. For this reason, free neutrons are normally considered effectively ionizing radiation, at any energy (see neutron radiation). Examples of other ionizing particles are alpha particles, beta particles, and cosmic rays. The radiations cause ionization due to the kinetic energy involved in the production of the individual particles, which inevitably exceed the threshold of 10 or 33 eV, and commonly exceed thousands or even millions of eV of energy.
Ionizing radiation is ubiquitous in the environment, and comes from naturally occurring radioactive materials and cosmic rays. Common artificial sources are artificially produced radioisotopes, X-ray tubes and particle accelerators. Ionizing radiation is invisible and not directly detectable by human senses, so instruments such as Geiger counters are usually required to detect its presence. In some cases it may lead to secondary emission of visible light upon interaction with matter, such as in Cherenkov radiation and radioluminescence. It has many practical uses in medicine, research, construction, and other areas, but presents a health hazard if used improperly. Exposure to ionizing radiation causes damage to living tissue, and can result in mutation, radiation sickness, cancer, and death.
Various types of ionizing radiation may be produced by radioactive decay, nuclear fission and nuclear fusion, and by particle accelerators and naturally occurring cosmic rays. Muons and many types of mesons (in particular charged pions) are also ionizing.
In order for a stable particle to be directly ionizing, it must both have a high enough energy and interact with the atoms of a target. Exceptions are positrons that are common artificial sources of ionizing radiation in medical PET scans, and this is due to the energy of annihilation with electrons in ordinary matter, which produces secondary gamma radiation. Neutrons are indirectly ionizing, and are discussed below.
Photons interact electromagnetically with charged particles, so photons of sufficiently high energy also are ionizing. The energy at which this begins to happen with photons (light) lies in the high-frequency end of the ultraviolet (UV) region of the electromagnetic spectrum. As noted above, most UV is not ionizing, but all UV may cause molecular damage in a somewhat similar way, and thus all UV (like ionizing radiation) is more biologically harmful than expected from its heating effect and simple energy deposition.
Charged particles such as electrons, positrons, muons, protons, alpha particles, and heavy atomic nuclei from accelerators or cosmic rays also interact electromagnetically with electrons of an atom or molecule, and all may cause ionization. Muons contribute to background radiation due to cosmic rays, but by themelves are thought to be of little hazard-importance, due to their relatively low dose. Pions (another very short-lived sometimes-charged particle) may be produced in large amounts in the largest particle accelerators. Pions are not a theoretical biological hazard except near such operating accelerator machines, which are then subject to heavy security.
Neutrons, on the other hand, having zero electrical charge, do not interact electromagnetically with electrons, and so they cannot directly cause ionization by this mechanism. However, fast neutrons will interact with the protons in hydrogen (in the manner of a billiard ball hitting another, head on, sending it away with all of the first ball's energy of motion), and this mechanism produces proton radiation (fast protons). These protons are ionizing because they are of high energy, are charged, and interact with the electrons in matter.
Neutrons of all energies are radioactive, and thus produce ionization via decay (in practice this is a small part of their hazard). A neutron can also interact with other atomic nuclei, depending on the nucleus and the neutron's velocity; these reactions happen with fast neutrons and slow neutrons, depending on the situation (see neutron temperature). Neutron interactions with most types of matter in this manner usually produce radioactive nuclei, which produce ionizing radiation when they decay (see neutron activation). In fissile materials, secondary neutrons may produce nuclear chain reactions, sometimes causing a larger amount of ionization.
An ionization event normally produces a positive atomic ion and an electron. High-energy beta particles may produce bremsstrahlung as they pass through matter, or secondary electrons (δ-electrons); both can ionize in turn. Energetic Beta-particles, like those emitted by 32P, are quickly decelerated by surrounding matter. The energy lost to deceleration is emitted in the form of X-rays called "bremsstrahlung," which translates to "braking radiation". Bremsstrahlung is of concern when shielding beta emitters. The intensity of bremsstrahlung increases with the increase in energy of the electrons and the atomic number of the absorbing medium.
Unlike alpha or beta particles (see particle radiation), gamma rays do not ionize all along their path, but rather interact with matter in one of three ways: the photoelectric effect, the Compton effect, and pair production. By way of example, the figure shows Compton effect: two Compton scatterings that happen sequentially. In every scattering event, the gamma ray transfers energy to an electron, and it continues on its path in a different direction and with reduced energy.
In the same figure, the neutron collides with a proton of the target material, and then becomes a fast recoil proton that ionizes in turn. At the end of its path, the neutron is captured by a nucleus in an (n,γ)-reaction that leads to the emission of a neutron capture photon. As noted above, such photons always have enough energy to qualify as ionizing radiation.
The units used to measure ionizing radiation are rather complex. The ionizing effects of radiation are measured by units of exposure:
However, the amount of damage done to matter (especially living tissue) by ionizing radiation is more closely related to the amount of energy deposited rather than the charge. This is called the absorbed dose.
Equal doses of different types or energies of radiation cause different amounts of damage to living tissue. For example, 1 Gy of alpha radiation causes about 20 times as much damage as 1 Gy of X-rays. Therefore, the equivalent dose was defined to give an approximate measure of the biological effect of radiation. It is calculated by multiplying the absorbed dose by a weighting factor WR, which is different for each type of radiation (see above table). This weighting factor is also called the Q (quality factor), or RBE (relative biological effectiveness of the radiation).
For comparison, the average 'background' dose of natural radiation received by a person per day, based on 2000 UNSCEAR estimate, makes BRET 6.6 μSv (660 μrem). However local exposures vary, with the yearly average in the US being around 3.6 mSv (360 mrem), and in a small area in India as high as 30 mSv (3 rem). The lethal full-body dose of radiation for a human is around 4–5 Sv (400–500 rem).
Ionizing radiation has many uses, including killing cancerous cells and power generation. However, although ionizing radiation has many applications, overuse can be hazardous to human health. For example, at one time, assistants in shoe shops used X-rays to check a child's shoe size, but this practice was halted when it was discovered that ionizing radiation is dangerous.
Nuclear reactors produce large quantities of ionizing radiation as a byproduct of fission during operation. In addition, they produce highly radioactive nuclear waste, which will emit ionizing radiation for thousands of years for some of the fission byproducts. The safe disposal of this waste in a way that protects future generations from radiation exposure is currently imperfect and remains a highly controversial issue.
Radiation emissions from high level nuclear waste decrease extremely slowly, which requires long term containment and storage for thousands of years before it is considered safe. During normal conditions, radioactive emissions from nuclear power plants are generally lower than coal-burning plants; though several high profile nuclear accidents have released dangerous levels of radioactivity.
Since some ionizing radiation (principly gamma) can penetrate matter, they are used for a variety of measuring methods.
X-rays and gamma rays are used to make images of the inside of solid products, as a means of nondestructive testing and inspection. The piece to be radiographed is placed between the source and a photographic film in a cassette. After a certain exposure time, the film is developed and it shows internal defects of the material if there are any.
The largest use of ionizing radiation in medicine is in medical radiography to make images of the inside of the human body using x-rays. This is the largest artificial source of radiation exposure for humans. Radiation is also used to treat diseases in radiation therapy. Tracer methods (mentioned above) are used in nuclear medicine to diagnose diseases, and widely used in biological research.
In biology and agriculture, radiation is used to induce mutations to produce new or improved species. Another use in insect control is the sterile insect technique, where male insects are sterilized by radiation and released, so they have no offspring, to reduce the population.
In industrial and food applications, radiation is used for sterilization of tools and equipment. An advantage is that the object may be sealed in plastic before sterilization. An emerging use in food production is the sterilization of food using food irradiation.
Detractors of food irradiation have concerns about the health hazards of induced radioactivity. Also, a report for the American Council on Science and Health entitled "Irradiated Foods" states: "The types of radiation sources approved for the treatment of foods have specific energy levels well below that which would cause any element in food to become radioactive. Food undergoing irradiation does not become any more radioactive than luggage passing through an airport X-ray scanner or teeth that have been X-rayed." 
External exposure is exposure which occurs when the radioactive source (or other radiation source) is outside (and remains outside) the organism which is exposed. Examples of external exposure include:
One of the key points is that external exposure is often relatively easy to estimate, and the irradiated objects do not become radioactive, except for a case where the radiation is an intense neutron beam which causes activation of the object. It is possible for an object to be contaminated on the outer surfaces; assuming that no radioactivity enters the object it is still a case of external exposure and it is normally the case that decontamination is relatively easy.
Internal exposure occurs when the radioactive material enters the organism, and the radioactive atoms become incorporated into the organism. Below are a series of examples of internal exposure.
When radioactive compounds enter the human body, the effects are different from those resulting from exposure to an external radiation source. Especially in the case of alpha radiation, which normally does not penetrate the skin, the exposure can be much more damaging after ingestion or inhalation. The radiation exposure is normally expressed as a committed effective dose equivalent (CEDE).
The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) itemized type of exposures and reported exposure rate of each segment.
|Natural Sources||Normal occurances||Cosmic radiation|
|Enhanced sources||Metal mining and smelting|
|Coal mining and power production from coal|
|Oil and gas drilling|
|Rare earth and titanium dioxide industries|
|Zirconium and ceramics industries|
|Application of radium and thorium|
|Other exposure situations|
|Man-made sources||Peaceful purposes||Nuclear power production|
|Transport of nuclear and radioactive material|
|Application other than nuclear power|
|Military purposes||Nuclear tests|
|Residues in the environment. Nuclear fallout|
|Exposure from accidents|
|Occupational Radiation Exposure|
|Natural Sources||Cosmic ray exposures of aircrew and space crew|
|Exposures in extractive and processing industries|
|Gas and oil extraction industries|
|Radon exposure in workplaces other than mines|
|Man-made sources||Peaceful purposes||Nuclear power industries|
|Medical uses of radiation|
|Industrial uses of radiation|
|Military purposes||Other exposed workers|
|Source UNSCEAR 2008 Annex B retrieved 2011-7-4|
Natural and artificial radiation sources are similar in their effects on matter.
The average exposure for Americans is about 360 mrem (3.6 mSv) per year, 81 percent of which comes from natural sources of radiation. The remaining 19 percent results from exposure to human-made radiation sources such as medical X-rays, most of which is deposited in people having had CT scans. However, in some areas, the average background dose can be over 1,000 mrem (10 mSv) per year. An important source of natural radiation is radon gas, which seeps continuously from bedrock but can, because of its high density, accumulate in poorly ventilated houses.
The background rate for radiation varies considerably with location, being as low as 1.5 mSv/a (1.5 mSv per year) in some areas and over 100 mSv/a in others. People in some parts of Ramsar, a city in northern Iran, receive an annual absorbed dose from background radiation that is up to 260 mSv/a. Despite having lived for many generations in these high-background areas, inhabitants of Ramsar show no significant cytogenetic differences compared to people in normal background areas. This has led to the suggestion that high but steady levels of radiation are easier for humans to sustain than sudden radiation bursts.
The Earth, and all living things on it, are constantly bombarded by radiation from outside our solar system. This cosmic radiation consists of positively-charged ions from protons to iron nuclei. The energy of this radiation can far exceed that which humans can create even in the largest particle accelerators (see ultra-high-energy cosmic ray). This radiation interacts in the atmosphere to create secondary radiation that rains down, including x-rays, muons, protons, alpha particles, pions, electrons, and neutrons.
The dose from cosmic radiation is largely from muons, neutrons, and electrons, with a dose rate that varies in different parts of the world and based largely on the geomagnetic field, altitude, and solar cycle. The cosmic-radiation dose rate on airplanes is so high that, according to the United Nations UNSCEAR 2000 Report (see links at bottom), airline flight crew workers receive more dose on average than any other worker, including those in nuclear power plants.
Most materials on Earth contain some radioactive atoms, even if in small quantities. Most of the dose received from these sources is from gamma-ray emitters in building materials, or rocks and soil when outside. The major radionuclides of concern for terrestrial radiation are isotopes of potassium, uranium, and thorium. Each of these sources has been decreasing in activity since the birth of the Earth.
All Earthly materials that are the building-blocks of life contain a radioactive component. As humans, plants, and animals consume food, air, and water, an inventory of radioisotopes builds up within the organism (see banana equivalent dose). Some radionuclides, like potassium-40, emit a high-energy gamma ray that can be measured by sensitive electronic radiation measurement systems. Other radionuclides, like carbon-14, can be used to date the remains of long-dead organisms (such as wood that is thousands of years old). These internal radiation sources contribute to an individual's total radiation dose from natural background radiation.
Radon-222 is a gas produced by the decay of radium-226. Both are a part of the natural uranium decay chain. Uranium is found in soil throughout the world in varying concentrations. Since radon is a gas, it can accumulate in homes. Accumulation is dependent upon home location as well as building methods. Among non-smokers, Radon is the number one cause of lung cancer and, overall, the second leading cause.
Above the background level of radiation exposure, the U.S. Nuclear Regulatory Commission (NRC) requires that its licensees limit human-made radiation exposure for individual members of the public to 100 mrem (1 mSv) per year, and limit occupational radiation exposure to adults working with radioactive material to 5,000 mrem (50 mSv) per year. Occupationally exposed individuals are exposed according to the sources with which they work. The radiation exposure of these individuals is carefully monitored with the use of pocket-pen-sized instruments called dosimeters.
Examples of industries where occupational exposure is a concern include:
Medical procedures, such as diagnostic X-rays, nuclear medicine, and radiation therapy are by far the most significant source of human-made radiation exposure to the general public. Some of the major radionuclides used are I-131, Tc-99, Co-60, Ir-192, and Cs-137. These are rarely released into the environment. The public also is exposed to radiation from consumer products, such as tobacco (polonium-210), building materials, combustible fuels (gas, coal, etc.), ophthalmic glass, televisions, luminous watches and dials (tritium), airport X-ray systems, smoke detectors (americium), road construction materials, electron tubes, fluorescent lamp starters, and lantern mantles (thorium). A typical dose for radiation therapy might be 7 Gy spread daily (on weekdays) over two months.
Of lesser magnitude, members of the public are exposed to radiation from the nuclear fuel cycle, which includes the entire sequence from mining and milling of uranium to the disposal of the spent fuel, as well as the coal power cycle due to the release and emission of radioactive contaminants that were trapped in the coal. The effects of such exposure have not been reliably measured due to the extremely low doses involved. Estimates of exposure are low enough that proponents of nuclear power liken them to the mutagenic power of wearing trousers for two extra minutes per year (because heat causes mutation). Opponents use a cancer per dose model to assert that such activities cause several hundred cases of cancer per year, an application of the controversial Linear no-threshold model (LNT).
In a nuclear war, gamma rays from fallout of nuclear weapons would probably cause the largest number of casualties. Immediately downwind of targets, doses would exceed 300 Gy per hour. As a reference, 4.5 Gy (around 15,000 times the average annual background rate) is fatal to half of a normal population, without medical treatment.
Ionizing radiation is generally harmful and potentially lethal to living things, but low or controlled doses can have beneficial effects. Its most common impact is the induction of cancer with a latent period of years or decades after exposure. High doses can cause visually dramatic radiation burns, and/or rapid fatality through acute radiation syndrome. Controlled doses are used for medical imaging and radiotherapy to improve health. Some scientists suspect that low doses may have a mild hormetic effect that can improve health. Sometimes the presence of a radiation hazard can be a boon to wildlife by reducing human interference in their habitat.
Some effects of ionizing radiation on human health are stochastic, meaning that their probability of occurrence increases with dose, while the severity is independent of dose. Radiation-induced cancer, teratogenesis, cognitive decline, and heart disease are all examples of stochastic effects. Other conditions such as radiation burns, acute radiation syndrome, chronic radiation syndrome, and radiation-induced thyroiditis are deterministic, meaning they reliably occur above a threshold dose, and their severity increases with dose. Deterministic effects are not necessarily more or less serious than stochastic effects; either can ultimately lead to a temporary nuisance or a fatality.
Quantitative data on the effects of ionizing radiation on human health is relatively limited compared to other medical conditions because of the low number of cases to date, and because of the stochastic nature of some of the effects. Stochastic effects can only be measured through large epidemiological studies where enough data has been collected to remove confounding factors such as smoking habits and other lifestyle factors. The richest source of high-quality data comes from the study of Japanese atomic bomb survivors. In vitro and animal experiments are informative, but radioresistance varies greatly across species.
The consensus of the nuclear industry, regulators and governments regarding radiation health effects is expressed by the International Commission on Radiological Protection. (ICRP) Other important organizations studying the topic include
Radiation has always been present in the environment and in our bodies. The human body cannot sense ionizing radiation, but a range of instruments that are capable of detecting even very low levels of radiation from natural and man-made sources exists.
Dosimeters measure an absolute dose received over a period of time. Ion-chamber dosimeters resemble pens, and can be clipped to one's clothing. Film-badge dosimeters enclose a piece of photographic film, which will become exposed as radiation passes through it. Ion-chamber dosimeters must be periodically recharged, and the result logged. Film-badge dosimeters must be developed as photographic emulsion so the exposures can be counted and logged; once developed, they are discarded. Another type of dosimeter is the TLD (Thermoluminescent Dosimeter). These dosimeters contain crystals that emit visible light when heated, in direct proportion to their total radiation exposure. Like ion-chamber dosimeters, TLDs can be re-used after they have been 'read'.
There are three standard ways to limit exposure:
Some generally accepted thicknesses of attenuating material are 5 mm of aluminum for most beta particles, and 3 inches of lead for gamma radiation.
Containment: Radioactive materials are confined in the smallest possible space and kept out of the environment. Radioactive isotopes for medical use, for example, are dispensed in closed handling facilities, while nuclear reactors operate within closed systems with multiple barriers that keep the radioactive materials contained. Rooms have a reduced air pressure so that any leaks occur into the room and not out of it.
In a nuclear war, an effective fallout shelter reduces human exposure at least 1,000 times. Other civil defense measures can help reduce exposure of populations by reducing ingestion of isotopes and occupational exposure during war time. One of these available measures could be the use of potassium iodide (KI) tablets, which effectively block the uptake of radioactive iodine into the human thyroid gland.
During human spaceflights, in particular flights beyond low Earth orbit, astronauts are exposed to both galactic cosmic radiation (GCR) and possibly solar particle event (SPE) radiation. Evidence indicates past SPE radiation levels that would have been lethal for unprotected astronauts. GCR levels that might lead to acute radiation poisoning are not as well-understood.
Air travel exposes people on aircraft to increased radiation from space as compared to sea level, including cosmic rays and from solar flare events. Software programs such as Epcard, CARI, SIEVERT, PCAIRE are attempts to simulate exposure by aircrews and passengers. An example of a measured dose (not simulated dose), is 6 μSv per hour from London Heathrow to Tokyo Narita on a high-latitude polar route. However, dosages can vary, such as during periods of high solar activity. The United States FAA requires airlines to provide flight crew with information about cosmic radiation, and an ICRP recommendation for the general public is no more than 1 mSv per year. In addition, many airlines do not allow pregnant flightcrew members, to comply with a European Directive. The FAA has a recommended limit of 1 mSv total for a pregnancy, and no more than 0.5 mSv per month. Information originally based on Fundamentals of Aerospace Medicine published in 2008.