In electronics, noise is an unwanted disturbance in an electrical signal.
Noise generated by electronic devices varies greatly as it is produced by several different effects.
In particular, noise is inherent in physics and central to thermodynamics. Any conductor with electrical resistance will generate thermal noise inherently. The final elimination of thermal noise in electronics can only be achieved cryogenically, and even then quantum noise would remain inherent.
While noise is generally unwanted, it can serve a useful purpose in some applications, such as random number generation or dither.
Noise types
Different types of noise are generated by different devices and different processes. Thermal noise is unavoidable at non-zero temperature (see fluctuation-dissipation theorem), while other types depend mostly on device type (such as shot noise, which needs a steep potential barrier) or manufacturing quality and semiconductor defects, such as conductance fluctuations, including 1/f noise.
Johnson–Nyquist noise (more often thermal noise) is unavoidable, and generated by the random thermal motion of charge carriers (usually electrons), inside an electrical conductor, which happens regardless of any applied voltage.
Shot noise in electronic devices results from unavoidable random statistical fluctuations of the electric current
when the charge carriers (such as electrons) traverse a gap. If
electrons flow across a barrier, then they have discrete arrival times.
Those discrete arrivals exhibit shot noise. Typically, the barrier in a
diode is used.
Shot noise is similar to the noise created by rain falling on a tin
roof. The flow of rain may be relatively constant, but the individual
raindrops arrive discretely.
The root-mean-square value of the shot noise current in is given by the Schottky formula.
where I is the DC current, q is the charge of an electron, and ΔB is the bandwidth in hertz. The Schottky formula assumes independent arrivals.
Vacuum tubes
exhibit shot noise because the electrons randomly leave the cathode and
arrive at the anode (plate). A tube may not exhibit the full shot noise
effect: the presence of a space charge tends to smooth out the arrival times (and thus reduce the randomness of the current). Pentodes and screen-grid tetrodes exhibit more noise than triodes because the cathode current splits randomly between the screen grid and the anode.
Conductors and resistors typically do not exhibit shot noise because the electrons thermalize
and move diffusively within the material; the electrons do not have
discrete arrival times. Shot noise has been demonstrated in mesoscopic resistors when the size of the resistive element becomes shorter than the electron–phonon scattering length.
Partition noise
Where current divides between two (or more) paths, noise occurs as a result of random fluctuations that occur during this division.
For this reason, a transistor will have more noise than the combined shot noise from its two PN junctions.
Flicker noise, also known as 1/f noise, is a signal or process with a frequency spectrum that falls off steadily into the higher frequencies, with a pink spectrum. It occurs in almost all electronic devices and results from a variety of effects.
Burst noise consists of sudden step-like transitions between two or
more discrete voltage or current levels, as high as several hundred microvolts,
at random and unpredictable times. Each shift in offset voltage or
current lasts for several milliseconds to seconds. It is also known as popcorn noise for the popping or crackling sounds it produces in audio circuits.
Transit-time noise
If
the time taken by the electrons to travel from emitter to collector in a
transistor becomes comparable to the period of the signal being
amplified, that is, at frequencies above VHF
and beyond, the transit-time effect takes place and the noise input
impedance of the transistor decreases. From the frequency at which this
effect becomes significant, it increases with frequency and quickly
dominates other sources of noise.
While noise may be generated in the electronic circuit itself,
additional noise energy can be coupled into a circuit from the external
environment, by inductive coupling or capacitive coupling, or through the antenna of a radio receiver.
Phenomenon in which a signal transmitted in one circuit or channel
of a transmission system creates undesired interference onto a signal in
another channel.
Also called static noise, it is caused by lightning discharges in thunderstorms and other electrical disturbances occurring in nature, such as corona discharge.
Industrial noise
Sources such as automobiles, aircraft, ignition electric motors and switching gear, High voltage wires and fluorescent lamps cause industrial noise. These noises are produced by the discharge present in all these operations.
Solar noise
Noise that originates from the Sun is called solar noise. Under normal conditions, there is approximately constant radiation from the Sun due to its high temperature, but solar storms can cause a variety of electrical disturbances. The intensity of solar noise varies over time in a solar cycle.
Distant stars generate noise called cosmic noise. While these stars are too far away to individually affect terrestrial communications systems,
their large number leads to appreciable collective effects. Cosmic
noise has been observed in a range from 8 MHz to 1.43 GHz, the latter
frequency corresponding to the 21-cm hydrogen line.
Apart from man-made noise, it is the strongest component over the range
of about 20 to 120 MHz. Little cosmic noise below 20MHz penetrates the
ionosphere, while its eventual disappearance at frequencies in excess of
1.5 GHz is probably governed by the mechanisms generating it and its
absorption by hydrogen in interstellar space.
Mitigation
In
many cases noise found on a signal in a circuit is unwanted. There are
many different noise reduction techniques that can reduce the noise
picked up by a circuit.
Faraday cage – A Faraday cage
enclosing a circuit can be used to isolate the circuit from external
noise sources. A Faraday cage cannot address noise sources that
originate in the circuit itself or those carried in on its inputs,
including the power supply.
Capacitive coupling – Capacitive coupling
allows an AC signal from one part of the circuit to be picked up in
another part through the interaction of electric fields. Where coupling
is unintended, the effects can be addressed through improved circuit
layout and grounding.
Ground loops – When grounding a circuit, it is important to avoid ground loops.
Ground loops occur when there is a voltage difference between two
ground connections. A good way to fix this is to bring all the ground
wires to the same potential in a ground bus.
Shielding cables – A shielded cable
can be thought of as a Faraday cage for wiring and can protect the
wires from unwanted noise in a sensitive circuit. The shield must be
grounded to be effective. Grounding the shield at only one end can avoid
a ground loop on the shield.
Twisted pair wiring – Twisting wires
in a circuit will reduce electromagnetic noise. Twisting the wires
decreases the loop size in which a magnetic field can run through to
produce a current between the wires. Small loops may exist between wires
twisted together, but the magnetic field going through these loops
induces a current flowing in opposite directions in alternate loops on
each wire and so there is no net noise current.
Notch filters – Notch filters or band-rejection filters are useful for eliminating a specific noise frequency. For example, power lines within a building run at 50 or 60 Hz line frequency. A sensitive circuit will pick up this frequency as noise. A notch filter tuned to the line frequency can remove the noise.
Thermal noise can be reduced by cooling of circuits - this is
typically only employed in high accuracy high-value applications such as
radio telescopes.
A noise signal is typically considered as a linear addition to a
useful information signal. Typical signal quality measures involving
noise are signal-to-noise ratio (SNR or S/N), signal-to-quantization noise ratio (SQNR) in analog-to-digital conversion and compression, peak signal-to-noise ratio (PSNR) in image and video coding and noise figure in cascaded amplifiers. In a carrier-modulated passband analogue communication system, a certain carrier-to-noise ratio
(CNR) at the radio receiver input would result in a certain
signal-to-noise ratio in the detected message signal. In a digital
communications system, a certain Eb/N0 (normalized signal-to-noise ratio) would result in a certain bit error rate.
Telecommunication systems strive to increase the ratio of signal level
to noise level in order to effectively transfer data. Noise in
telecommunication systems is a product of both internal and external
sources to the system.
Noise is a random process, characterized by stochastic properties such as its variance, distribution, and spectral density. The spectral distribution of noise can vary with frequency, so its power density is measured in watts per hertz (W/Hz). Since the power in a resistive
element is proportional to the square of the voltage across it, noise
voltage (density) can be described by taking the square root of the
noise power density, resulting in volts per root hertz (). Integrated circuit devices, such as operational amplifiers commonly quote equivalent input noise level in these terms (at room temperature).
Dither
If the noise source is correlated with the signal, such as in the case of quantisation error, the intentional introduction of additional noise, called dither,
can reduce overall noise in the bandwidth of interest. This technique
allows retrieval of signals below the nominal detection threshold of an
instrument. This is an example of stochastic resonance.
Background radiation is a measure of the level of ionizing radiation present in the environment at a particular location which is not due to deliberate introduction of radiation sources.
Background radiation is defined by the International Atomic Energy Agency
as "Dose or the dose rate (or an observed measure related to the dose
or dose rate) attributable to all sources other than the one(s)
specified.
A distinction is thus made between the dose which is already in a
location, which is defined here as being "background", and the dose due
to a deliberately introduced and specified source. This is important
where radiation measurements are taken of a specified radiation source,
where the existing background may affect this measurement. An example
would be measurement of radioactive contamination in a gamma radiation
background, which could increase the total reading above that expected
from the contamination alone.
However, if no radiation source is specified as being of concern,
then the total radiation dose measurement at a location is generally
called the background radiation, and this is usually the case where an ambient dose rate is measured for environmental purposes.
Background dose rate examples
Background radiation varies with location and time, and the following table gives examples:
Average annual human exposure to ionizing radiation in millisieverts (mSv) per year
peak of 0.11 mSv in 1963 and declining since; higher near sites
Occupational exposure
0.005
0.005
0.01
worldwide average to workers only is 0.7 mSv, mostly due to radon in mines; US is mostly due to medical and aviation workers.
Chernobyl accident
0.002
–
0.01
peak of 0.04 mSv in 1986 and declining since; higher near site
Nuclear fuel cycle
0.0002
0.001
up to 0.02 mSv near sites; excludes occupational exposure
Other
–
0.003
Industrial, security, medical, educational, and research
sub total (artificial)
0.61
3.14
2.33
Total
3.01
6.24
3.83
millisieverts per year
Natural background radiation
The weather station outside of the Atomic Testing Museum on a hot summer day. Displayed background gamma radiation level is 9.8 μR/h (0.82 mSv/a) This is very close to the world average background radiation of 0.87 mSv/a from cosmic and terrestrial sources.Cloud chambers
used by early researchers first detected cosmic rays and other
background radiation. They can be used to visualize the background
radiation
Radioactive material is found throughout nature. Detectable amounts occur naturally in soil, rocks, water, air, and vegetation, from which it is inhaled and ingested into the body. In addition to this internal exposure, humans also receive external exposure from radioactive materials that remain outside the body and from cosmic radiation from space. The worldwide average natural dose to humans is about 2.4 mSv (240 mrem) per year. This is four times the worldwide average artificial radiation exposure, which in 2008 amounted to about 0.6 millisieverts (60 mrem)
per year. In some developed countries, like the US and Japan,
artificial exposure is, on average, greater than the natural exposure,
due to greater access to medical imaging.
In Europe, average natural background exposure by country ranges from
under 2 mSv (200 mrem) annually in the United Kingdom to more than 7 mSv
(700 mrem) annually for some groups of people in Finland.
"Exposure to radiation from natural sources is an inescapable
feature of everyday life in both working and public environments. This
exposure is in most cases of little or no concern to society, but in
certain situations the introduction of health protection measures needs
to be considered, for example when working with uranium and thorium ores
and other Naturally Occurring Radioactive Material (NORM). These situations have become the focus of greater attention by the Agency in recent years."
Terrestrial radiation, for the purpose of the table above, only includes sources that remain external to the body. The major radionuclides of concern are potassium, uranium and thorium and their decay products, some of which, like radium and radon are intensely radioactive but occur in low concentrations. Most of these sources have been decreasing, due to radioactive decay
since the formation of the Earth, because there is no significant
amount currently transported to the Earth. Thus, the present activity on
Earth from uranium-238 is only half as much as it originally was because of its 4.5 billion year half-life, and potassium-40
(half-life 1.25 billion years) is only at about 8% of original
activity. But during the time that humans have existed the amount of
radiation has decreased very little.
Many shorter half-life (and thus more intensely radioactive)
isotopes have not decayed out of the terrestrial environment because of
their on-going natural production. Examples of these are radium-226 (decay product of thorium-230 in decay chain of uranium-238) and radon-222 (a decay product of radium-226 in said chain).
Thorium and uranium (and their daughters) primarily undergo alpha and beta decay, and are not easily detectable. However, many of their daughter products are strong gamma emitters. Thorium-232 is detectable via a 239 keV peak from lead-212, 511, 583 and 2614 keV from thallium-208, and 911 and 969 keV from actinium-228. Uranium-238 manifests as 609, 1120, and 1764 keV peaks of bismuth-214 (cf. the same peak for atmospheric radon). Potassium-40 is detectable directly via its 1461 keV gamma peak.
The level over the sea and other large bodies of water tends to
be about a tenth of the terrestrial background. Conversely, coastal
areas (and areas by the side of fresh water) may have an additional
contribution from dispersed sediment.
Airborne sources
The biggest source of natural background radiation is airborne radon, a radioactive gas that emanates from the ground. Radon and its isotopes, parent radionuclides, and decay products all contribute to an average inhaled dose of 1.26 mSv/a (millisievert per year).
Radon is unevenly distributed and varies with weather, such that much
higher doses apply to many areas of the world, where it represents a significant health hazard.
Concentrations over 500 times the world average have been found inside
buildings in Scandinavia, the United States, Iran, and the Czech
Republic.
Radon is a decay product of uranium, which is relatively common in the
Earth's crust, but more concentrated in ore-bearing rocks scattered
around the world. Radon seeps out of these ores into the atmosphere or into ground water or infiltrates into buildings. It can be inhaled into the lungs, along with its decay products, where they will reside for a period of time after exposure.
Although radon is naturally occurring, exposure can be enhanced
or diminished by human activity, notably house construction. A poorly
sealed dwelling floor, or poor basement ventilation, in an otherwise
well insulated house can result in the accumulation of radon within the
dwelling, exposing its residents to high concentrations. The widespread
construction of well insulated and sealed homes in the northern
industrialized world has led to radon becoming the primary source of
background radiation in some localities in northern North America and
Europe. Basement sealing and suction ventilation reduce exposure. Some building materials, for example lightweight concrete with alum shale, phosphogypsum and Italian tuff, may emanate radon if they contain radium and are porous to gas.
Radiation exposure from radon is indirect. Radon has a short half-life (4 days) and decays into other solid particulate radium-series
radioactive nuclides. These radioactive particles are inhaled and
remain lodged in the lungs, causing continued exposure. Radon is thus
assumed to be the second leading cause of lung cancer after smoking, and accounts for 15,000 to 22,000 cancer deaths per year in the US alone. However, the discussion about the opposite experimental results is still going on.
About 100,000 Bq/m3 of radon was found in Stanley Watras's basement in 1984. He and his neighbours in Boyertown, Pennsylvania,
United States may hold the record for the most radioactive dwellings in
the world. International radiation protection organizations estimate
that a committed dose may be calculated by multiplying the equilibrium equivalent concentration (EEC) of radon by a factor of 8 to 9 nSv·m3/Bq·h and the EEC of thoron by a factor of 40 nSv·m3/Bq·h.
Most of the atmospheric background is caused by radon and its decay products. The gamma spectrum shows prominent peaks at 609, 1120, and 1764 keV, belonging to bismuth-214,
a radon decay product. The atmospheric background varies greatly with
wind direction and meteorological conditions. Radon also can be released
from the ground in bursts and then form "radon clouds" capable of
traveling tens of kilometers.
Estimate
of the maximum dose of radiation received at an altitude of 12 km 20
January 2005, following a violent solar flare. The doses are expressed
in microsieverts per hour.
The Earth and all living things on it are constantly bombarded by
radiation from outer space. This radiation primarily consists of
positively charged ions from protons to iron and larger nuclei derived from outside the Solar System. This radiation interacts with atoms in the atmosphere to create an air shower of secondary radiation, including X-rays, muons, protons, alpha particles, pions, electrons, and neutrons.
The immediate dose from cosmic radiation is largely from muons,
neutrons, and electrons, and this dose varies in different parts of the
world based largely on the geomagnetic field and altitude. For example, the city of Denver in the United States (at 1650 meters elevation) receives a cosmic ray dose roughly twice that of a location at sea level. This radiation is much more intense in the upper troposphere, around 10 km altitude, and is thus of particular concern for airline
crews and frequent passengers, who spend many hours per year in this
environment. During their flights airline crews typically get an
additional occupational dose between 2.2 mSv (220 mrem) per year and 2.19 mSv/year, according to various studies.
Similarly, cosmic rays cause higher background exposure in astronauts than in humans on the surface of Earth. Astronauts in low orbits, such as in the International Space Station or the Space Shuttle, are partially shielded by the magnetic field of the Earth, but also suffer from the Van Allen radiation belt which accumulates cosmic rays and results from the Earth's magnetic field. Outside low Earth orbit, as experienced by the Apollo astronauts who traveled to the Moon,
this background radiation is much more intense, and represents a
considerable obstacle to potential future long term human exploration of
the Moon or Mars.
Cosmic rays also cause elemental transmutation in the atmosphere, in which secondary radiation generated by the cosmic rays combines with atomic nuclei in the atmosphere to generate different nuclides. Many so-called cosmogenic nuclides can be produced, but probably the most notable is carbon-14, which is produced by interactions with nitrogen
atoms. These cosmogenic nuclides eventually reach the Earth's surface
and can be incorporated into living organisms. The production of these
nuclides varies slightly with short-term variations in solar cosmic ray
flux, but is considered practically constant over long scales of
thousands to millions of years. The constant production, incorporation
into organisms and relatively short half-life of carbon-14 are the principles used in radiocarbon dating of ancient biological materials, such as wooden artifacts or human remains.
The cosmic radiation at sea level usually manifests as 511 keV gamma rays from annihilation of positrons
created by nuclear reactions of high energy particles and gamma rays.
At higher altitudes there is also the contribution of continuous bremsstrahlung spectrum.
Food and water
Two
of the essential elements that make up the human body, namely potassium
and carbon, have radioactive isotopes that add significantly to our
background radiation dose. An average human contains about 17 milligrams
of potassium-40 (40K) and about 24 nanograms (10−9 g) of carbon-14 (14C),
(half-life 5,730 years). Excluding internal contamination by external
radioactive material, these two are the largest components of internal
radiation exposure from biologically functional components of the human
body. About 4,000 nuclei of 40K decay per second, and a similar number of 14C. The energy of beta particles produced by 40K is about 10 times that from the beta particles from 14C decay.
14C is present in the human body at a level of about 3700 Bq (0.1 μCi) with a biological half-life of 40 days. This means there are about 3700 beta particles per second produced by the decay of 14C. However, a 14C atom is in the genetic information of about half the cells, while potassium is not a component of DNA. The decay of a 14C atom inside DNA in one person happens about 50 times per second, changing a carbon atom to one of nitrogen.
The global average internal dose from radionuclides other than
radon and its decay products is 0.29 mSv/a, of which 0.17 mSv/a comes
from 40K, 0.12 mSv/a comes from the uranium and thorium series, and 12 μSv/a comes from 14C.
Areas with high natural background radiation
Some areas have greater dosage than the country-wide averages. In the world in general, exceptionally high natural background locales include Ramsar in Iran, Guarapari in Brazil, Karunagappalli in India, Arkaroola in Australia and Yangjiang in China.
The highest level of purely natural radiation ever recorded on the Earth's surface was 90 µGy/h on a Brazilian black beach (areia preta in Portuguese) composed of monazite.
This rate would convert to 0.8 Gy/a for year-round continuous exposure,
but in fact the levels vary seasonally and are much lower in the
nearest residences. The record measurement has not been duplicated and
is omitted from UNSCEAR's latest reports. Nearby tourist beaches in Guarapari and Cumuruxatiba were later evaluated at 14 and 15 µGy/h. Note that the values quoted here are in Grays.
To convert to Sieverts (Sv) a radiation weighting factor is required;
these weighting factors vary from 1 (beta & gamma) to 20 (alpha
particles).
The highest background radiation in an inhabited area is found in Ramsar,
primarily due to the use of local naturally radioactive limestone as a
building material. The 1000 most exposed residents receive an average
external effective radiation dose of 6 mSv (600 mrem) per year, six times the ICRP recommended limit for exposure to the public from artificial sources.
They additionally receive a substantial internal dose from radon.
Record radiation levels were found in a house where the effective dose
due to ambient radiation fields was 131 mSv (13.1 rem) per year, and the
internal committed dose from radon was 72 mSv (7.2 rem) per year. This unique case is over 80 times higher than the world average natural human exposure to radiation.
Epidemiological studies are underway to identify health effects
associated with the high radiation levels in Ramsar. It is much too
early to draw unambiguous statistically significant conclusions. While so far support for beneficial effects of chronic radiation (like longer lifespan) has been observed in few places only,
a protective and adaptive effect is suggested by at least one study
whose authors nonetheless caution that data from Ramsar are not yet
sufficiently strong to relax existing regulatory dose limits.
However, the recent statistical analyses discussed that there is no
correlation between the risk of negative health effects and elevated
level of natural background radiation.
Photoelectric
Background
radiation doses in the immediate vicinity of particles of high atomic
number materials, within the human body, have a small enhancement due to
the photoelectric effect.
Neutron background
Most
of the natural neutron background is a product of cosmic rays
interacting with the atmosphere. The neutron energy peaks at around
1 MeV and rapidly drops above. At sea level, the production of neutrons
is about 20 neutrons per second per kilogram of material interacting
with the cosmic rays (or, about 100–300 neutrons per square meter per
second). The flux is dependent on geomagnetic latitude, with a maximum
near the magnetic poles. At solar minimums, due to lower solar magnetic
field shielding, the flux is about twice as high vs the solar maximum.
It also dramatically increases during solar flares. In the vicinity of
larger heavier objects, e.g. buildings or ships, the neutron flux
measures higher; this is known as "cosmic ray induced neutron
signature", or "ship effect" as it was first detected with ships at sea.
Artificial background radiation
Displays
showing ambient radiation fields of 0.120–0.130 μSv/h (1.05–1.14 mSv/a)
in a nuclear power plant. This reading includes natural background from
cosmic and terrestrial sources.
Atmospheric nuclear testing
Per capita thyroid doses in the continental United States resulting from all exposure routes from all atmospheric nuclear tests conducted at the Nevada Test Site from 1951 to 1962.Atmospheric 14C, New Zealand and Austria.
The New Zealand curve is representative for the Southern Hemisphere,
the Austrian curve is representative for the Northern Hemisphere.
Atmospheric nuclear weapon tests almost doubled the concentration of 14C in the Northern Hemisphere.
Frequent above-ground nuclear explosions between the 1940s and 1960s scattered a substantial amount of radioactive contamination.
Some of this contamination is local, rendering the immediate
surroundings highly radioactive, while some of it is carried longer
distances as nuclear fallout;
some of this material is dispersed worldwide. The increase in
background radiation due to these tests peaked in 1963 at about 0.15 mSv
per year worldwide, or about 7% of average background dose from all
sources. The Limited Test Ban Treaty
of 1963 prohibited above-ground tests, thus by the year 2000 the
worldwide dose from these tests has decreased to only 0.005 mSv per
year.
However, background radiation for occupational doses
includes radiation that is not measured by radiation dose instruments in
potential occupational exposure conditions. This includes both offsite
"natural background radiation" and any medical radiation doses. This
value is not typically measured or known from surveys, such that
variations in the total dose to individual workers is not known. This
can be a significant confounding factor in assessing radiation exposure
effects in a population of workers who may have significantly different
natural background and medical radiation doses. This is most significant
when the occupational doses are very low.
At an IAEA conference in 2002, it was recommended that occupational doses below 1–2 mSv per year do not warrant regulatory scrutiny.
Nuclear accidents
Radiation
level in a range of situations, from normal activities up to the
nuclear accidents. Each step up the scale indicates a tenfold increase
in radiation level.
Under normal circumstances, nuclear reactors release small amounts of
radioactive gases, which cause small radiation exposures to the public.
Events classified on the International Nuclear Event Scale
as incidents typically do not release any additional radioactive
substances into the environment. Large releases of radioactivity from
nuclear reactors are extremely rare. To the present day, there were two
major civilian accidents – the Chernobyl accident and the Fukushima I nuclear accidents – which caused substantial contamination. The Chernobyl accident was the only one to cause immediate deaths.
Total doses from the Chernobyl accident ranged from 10 to 50 mSv
over 20 years for the inhabitants of the affected areas, with most of
the dose received in the first years after the disaster, and over 100
mSv for liquidators. There were 28 deaths from acute radiation syndrome.
Total doses from the Fukushima I accidents were between 1 and 15
mSv for the inhabitants of the affected areas. Thyroid doses for
children were below 50 mSv. 167 cleanup workers received doses above 100
mSv, with 6 of them receiving more than 250 mSv (the Japanese exposure
limit for emergency response workers).
Non-civilian: In addition to the civilian accidents described above, several accidents at early nuclear weapons facilities – such as the Windscale fire, the contamination of the Techa River by the nuclear waste from the Mayak compound, and the Kyshtym disaster
at the same compound – released substantial radioactivity into the
environment. The Windscale fire resulted in thyroid doses of 5–20 mSv
for adults and 10–60 mSv for children. The doses from the accidents at Mayak are unknown.
Per UNECE life-cycle assessment, nearly all sources of energy result in some level of occupational and public exposure to radionuclides as result of their manufacturing or operations. The following table uses man·Sievert/GW-annum:
Source
Public
Occupational
Nuclear power
0.43
4.5
Coal (modern)
0.7
11
Coal (older)
1.4
11
Natural gas
0.1
0.02
Oil
0.0003
0.15
Geothermal
1–20
0.05
Solar power
0.8
Wind power
0.1
Biomass
0.01
Coal burning
Coal plants emit radiation in the form of radioactive fly ash which is inhaled and ingested by neighbours, and incorporated into crops. A 1978 paper from Oak Ridge National Laboratory
estimated that coal-fired power plants of that time may contribute a
whole-body committed dose of 19 µSv/a to their immediate neighbours in a
radius of 500 m. The United Nations Scientific Committee on the Effects of Atomic Radiation's
1988 report estimated the committed dose 1 km away to be 20 µSv/a for
older plants or 1 µSv/a for newer plants with improved fly ash capture,
but was unable to confirm these numbers by test.
When coal is burned, uranium, thorium and all the uranium daughters
accumulated by disintegration – radium, radon, polonium – are released.
Radioactive materials previously buried underground in coal deposits
are released as fly ash or, if fly ash is captured, may be incorporated
into concrete manufactured with fly ash.
Other sources of dose uptake
Medical
The global average human exposure to artificial radiation is 0.6 mSv/a, primarily from medical imaging. This medical component can range much higher, with an average of 3 mSv per year across the USA population.
Other human contributors include smoking, air travel, radioactive
building materials, historical nuclear weapons testing, nuclear power
accidents and nuclear industry operation.
A typical chest x-ray delivers 20 µSv (2 mrem) of effective dose. A dental x-ray delivers a dose of 5 to 10 µSv. A CT scan
delivers an effective dose to the whole body ranging from 1 to 20 mSv
(100 to 2000 mrem). The average American receives about 3 mSv of
diagnostic medical dose per year; countries with the lowest levels of
health care receive almost none. Radiation treatment for various
diseases also accounts for some dose, both in individuals and in those
around them.
Consumer items
Cigarettes contain polonium-210, originating from the decay products of radon, which stick to tobacco leaves.
Heavy smoking results in a radiation dose of 160 mSv/year to localized
spots at the bifurcations of segmental bronchi in the lungs from the
decay of polonium-210. This dose is not readily comparable to the
radiation protection limits, since the latter deal with whole body
doses, while the dose from smoking is delivered to a very small portion
of the body.
Radiation metrology
In a radiation metrology laboratory, background radiation
refers to the measured value from any incidental sources that affect an
instrument when a specific radiation source sample is being measured.
This background contribution, which is established as a stable value by
multiple measurements, usually before and after sample measurement, is
subtracted from the rate measured when the sample is being measured.
This is in accordance with the International Atomic Energy Agency
definition of background as being "Dose or dose rate (or an observed
measure related to the dose or dose rate) attributable to all sources
other than the one(s) specified.
The same issue occurs with radiation protection instruments,
where a reading from an instrument may be affected by the background
radiation. An example of this is a scintillation detector
used for surface contamination monitoring. In an elevated gamma
background the scintillator material will be affected by the background
gamma, which will add to the reading obtained from any contamination
which is being monitored. In extreme cases it will make the instrument
unusable as the background swamps the lower level of radiation from the
contamination. In such instruments the background can be continually
monitored in the "Ready" state, and subtracted from any reading obtained
when being used in "Measuring" mode.
Regular Radiation measurement is carried out at multiple levels.
Government agencies compile radiation readings as part of environmental
monitoring mandates, often making the readings available to the public
and sometimes in near-real-time. Collaborative groups and private
individuals may also make real-time readings available to the public.
Instruments used for radiation measurement include the Geiger–Müller tube and the Scintillation detector.
The former is usually more compact and affordable and reacts to
several radiation types, while the latter is more complex and can detect
specific radiation energies and types. Readings indicate radiation
levels from all sources including background, and real-time readings are
in general unvalidated, but correlation between independent detectors
increases confidence in measured levels.
List of near-real-time government radiation measurement sites, employing multiple instrument types:
A dirty bomb or radiological dispersal device is a radiological weapon
that combines radioactive material with conventional explosives. The
purpose of the weapon is to contaminate the area around the dispersal
agent/conventional explosion with radioactive material, serving
primarily as an area denial device against civilians.[1][2][3] It is not to be confused with a nuclear explosion, such as a fission bomb, which produces blast effects far in excess of what is achievable by the use of conventional explosives. Unlike the cloud of radiation from a typical fission bomb, a dirty bomb's radiation can be dispersed only within a few hundred meters or a few miles of the explosion.
Dirty bombs have never been used, only tested. They are designed
to disperse radioactive material over a certain area. They act through
the effects of radioactive contamination on the environment and related health effects of radiation poisoning in the affected populations. The containment and decontamination
of victims, as well as decontamination of the affected area require
considerable time and expenses, rendering areas partly unusable and
causing economic damage. Dirty bombs might be used to create mass panic as a weapon of terror.
Effect of a dirty bomb explosion
When dealing with the implications of a dirty bomb attack, there are two main areas to be addressed: the civilian impact, not only dealing with immediate casualties and long term health issues, but also the psychological
effect, and the economic impact. With no prior event of a dirty bomb
detonation, it is considered difficult to predict the impact. Several
analyses have predicted that radiological dispersal devices will neither
sicken nor kill many people.
Differences between dirty bombs and fission bombs
Dirty bomb
Explosives combined with radioactive materials
Detonation vaporizes or aerosolizes radioactive material and propels it into the air
Not a nuclear detonation
Fission bomb
Caused by an uncontrolled nuclear chain reaction with highly enriched uranium or weapons-grade plutonium
Sphere of fissile material (pit) surrounded by explosive lenses
Initial explosion produces imploding shock wave that compresses pit inward, creating supercritical mass by increasing the density of fissile material. Neutrons from either modulated neutron initiator or external neutron generator start chain reaction in compressed pit
Resulting fission chain reaction causes bomb to explode with tremendous force: nuclear detonation
Source: Adapted from Levi MA, Kelly HC. "Weapons of mass disruption". Sci Am. 2002 Nov;287(5):76-81.
One example is the radiological accident occurring in Goiânia, Brazil, between September 1987 and March 1988: Two metal scavengers broke into an abandoned radiotherapy clinic and removed a teletherapy source capsule containing powdered caesium-137 with an activity of 50 TBq.
They brought it back to the home of one of the men to take it apart and
sell as scrap metal. Later that day both men were showing acute signs
of radiation illness with vomiting and one of the men had a swollen hand and diarrhea. A few days later one of the men punctured the 1-millimetre-thick (0.039 in) thick window of the capsule, allowing the caesium chloride
powder to leak out and when realizing the powder glowed blue in the
dark, brought it back home to his family and friends to show it off.
After two weeks of spread by contact contamination causing an increasing
number of adverse health effects, the correct diagnosis of acute radiation sickness
was made at a hospital and proper precautions could be put into
procedure. By this time 249 people were contaminated, 151 exhibited both
external and internal contamination of which 20 people were seriously
ill and five people died.
The Goiânia incident to some extent predicts the contamination
pattern if it is not immediately realized that the explosion spread
radioactive material, but also how fatal even very small amounts of
ingested radioactive powder can be. This raises worries of terrorists using powdered alpha emitting material, that if ingested can pose a serious health risk, as in the case of Alexander Litvinenko, who was poisoned by tea with polonium-210. "Smoky bombs" based on alpha emitters might easily be just as dangerous as beta or gamma emitting dirty bombs.
Public perception of risks
Although
the exposure might be minimal, many people find radiation exposure
especially frightening because it is something they cannot see or feel,
and it therefore becomes an unknown source of danger. When United States Attorney General John Ashcroft on June 10, 2002, announced the arrest of José Padilla, allegedly plotting to detonate such a weapon, he said:
[A] radioactive "dirty bomb" ...
spreads radioactive material that is highly toxic to humans and can
cause mass death and injury.
— Attorney General John Ashcroft
This public fear of radiation also plays a big role in why the costs
of a radiological dispersal device impact on a major metropolitan area
(such as lower Manhattan) might be equal to or even larger than that of
the 9/11 attacks. Assuming the radiation levels are not too high and the area does not need to be abandoned such as the town of Pripyat near the Chernobyl reactor,
an expensive and time-consuming cleanup procedure will begin. This will
mainly consist of tearing down highly contaminated buildings, digging
up contaminated soil and quickly applying sticky substances to remaining
surfaces so that radioactive particles adhere before radioactivity
penetrates the building materials. These procedures are the current state of the art for radioactive contamination
cleanup, but some experts say that a complete cleanup of external
surfaces in an urban area to current decontamination limits may not be
technically feasible.
Loss of working hours will be vast during cleanup, but even after the
radiation levels reduce to an acceptable level, there might be residual
public fear of the site including possible unwillingness to conduct
business as usual in the area. Tourist traffic is likely never to resume.
Since the 9/11 attacks, the fear of terrorist groups using dirty bombs has increased, which has been frequently reported in the media. The meaning of terrorism used here, is described by the U.S. Department of Defense's
definition, which is "the calculated use of unlawful violence or threat
of unlawful violence to inculcate fear; intended to coerce or to
intimidate governments or societies in the pursuit of goals that are
generally political, religious, or ideological."
Constructing and obtaining material for a dirty bomb
In
order for a terrorist organization to construct and detonate a dirty
bomb, it must acquire radioactive material. Possible radiological
dispersal device material could come from the millions of radioactive
sources used worldwide in the industry, for medical purposes and in
academic applications mainly for research. Of these sources, only nine reactor-produced isotopes stand out as being suitable for radiological terror: americium-241, californium-252, caesium-137, cobalt-60, iridium-192, plutonium-238, polonium-210, radium-226 and strontium-90, and even from these it is possible that radium-226 and polonium-210 do not pose a significant threat. Of these sources the U.S. Nuclear Regulatory Commission has estimated that within the U.S., approximately one source is lost, abandoned or stolen every day of the year. Within the European Union the annual estimate is 70.
There exist thousands of such "orphan" sources scattered throughout the
world, but of those reported lost, no more than an estimated 20 percent
can be classified as potential high security concerns if used in a
radiological dispersal device. Russia is believed to house thousands of orphan sources, which were lost following the collapse of the Soviet Union.
A large but unknown number of these sources probably belong to the high
security risk category. These include the beta-emitting strontium-90
sources used as radioisotope thermoelectric generators for beacons in lighthouses in remote areas of Russia. In December 2001, three Georgian
woodcutters stumbled over such a power generator and dragged it back to
their camp site to use it as a heat source. Within hours they suffered
from acute radiation sickness and sought hospital treatment. The International Atomic Energy Agency (IAEA) later stated that it contained approximately 40 kilocuries (1.5 PBq) of strontium, equivalent to the amount of radioactivity released immediately after the Chernobyl accident (though the total radioactivity release from Chernobyl was 2500 times greater at around 100 MCi (3,700 PBq)).
Although a terrorist organization might obtain radioactive material through the "black market",
and there has been a steady increase in illicit trafficking of
radioactive sources from 1996 to 2004, these recorded trafficking
incidents mainly refer to rediscovered orphan sources without any sign
of criminal activity, and it has been argued that there is no conclusive evidence for such a market.
In addition to the hurdles of obtaining usable radioactive material,
there are several conflicting requirements regarding the properties of
the material the terrorists need to take into consideration: First, the
source should be "sufficiently" radioactive to create direct
radiological damage at the explosion or at least to perform societal
damage or disruption. Second, the source should be transportable with
enough shielding to protect the carrier, but not so much that it will be
too heavy to maneuver. Third, the source should be sufficiently
dispersible to effectively contaminate the area around the explosion.
Possibility of use by terrorist groups
The first attempt of radiological terror was reportedly carried out in November 1995 by a group of Chechen separatists, who buried a caesium-137 source wrapped in explosives at the Izmaylovsky Park in Moscow. A Chechen rebel leader alerted the media, the bomb was never activated, and the incident amounted to a mere publicity stunt.
In December 1998, a second attempt was announced by the Chechen
Security Service, who discovered a container filled with radioactive
materials attached to an explosive mine. The bomb was hidden near a
railway line in the suburban area Argun, ten miles east of the Chechen capital of Grozny. The same Chechen separatist group was suspected to be involved.
On 8 May 2002, José Padilla
(a.k.a. Abdulla al-Muhajir) was arrested on suspicion that he was an
al-Qaeda terrorist planning to detonate a dirty bomb in the U.S. This
suspicion was raised by information obtained from an arrested terrorist
in U.S. custody, Abu Zubaydah,
who under interrogation revealed that the organization was close to
constructing a dirty bomb. Although Padilla had not obtained radioactive
material or explosives at the time of arrest, law enforcement
authorities uncovered evidence that he was on reconnaissance for usable
radioactive material and possible locations for detonation.
It has been doubted whether José Padilla was preparing such an attack,
and it has been claimed that the arrest was highly politically
motivated, given the pre-9/11 security lapses by the CIA and FBI.
In 2006, Dhiren Barot
from North London pleaded guilty of conspiring to murder people in the
United Kingdom and United States using a radioactive dirty bomb. He
planned to target underground car parks within the UK and buildings in the U.S. such as the International Monetary Fund, World Bank buildings in Washington D.C., the New York Stock Exchange, Citigroup buildings and the Prudential Financial buildings in Newark, New Jersey. He also faces 12 other charges including, conspiracy to commit public nuisance,
seven charges of making a record of information for terrorist purposes
and four charges of possessing a record of information for terrorist
purposes. Experts say if the plot to use the dirty bomb was carried out
"it would have been unlikely to cause deaths, but was designed to affect
about 500 people".
In January 2009, a leaked FBI report described the results of a search of the Maine home of James G. Cummings, a white supremacist
who had been shot and killed by his wife. Investigators found four
one-gallon containers of 35 percent hydrogen peroxide, uranium, thorium,
lithium metal, aluminum powder, beryllium,
boron, black iron oxide and magnesium as well as literature on how to
build dirty bombs and information about caesium-137, strontium-90 and
cobalt-60, radioactive materials. Officials confirmed the veracity of the report but stated that the public was never at risk.
In July 2014, ISIS militants seized 88 pounds (40 kg) of uranium compounds from Mosul University.
The material was unenriched and so could not be used to build a
conventional fission bomb, but a dirty bomb is a theoretical
possibility. However, uranium's relatively low radioactivity makes it a
poor candidate for use in a dirty bomb.
Terrorist organizations may also capitalize on the fear of radiation
to create weapons of mass disruption rather than weapons of mass
destruction. A fearful public response may in itself accomplish the
goals of a terrorist organization to gain publicity or destabilize
society.
Even simply stealing radioactive materials may trigger a panic reaction
from the general public. Similarly, a small-scale release of
radioactive materials or a threat of such a release may be considered
sufficient for a terror attack.
Particular concern is directed towards the medical sector and
healthcare sites which are "intrinsically more vulnerable than
conventional licensed nuclear sites".
Opportunistic attacks may range to even kidnapping patients whose
treatment involve radioactive materials. Of note is the public reaction
to the Goiânia accident,
in which over 100,000 people admitted themselves to monitoring, while
only 49 were admitted to hospitals. Other benefits to a terrorist
organization of a dirty bomb include economic disruption in the area
affected, abandonment of affected assets (such a buildings, subways) due
to public concern, and international publicity useful for recruitment.
Tests
Israel
carried out a four-year series of tests on nuclear explosives to measure
the effects were hostile forces ever to use them against Israel, Haaretz
reported in 2015. According to the report, high-level radiation was
measured only at the center of the explosions, while the level of
dispersal of radiation by particles carried by the wind (fallout) was
low. The bombs reportedly did not pose a significant danger beyond their
psychological effect.
Detection and prevention
Dirty bombs may be prevented by detecting illicit radioactive materials in shipping with tools such as a Radiation Portal Monitor. Similarly, unshielded radioactive materials may be detected at checkpoints by Geiger Counters, gamma-ray detectors, and even Customs and Border Patrol (CBS) pager-sized radiation detectors.
Hidden materials may also be detected by x-ray inspection and heat
emitted may be picked up by infrared detectors. Such devices, however,
may be circumvented by simply transporting materials across unguarded
stretches of coastline or other barren border areas.
One proposed method for detecting shielded Dirty Bombs is Nanosecond Neutron Analysis (NNA).
Designed originally for the detection of explosives and hazardous
chemicals, NNA is also applicable to fissile materials. NNA determines
what chemicals are present in an investigated device by analyzing
emitted γ-emission neutrons and α-particles created from a reaction in
the neutron generator. The system records the temporal and spatial
displacement of the neutrons and α-particles within separate 3D regions.
A prototype dirty-bomb detection device created with NNA is
demonstrated to be able to detect uranium from behind a 5 cm-thick lead
wall.
Other radioactive material detectors include Radiation Assessment and
Identification (RAID) and Sensor for Measurement and Analysis of
Radiation Transients, both developed by Sandia National Laboratories. Sodium iodidescintillator based aerial radiation detection systems are capable to detect International Atomic Energy Agency (IAEA) defined dangerous quantities of radioactive material and have been deployed by the New York City Police Department (NYPD) Counterterrorism Bureau.
The IAEA recommends certain devices be used in tandem at country
borders to prevent transfer of radioactive materials, and thus the
building of dirty bombs.
They define the four main goals of radiation detection instruments as
detection, verification, assessment and localization, and identification
as a means to escalate a potential radiological situation. The IAEA
also defines the following types of instruments:
Pocket-Type Instruments: these instruments provide a low-power,
mobile option to detection that allows for security officers to
passively scan an area for radioactive materials. These devices should
be easily worn, should have an alarm threshold of three times normal
radiation levels, and should have a long battery life - over 800 hours.
Handheld Instruments: these instruments may be used to detect all
types of radiation (including neutron) and may be used to search
specific targets flexibly. These instruments should aim for ease of use
and speed, ideally weighing less than 2 kg and being able to make
measurements in less than a second.
Fixed, installed instruments: these instruments provide a
continuous, automatic detection system that can monitor pedestrians and
vehicles that pass through. To work effectively pedestrians and vehicles
should be led close to the detectors, as performance is directly
related to range.
Legislative and regulatory actions can also be used to prevent access
to materials needed to create a dirty bomb. Examples include the 2006
U.S. Dirty Bomb Bill, the Yucca Flats proposal, and the Nunn-Lungar act.
Similarly, close monitoring and restrictions of radioactive materials
may provide security for materials in vulnerable private-sector
applications, most notably in the medical sector where such materials
are used for treatments. Suggestions for increased security include isolation of materials in remote locations and strict limitation of access.
One way to mitigate a major effect of a radiological weapons may
also be to educate the public on the nature of radioactive materials. As
one of the major concerns of a dirty bomb is the public panic proper
education may prove a viable counter-measure. Education on radiation is considered by some to be "the most neglected issue related to radiological terrorism".
The dangers of a dirty bomb come from the initial blast and the radioactive materials To mitigate the risk of radiation exposure, FEMA suggests the following guidelines:
Covering the mouth/nose with cloth to reduce risk of breathing in radioactive materials.
Avoiding touching materials touched by the explosion.
Quickly relocating inside to shield from radiation.
Remove and pack up clothes. Keep clothes until instructed by authorities how to dispose of them.
Keep radioactive dust outside.
Remove all dust possible by showering with soap and water.
Avoid taking potassium iodide, as it only prevents effects from radioactive iodine and may instead cause a dangerous reaction.
Treatment
As of 2023,
research is under way to find radioactive decontanimation drugs to
remove radioactive elements from the body. One drug candidate under
investigation is HOPO 14-1.
In popular culture
In the 2004 TV movie Dirty War London was struck by a dirty bomb.
The crime drama television series Numb3rs has an episode that revolves around a dirty bomb (season 1, episode 10).
In a two-part 2011 episode of Castle, former U.S. soldiers plot to detonate a dirty bomb in New York City and frame a Syrian immigrant for the crime.
In the 2012 series finale of Flashpoint, an officer is poisoned by caesium from a dirty bomb and is administered Prussian blue to assist in recovery.
In the 2013 Indian movie Vishwaroopam, the plot revolves around a dirty bomb developed by scraping caesium from oncological equipment to trigger a blast in New York City.
In the 2019 BBC drama Years and Years, Leeds and Bristol is struck by a dirty bomb in a terrorist attack, it is hinted at that it was organised by the British government.
In the 2020 video game Call of Duty: Black Ops Cold War, there is a game mode in the Multiplayer mode where squads compete to collect uranium and use it to plant dirty bombs on enemy positions.
