Ionizing radiation

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Overview

Ionizing radiation is energetic particles or waves that have the potential to ionize an atom or molecule through atomic interactions. It is a function of the energy of the individual particles or waves, and not a function of the number of particles or waves present. A large flood of particles or waves will not cause ionization if the individual particles or waves are not energetic enough. These ionizations, if enough occur, can be destructive to biological organisms, and can cause DNA damage in individual cells. Extensive doses of ionizing radiation have been shown to have a mutating effect to future generations of the individual receiving the dose. Examples of ionizing radiation are energetic Beta particles, neutrons, alpha particles and energetic photons (UV and above). The amount of energy required to ionize an atom or molecule may widely vary. X-rays and gamma rays will ionize almost any molecule or atom; Far ultraviolet, near ultraviolet and visible light are ionizing to very few molecules; microwaves and radio waves are non-ionizing radiation.

Visible light is so ubiquitous that molecules that are ionized by it will often react nearly spontaneously unless protected by materials that block the visible spectrum. Examples include photographic film and some molecules involved in photosynthesis.

Ionizing radiation has many practical uses in medicine, research, construction, etc. It also presents a health hazard to humans if used improperly. Both aspects are discussed below.

Types of radiation

Alpha radiation consists of helium-4 nuclei and is stopped by a sheet of paper. Beta radiation, consisting of electrons, is halted by an aluminium plate. Gamma radiation, consisting of energetic photons, is eventually absorbed as it penetrates a dense material.

Ionizing radiation is produced by radioactive decay, nuclear fission and nuclear fusion, by extremely hot objects (the hot sun, e.g., produces ultraviolet), and by particle accelerators that may produce, e.g., fast electrons or protons or bremsstrahlung or synchrotron radiation.

In order for radiation to be ionizing, the particles must both have a high enough energy and interact with the atom. Photons interact strongly with charged particles, so photons of sufficiently high energy are ionizing. The energy at which this begins to happen is in the ultraviolet region; sunburn is one of the effects of this ionization. Charged particles such as electrons, positrons, and alpha particles also interact strongly with electrons. Neutrons, on the other hand, do not interact strongly with electrons, and so they cannot directly ionize atoms by this mechanism. However, fast neutrons will interact with the protons in hydrogen (in the manner of a billiard ball hitting another, sending it away with all of the first ball's energy of motion), and this mechanism produces proton radiation (fast protons). These are ionizing because of the strong interaction of the charged proton with the electrons in matter. Neutrons can also interact with atomic nuclei, depending on the nucleus and their velocity; these reactions happen with fast neutrons and slow neutrons, depending on the situation. Neutron interaction with nuclei in this manner often produces radioactive nuclei, which produce ionizing radiation when they decay.

Types of radiation.png

In the picture at left, gamma quanta are represented by wavy lines, charged particles and neutrons by straight lines. The little circles show where ionization processes occur.

An ionization event normally produces a positive atomic ion and an electron. High energy beta particles may produce bremsstrahlung when passing through matter, or secondary electrons (δ-electrons); both can ionize in turn.

Gamma quanta do not ionize all along their path like alpha or beta particles (see particle radiation. They interact by one of three effects: photoelectric effect, Compton effect, or pair production. By way of example, the figure shows Compton effect: two Compton scatterings that happen sequentially. In every scattering event, the gamma quantum transfers energy to an electron, and it continues on its path in a different direction with reduced energy.

In the figure, the neutron collides with a proton of the material which then becomes a fast recoil proton that ionizes in turn. At the end of its path, the neutron is captured by some nucleus in an (n,γ)-reaction that leads to a neutron capture photon.

The negatively charged electrons and positively charged ions created by ionizing radiation may cause damage in living tissue. If the dose is sufficient, the effect may be seen almost immediately, in the form of radiation poisoning. Lower doses may cause cancer or other long-term problems. The effect of the very low doses encountered in normal circumstances (from both natural and artificial sources, like cosmic rays, medical X-rays and nuclear power plants) is a subject of current debate. A 2005 report released by the National Research Council (the BEIR VII report, summarized in [2]) indicated that the overall cancer risk associated with background sources of radiation was relatively low.

Radioactive materials usually release alpha particles which are the nuclei of helium, beta particles, which are quickly moving electrons or positrons, or gamma rays. Alpha and beta rays can often be shielded by a piece of paper or a sheet of aluminium, respectively. They cause most damage when they are emitted inside the human body. Gamma rays are less ionizing than either alpha or beta rays, but protection against them requires thicker shielding. They produce damage similar to that caused by X-rays such as burns, and cancer through mutations. Human biology resists germline mutation by either correcting the changes in the DNA or inducing apoptosis in the mutated cell.

Non-ionizing radiation is thought to be essentially harmless below the levels that cause heating. Ionizing radiation is dangerous in direct exposure, although the degree of danger is a subject of debate. Humans and animals can also be exposed to ionizing radiation internally: if radioactive isotopes are present in the environment, they may be taken into the body. For example, radioactive iodine is treated as normal iodine by the body and used by the thyroid; its accumulation there often leads to thyroid cancer. Some radioactive elements also bioaccumulate.

Uses of ionizing radiation

Ionizing radiation has many uses. An X-ray is ionizing radiation, and ionizing radiation can be used in medicine to kill cancerous cells. However, although ionizing radiation has many uses, the overuse of it can be hazardous to human health. Shop assistants in shoe shops used to use an X-ray machine to check a child's shoe size, but when it was discovered that ionizing radiation was dangerous these machines were promptly removed.

Technical uses of ionizing radiation

Since they are able to penetrate matter, ionizing radiations are used for a variety of measuring methods.

Radiography by means of gamma or x rays
This is a method used in industrial production. The piece to be radiographed is placed between the source and a photographic film in a cassette. After a certain exposition time, the film is developed and it shows internal defects of the material if there are any.
Gauges
Gauges use the exponential absorption law of gamma rays
  • Level indicators: Source and detector are placed at opposite sides of a container, indicating the presence or absence of material in the horizontal radiation path. Beta or gamma sources are used, depending on the thickness and the density of the material to be measured. The method is used for containers of liquids or of grainy substances
  • Thickness gauges: if the material is of constant density, the signal measured by the radiation detector depends on the thickness of the material. This is useful for continuous production, like of paper, rubber, etc.
Applications using ionization of gases by radiation
  • To avoid the build-up of static electricity in production of paper, plastics, synthetic textiles, etc., a ribbon-shaped source of the alpha emitter 241Am can be placed close to the material at the end of the production line. The source ionises the air to remove electric charges on the material.
  • Smoke detector: Two ionisation chambers are placed next to each other. Both contain a small source of 241Am that gives rise to a small constant current. One is closed and serves for comparison, the other is open to ambient air; it has a gridded electrode. When smoke enters the open chamber, the current is disrupted as the smoke particles attach to the charged ions and restore them to a neutral electrical state. This reduces the current in the open chamber. When the current drops below a certain threshold, the alarm is triggered.
  • Radioactive tracers for industry: Since radioactive isotopes behave, chemically, mostly like the inactive element, the behavior of a certain chemical substance can be followed by tracing the radioactivity. Examples:
    • Adding a gamma tracer to a gas or liquid in a closed system makes it possible to find a hole in a tube.
    • Adding a tracer to the surface of the component of a motor makes it possible to measure wear by measuring the activity of the lubricating oil.

Biological and medical applications of ionizing radiation

In biology, one uses mainly the fact that radiation sterilizes, and that it enhances mutations. For example, mutations may be induced by radiation to produce new or improved species. A very promising field is the sterile insect technique, where male insects are sterilized and liberated in the chosen field, so that they have no descendants, and the population is reduced.

Radiation is also useful in sterilizing medical hardware or food. The advantage for medical hardware is that the object may be sealed in plastic before sterilization. For food, there are strict regulations to prevent the occurrence of induced radioactivity. The growth of a seedling may be enhanced by radiation, but excessive radiation will hinder growth.

Electrons, x rays, gamma rays or atomic ions may be used in radiation therapy to treat malignant tumors (cancer).

Tracer methods are used in nuclear medicine in a way analogous to the technical uses mentioned above.

Natural background radiation

Natural background radiation comes from four primary sources: cosmic radiation, solar radiation, external terrestrial sources, and radon.

Cosmic radiation

The earth, and all living things on it, are constantly bombarded by radiation from outside our solar system of positively charged ions from protons to iron nuclei. The energy of this radiation can far exceed energies that humans can create even in the largest particle accelerators. 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. The dose rate from cosmic radiation varies in different parts of the world based largely on the geomagnetic field, altitude, and solar cycle. The dose rate from cosmic radiation on aeroplanes is so high that, according to the United Nations UNSCEAR 2000 Report (see links at bottom), airline workers receive more dose on average than any other worker, including nuclear power plant workers.

Solar radiation

While most solar radiation is electromagnetic radiation, the sun also produces particle radiation, solar particles, which vary with the solar cycle. They are mostly protons; these are relatively low in energy (10-100 keV). The average composition is similar to that of the Sun itself. This represents significantly lower energy particles than come form cosmic rays. Solar particles vary widely in their intensity and spectrum, increasing in strength after some solar events such as solar flares. Further, an increase in the intensity of solar cosmic rays is often followed by a decrease in the galactic cosmic rays, called a Forbush decrease after their discoverer, the physicist Scott Forbush. These decreases are due to the solar wind which carries the sun's magnetic field out further to shield the earth more thoroughly from cosmic radiation.

The ionizing component of solar radiation is negligible relative to other forms of radiation on Earth's surface.

External terrestrial sources

Most material on earth contains some radioactive atoms, if in small quantities. But most of terrestrial non-radon-dose one receives from these sources is from gamma-ray emitters in the walls and floors when inside the house or rocks and soil when outside. The major radionuclides of concern for terrestrial radiation are potassium, uranium and thorium. Each of these sources has been decreasing in activity since the birth of the Earth so that our present dose from potassium-40 is about ½ what it would have been at the dawn of life on Earth.

Radon

Radon-222 is produced by the decay of radium-226 which is present wherever uranium is found. Since radon is a gas, it seeps out of uranium-containing soils found across most of the world and may concentrate in well-sealed homes. It is often the single largest contributor to an individual's background radiation dose and is certainly the most variable from location to location. Radon gas could be the second largest cause of lung cancer in America, after smoking.[3]

Human-made radiation sources

Natural and artificial radiation sources are identical in their nature and their effect. Above the background level of radiation exposure, the U.S. Nuclear Regulatory Commission (NRC) requires that its licensees limit human-made radiation exposure to 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.

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 who have CAT scans. This compares with the average dose received by people in the UK of about 2.2 mSv. One 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 varies considerably with location, being as low as 1.5 mSv/a in some areas and over 100 mSv/a in others. People in some areas of Ramsar, a city in northern Iran, receive an annual radiation 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 the body can sustain much higher steady levels of radiation than sudden bursts.

Some human-made radiation sources affect the body through direct radiation, while others take the form of radioactive contamination and irradiate the body from the inside.

By far, the most significant source of human-made radiation exposure to the general public is from medical procedures, such as diagnostic X-rays, nuclear medicine, and radiation therapy. Some of the major radionuclides used are I-131, Tc-99, Co-60, Ir-192, Cs-137. These are rarely released into the environment.

In addition, members of the public are 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, lantern mantles (thorium), etc.

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. The effects of such exposure have not been reliably measured. 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 prove that such activities cause several hundred cases of cancer per year.

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.

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.

Some of the radionuclides of concern include cobalt-60, caesium-137, americium-241 and iodine-131. Examples of industries where occupational exposure is a concern include:

  • Airline crew (the most exposed population)
  • Fuel cycle
  • Industrial radiography
  • Nuclear medicine and medical radiology departments (including nuclear oncology)
  • Nuclear power plants
  • Research laboratories (government, university and private)

Biological effects of ionizing radiation

The biological effects of radiation are thought of in terms of their effect on living cells. For low levels of radiation exposure, the biological effects are so small they may not be detected in epidemiological studies. The body repairs many types of radiation and chemical damage. Biological effects of radiation on living cells may result in a variety of outcomes, including:

  1. Cells experience DNA damage and are able to detect and repair the damage.
  2. Cells experience DNA damage and are unable to repair the damage. These cells may go through the process of programmed cell death, or apoptosis, thus eliminating the potential genetic damage from the larger tissue.
  3. Cells experience a nonlethal DNA mutation that is passed on to subsequent cell divisions. This mutation may contribute to the formation of a cancer.

Other observations at the tissue level are more complicated. These include:

  1. In some cases, a small radiation dose reduces the impact of a subsequent, larger radiation dose. This has been termed an 'adaptive response' and is related to hypothetical mechanisms of hormesis.

Hormesis

the notion of radiation hormesis has been rejected by the National Research Council. "The scientific research base shows that there is no threshold of exposure below which low levels of ionizing radiation can be demonstrated to be harmless or beneficial. The health risks – particularly the development of solid cancers in organs – rise proportionally with exposure" says Richard R. Monson, associate dean for professional education and professor of epidemiology, Harvard School of Public Health, Boston.[2] See the National Academies Press book.[3].

  1. Cells that are not 'hit' by a radiation track but are located nearby may express damage or alterations in normal function, presumably after communication between the 'hit' cell and neighboring cells occurs. This has been termed the 'bystander effect'.
  2. The progeny of a cell that survives radiation exposure may have increased probabilities for mutation. This has been termed 'genomic instability'.

Chronic radiation exposure

Exposure to ionizing radiation over an extended period of time is called chronic exposure. The natural background radiation is chronic exposure, but a normal level is difficult to determine due to variations. Geographic location and occupation often affect chronic exposure.

Acute radiation exposure

Acute radiation exposure is an exposure to ionizing radiation which occurs during a short period of time. There are routine brief exposures, and the boundary at which it becomes significant is difficult to identify. Extreme examples include

  • Instantaneous flashes from nuclear explosions.
  • Exposures of minutes to hours during handling of highly radioactive sources.
  • Laboratory and manufacturing accidents.
  • Intentional and accidental high medical doses.

The effects of acute events are more easily studied than those of chronic exposure. Chronic exposure is reactant.

Radiation levels

The associations between ionizing radiation exposure and the development of cancer are mostly based on populations exposed to relatively high levels of ionizing radiation, such as Japanese atomic bomb survivors, and recipients of selected diagnostic or therapeutic medical procedures.

Cancers associated with high dose exposure include leukemia, thyroid, breast, bladder, colon, liver, lung, esophagus, ovarian, multiple myeloma, and stomach cancers. United States Department of Health and Human Services literature also suggests a possible association between ionizing radiation exposure and prostate, nasal cavity/sinuses, pharyngeal and laryngeal, and pancreatic cancer.

The period of time between radiation exposure and the detection of cancer is known as the latent period. Those cancers that may develop as a result of radiation exposure are indistinguishable from those that occur naturally or as a result of exposure to other chemical carcinogens. Furthermore, National Cancer Institute literature indicates that other chemical and physical hazards and lifestyle factors, such as smoking, alcohol consumption, and diet, significantly contribute to many of these same diseases.

Although radiation may cause cancer at high doses and high dose rates, public health data regarding lower levels of exposure, below about 1,000 mrem (10 mSv), are harder to interpret. To assess the health impacts of lower radiation doses, researchers rely on models of the process by which radiation causes cancer; several models have emerged which predict differing levels of risk.

Studies of occupational workers exposed to chronic low levels of radiation, above normal background, have provided mixed evidence regarding cancer and transgenerational effects. Cancer results, although uncertain, are consistent with estimates of risk based on atomic bomb survivors and suggest that these workers do face a small increase in the probability of developing leukemia and other cancers. One of the most recent and extensive studies of workers was published by Cardis et al. in 2005 [4].

The linear dose-response model suggests that any increase in dose, no matter how small, results in an incremental increase in risk. The linear no-threshold model (LNT) hypothesis is accepted by the Nuclear Regulatory Commission (NRC) and the EPA and its validity has been reaffirmed by a National Academy of Sciences Committee. (See the BEIR VII report, summarized in [5].) Under this model, about 1% of a population would develop cancer in their lifetime as a result of ionizing radiation from background levels of natural and manmade sources.

Ionizing radiation damages tissue by causing ionization, which disrupts molecules directly and also produces highly reactive free radicals, which attack nearby cells. The net effect is that biological molecules suffer local disruption; this may exceed the body's capacity to repair the damage and may also cause mutations in cells currrently undergoing replication.

Two widely studies instances of large-scale exposure to high doses of ionizing radiation are: atomic bomb survivors in 1945; and emergency workers responding to the 1986 Chernobyl accident.

Approximately 134 plant workers and firefighters engaged at the Chernobyl power plant received high radiation doses (70,000 to 1,340,000 mrem or 700 to 13,400 mSv) and suffered from acute radiation sickness. Of these, 28 died from their radiation injuries.

Longer term effects of the Chernobyl accident have also been studied. There is a clear link (see the UNSCEAR 2000 Report, Volume 2: Effects) between the Chernobyl accident and the unusually large number, approximately 1,800, of thyroid cancers reported in contaminated areas, mostly in children. These were fatal in some cases. Other health effects of the Chernobyl accident are subject to current debate.

Ionizing radiation level examples

Recognized effects of acute radiation exposure are described in the article on radiation poisoning. The exact units of measurement vary, but light radiation sickness begins at about 50–100 rad (0.5–1 gray (Gy), 0.5–1 Sv, 50–100 rem, 50,000–100,000 mrem).

Although the SI unit of radiation dose equivalent is the sievert, chronic radiation levels and standards are still often given in millirems, 1/1000th of a rem (1 mrem = 0.01 mSv).

The following table includes some short-term dosages for comparison purposes.

Level (mSv) Example
0.001-0.01 Hourly cosmic dose on high-altitude flight, depends on position and solar sunspot phase.[4]
0.01 Annual USA dose from nuclear fuel and nuclear power plants [5]
0.01 Daily natural background radiation, including radon [6]
0.1 Average annual USA dose from consumer products [5]
0.15 /a USA EPA cleanup standard
0.25 /a USA NRC cleanup standard for individual sites/sources
0.27 Annual USA dose from natural cosmic radiation (0.16 coastal plain, 0.63 eastern Rocky Mountains) [5]
0.28 Annual USA dose from natural terrestrial sources [5]
0.39 /a Global level of human internal radiation due to radioactive potassium
0.46 Estimated largest off-site dose possible from March 28 1979 Three Mile Island accident
0.48 /d
(175 /a)
USA NRC public area exposure limit
0.66 Average annual USA dose from human-made sources [6]
1 /a USA NRC total exposure limit for the public
1.1 /a 1980 average USA radiation worker occupational dose [6]
2 /a USA average medical and natural background [6]

Human internal radiation due to radon, varies with radon levels [5]

2.2 Average dose from upper gastrointestinal diagnostic X-ray series
3 /a USA average dose from all natural sources [6]
3.66 /a USA average from all sources, including medical diagnostic radiation doses
few /a Estimate of cobalt-60 contamination within about 0.5 mile of dirty bomb
5 /a USA NRC occupational limit for minors (10% of adult limit)
USA NRC limit for visitors
Orvieto town, Italy, natural [7]
5 over 9 months USA NRC occupational limit for pregnant women
6.4 /a High Background Radiation Area (HBRA) of Yangjiang, China [8]
7.6 /a Fountainhead Rock Place, Santa Fe, NM natural
10–50 USA EPA nuclear accident emergency action level [6]
50 USA NRC annual occupational limit (10 CFR 20)
100 acute USA EPA acute dose level estimated to increase cancer risk 0.8% [6]
120 30-year exposure, Ural mountains, lower cancer mortality rate[9]
150 USA NRC annual occupational eye lens exposure limit
175 Guarapari, Brazil annual natural radiation sources [10]
250 acute USA EPA voluntary maximum dose for emergency non-life-saving work [6]
260 Ramsar, Iran, annual natural background peak dose [11]
500 USA NRC occupational whole skin, limb skin, or single organ exposure limit
30-year exposure, Ural mountains, (exposed population lower

cancer mortality rate) [12]

750 acute USA EPA voluntary maximum dose for emergency life-saving work [6]
500–1000 acute Low-level radiation sickness due to short-term exposure
World War II nuclear bomb victims

Monitoring and controlling exposure to ionizing radiation

Radiation has always been present in the environment and in our bodies. The human body cannot sense ionizing radiation, but a range of instruments exists which are capable of detecting even very low levels of radiation from natural and man-made sources.

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.

Geiger counters and scintillation counters measure the dose rate of ionizing radiation directly.

There are four standard ways to limit exposure:

Time: For people who are exposed to radiation in addition to natural background radiation, limiting or minimizing the exposure time will reduce the dose from the radiation source.

Distance: Radiation intensity decreases sharply with distance, according to an inverse square law.

Shielding: Barriers of lead, concrete, or water give effective protection from radiation formed of energetic particles such as gamma rays and neutrons. Som radioactive materials are stored or handled underwater or by remote control in rooms constructed of thick concrete or lined with lead. There are special plastic shields which stop beta particles and air will stop alpha particles. Shielding can be designed using halving thicknesses, the thickness of material that reduces the radiation by half. Halving thicknesses for gamma rays are discussed in the article gamma rays.

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 which 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. Most people can accept doses as high as 1 Gy, distributed over several months, although with increased risk of cancer later in life. 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 dangerous radioactive iodine into the human thyroid gland.

See also

References

  1. This symbol is included in ISO 21482:2007. ISO International Standards are protected by copyright and may be purchased from ISO or its members (please visit www.iso.org for more information). ISO has not reviewed the accuracy or veracity of this information.
  2. "Low Levels of Ionizing Radiation May Cause Harm". the national academies. Retrieved 18 March. 
  3. Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2. ISBN -309-09156-X. 
  4. UNSCEAR 2000 report, Volume 1, Annex B, ``Exposures from natural radiation sources, pp 88. See figure 3. available online at [1]
  5. 5.0 5.1 5.2 5.3 5.4 Oak Ridge National Laboratory (http://www.ornl.gov/sci/env_rpt/aser95/appa.htm)
  6. 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 Oak Ridge National Laboratory (http://www.ornl.gov/sci/env_rpt/aser95/tb-a-2.pdf)


External links

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