Ionizing radiation
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Ionizing radiation is a type of particle radiation in which an individual particle, such as a photon, electron, or helium nucleus, carries enough energy to ionize an atom or molecule, that is, to completely remove an electron from its orbit. If the individual particles do not carry this amount of energy, it is essentially impossible for even a large flood of particles to cause ionization. These ionizations, if enough occur, can be very destructive to living tissue, and can cause DNA damage and mutations.
The composition of ionizing radiation can vary. Electromagnetic radiation can cause ionization if the energy per photon, or frequency, is high enough, and thus the wavelength is short enough. Far ultraviolet, X-rays, and gamma rays are all ionizing radiation, while visible light, microwaves, and radio waves are non-ionizing radiation. Ionizing radiation may also consist of fast-moving particles such as electrons, positrons, or small atomic nuclei.
Where exactly the line between ionizing and non-ionizing radiation should be drawn is very tricky because some molecules can be ionized with very little energy. However, visible light is so common that molecules that are ionized by it need to be specially protected by materials that block the visible spectrum.
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Types of radiation
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Ionizing radiation is produced by radioactive decay, nuclear fission and nuclear fusion, in extremely hot objects, they produce thermal or blackbody radiation, and accelerated charges produce bremsstrahlung or synchrotron radiation.
In order for radiation to be ionizing, the particles must both have a high enough energy and interact with electrons. 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. They can 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 radiation often produces radioactive nuclei, which produce ionizing radiation when they decay.
The negatively charged electrons and positively charged nuclei 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 [1]) 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 aborting most mutated conceptuses.
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.
Example: Electromagnetic radiation
The energy of a photon (i.e., a quantum of electromagnetic radiation) is given by the Planck equation:
- <math>E = h \nu</math>
where
- <math>E</math> is the energy of the photon
- <math>h</math> is Planck's constant
- <math>\nu</math> is the frequency of the photon
The wavelength of a photon is related to its frequency by the equation of a wave's velocity:
- <math>c = \lambda \nu</math>
where
- <math>c</math> is the speed of light
- <math>\lambda</math> is the wavelength of light
Plugging back in and solving for the wavelength, we get,
- <math>\lambda = h c/E</math>
The elements with the lowest and highest ionization potential are cesium (3.89 eV) and helium (24.6 eV), respectively. Compounds can have low ionization potentials as well. For example, PMMA has an ionization potential of 8.1 eV. Photons with energies less than 3.89 eV (λ > 318.8 nm) are non-ionizing radiation, photons with energies greater than 24.6 eV (λ < 50.4 nm) are ionizing radiation, and photons with energies between 3.89 eV and 24.6 eV may be either ionizing or non-ionizing radiation depending on the nature of material (e.g., cesium or helium). Visible light corresponds to photons with energies from 1.77 eV (λ = 700.6 nm) to 3.10 eV (λ = 400 nm) and are thus non-ionizing electromagnetic radiation. Ultraviolet (UV) radiation spans the energy range from 3.10 eV (UV-A) to 12.4 eV (UV-C, λ = 100 nm). Because UV radiation, especially UV-C, exceeds the ionization energy of many of the elements, it is often considered ionizing radiation rather than non-ionizing radiation.
Sources of ionizing radiation
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. 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 and altitude. The dose rate from cosmic radiation on airplanes is so high that, according to the UNSCEAR, airline workers receive more dose on average than any other worker, including those who work in nuclear power plants.
Solar radiation
While most solar radiation is electro-magnetic radiation, the sun also produces particle radiation, solar cosmic rays, which vary with the solar cycle. Most are made of protons; these rays are relatively low in energy (10-100 keV). The average composition is similar to that of the Sun itself. High energy (Mev and above) cosmic rays come mainly from outside the solar system, while the particles in the solar case are energized near the Sun's surface by the action of magnetic fields. Solar cosmic rays 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 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 with its entrained magnetic field sweeping some of the galactic cosmic rays outwards, away from the Sun and Earth.
External terrestrial sources
Radioactive material is found throughout nature. It occurs naturally in the soil, rocks, water, air and vegetation. 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 gas 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 in the United States.
Man-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 man-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 man-made radiation sources such as medical X-rays, most of which is deposited in people who have CAT scans. 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 as over as 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 man-made radiation sources affect man through direct radiation, while others take the form of radioactive contamination and irradiate man from the inside.
By far, the most significant source of man-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.
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
- Radiology Departments (Medical)
- Radiation Oncology Departments
- Nuclear power plant
- Nuclear medicine Departments
- National (government) and university Research Laboratories
The effects of ionizing radiation on animals
We tend to think of biological effects of radiation 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:
- Cells experience DNA damage and are able to detect and repair the damage.
- 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.
- 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:
- 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.
- 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'.
- 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. 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 radioactive material.
- Laboratory and manufacturing accidents.
- Intentional and accidental high medical doses.
The effects of acute events are more easily studied than those of chronic exposure.
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 [2].
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 [3].) 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.
All ionizing radiation attacks living 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. Very high doses of radiation disrupt cells by wrecking large amounts of cellular machinery. Lower doses also wreck cellular machinery, but the damage can be effectively repaired, or doses sufficient to destroy cells outright affect cells in the process of replication more severely.
This syndrome was observed in many atomic bomb survivors in 1945 and emergency workers responding to the 1986 Chernobyl accident.
Approximately 134 plant workers and firefighters battling the fire 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 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 |
|---|---|
| 0.004 | Hourly cosmic dose on high-altitude flight [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 [7] |
| 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) [8] |
| 0.28 | Annual USA dose from natural terrestrial sources [9] |
| 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 [10] |
| 1 /a | USA NRC total exposure limit for the public |
| 1.1 /a | 1980 average USA radiation worker occupational dose [11] |
| 2 /a | USA average medical and natural background [12] Human internal radiation due to radon, varies with radon levels [13] |
| 2.2 | Average dose from upper gastrointestinal diagnostic X-ray series |
| 3 /a | USA average dose from all natural sources [14] |
| 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 [15] |
| 5 over 9 months | USA NRC occupational limit for pregnant women |
| 6.4 /a | High Background Radiation Area (HBRA) of Yangjiang, China [16] |
| 7.6 /a | Fountainhead Rock Place, Santa Fe, NM natural |
| 10–50 | USA EPA nuclear accident emergency action level [17] |
| 15 /a | Taiwan cobalt-60 10-year exposure, 97% lower cancer than population[18] |
| 50 | USA NRC annual occupational limit (10 CFR 20) |
| 100 acute | USA EPA acute dose level estimated to increase cancer risk 0.8% [19] |
| 120 | 30-year exposure, Ural mountains, lower cancer mortality rate[20] |
| 150 | USA NRC annual occupational eye lens exposure limit |
| 175 | Guarapari, Brazil annual natural radiation sources [21] |
| 250 acute | USA EPA voluntary maximum dose for emergency non-life-saving work [22] |
| 260 | Ramsar, Iran, annual natural background peak dose [23] |
| 500 | USA NRC occupational whole skin, limb skin, or single organ exposure limit 30-year exposure, Ural mountains, (exposed population lower cancer mortality rate) [24] |
| 750 acute | USA EPA voluntary maximum dose for emergency life-saving work [25] |
| 500–1000 acute | Low-level radiation sickness due to short-term exposure World War II nuclear bomb victims |
Minimizing health effects of ionizing radiation
Although exposure to ionizing radiation carries a risk, it is impossible to completely avoid exposure. Radiation has always been present in the environment and in our bodies. We can, however, avoid undue exposure.
Although people cannot sense ionizing radiation, there is a range of simple, sensitive instruments capable of detecting minute amounts 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. Badge dosimeters must be developed as photographic emulsion so the exposures can be counted and logged; once developed, they are discarded.
Geiger counters and scintillometers measure the dose rate of ionizing radiation directly.
In addition, there are four ways in which we can protect ourselves:
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: In the same way that the heat from a fire is less intense the further away you are, so the intensity of the radiation decreases the further you are form the source of the radiation. The dose decreases dramatically as you increase your distance from the source.
Shielding: Barriers of lead, concrete, or water give good protection from penetrating radiation such as gamma rays and neutrons. This is why certain 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. Inserting the proper shield between you and the radiation source will greatly reduce or eliminate the extra radiation dose.
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 100 R, 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
- Civil defense
- Electromagnetic radiation
- Fallout shelter
- Gamma ray
- Hormesis
- Irradiated mail
- Non-ionizing radiation
- Nuclear war
- Nuclear weapon
- Particle radiation
- Petkau effect
- Radiant energy
- Radiation poisoning
- Radiation therapy
- Radioactivity
- Radiobiology
- Radioresistance
- Radiosensitivity
External links
- The Nuclear Regulatory Commission regulates most commercial radiation sources and non-medical exposures in the US:
- Biological Effects of Low Level Exposures: Radiation Hormesis
- UNSCEAR 2000 Report, Volume 1:Sources
- UNSCEAR 2000 Report, Volume 2:Effects
- Beginners Guide to Ionising Radiation Measurement
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