From Wikipedia, the free encyclopedia Radiation
Radiation
The question of harm to biological systems due to
low-power ionizing and non-ionization radiation is not
settled. Controversy continues about possible non-heat-
ing effects of low-power non-ionizing radiation, such as
non-heating microwave and radio wave exposure. Non-
ionizing radiation is usually considered to have no com-
pletely safe lower limit, although at some energy levels,
new exposures do not add appreciably to background ra-
diation. The evidence that small amounts of some types
of ionizing radiation might confer a net health benefit in
Illustration of the relative abilities of three different types of some situations is called radiation hormesis.
ionizing radiation to penetrate solid matter. Alpha particles (α)
are stopped by a sheet of paper while beta particles (β) are
stopped by an aluminium plate. Gamma radiation (γ) is damp- Ionizing radiation
ened when it penetrates matter.
Main article: Ionizing radiation
Radiation with sufficiently high energy can ionize atoms.
In physics, radiation is a process in which energetic par-
Most often, this occurs when an electron is stripped (or
ticles or energetic waves travel through a medium or
"knocked out") from an electron shell, which leaves the
space. Two types of radiation are commonly differentiat-
atom with a net positive charge. Because cells and more
ed in the way they interact with normal chemical matter:
importantly the DNA can be damaged, this ionization can
ionizing and non-ionizing radiation. The word radiation
result in an increased chance of cancer. An individual cell
is often colloquially used in reference to ionizing radia-
is made of trillions of atoms. The probability of ionizing
tion (i.e., radiation having sufficient energy to ionize an
radiation causing cancer is dependent upon the absorbed
atom), but the term radiation may correctly also refer to
dose of the radiation, as adjusted for the damaging ten-
non-ionizing radiation (e.g., radio waves, heat or visible
dency of the type of radiation (equivalent dose) and the
light). The particles or waves radiate (i.e., travel outward
sensitivity of the organism or tissue being irradiated (ef-
in all directions) from a source. This aspect leads to a sys-
fective dose).
tem of measurements and physical units that are applic-
Roughly speaking, photons and particles with ener-
able to all types of radiation.
gies above about 10 electron volts (eV) are ionizing. Al-
Both ionizing and non-ionizing radiation can be
pha particles, beta particles, cosmic rays, gamma rays,
harmful to organisms and can result in changes to the
and X-ray radiation all carry energy high enough to ion-
natural environment. In general, however, ionizing radi-
ize atoms. In addition, free neutrons are also ionizing,
ation is far more harmful to living organisms per unit of
since their interactions with matter are inevitably more
energy deposited than non-ionizing radiation, since the
energetic than this threshold.
ions that are produced by ionizing radiation, even at low
Ionizing radiation comes from radioactive materials,
radiation powers, have the potential to cause DNA dam-
X-ray tubes, particle accelerators, and is present in the
age. By contrast, most non-ionizing radiation is harmful
environment. It is invisible and not directly detectable
to organisms only in proportion to the thermal energy
by human senses, so instruments such as Geiger counters
deposited, and is conventionally considered harmless at
are usually required to detect its presence. In some cases,
low powers which do not produce significant tempera-
it may lead to secondary emission of visible light upon in-
ture rise. Ultraviolet radiation in some aspects occupies
teraction with matter, as in Cherenkov radiation and ra-
a middle ground, in having some features of both ion-
dioluminescence. It has many practical uses in medicine,
izing and non-ionizing radiation. Although nearly all of
research, construction, and other areas, but presents a
the ultraviolet spectrum of radiation is non-ionizing, at
health hazard if used improperly. Exposure to radiation
the same time ultraviolet radiation does far more damage
causes damage to living tissue, resulting in skin burns,
to many molecules in biological systems than is account-
radiation sickness and death at high doses and cancer,[1]
ed for by heating effects (an example is sunburn). These
tumors and genetic damage at low doses.
properties derive from ultraviolet’s power to alter chem-
Electromagnetic radiation (sometimes abbreviated
ical bonds, even without having quite enough energy to
EMR) takes the form of self-propagating waves in a vac-
ionize atoms.
uum or in matter. EM radiation has an electric and mag-
1
From Wikipedia, the free encyclopedia Radiation
netic field component which oscillate in phase perpen- Alpha radiation is dangerous when alpha-emitting ra-
dicular to each other and to the direction of energy prop- dioisotopes are ingested (breathed or swallowed). This
agation. Electromagnetic radiation is classified into types brings the radioisotope close enough to tissue for the al-
according to the frequency of the wave, these types in- pha radiation to damage cells. Per unit of energy, alpha
clude (in order of increasing frequency): radio waves, particles are at least 20 times more effective at cell-dam-
microwaves, terahertz radiation, infrared radiation, vis- age as gamma rays and X-rays. See relative biological ef-
ible light, ultraviolet radiation, X-rays and gamma rays. fectiveness for a discussion of this. Examples of highly
Of these, radio waves have the longest wavelengths and poisonous alpha-emitters are radium, radon, and poloni-
gamma rays have the shortest. A small window of fre- um.
quencies, called visible spectrum or light, is sensed by the
eye of various organisms. Beta
Ionizing electromagnetic radiation is that for which Main article: Beta decay
the photons making up the radiation have energies larg- Beta-minus (β−) radiation consists of an energetic elec-
er than about 10 electron volts. The ability of an electro- tron. It is more ionizing than alpha radiation, but less
magnetic wave (photons) to ionize an atom or molecule than gamma. Beta radiation from radioactive decay can
thus depends on its frequency, which determines the en- be stopped with a few centimeters of plastic or a few mil-
ergy of a photon of the radiation. An energy of 10 eV is limeters of metal. It occurs when a neutron decays into
about 1.6×10−18 joules, which is a typical binding energy a proton in a nucleus, releasing the beta particle and an
of an outer electron to an atom or organic molecule.[2] antineutrino. Beta radiation from linac accelerators is far
This corresponds with a frequency of 2.4×1015 Hz, and a more energetic and penetrating than natural beta radia-
wavelength of 125 nm (this is in far ultraviolet).[3] Radia- tion. It is sometimes used therapeutically in radiotherapy
tion on the short-wavelength end of the electromagnet- to treat superficial tumors.
ic spectrum, and above 125 nm, is ionizing. This includes Beta-plus (β+) radiation is the emission of positrons.
extreme ultraviolet, X-rays, and gamma rays. Because these are antimatter particles, they annihilate
Most of the ultraviolet spectrum (which begins above any matter nearby, releasing gamma photons.
energies of 3.1 eV (400 nm) is non-ionizing, but is still
biologically hazardous due the ability of single photons Neutron
of this energy to cause electronic excitation in biological
Main articles: Neutron radiation and Neutron tempera-
molecules, and thus damage them by means of unwanted
ture
reactions. An example is formation of pyrimidine dimers
Neutrons are categorized according to their speed. Neu-
in DNA. This property gives the ultraviolet spectrum
tron radiation consists of free neutrons. These neutrons
some of the dangers of ionizing radiation in biological
may be emitted during either spontaneous or induced
systems, without actual ionization occurring. In contrast
nuclear fission, nuclear fusion processes, or from any
visible light and longer-wavelength electromagnetic ra-
other nuclear reactions.
diation, such as infrared, microwaves, and radio waves,
Neutrons are the only type of ionizing radiation that
consists of photons with too little energy to cause dam-
can make other objects, or material, radioactive. This
aging molecular excitation, and thus this radiation is far
process, called neutron activation, is the primary method
less hazardous per unit of energy.
used to produce radioactive sources for use in medical,
academic, and industrial applications. Even comparative-
Alpha ly low speed thermal neutrons, which do not carry
Main article: Alpha decay enough kinetic energy individually to be ionizing, will
Alpha particles travel at speeds in excess of 5% of the cause neutron activation (in fact, they cause it more effi-
speed of light, but they interact with matter very heavily, ciently). Such neutrons are "indirectly ionizing."
and thus at their usual velocities only penetrate a few Neutrons do not ionize atoms in the same way that
centimeters of air, or a few millimeters of low density charged particles such as protons and electrons do (excit-
material (such as the thin mica material which is spe- ing an electron), because neutrons have no charge. How-
cially placed in some Geiger counter tubes to allow alpha ever, both slow and fast neutrons react with the atomic
particles in). This means that alpha particles from or- nuclei of many elements upon collision with nuclei, cre-
dinary alpha decay do not penetrate skin and cause no ating unstable isotopes and therefore inducing radioac-
damage to tissues below. Some very high energy alpha tivity in a previously non-radioactive material. This is
particles compose about 10% of cosmic rays, and these neutron activation.
are capable of penetrating the body and even thin metal In addition, high-energy (high-speed) neutrons have
plates. However, they are of danger only to astronauts, the ability to directly ionize atoms. One mechanism by
since they are deflected by the Earth’s magnetic field and which high energy neutrons ionize atoms is to strike the
then stopped by its atmosphere. nucleus of an atom and knock the atom out of a molecule,
2
From Wikipedia, the free encyclopedia Radiation
leaving one or more electrons behind as the chemical stopped by a sufficiently thick layer of material, where
bond is broken. This leads to production of chemical free stopping power of the material per given area depends
radicals. In addition, very high energy neutrons can mostly (but not entirely) on its total mass, whether the
cause ionizing radiation by "neutron spallation" or material is of high or low density. However, as is the case
knockout, wherein neutrons cause emission of high-en- with X-rays, materials with high atomic number such as
ergy protons from atomic nuclei (especially hydrogen lead or depleted uranium add a modest (typically 20% to
nuclei) on impact. The last process imparts most of the 30%) amount of stopping power over an equal mass of
neutron’s energy to the proton, much like one billiard less-dense and lower atomic weight materials (such as
ball striking another. The charged protons, and other water or concrete).
products from such reactions are directly ionizing.
High-energy neutrons are very penetrating and can
travel great distances in air (hundreds or even thousands
Non-ionizing radiation
of meters) and moderate distances (several meters) in Main article: Non-ionizing radiation
common solids. They typically require hydrogen rich The energy of non-ionizing radiation is less and instead
shielding, such as concrete or water, to block them with- of producing charged ions when passing through matter,
in distances of less than a meter. A common source of the electromagnetic radiation has only sufficient energy
neutron radiation occurs inside a nuclear reactor, where to change the rotational, vibrational or electronic va-
meters-thick water layer is used as effective shielding. lence configurations of molecules and atoms. The effect
of non-ionizing forms of radiation on living tissue has on-
X-ray ly recently been studied. Nevertheless, different biologi-
Main article: X-ray cal effects are observed for different types of non-ioniz-
X-rays are electromagnetic waves with a wavelength ing radiation.[1][4]
smaller than about 10 nanometers. A smaller wavelength Even "non-ionizing" radiation is capable of causing
corresponds to a higher energy according to the equation thermal-ionization if it deposits enough heat to raise
E=hc/λ. ("E" is Energy; "h" is Planck’s constant; "c" is the temperatures to ionization energies. These reactions oc-
speed of light; "λ" is wavelength.) A "packet" of electro- cur at far higher energies than with non-ionization ra-
magnetic waves is called a photon. When an X-ray pho- diation, which requires only single particles to ionize. A
ton collides with an atom, the atom may absorb the ener- familiar example of thermal ionization is the flame-ion-
gy of the photon and boost an electron to a higher orbital ization of a common fire, and the browning (chemical
level or if the photon is very energetic, it may knock an process) reactions in common food items induced by in-
electron from the atom altogether, causing the atom to frared radiation, during broiling-type cooking.
ionize. Generally, a larger atom is more likely to absorb
an X-ray photon, since larger atoms have greater energy Non-ionizing electromagnetic radiation
differences between orbital electrons. Soft tissue in the Main article: Electromagnetic radiation
human body is composed of smaller atoms than the calci- The non-ionizing portion of electromagnetic radiation
um atoms that make up bone, hence there is a contrast in consists of electromagnetic waves that (as individual
the absorption of X-rays. X-ray machines are specifically quanta or particles, see photon) are not energetic enough
designed to take advantage of the absorption difference to detach electrons from atoms or molecules, ionizing
between bone and soft tissue, allowing physicians to ex- them. These include radio waves, microwaves, infrared,
amine structure in the human body. and (sometimes) visible light. (Ultraviolet light, X-rays
and gamma-rays are regarded as ionizing.) The occur-
Gamma rence of ionization depends on the energy of the individ-
Main article: Gamma ray ual particles or waves, and not on their number. An in-
Gamma (γ) radiation consists of photons with a frequen- tense flood of particles or waves will not cause ionization
cy of greater than 1019 Hz.[1] Gamma radiation occurs if these particles or waves do not carry enough energy to
to rid the decaying nucleus of excess energy after it has be ionizing, unless they raise the temperature of a body
emitted either alpha or beta radiation. Both alpha and to a point high enough to ionize small fractions of atoms
beta particles have an electric charge and mass, and thus or molecules by the process of thermal-ionization (this
are quite likely to interact with other atoms in their path. requires relatively extreme radiation energies, however).
Gamma radiation is composed of photons, and photons The electromagnetic spectrum is the range of all pos-
have neither mass nor electric charge. Gamma radiation sible electromagnetic radiation frequencies.[1] The elec-
penetrates much further through matter than either al- tromagnetic spectrum (usually just spectrum) of an ob-
pha or beta radiation. ject is the characteristic distribution of electromagnetic
Gamma rays, which are highly energetic photons, radiation emitted by, or absorbed by, that particular ob-
penetrate deeply and are difficult to stop. They can be ject.
3
From Wikipedia, the free encyclopedia Radiation
tion includes both UHF and EHF (millimeter waves), and
various sources use different boundaries.[1] In all cases,
microwave includes the entire SHF band (3 to 30 GHz,
or 10 to 1 cm) at minimum, with RF engineering often
putting the lower boundary at 1 GHz (30 cm), and the up-
per around 100 GHz (3mm).
Radio waves
Main article: Radio waves
Radio waves are a type of electromagnetic radiation with
wavelengths in the electromagnetic spectrum longer
than infrared light. Like all other electromagnetic waves,
they travel at the speed of light. Naturally occurring ra-
dio waves are made by lightning, or by astronomical ob-
jects. Artificially generated radio waves are used for fixed
and mobile radio communication, broadcasting, radar
and other navigation systems, satellite communication,
computer networks and innumerable other applications.
Different frequencies of radio waves have different prop-
agation characteristics in the Earth’s atmosphere; long
waves may cover a part of the Earth very consistently,
shorter waves can reflect off the ionosphere and travel
around the world, and much shorter wavelengths bend
or reflect very little and travel on a line of sight.
Very low frequency (VLF)
Very low frequency or VLF refers to radio frequencies
(RF) in the range of 3 to 30 kHz. Since there is not much
The electromagnetic spectrum bandwidth in this band of the radio spectrum, only the
very simplest signals are used, such as for radio naviga-
Visible light tion. Also known as the myriameter band or myriameter
Main article: Light wave as the wavelengths range from ten to one myriam-
Light, or visible light, is a very narrow range of electro- eter (an obsolete metric unit equal to 10 kilometers)
magnetic radiation of a wavelength that is visible to the
Extremely low frequency (ELF)
human eye (about 400–700 nm), or up to 380–750 nm.[1]
More broadly, physicists refer to light as electromagnetic Extremely low frequency (ELF) is a term used to describe
radiation of all wavelengths, whether visible or not. radiation frequencies from 3 to 30 Hz. In atmosphere sci-
ence, an alternative definition is usually given, from 3 Hz
Infrared to 3 kHz.[1] In the related magnetosphere science, the
Main article: Infrared lower frequency electromagnetic oscillations (pulsations
Infrared (IR) light is electromagnetic radiation with a occurring below ~3 Hz) are considered to lie in the ULF
wavelength between 0.7 and 300 micrometers, which range, which is thus also defined differently from the ITU
equates to a frequency range between approximately 1 Radio Bands.
and 430 THz. IR wavelengths are longer than that of visi-
Thermal radiation (heat)
ble light, but shorter than that of terahertz radiation mi-
crowaves. Bright sunlight provides an irradiance of just Main article: Thermal radiation
over 1 kilowatt per square meter at sea level. Of this en- Thermal radiation, a common synonym for infra-red
ergy, 527 watts is infrared radiation, 445 watts is visible when it occurs at temperatures often encountered on
light, and 32 watts is ultraviolet radiation.[1] Earth, is the process by which the surface of an object ra-
diates its thermal energy in the form of electromagnetic
Microwave waves. Infrared radiation from a common household ra-
Main article: Microwave diator or electric heater is an example of thermal radi-
Microwaves are electromagnetic waves with wave- ation, as is the heat and light (IR and visible EM waves)
lengths ranging from as long as one meter to as short emitted by a glowing incandescent light bulb. Thermal
as one millimeter, or equivalently, with frequencies be- radiation is generated when heat from the movement of
tween 300 MHz (0.3 GHz) and 300 GHz. This broad defini- charged particles within atoms is converted to electro-
4
From Wikipedia, the free encyclopedia Radiation
magnetic radiation. The emitted wave frequency of the Alpha particles, beta particles and gamma ray radia-
thermal radiation is a probability distribution depend- tion were discovered by Ernest Rutherford through sim-
ing only on temperature, and for a black body is given ple experimentation. Rutherford used a generic radioac-
by Planck’s law of radiation. Wien’s law gives the most tive source and determined that the rays produced by
likely frequency of the emitted radiation, and the Ste- the source struck three distinct areas on a screen of re-
fan–Boltzmann law gives the heat intensity. active material: one of them corresponding to a positive
Parts of the electromagnetic spectrum of thermal ra- charge (alpha), one of them being negative (beta), and
diation may be ionizing, if the object emitting the radi- one of them being neutral (gamma). He calculated the
ation is hot enough (has a high enough temperature). A magnitude of the charge by their location. Using this
common example of such radiation is sunlight, which is data, Rutherford concluded that this radiation consisted
thermal radiation from the Sun’s photosphere and which of three different types, and named them after the first
contains enough ultraviolet light to cause ionization in three letters of the Greek alphabet alpha, beta, and gam-
many molecules and atoms. An extreme example is the ma.
flash from the detonation of a nuclear weapon, which In December 1899, Marie Curie and Pierre Curie dis-
emits a large number of ionizing X-rays purely as a prod- covered radium in pitchblende. This new element was
uct of heating the atmosphere around the bomb to ex- two million times more radioactive than uranium, as de-
tremely high temperatures. scribed by Madam Curie.
As noted above, even low-frequency thermal radia-
tion may cause temperature-ionization whenever it de-
posits sufficient thermal energy to raises temperatures
Uses of radiation
to a high enough level. Common examples of this are the
ionization (plasma) seen in common flames, and the mol- In medicine
ecular changes caused by the "browning" in food-cook- Main articles: Medical radiography and Medical radiation
ing, which is a chemical process that begins with a large scientist
component of ionization. Radiation and radioactive substances are used for diag-
nosis, treatment, and research. X-rays, for example, pass
Black body radiation through muscles and other soft tissue but are stopped by
Main article: Black Body Radiation dense materials. This property of X-rays enables doctors
Black body radiation is radiation from an idealized radiator to find broken bones and to locate cancers that might be
that emits at any temperature the maximum possible growing in the body[5] . Doctors also find certain diseas-
amount of radiation at any given wavelength. A black es by injecting a radioactive substance and monitoring
body will also absorb the maximum possible incident ra- the radiation given off as the substance moves through
diation at any given wavelength. The radiation emitted the body[6] . Radiation used for cancer treatment is called
covers the entire electromagnetic spectrum and the in- ionizing radiation because it forms ions in the cells of the
tensity (power/unit-area) at a given frequency is dictat- tissues it passes through as it dislodges electrons from
ed by Planck’s law of radiation. A black body at temper- atoms. This can kill cells or change genes so the cells can-
atures at or below room temperature would thus appear not grow. Other forms of radiation such as radio waves,
absolutely black as it would not reflect any light. The- microwaves, and light waves are called non-ionizing.
oretically a black body emits electromagnetic radiation They don’t have as much energy and are not able to ion-
over the entire spectrum from very low frequency radio ize cells.
waves to x-rays. The frequency at which the black body
radiation is at maximum is given by Wien’s displacement In communication
law. All modern communication systems use forms of electro-
magnetic radiation. Variations in the intensity of the ra-
Discovery diation represent changes in the sound, pictures, or oth-
er information being transmitted. For example, a human
Wilhelm Röntgen discovered and named X-rays when ex- voice can be sent as a radio wave or microwave by mak-
perimenting with a vacuum and a tube, he noticed a fluo- ing the wave vary to correspond variations in the voice.
rescence on a nearby plate of coated glass. In one month,
he discovered the main properties of X-rays that we un- In science
derstand to this day. Henri Becquerel found that uranium
Researchers use radioactive atoms to determine the age
salts caused fogging of an unexposed photographic plate,
of materials that were once part of a living organism. The
and Marie Curie discovered that only certain elements
age of such materials can be estimated by measuring the
gave off these rays of energy. She named this behavior
amount of radioactive carbon they contain in a process
radioactivity.
called radiocarbon dating. Environmental scientists use
5
From Wikipedia, the free encyclopedia Radiation
radioactive atoms known as tracer atoms to identify the
pathways taken by pollutants through the environment.
References
Radiation is used to determine the composition of [1] ^ Kwan-Hoong Ng (20–22 October 2003). "Non-
materials in a process called neutron activation analysis. Ionizing Radiations – Sources, Biological Effects,
In this process, scientists bombard a sample of a sub- Emissions and Exposures". Proceedings of the
stance with particles called neutrons. Some of the atoms International Conference on Non-Ionizing Radiation at
in the sample absorb neutrons and become radioactive. UNITEN ICNIR2003 Electromagnetic Fields and Our
The scientists can identify the elements in the sample by Health. http://www.who.int/peh-emf/meetings/
studying the emitted radiation. archive/en/keynote3ng.pdf.
[2] Ionization energy. chemguide.co.uk. The ionization
energies of hydrogen and oxygen (first ionization)
See also are both about 14 eV
• Background radiation, which actually refers to the [3] Questions and Answers about Biological Effects and
background ionizing radiation Potential Hazards of Radiofrequency
• Čerenkov radiation Electromagnetic Fields. Office of Engineering and
• Cosmic microwave background radiation, 3 K Technology. Bulletin 56, Fourth Edition, August
blackbody radiation that fills the Universe 1999.
• Electromagnetic spectrum [4] John E. Moulder. "Static Electric and Magnetic
• Hawking radiation Fields and Human Health".
• Ionizing radiation http://web.archive.org/web/20070714054650/
• Banana equivalent dose http://www.mcw.edu/gcrc/cop/static-fields-
• Non-ionizing radiation cancer-faq/toc.html.
• Radiant energy, radiation by a source into the [5] "Radiography Wikipedia article". Radiography
surrounding environment. Wikipedia article. http://en.wikipedia.org/wiki/
• Radiation damage – adverse effects on materials and Radiography.
devices [6] "Nuclear Medicine". Nuclear Medicine Wikipedia
• Radiation hardening – making devices resistant to article. http://en.wikipedia.org/wiki/
failure in high radiation environments Nuclear_medicine.
• Radiation hormesis – dosage threshold damage
theory
• Radiation poisoning – adverse effects on life forms
External links
• Radioactive contamination • Radiation on In Our Time at the BBC. (listen now)
• Radioactive decay • Health Physics Society Public Education Website
• Radiation Protection Convention, 1960 – by • Ionizing Radiation and Radon from World Health
International Labour Organization Organization
Retrieved from "http://en.wikipedia.org/w/index.php?title=Radiation&oldid=473608836"
Categories:
• Radiation
• Physics
• Fundamental physics concepts
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