Two energies of radiation are commonly differentiated by the way they interact with normal chemical matter: ionizing and non-ionizing radiation. The word radiation is often colloquially used in reference to ionizing radiation (i.e., radiation having sufficient energy to ionize an atom), but the term radiation may correctly also refer to non-ionizing radiation (e.g., radio waves, heat or visible light). The particles or waves radiate (i.e., travel outward in all directions) from a source. This aspect leads to a system of measurements and physical units that are applicable to all types of radiation. Because radiation radiates through space and its energy is conserved in vacuum, the power of all types of radiation follows an inverse-square law of power with regard to distance from its source.
Both ionizing and non-ionizing radiation can be harmful to organisms and can result in changes to the natural environment. In general, however, ionizing radiation is far more harmful to living organisms per unit of energy deposited than non-ionizing radiation, since the ions that are produced by ionizing radiation, even at low radiation powers, have the potential to cause DNA damage. By contrast, most non-ionizing radiation is harmful to organisms only in proportion to the thermal energy deposited, and is conventionally considered harmless at low powers which do not produce significant temperature rise. Ultraviolet radiation in some aspects occupies a middle ground, in having some features of both ionizing and non-ionizing radiation. Although nearly all of the ultraviolet spectrum of radiation is non-ionizing, at the same time ultraviolet radiation does far more damage to many molecules in biological systems than is accounted for by heating effects (an example is sunburn). These properties derive from ultraviolet's power to alter chemical bonds, even without having quite enough energy to ionize atoms.
The question of harm to biological systems due to low-power ionizing and non-ionization radiation is not settled. Controversy continues about possible non-heating effects of low-power non-ionizing radiation, such as non-heating microwave and radio wave exposure. Non-ionizing radiation is usually considered to have a safe lower limit, especially as thermal radiation is unavoidable and ubiquitous. By contrast, ionizing radiation is conventionally considered to have no completely safe lower limit, although at some energy levels, new exposures do not add appreciably to background radiation. The evidence that small amounts of some types of ionizing radiation might confer a net health benefit in some situations, is called radiation hormesis.
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Ionizing radiation
Main article: Ionizing radiation
Radiation with sufficiently high energy can ionize atoms.
Most often, this occurs when an electron is stripped (or "knocked out")
from an electron shell, which leaves the atom with a net positive
charge. Because cells and more importantly the DNA can be damaged, this ionization can result in an increased chance of cancer, and thus "ionizing radiation" is somewhat artificially separated out of particle and electromagnetic radiation, simply due to its larger potential for biological damage per unit of energy. An individual cell is made of trillions
of atoms, only a small fraction of which will be ionized at low
radiation powers. The probability of ionizing radiation causing cancer
is dependent upon the absorbed dose of the radiation, as adjusted for the damaging tendency of the type of radiation (equivalent dose) and the sensitivity of the organism or tissue being irradiated (effective dose).Roughly speaking, photons and particles with energies above about 10 electron volts (eV) are ionizing. Alpha particles, beta particles, cosmic rays, gamma rays, and X-ray radiation all carry energy high enough to ionize atoms. In addition, free neutrons are also ionizing, since their interactions with matter are inevitably more energetic than this threshold.
Ionizing radiation comes from radioactive materials, X-ray tubes, particle accelerators, and is present in the environment. It is invisible and not directly detectable by human senses, so instruments such as Geiger counters are usually required to detect its presence. In some cases, it may lead to secondary emission of visible light upon interaction with matter, as in Cherenkov radiation and radioluminescence. It has many practical uses in medicine, research, construction, and other areas, but presents a health hazard if used improperly. Exposure to radiation causes damage to living tissue, resulting in skin burns, radiation sickness and death at high doses and cancer,[1] tumors and genetic damage at low doses.
Electromagnetic radiation (sometimes abbreviated EMR) takes the form of self-propagating waves in a vacuum or in matter. EM radiation has an electric and magnetic field component which oscillate in phase perpendicular to each other and to the direction of energy propagation. Electromagnetic radiation is classified into types according to the frequency of the wave, these types include (in order of increasing frequency): radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. Of these, radio waves have the longest wavelengths and gamma rays have the shortest. A small window of frequencies, called visible spectrum or light, is sensed by the eye of various organisms.
Ionizing electromagnetic radiation is that for which the photons making up the radiation have energies larger than about 10 electron volts. The ability of an electromagnetic wave (photons) to ionize an atom or molecule thus depends on its frequency, which determines the energy of a photon of the radiation. An energy of 10 eV is about 1.6×10−18 joules, which is a typical binding energy of an outer electron to an atom or organic molecule.[2] This corresponds with a frequency of 2.4×1015 Hz, and a wavelength of 125 nm (this is in far ultraviolet).[3] Radiation on the short-wavelength end of the electromagnetic spectrum, and above 125 nm, is ionizing. This includes extreme ultraviolet, X-rays, and gamma rays.
Most of the ultraviolet spectrum (which begins above energies of 3.1 eV (400 nm)) is non-ionizing, but is still biologically hazardous due the ability of single photons of this energy to cause electronic excitation in biological molecules, and thus damage them by means of unwanted reactions. An example is formation of pyrimidine dimers in DNA. This property gives the ultraviolet spectrum some of the dangers of ionizing radiation in biological systems, without actual ionization occurring. In contrast visible light and longer-wavelength electromagnetic radiation, such as infrared, microwaves, and radio waves, consists of photons with too little energy to cause damaging molecular excitation, and thus this radiation is far less hazardous per unit of energy.
Alpha
Main article: Alpha decay
Alpha particles are helium-4 nuclei
(two protons and two neutrons). They interact with matter very heavily,
and at their usual velocities only penetrate a few centimeters of air,
or a few millimeters of low density material (such as the thin mica
material which is specially placed in some Geiger counter tubes to allow alpha particles in). This means that alpha particles from ordinary alpha decay do not penetrate skin and cause no damage to tissues below. Some very high energy alpha particles compose about 10% of cosmic rays,
and these are capable of penetrating the body and even thin metal
plates. However, they are of danger only to astronauts, since they are
deflected by the Earth's magnetic field and then stopped by its
atmosphere.Alpha radiation is dangerous when alpha-emitting radioisotopes are ingested (breathed or swallowed). This brings the radioisotope close enough to tissue for the alpha radiation to damage cells. Per unit of energy, alpha particles are at least 20 times more effective at cell-damage as gamma rays and X-rays. See relative biological effectiveness for a discussion of this. Examples of highly poisonous alpha-emitters are radium, radon, and polonium.
Beta
Main article: Beta decay
Beta-minus (β−) radiation consists of an energetic electron. It is more ionizing than alpha radiation, but less than gamma. Beta radiation from radioactive decay
can be stopped with a few centimeters of plastic or a few millimeters
of metal. It occurs when a neutron decays into a proton in a nucleus,
releasing the beta particle and an antineutrino. Beta radiation from linac accelerators is far more energetic and penetrating than natural beta radiation. It is sometimes used therapeutically in radiotherapy to treat superficial tumors.Beta-plus (β+) radiation is the emission of positrons, which are antimatter electrons. When a positron slows down to speeds similar to those of electrons in the material, the positron will annihilate an electron, releasing two gamma photons in the process. Those two gamma photons will be traveling in (approximately) opposite directions.
Neutron
Main articles: Neutron radiation and Neutron temperature
Neutrons are categorized according to their speed. Neutron radiation consists of free neutrons. These neutrons may be emitted during either spontaneous or induced nuclear fission, nuclear fusion processes, or from any other nuclear reactions.Neutrons are the only type of ionizing radiation that can make other objects, or material, radioactive. This process, called neutron activation, is the primary method used to produce radioactive sources for use in medical, academic, and industrial applications. Even comparatively low speed thermal neutrons, which do not carry enough kinetic energy individually to be ionizing, will cause neutron activation (in fact, they cause it more efficiently). Such neutrons are "indirectly ionizing."
Neutrons do not ionize atoms in the same way that charged particles such as protons and electrons do (exciting an electron), because neutrons have no charge. However, both slow and fast neutrons react with the atomic nuclei of many elements upon collision with those nuclei, creating unstable isotopes and therefore inducing radioactivity in a previously non-radioactive material. This is neutron activation.
In addition, high-energy (high-speed) neutrons have the ability to directly ionize atoms. One mechanism by which high energy neutrons ionize atoms is to strike the nucleus of an atom and knock the atom out of a molecule, leaving one or more electrons behind as the chemical bond is broken. This leads to production of chemical free radicals. In addition, very high energy neutrons can cause ionizing radiation by "neutron spallation" or knockout, wherein neutrons cause emission of high-energy protons from atomic nuclei (especially hydrogen nuclei) on impact. The last process imparts most of the neutron's energy to the proton, much like one billiard ball striking another. The charged protons, and other products from such reactions are directly ionizing.
High-energy neutrons are very penetrating and can travel great distances in air (hundreds or even thousands of meters) and moderate distances (several meters) in common solids. They typically require hydrogen rich shielding, such as concrete or water, to block them within distances of less than a meter. A common source of neutron radiation occurs inside a nuclear reactor, where a meters-thick water layer is used as effective shielding.
X-ray
Main article: X-ray
X-rays are electromagnetic waves with a wavelength smaller than about 10 nanometers. A smaller wavelength corresponds to a higher energy according to the equation E=hc/λ.
("E" is Energy; "h" is Planck's constant; "c" is the speed of light;
"λ" is wavelength.) A "packet" of electromagnetic waves is called a photon.
When an X-ray photon collides with an atom, the atom may absorb the
energy of the photon and boost an electron to a higher orbital level or
if the photon is very energetic, it may knock an electron from the atom
altogether, causing the atom to ionize. Generally, a larger atom is more
likely to absorb an X-ray photon, since larger atoms have greater
energy differences between orbital electrons. Soft tissue in the human
body is composed of smaller atoms than the calcium atoms that make up
bone, hence there is a contrast in the absorption of X-rays. X-ray
machines are specifically designed to take advantage of the absorption
difference between bone and soft tissue, allowing physicians to examine
structure in the human body.Gamma
Main article: Gamma ray
Gamma (γ) radiation consists of photons with a frequency of greater than 1019 Hz.[1] Gamma radiation occurs to rid the decaying nucleus
of excess energy after it has emitted either alpha or beta radiation.
Both alpha and beta particles have an electric charge and mass, and thus
are quite likely to interact with other atoms in their path. Gamma
radiation is composed of photons, which have neither mass nor electric
charge. Gamma radiation penetrates much further through matter than
either alpha or beta radiation.Gamma rays, which are highly energetic photons, penetrate deeply and are difficult to stop. They can be stopped by a sufficiently thick layer of material, where stopping power of the material per given area depends mostly (but not entirely) on its total mass, whether the material is of high or low density. However, as is the case with X-rays, materials with high atomic number such as lead or depleted uranium add a modest (typically 20% to 30%) amount of stopping power over an equal mass of less-dense and lower atomic weight materials (such as water or concrete).
Non-ionizing radiation
Main article: Non-ionizing radiation
The energy of non-ionizing radiation is less and instead of producing
charged ions when passing through matter, the electromagnetic radiation
has only sufficient energy to change the rotational, vibrational or
electronic valence configurations of molecules and atoms. The effect of
non-ionizing forms of radiation on living tissue has only recently been
studied. Nevertheless, different biological effects are observed for
different types of non-ionizing radiation.[1][4]Even "non-ionizing" radiation is capable of causing thermal-ionization if it deposits enough heat to raise temperatures to ionization energies. These reactions occur at far higher energies than with ionization radiation, which requires only single particles to ionize. A familiar example of thermal ionization is the flame-ionization of a common fire, and the browning (chemical process) reactions in common food items induced by infrared radiation, during broiling-type cooking.
Non-ionizing electromagnetic radiation
Main article: Electromagnetic radiation
The non-ionizing portion of electromagnetic radiation consists of
electromagnetic waves that (as individual quanta or particles, see photon)
are not energetic enough to detach electrons from atoms or molecules,
ionizing them. These include radio waves, microwaves, infrared, and
(sometimes) visible light. (Ultraviolet light, X-rays and gamma-rays are
regarded as ionizing.) The occurrence of ionization depends on the
energy of the individual particles or waves, and not on their number. An
intense flood of particles or waves will not cause ionization if these
particles or waves do not carry enough energy to be ionizing, unless
they raise the temperature of a body to a point high enough to ionize
small fractions of atoms or molecules by the process of
thermal-ionization (this requires relatively extreme radiation energies,
however).
The electromagnetic spectrum is the range of all possible electromagnetic radiation frequencies.[1] The electromagnetic spectrum (usually just spectrum) of an object is the characteristic distribution of electromagnetic radiation emitted by, or absorbed by, that particular object.
Visible light
Main article: Light
Light, or visible light, is a very narrow range of electromagnetic
radiation of a wavelength that is visible to the human eye (about
400–700 nm), or up to 380–750 nm.[1] More broadly, physicists refer to light as electromagnetic radiation of all wavelengths, whether visible or not.Infrared
Main article: Infrared
Infrared (IR) light is electromagnetic radiation with a wavelength
between 0.7 and 300 micrometers, which equates to a frequency range
between approximately 1 and 430 THz. IR wavelengths are longer than that
of visible light, but shorter than that of terahertz radiation
microwaves. Bright sunlight provides an irradiance of just over 1
kilowatt per square meter at sea level. Of this energy, 527 watts is
infrared radiation, 445 watts is visible light, and 32 watts is
ultraviolet radiation.[1]Microwave
Main article: Microwave
Microwaves are electromagnetic waves with wavelengths ranging from as
long as one meter to as short as one millimeter, or equivalently, with
frequencies between 300 MHz (0.3 GHz) and 300 GHz. This broad definition
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 upper 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 radio waves are made by lightning, or by
astronomical objects. 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 propagation 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 bandwidth in this band of the radio spectrum, only the very simplest signals are used, such as for radio navigation. Also known as the myriameter band or myriameter wave as the wavelengths range from ten to one myriameter (an obsolete metric unit equal to 10 kilometers)Extremely low frequency (ELF)
Extremely low frequency (ELF) is radiation frequencies from 3 to 30 Hz. In atmosphere science, an alternative definition is usually given, from 3 Hz to 3 kHz.[1] In the related magnetosphere science, the lower frequency electromagnetic oscillations (pulsations occurring below ~3 Hz) are considered to lie in the ULF range, which is thus also defined differently from the ITU Radio Bands.Thermal radiation (heat)
Main article: Thermal radiation
Thermal radiation is a common synonym for approximately black body radiation spontaneously emitted from objects, that is chiefly infrared
electromagnetic radiation at temperatures often encountered on Earth.
Thermal radiation refers not only to the radiation itself, but also the
process by which the surface of an object radiates its thermal energy
in the form black body radiation. Infrared or red radiation from a
common household radiator or electric heater is an example of thermal
radiation, as is the heat and light (IR and visible EM waves) emitted by
a glowing incandescent light bulb. Thermal radiation is generated when
heat from the movement of charged particles within atoms is converted to
electromagnetic radiation. The emitted wave frequency of the thermal
radiation is a probability distribution depending only on temperature,
and for a black body is given by Planck's law of radiation. Wien's displacement law gives the most likely frequency of the emitted radiation, and the Stefan–Boltzmann law gives the heat radiation intensity.Parts of the electromagnetic spectrum of thermal radiation may be ionizing, if the object emitting the radiation is hot enough (has a high enough temperature). A common example of such radiation is sunlight, which is thermal radiation from the Sun's photosphere and which contains enough ultraviolet light (before being filtered by the Earth's atmosphere) to cause ionization in many molecules and atoms. An extreme example is the flash from the detonation of a nuclear weapon, which emits a large number of ionizing X-rays purely as a product of heating the atmosphere around the bomb to extremely high temperatures.
As noted above, even low-frequency thermal radiation may cause temperature-ionization whenever it deposits sufficient thermal energy to raises temperatures to a high enough level. Common examples of this are the ionization (plasma) seen in common flames, and the molecular changes caused by the "browning" in food-cooking, which is a chemical process that begins with a large component of ionization.
Black body radiation
Main article: Black Body Radiation
Black body radiation
is radiation from an idealized radiator that emits at any temperature
the maximum possible amount of radiation at any given wavelength. A black body
will also absorb the maximum possible incident radiation at any given
wavelength. The radiation emitted covers the entire electromagnetic
spectrum and the intensity (power/unit-area) at a given frequency is
dictated by Planck's law of radiation. A black body
at temperatures at or below room temperature would thus appear
absolutely black as it would not reflect any light. Theoretically a
black body emits electromagnetic radiation over the entire spectrum from
very low frequency radio waves to x-rays. The frequency at which the
black body radiation is at maximum is given by Wien's displacement law.Discovery
Electromagnetic radiation of wavelengths other than light were discovered in the early 19th century. The discovery of infrared radiation is ascribed to William Herschel, the astronomer. Herschel published his results in 1800 before the Royal Society of London. Herschel, like Ritter used a prism to refract light from the Sun and detected the infrared, beyond the red part of the spectrum, through an increase in the temperature recorded on a thermometer.In 1801, the German physicist Johann Wilhelm Ritter made the discovery of ultraviolet by noting that the rays from a prism darkened silver chloride preparations more quickly than violet light. Ritter's experiments were an early precursor to what would become photography. Ritter noted that the UV rays were capable of causing chemical reactions.
Radio waves were not detected first from a natural source, but were rather produced deliberately an artificially by the German scientist Heinrich Hertz in 1887, using electrical circuits calculated to produce oscillations in the radio frequency range, following recipes suggested by the equations of James Clerk Maxwell.
Wilhelm Röntgen discovered and named X-rays. After experimenting with high voltages applied to an evaccuated tube on 8 November 1895, he noticed a fluorescence on a nearby plate of coated glass. In one month, he discovered the main properties of X-rays that we understand to this day. Henri Becquerel found that uranium salts caused fogging of an unexposed photographic plate, and Marie Curie discovered that only certain elements gave off these rays of energy. She named this behavior radioactivity. In December 1899, Marie Curie and Pierre Curie discovered radium in pitchblende. This new element was two million times more radioactive than uranium, as described by Madam Curie.
Alpha rays (alpha particles) and beta rays (beta particles) were differentiated by Ernest Rutherford through simple experimentation in 1899. Rutherford used a generic pitchblende radioactive source and determined that the rays produced by the source had differing penetrations in materials. One type had short pentration and a positive charge, which Rutherford named alpha and the other was more penetrating with a negative charge, and this type Rutherford named beta. Henri Becquerel shortly proved that beta rays are fast electrons, while Rutherford and a colleague eventually proved that alpha particles are ionized helium. In 1900 the French scientist Paul Villard discovered a third neutrally charged and especially penetrating type of radiation from radium, and after he described it, Rutherford realized it must be yet a third type of radiation, which in 1903 Rutherford named gamma rays.
Cosmic ray radiations striking the Earth from outer space were finally definitively recognized and proven to exist in 1912, as the scientist Victor Hess carried electrometer to various altitudes in a free balloon flight. The nature of these radiations was only gradually understood in later years.
Neutron radiation was discovered with the neutron by Chadwick, in 1932. A number of other high energy particulate radiations such as positrons, muons, and pions were discovered by cloud chamber examination of cosmic ray reactions shortly thereafter, and others types of particle radiation were produced artificially in particle accelerators, through the last half of the twentieth century.
Uses of radiation
In medicine
Main articles: Medical radiography and Medical radiation scientist
Radiation and radioactive substances are used for diagnosis,
treatment, and research. X-rays, for example, pass through muscles and
other soft tissue but are stopped by dense materials. This property of
X-rays enables doctors to find broken bones and to locate cancers that
might be growing in the body.[5]
Doctors also find certain diseases by injecting a radioactive substance
and monitoring the radiation given off as the substance moves through
the body.[6]
Radiation used for cancer treatment is called ionizing radiation
because it forms ions in the cells of the tissues it passes through as
it dislodges electrons from atoms. This can kill cells or change genes
so the cells cannot grow. Other forms of radiation such as radio waves,
microwaves, and light waves are called non-ionizing. They don't have as
much energy and are not able to ionize cells.In communication
All modern communication systems use forms of electromagnetic radiation. Variations in the intensity of the radiation represent changes in the sound, pictures, or other information being transmitted. For example, a human voice can be sent as a radio wave or microwave by making the wave vary to correspond variations in the voice.In science
Researchers use radioactive atoms to determine the age of materials that were once part of a living organism. The age of such materials can be estimated by measuring the amount of radioactive carbon they contain in a process called radiocarbon dating. Environmental scientists use radioactive atoms known as tracer atoms to identify the pathways taken by pollutants through the environment.Radiation is used to determine the composition of materials in a process called neutron activation analysis. In this process, scientists bombard a sample of a substance with particles called neutrons. Some of the atoms in the sample absorb neutrons and become radioactive. The scientists can identify the elements in the sample by studying the emitted radiation.
See also
- Background radiation, which actually refers to the background ionizing radiation
- Čerenkov radiation
- Cosmic microwave background radiation, 3 K blackbody radiation that fills the Universe
- Electromagnetic spectrum
- Hawking radiation
- Ionizing radiation
- Banana equivalent dose
- Non-ionizing radiation
- Radiant energy, radiation by a source into the surrounding environment.
- Radiation damage – adverse effects on materials and devices
- Radiation hardening – making devices resistant to failure in high radiation environments
- Radiation hormesis – dosage threshold damage theory
- Radiation poisoning – adverse effects on life forms
- Radiation properties
- Radioactive contamination
- Radioactive decay
- Radiation Protection Convention, 1960 – by International Labour Organization
References
- ^ a b c d e f g h Kwan-Hoong Ng (20–22 October 2003). "Non-Ionizing Radiations – Sources, Biological Effects, Emissions and Exposures". Proceedings of the International Conference on Non-Ionizing Radiation at UNITEN ICNIR2003 Electromagnetic Fields and Our Health.
- ^ Ionization energy. chemguide.co.uk. The ionization energies of hydrogen and oxygen (first ionization) are both about 14 eV
- ^ Questions and Answers about Biological Effects and Potential Hazards of Radiofrequency Electromagnetic Fields. Office of Engineering and Technology. Bulletin 56, Fourth Edition, August 1999.
- ^ John E. Moulder. "Static Electric and Magnetic Fields and Human Health".
- ^ "Radiography Wikipedia article". Radiography Wikipedia article.
- ^ "Nuclear Medicine". Nuclear Medicine Wikipedia article.
External links
- Radiation on In Our Time at the BBC. (listen now)
- Health Physics Society Public Education Website
- Ionizing Radiation and Radon from World Health Organization
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