Как пишется гамма излучение

гамма-излучение

Синонимы:

A gamma ray, also known as gamma radiation (symbol γ or ), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically shorter than those of X-rays. With frequencies above 30 exahertz (3×10¹⁹ Hz), it imparts the highest photon energy. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. In 1903, Ernest Rutherford named this radiation gamma rays based on their relatively strong penetration of matter; in 1900 he had already named two less penetrating types of decay radiation (discovered by Henri Becquerel) alpha rays and beta rays in ascending order of penetrating power.

Gamma rays from radioactive decay are in the energy range from a few kiloelectronvolts (keV) to approximately 8 megaelectronvolts (MeV), corresponding to the typical energy levels in nuclei with reasonably long lifetimes. The energy spectrum of gamma rays can be used to identify the decaying radionuclides using gamma spectroscopy. Very-high-energy gamma rays in the 100–1000 teraelectronvolt (TeV) range have been observed from sources such as the Cygnus X-3 microquasar.

Natural sources of gamma rays originating on Earth are mostly a result of radioactive decay and secondary radiation from atmospheric interactions with cosmic ray particles. However, there are other rare natural sources, such as terrestrial gamma-ray flashes, which produce gamma rays from electron action upon the nucleus. Notable artificial sources of gamma rays include fission, such as that which occurs in nuclear reactors, and high energy physics experiments, such as neutral pion decay and nuclear fusion.

Gamma rays and X-rays are both electromagnetic radiation, and since they overlap in the electromagnetic spectrum, the terminology varies between scientific disciplines. In some fields of physics, they are distinguished by their origin: Gamma rays are created by nuclear decay while X-rays originate outside the nucleus. In astrophysics, gamma rays are conventionally defined as having photon energies above 100 keV and are the subject of gamma ray astronomy, while radiation below 100 keV is classified as X-rays and is the subject of X-ray astronomy.

Gamma rays are ionizing radiation and are thus hazardous to life. Due to their high penetration power, they can damage bone marrow and internal organs. Unlike alpha and beta rays, they easily pass through the body and thus pose a formidable radiation protection challenge, requiring shielding made from dense materials such as lead or concrete. On Earth, the magnetosphere protects life from most types of lethal cosmic radiation other than gamma rays.

History of discovery

The first gamma ray source to be discovered was the radioactive decay process called gamma decay. In this type of decay, an excited nucleus emits a gamma ray almost immediately upon formation.^{[note 1]} Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium. Villard knew that his described radiation was more powerful than previously described types of rays from radium, which included beta rays, first noted as «radioactivity» by Henri Becquerel in 1896, and alpha rays, discovered as a less penetrating form of radiation by Rutherford, in 1899. However, Villard did not consider naming them as a different fundamental type.^[1][2] Later, in 1903, Villard’s radiation was recognized as being of a type fundamentally different from previously named rays by Ernest Rutherford, who named Villard’s rays «gamma rays» by analogy with the beta and alpha rays that Rutherford had differentiated in 1899.^[3] The «rays» emitted by radioactive elements were named in order of their power to penetrate various materials, using the first three letters of the Greek alphabet: alpha rays as the least penetrating, followed by beta rays, followed by gamma rays as the most penetrating. Rutherford also noted that gamma rays were not deflected (or at least, not easily deflected) by a magnetic field, another property making them unlike alpha and beta rays.

Gamma rays were first thought to be particles with mass, like alpha and beta rays. Rutherford initially believed that they might be extremely fast beta particles, but their failure to be deflected by a magnetic field indicated that they had no charge.^[4] In 1914, gamma rays were observed to be reflected from crystal surfaces, proving that they were electromagnetic radiation.^[4] Rutherford and his co-worker Edward Andrade measured the wavelengths of gamma rays from radium, and found they were similar to X-rays, but with shorter wavelengths and thus, higher frequency. This was eventually recognized as giving them more energy per photon, as soon as the latter term became generally accepted. A gamma decay was then understood to usually emit a gamma photon.

Sources

Natural sources of gamma rays on Earth include gamma decay from naturally occurring radioisotopes such as potassium-40, and also as a secondary radiation from various atmospheric interactions with cosmic ray particles. Some rare terrestrial natural sources that produce gamma rays that are not of a nuclear origin, are lightning strikes and terrestrial gamma-ray flashes, which produce high energy emissions from natural high-energy voltages. Gamma rays are produced by a number of astronomical processes in which very high-energy electrons are produced. Such electrons produce secondary gamma rays by the mechanisms of bremsstrahlung, inverse Compton scattering and synchrotron radiation. A large fraction of such astronomical gamma rays are screened by Earth’s atmosphere. Notable artificial sources of gamma rays include fission, such as occurs in nuclear reactors, as well as high energy physics experiments, such as neutral pion decay and nuclear fusion.

A sample of gamma ray-emitting material that is used for irradiating or imaging is known as a gamma source. It is also called a radioactive source, isotope source, or radiation source, though these more general terms also apply to alpha and beta-emitting devices. Gamma sources are usually sealed to prevent radioactive contamination, and transported in heavy shielding.

Radioactive decay (gamma decay)

Gamma rays are produced during gamma decay, which normally occurs after other forms of decay occur, such as alpha or beta decay. A radioactive nucleus can decay by the emission of an
α
or
β
particle. The daughter nucleus that results is usually left in an excited state. It can then decay to a lower energy state by emitting a gamma ray photon, in a process called gamma decay.

The emission of a gamma ray from an excited nucleus typically requires only 10⁻¹² seconds. Gamma decay may also follow nuclear reactions such as neutron capture, nuclear fission, or nuclear fusion. Gamma decay is also a mode of relaxation of many excited states of atomic nuclei following other types of radioactive decay, such as beta decay, so long as these states possess the necessary component of nuclear spin. When high-energy gamma rays, electrons, or protons bombard materials, the excited atoms emit characteristic «secondary» gamma rays, which are products of the creation of excited nuclear states in the bombarded atoms. Such transitions, a form of nuclear gamma fluorescence, form a topic in nuclear physics called gamma spectroscopy. Formation of fluorescent gamma rays are a rapid subtype of radioactive gamma decay.

In certain cases, the excited nuclear state that follows the emission of a beta particle or other type of excitation, may be more stable than average, and is termed a metastable excited state, if its decay takes (at least) 100 to 1000 times longer than the average 10⁻¹² seconds. Such relatively long-lived excited nuclei are termed nuclear isomers, and their decays are termed isomeric transitions. Such nuclei have half-lifes that are more easily measurable, and rare nuclear isomers are able to stay in their excited state for minutes, hours, days, or occasionally far longer, before emitting a gamma ray. The process of isomeric transition is therefore similar to any gamma emission, but differs in that it involves the intermediate metastable excited state(s) of the nuclei. Metastable states are often characterized by high nuclear spin, requiring a change in spin of several units or more with gamma decay, instead of a single unit transition that occurs in only 10⁻¹² seconds. The rate of gamma decay is also slowed when the energy of excitation of the nucleus is small.^[5]

An emitted gamma ray from any type of excited state may transfer its energy directly to any electrons, but most probably to one of the K shell electrons of the atom, causing it to be ejected from that atom, in a process generally termed the photoelectric effect (external gamma rays and ultraviolet rays may also cause this effect). The photoelectric effect should not be confused with the internal conversion process, in which a gamma ray photon is not produced as an intermediate particle (rather, a «virtual gamma ray» may be thought to mediate the process).

Decay schemes

One example of gamma ray production due to radionuclide decay is the decay scheme for cobalt-60, as illustrated in the accompanying diagram. First, ⁶⁰
Co
decays to excited ⁶⁰
Ni
by beta decay emission of an electron of 0.31 MeV. Then the excited ⁶⁰
Ni
decays to the ground state (see nuclear shell model) by emitting gamma rays in succession of 1.17 MeV followed by 1.33 MeV. This path is followed 99.88% of the time:

60 27Co	→	60 28Ni*	+	e−	+	ν e	+	γ	+	1.17 MeV
60 28Ni*	→	60 28Ni					+	γ	+	1.33 MeV

Another example is the alpha decay of ²⁴¹
Am
to form ²³⁷
Np
; which is followed by gamma emission. In some cases, the gamma emission spectrum of the daughter nucleus is quite simple, (e.g. ⁶⁰
Co
/⁶⁰
Ni
) while in other cases, such as with (²⁴¹
Am
/²³⁷
Np
and ¹⁹²
Ir
/¹⁹²
Pt
), the gamma emission spectrum is complex, revealing that a series of nuclear energy levels exist.

Particle physics

Gamma rays are produced in many processes of particle physics. Typically, gamma rays are the products of neutral systems which decay through electromagnetic interactions (rather than a weak or strong interaction). For example, in an electron–positron annihilation, the usual products are two gamma ray photons. If the annihilating electron and positron are at rest, each of the resulting gamma rays has an energy of ~ 511 keV and frequency of ~ 1.24×10²⁰ Hz. Similarly, a neutral pion most often decays into two photons. Many other hadrons and massive bosons also decay electromagnetically. High energy physics experiments, such as the Large Hadron Collider, accordingly employ substantial radiation shielding.^{[citation needed]} Because subatomic particles mostly have far shorter wavelengths than atomic nuclei, particle physics gamma rays are generally several orders of magnitude more energetic than nuclear decay gamma rays. Since gamma rays are at the top of the electromagnetic spectrum in terms of energy, all extremely high-energy photons are gamma rays; for example, a photon having the Planck energy would be a gamma ray.

Other sources

A few gamma rays in astronomy are known to arise from gamma decay (see discussion of SN1987A), but most do not.

Photons from astrophysical sources that carry energy in the gamma radiation range are often explicitly called gamma-radiation. In addition to nuclear emissions, they are often produced by sub-atomic particle and particle-photon interactions. Those include electron-positron annihilation, neutral pion decay, bremsstrahlung, inverse Compton scattering, and synchrotron radiation.

Laboratory sources

In October 2017, scientists from various European universities proposed a means for sources of GeV photons using lasers as exciters through a controlled interplay between the cascade and anomalous radiative trapping.^[6]

Terrestrial thunderstorms

Thunderstorms can produce a brief pulse of gamma radiation called a terrestrial gamma-ray flash. These gamma rays are thought to be produced by high intensity static electric fields accelerating electrons, which then produce gamma rays by bremsstrahlung as they collide with and are slowed by atoms in the atmosphere. Gamma rays up to 100 MeV can be emitted by terrestrial thunderstorms, and were discovered by space-borne observatories. This raises the possibility of health risks to passengers and crew on aircraft flying in or near thunderclouds.^[7]

Solar flares

The most effusive solar flares emit across the entire EM spectrum, including γ-rays. The first confident observation occurred in 1972.^[8]

Cosmic rays

Extraterrestrial, high energy gamma rays include the gamma ray background produced when cosmic rays (either high speed electrons or protons) collide with ordinary matter, producing pair-production gamma rays at 511 keV. Alternatively, bremsstrahlung are produced at energies of tens of MeV or more when cosmic ray electrons interact with nuclei of sufficiently high atomic number (see gamma ray image of the Moon near the end of this article, for illustration).

Pulsars and magnetars

The gamma ray sky (see illustration at right) is dominated by the more common and longer-term production of gamma rays that emanate from pulsars within the Milky Way. Sources from the rest of the sky are mostly quasars. Pulsars are thought to be neutron stars with magnetic fields that produce focused beams of radiation, and are far less energetic, more common, and much nearer sources (typically seen only in our own galaxy) than are quasars or the rarer gamma-ray burst sources of gamma rays. Pulsars have relatively long-lived magnetic fields that produce focused beams of relativistic speed charged particles, which emit gamma rays (bremsstrahlung) when those strike gas or dust in their nearby medium, and are decelerated. This is a similar mechanism to the production of high-energy photons in megavoltage radiation therapy machines (see bremsstrahlung). Inverse Compton scattering, in which charged particles (usually electrons) impart energy to low-energy photons boosting them to higher energy photons. Such impacts of photons on relativistic charged particle beams is another possible mechanism of gamma ray production. Neutron stars with a very high magnetic field (magnetars), thought to produce astronomical soft gamma repeaters, are another relatively long-lived star-powered source of gamma radiation.

Quasars and active galaxies

More powerful gamma rays from very distant quasars and closer active galaxies are thought to have a gamma ray production source similar to a particle accelerator. High energy electrons produced by the quasar, and subjected to inverse Compton scattering, synchrotron radiation, or bremsstrahlung, are the likely source of the gamma rays from those objects. It is thought that a supermassive black hole at the center of such galaxies provides the power source that intermittently destroys stars and focuses the resulting charged particles into beams that emerge from their rotational poles. When those beams interact with gas, dust, and lower energy photons they produce X-rays and gamma rays. These sources are known to fluctuate with durations of a few weeks, suggesting their relatively small size (less than a few light-weeks across). Such sources of gamma and X-rays are the most commonly visible high intensity sources outside the Milky Way galaxy. They shine not in bursts (see illustration), but relatively continuously when viewed with gamma ray telescopes. The power of a typical quasar is about 10⁴⁰ watts, a small fraction of which is gamma radiation. Much of the rest is emitted as electromagnetic waves of all frequencies, including radio waves.

Gamma-ray bursts

The most intense sources of gamma rays, are also the most intense sources of any type of electromagnetic radiation presently known. They are the «long duration burst» sources of gamma rays in astronomy («long» in this context, meaning a few tens of seconds), and they are rare compared with the sources discussed above. By contrast, «short» gamma-ray bursts of two seconds or less, which are not associated with supernovae, are thought to produce gamma rays during the collision of pairs of neutron stars, or a neutron star and a black hole.^[9]

The so-called long-duration gamma-ray bursts produce a total energy output of about 10⁴⁴ joules (as much energy as the Sun will produce in its entire life-time) but in a period of only 20 to 40 seconds. Gamma rays are approximately 50% of the total energy output. The leading hypotheses for the mechanism of production of these highest-known intensity beams of radiation, are inverse Compton scattering and synchrotron radiation from high-energy charged particles. These processes occur as relativistic charged particles leave the region of the event horizon of a newly formed black hole created during supernova explosion. The beam of particles moving at relativistic speeds are focused for a few tens of seconds by the magnetic field of the exploding hypernova. The fusion explosion of the hypernova drives the energetics of the process. If the narrowly directed beam happens to be pointed toward the Earth, it shines at gamma ray frequencies with such intensity, that it can be detected even at distances of up to 10 billion light years, which is close to the edge of the visible universe.

Properties

Penetration of matter

Due to their penetrating nature, gamma rays require large amounts of shielding mass to reduce them to levels which are not harmful to living cells, in contrast to alpha particles, which can be stopped by paper or skin, and beta particles, which can be shielded by thin aluminium. Gamma rays are best absorbed by materials with high atomic numbers (Z) and high density, which contribute to the total stopping power. Because of this, a lead (high Z) shield is 20–30% better as a gamma shield than an equal mass of another low-Z shielding material, such as aluminium, concrete, water, or soil; lead’s major advantage is not in lower weight, but rather its compactness due to its higher density. Protective clothing, goggles and respirators can protect from internal contact with or ingestion of alpha or beta emitting particles, but provide no protection from gamma radiation from external sources.

The higher the energy of the gamma rays, the thicker the shielding made from the same shielding material is required. Materials for shielding gamma rays are typically measured by the thickness required to reduce the intensity of the gamma rays by one half (the half value layer or HVL). For example, gamma rays that require 1 cm (0.4 inch) of lead to reduce their intensity by 50% will also have their intensity reduced in half by 4.1 cm of granite rock, 6 cm (2.5 inches) of concrete, or 9 cm (3.5 inches) of packed soil. However, the mass of this much concrete or soil is only 20–30% greater than that of lead with the same absorption capability. Depleted uranium is used for shielding in portable gamma ray sources, but here the savings in weight over lead are larger, as a portable source is very small relative to the required shielding, so the shielding resembles a sphere to some extent. The volume of a sphere is dependent on the cube of the radius; so a source with its radius cut in half will have its volume (and weight) reduced by a factor of eight, which will more than compensate for uranium’s greater density (as well as reducing bulk).^{[clarification needed]} In a nuclear power plant, shielding can be provided by steel and concrete in the pressure and particle containment vessel, while water provides a radiation shielding of fuel rods during storage or transport into the reactor core. The loss of water or removal of a «hot» fuel assembly into the air would result in much higher radiation levels than when kept under water.

Matter interaction

When a gamma ray passes through matter, the probability for absorption is proportional to the thickness of the layer, the density of the material, and the absorption cross section of the material. The total absorption shows an exponential decrease of intensity with distance from the incident surface:

where x is the thickness of the material from the incident surface, μ= nσ is the absorption coefficient, measured in cm⁻¹, n the number of atoms per cm³ of the material (atomic density) and σ the absorption cross section in cm².

As it passes through matter, gamma radiation ionizes via three processes:

• The photoelectric effect: This describes the case in which a gamma photon interacts with and transfers its energy to an atomic electron, causing the ejection of that electron from the atom. The kinetic energy of the resulting photoelectron is equal to the energy of the incident gamma photon minus the energy that originally bound the electron to the atom (binding energy). The photoelectric effect is the dominant energy transfer mechanism for X-ray and gamma ray photons with energies below 50 keV (thousand electronvolts), but it is much less important at higher energies.
• Compton scattering: This is an interaction in which an incident gamma photon loses enough energy to an atomic electron to cause its ejection, with the remainder of the original photon’s energy emitted as a new, lower energy gamma photon whose emission direction is different from that of the incident gamma photon, hence the term «scattering». The probability of Compton scattering decreases with increasing photon energy. It is thought to be the principal absorption mechanism for gamma rays in the intermediate energy range 100 keV to 10 MeV. It is relatively independent of the atomic number of the absorbing material, which is why very dense materials like lead are only modestly better shields, on a per weight basis, than are less dense materials.
• Pair production: This becomes possible with gamma energies exceeding 1.02 MeV, and becomes important as an absorption mechanism at energies over 5 MeV (see illustration at right, for lead). By interaction with the electric field of a nucleus, the energy of the incident photon is converted into the mass of an electron-positron pair. Any gamma energy in excess of the equivalent rest mass of the two particles (totaling at least 1.02 MeV) appears as the kinetic energy of the pair and in the recoil of the emitting nucleus. At the end of the positron’s range, it combines with a free electron, and the two annihilate, and the entire mass of these two is then converted into two gamma photons of at least 0.51 MeV energy each (or higher according to the kinetic energy of the annihilated particles).

The secondary electrons (and/or positrons) produced in any of these three processes frequently have enough energy to produce much ionization themselves.

Additionally, gamma rays, particularly high energy ones, can interact with atomic nuclei resulting in ejection of particles in photodisintegration, or in some cases, even nuclear fission (photofission).

Light interaction

High-energy (from 80 GeV to ~10 TeV) gamma rays arriving from far-distant quasars are used to estimate the extragalactic background light in the universe: The highest-energy rays interact more readily with the background light photons and thus the density of the background light may be estimated by analyzing the incoming gamma ray spectra.^[10][11]

Gamma spectroscopy

Gamma spectroscopy is the study of the energetic transitions in atomic nuclei, which are generally associated with the absorption or emission of gamma rays. As in optical spectroscopy (see Franck–Condon effect) the absorption of gamma rays by a nucleus is especially likely (i.e., peaks in a «resonance») when the energy of the gamma ray is the same as that of an energy transition in the nucleus. In the case of gamma rays, such a resonance is seen in the technique of Mössbauer spectroscopy. In the Mössbauer effect the narrow resonance absorption for nuclear gamma absorption can be successfully attained by physically immobilizing atomic nuclei in a crystal. The immobilization of nuclei at both ends of a gamma resonance interaction is required so that no gamma energy is lost to the kinetic energy of recoiling nuclei at either the emitting or absorbing end of a gamma transition. Such loss of energy causes gamma ray resonance absorption to fail. However, when emitted gamma rays carry essentially all of the energy of the atomic nuclear de-excitation that produces them, this energy is also sufficient to excite the same energy state in a second immobilized nucleus of the same type.

Applications

Gamma rays provide information about some of the most energetic phenomena in the universe; however, they are largely absorbed by the Earth’s atmosphere. Instruments aboard high-altitude balloons and satellites missions, such as the Fermi Gamma-ray Space Telescope, provide our only view of the universe in gamma rays.

Gamma-induced molecular changes can also be used to alter the properties of semi-precious stones, and is often used to change white topaz into blue topaz.

Non-contact industrial sensors commonly use sources of gamma radiation in refining, mining, chemicals, food, soaps and detergents, and pulp and paper industries, for the measurement of levels, density, and thicknesses.^[12] Gamma-ray sensors are also used for measuring the fluid levels in water and oil industries.^[13] Typically, these use Co-60 or Cs-137 isotopes as the radiation source.

In the US, gamma ray detectors are beginning to be used as part of the Container Security Initiative (CSI). These machines are advertised to be able to scan 30 containers per hour.

Gamma radiation is often used to kill living organisms, in a process called irradiation. Applications of this include the sterilization of medical equipment (as an alternative to autoclaves or chemical means), the removal of decay-causing bacteria from many foods and the prevention of the sprouting of fruit and vegetables to maintain freshness and flavor.

Despite their cancer-causing properties, gamma rays are also used to treat some types of cancer, since the rays also kill cancer cells. In the procedure called gamma-knife surgery, multiple concentrated beams of gamma rays are directed to the growth in order to kill the cancerous cells. The beams are aimed from different angles to concentrate the radiation on the growth while minimizing damage to surrounding tissues.

Gamma rays are also used for diagnostic purposes in nuclear medicine in imaging techniques. A number of different gamma-emitting radioisotopes are used. For example, in a PET scan a radiolabeled sugar called fluorodeoxyglucose emits positrons that are annihilated by electrons, producing pairs of gamma rays that highlight cancer as the cancer often has a higher metabolic rate than the surrounding tissues. The most common gamma emitter used in medical applications is the nuclear isomer technetium-99m which emits gamma rays in the same energy range as diagnostic X-rays. When this radionuclide tracer is administered to a patient, a gamma camera can be used to form an image of the radioisotope’s distribution by detecting the gamma radiation emitted (see also SPECT). Depending on which molecule has been labeled with the tracer, such techniques can be employed to diagnose a wide range of conditions (for example, the spread of cancer to the bones via bone scan).

Health effects

Gamma rays cause damage at a cellular level and are penetrating, causing diffuse damage throughout the body. However, they are less ionising than alpha or beta particles, which are less penetrating.

Low levels of gamma rays cause a stochastic health risk, which for radiation dose assessment is defined as the probability of cancer induction and genetic damage.^[14] High doses produce deterministic effects, which is the severity of acute tissue damage that is certain to happen. These effects are compared to the physical quantity absorbed dose measured by the unit gray (Gy).^[15]

Body response

When gamma radiation breaks DNA molecules, a cell may be able to repair the damaged genetic material, within limits. However, a study of Rothkamm and Lobrich has shown that this repair process works well after high-dose exposure but is much slower in the case of a low-dose exposure.^[16]

Risk assessment

The natural outdoor exposure in the United Kingdom ranges from 0.1 to 0.5 µSv/h with significant increase around known nuclear and contaminated sites.^[17] Natural exposure to gamma rays is about 1 to 2 mSv per year, and the average total amount of radiation received in one year per inhabitant in the USA is 3.6 mSv.^[18] There is a small increase in the dose, due to naturally occurring gamma radiation, around small particles of high atomic number materials in the human body caused by the photoelectric effect.^[19]

By comparison, the radiation dose from chest radiography (about 0.06 mSv) is a fraction of the annual naturally occurring background radiation dose.^[20] A chest CT delivers 5 to 8 mSv. A whole-body PET/CT scan can deliver 14 to 32 mSv depending on the protocol.^[21] The dose from fluoroscopy of the stomach is much higher, approximately 50 mSv (14 times the annual background).

An acute full-body equivalent single exposure dose of 1 Sv (1000 mSv), or 1 Gy, will cause mild symptoms of acute radiation sickness, such as nausea and vomiting; and a dose of 2.0–3.5 Sv (2.0–3.5 Gy) causes more severe symptoms (i.e. nausea, diarrhea, hair loss, hemorrhaging, and inability to fight infections), and will cause death in a sizable number of cases —- about 10% to 35% without medical treatment. A dose of 5 Sv^[22] (5 Gy) is considered approximately the LD₅₀ (lethal dose for 50% of exposed population) for an acute exposure to radiation even with standard medical treatment. A dose higher than 5 Sv (5 Gy) brings an increasing chance of death above 50%. Above 7.5–10 Sv (7.5–10 Gy) to the entire body, even extraordinary treatment, such as bone-marrow transplants, will not prevent the death of the individual exposed (see radiation poisoning).^[23] (Doses much larger than this may, however, be delivered to selected parts of the body in the course of radiation therapy.)

For low-dose exposure, for example among nuclear workers, who receive an average yearly radiation dose of 19 mSv,^{[clarification needed]} the risk of dying from cancer (excluding leukemia) increases by 2 percent. For a dose of 100 mSv, the risk increase is 10 percent. By comparison, risk of dying from cancer was increased by 32 percent for the survivors of the atomic bombing of Hiroshima and Nagasaki.^[24]

Units of measurement and exposure

The following table shows radiation quantities in SI and non-SI units:

Ionizing radiation related quantities view ‧ talk ‧ edit

Quantity	Unit	Symbol	Derivation	Year	SI equivalent
Activity (A)	becquerel	Bq	s−1	1974	SI unit
curie	Ci	3.7 × 1010 s−1	1953	3.7×1010 Bq
rutherford	Rd	106 s−1	1946	1,000,000 Bq
Exposure (X)	coulomb per kilogram	C/kg	C⋅kg−1 of air	1974	SI unit
röntgen	R	esu / 0.001293 g of air	1928	2.58 × 10−4 C/kg
Absorbed dose (D)	gray	Gy	J⋅kg−1	1974	SI unit
erg per gram	erg/g	erg⋅g−1	1950	1.0 × 10−4 Gy
rad	rad	100 erg⋅g−1	1953	0.010 Gy
Equivalent dose (H)	sievert	Sv	J⋅kg−1 × WR	1977	SI unit
röntgen equivalent man	rem	100 erg⋅g−1 x WR	1971	0.010 Sv
Effective dose (E)	sievert	Sv	J⋅kg−1 × WR × WT	1977	SI unit
röntgen equivalent man	rem	100 erg⋅g−1 × WR × WT	1971	0.010 Sv

The measure of the ionizing effect of gamma and X-rays in dry air is called the exposure, for which a legacy unit, the röntgen was used from 1928. This has been replaced by kerma, now mainly used for instrument calibration purposes but not for received dose effect. The effect of gamma and other ionizing radiation on living tissue is more closely related to the amount of energy deposited in tissue rather than the ionisation of air, and replacement radiometric units and quantities for radiation protection have been defined and developed from 1953 onwards. These are:

• The gray (Gy), is the SI unit of absorbed dose, which is the amount of radiation energy deposited in the irradiated material. For gamma radiation this is numerically equivalent to equivalent dose measured by the sievert, which indicates the stochastic biological effect of low levels of radiation on human tissue. The radiation weighting conversion factor from absorbed dose to equivalent dose is 1 for gamma, whereas alpha particles have a factor of 20, reflecting their greater ionising effect on tissue.
• The rad is the deprecated CGS unit for absorbed dose and the rem is the deprecated CGS unit of equivalent dose, used mainly in the USA.

Distinction from X-rays

The conventional distinction between X-rays and gamma rays has changed over time. Originally, the electromagnetic radiation emitted by X-ray tubes almost invariably had a longer wavelength than the radiation (gamma rays) emitted by radioactive nuclei.^[25] Older literature distinguished between X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10⁻¹¹ m, defined as gamma rays.^[26] Since the energy of photons is proportional to their frequency and inversely proportional to wavelength, this past distinction between X-rays and gamma rays can also be thought of in terms of its energy, with gamma rays considered to be higher energy electromagnetic radiation than are X-rays.

However, since current artificial sources are now able to duplicate any electromagnetic radiation that originates in the nucleus, as well as far higher energies, the wavelengths characteristic of radioactive gamma ray sources vs. other types now completely overlap. Thus, gamma rays are now usually distinguished by their origin: X-rays are emitted by definition by electrons outside the nucleus, while gamma rays are emitted by the nucleus.^{[25][27][28][29]} Exceptions to this convention occur in astronomy, where gamma decay is seen in the afterglow of certain supernovas, but radiation from high energy processes known to involve other radiation sources than radioactive decay is still classed as gamma radiation.

For example, modern high-energy X-rays produced by linear accelerators for megavoltage treatment in cancer often have higher energy (4 to 25 MeV) than do most classical gamma rays produced by nuclear gamma decay. One of the most common gamma ray emitting isotopes used in diagnostic nuclear medicine, technetium-99m, produces gamma radiation of the same energy (140 keV) as that produced by diagnostic X-ray machines, but of significantly lower energy than therapeutic photons from linear particle accelerators. In the medical community today, the convention that radiation produced by nuclear decay is the only type referred to as «gamma» radiation is still respected.

Due to this broad overlap in energy ranges, in physics the two types of electromagnetic radiation are now often defined by their origin: X-rays are emitted by electrons (either in orbitals outside of the nucleus, or while being accelerated to produce bremsstrahlung-type radiation),^[31] while gamma rays are emitted by the nucleus or by means of other particle decays or annihilation events. There is no lower limit to the energy of photons produced by nuclear reactions, and thus ultraviolet or lower energy photons produced by these processes would also be defined as «gamma rays».^[32] The only naming-convention that is still universally respected is the rule that electromagnetic radiation that is known to be of atomic nuclear origin is always referred to as «gamma rays», and never as X-rays. However, in physics and astronomy, the converse convention (that all gamma rays are considered to be of nuclear origin) is frequently violated.

In astronomy, higher energy gamma and X-rays are defined by energy, since the processes that produce them may be uncertain and photon energy, not origin, determines the required astronomical detectors needed.^[33] High-energy photons occur in nature that are known to be produced by processes other than nuclear decay but are still referred to as gamma radiation. An example is «gamma rays» from lightning discharges at 10 to 20 MeV, and known to be produced by the bremsstrahlung mechanism.

Another example is gamma-ray bursts, now known to be produced from processes too powerful to involve simple collections of atoms undergoing radioactive decay. This is part and parcel of the general realization that many gamma rays produced in astronomical processes result not from radioactive decay or particle annihilation, but rather in non-radioactive processes similar to X-rays.^{[clarification needed]} Although the gamma rays of astronomy often come from non-radioactive events, a few gamma rays in astronomy are specifically known to originate from gamma decay of nuclei (as demonstrated by their spectra and emission half life). A classic example is that of supernova SN 1987A, which emits an «afterglow» of gamma-ray photons from the decay of newly made radioactive nickel-56 and cobalt-56. Most gamma rays in astronomy, however, arise by other mechanisms.

See also

• Annihilation
• Galactic Center GeV excess
• Gaseous ionization detectors
• Very-high-energy gamma ray
• Ultra-high-energy gamma ray

Notes

References

External links

• Basic reference on several types of radiation Archived 2018-04-25 at the Wayback Machine
• Radiation Q & A
• GCSE information
• Radiation information Archived 2010-06-11 at the Wayback Machine
• Gamma-ray bursts
• The Lund/LBNL Nuclear Data Search – Contains information on gamma-ray energies from isotopes.
• Mapping soils with airborne detectors
• The LIVEChart of Nuclides – IAEA with filter on gamma-ray energy
• Health Physics Society Public Education Website

A gamma ray, also known as gamma radiation (symbol γ or ), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically shorter than those of X-rays. With frequencies above 30 exahertz (3×10¹⁹ Hz), it imparts the highest photon energy. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. In 1903, Ernest Rutherford named this radiation gamma rays based on their relatively strong penetration of matter; in 1900 he had already named two less penetrating types of decay radiation (discovered by Henri Becquerel) alpha rays and beta rays in ascending order of penetrating power.

Gamma rays from radioactive decay are in the energy range from a few kiloelectronvolts (keV) to approximately 8 megaelectronvolts (MeV), corresponding to the typical energy levels in nuclei with reasonably long lifetimes. The energy spectrum of gamma rays can be used to identify the decaying radionuclides using gamma spectroscopy. Very-high-energy gamma rays in the 100–1000 teraelectronvolt (TeV) range have been observed from sources such as the Cygnus X-3 microquasar.

Natural sources of gamma rays originating on Earth are mostly a result of radioactive decay and secondary radiation from atmospheric interactions with cosmic ray particles. However, there are other rare natural sources, such as terrestrial gamma-ray flashes, which produce gamma rays from electron action upon the nucleus. Notable artificial sources of gamma rays include fission, such as that which occurs in nuclear reactors, and high energy physics experiments, such as neutral pion decay and nuclear fusion.

Gamma rays and X-rays are both electromagnetic radiation, and since they overlap in the electromagnetic spectrum, the terminology varies between scientific disciplines. In some fields of physics, they are distinguished by their origin: Gamma rays are created by nuclear decay while X-rays originate outside the nucleus. In astrophysics, gamma rays are conventionally defined as having photon energies above 100 keV and are the subject of gamma ray astronomy, while radiation below 100 keV is classified as X-rays and is the subject of X-ray astronomy.

Gamma rays are ionizing radiation and are thus hazardous to life. Due to their high penetration power, they can damage bone marrow and internal organs. Unlike alpha and beta rays, they easily pass through the body and thus pose a formidable radiation protection challenge, requiring shielding made from dense materials such as lead or concrete. On Earth, the magnetosphere protects life from most types of lethal cosmic radiation other than gamma rays.

History of discovery

The first gamma ray source to be discovered was the radioactive decay process called gamma decay. In this type of decay, an excited nucleus emits a gamma ray almost immediately upon formation.^{[note 1]} Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium. Villard knew that his described radiation was more powerful than previously described types of rays from radium, which included beta rays, first noted as «radioactivity» by Henri Becquerel in 1896, and alpha rays, discovered as a less penetrating form of radiation by Rutherford, in 1899. However, Villard did not consider naming them as a different fundamental type.^[1][2] Later, in 1903, Villard’s radiation was recognized as being of a type fundamentally different from previously named rays by Ernest Rutherford, who named Villard’s rays «gamma rays» by analogy with the beta and alpha rays that Rutherford had differentiated in 1899.^[3] The «rays» emitted by radioactive elements were named in order of their power to penetrate various materials, using the first three letters of the Greek alphabet: alpha rays as the least penetrating, followed by beta rays, followed by gamma rays as the most penetrating. Rutherford also noted that gamma rays were not deflected (or at least, not easily deflected) by a magnetic field, another property making them unlike alpha and beta rays.

Gamma rays were first thought to be particles with mass, like alpha and beta rays. Rutherford initially believed that they might be extremely fast beta particles, but their failure to be deflected by a magnetic field indicated that they had no charge.^[4] In 1914, gamma rays were observed to be reflected from crystal surfaces, proving that they were electromagnetic radiation.^[4] Rutherford and his co-worker Edward Andrade measured the wavelengths of gamma rays from radium, and found they were similar to X-rays, but with shorter wavelengths and thus, higher frequency. This was eventually recognized as giving them more energy per photon, as soon as the latter term became generally accepted. A gamma decay was then understood to usually emit a gamma photon.

Sources

Natural sources of gamma rays on Earth include gamma decay from naturally occurring radioisotopes such as potassium-40, and also as a secondary radiation from various atmospheric interactions with cosmic ray particles. Some rare terrestrial natural sources that produce gamma rays that are not of a nuclear origin, are lightning strikes and terrestrial gamma-ray flashes, which produce high energy emissions from natural high-energy voltages. Gamma rays are produced by a number of astronomical processes in which very high-energy electrons are produced. Such electrons produce secondary gamma rays by the mechanisms of bremsstrahlung, inverse Compton scattering and synchrotron radiation. A large fraction of such astronomical gamma rays are screened by Earth’s atmosphere. Notable artificial sources of gamma rays include fission, such as occurs in nuclear reactors, as well as high energy physics experiments, such as neutral pion decay and nuclear fusion.

A sample of gamma ray-emitting material that is used for irradiating or imaging is known as a gamma source. It is also called a radioactive source, isotope source, or radiation source, though these more general terms also apply to alpha and beta-emitting devices. Gamma sources are usually sealed to prevent radioactive contamination, and transported in heavy shielding.

Radioactive decay (gamma decay)

Gamma rays are produced during gamma decay, which normally occurs after other forms of decay occur, such as alpha or beta decay. A radioactive nucleus can decay by the emission of an
α
or
β
particle. The daughter nucleus that results is usually left in an excited state. It can then decay to a lower energy state by emitting a gamma ray photon, in a process called gamma decay.

The emission of a gamma ray from an excited nucleus typically requires only 10⁻¹² seconds. Gamma decay may also follow nuclear reactions such as neutron capture, nuclear fission, or nuclear fusion. Gamma decay is also a mode of relaxation of many excited states of atomic nuclei following other types of radioactive decay, such as beta decay, so long as these states possess the necessary component of nuclear spin. When high-energy gamma rays, electrons, or protons bombard materials, the excited atoms emit characteristic «secondary» gamma rays, which are products of the creation of excited nuclear states in the bombarded atoms. Such transitions, a form of nuclear gamma fluorescence, form a topic in nuclear physics called gamma spectroscopy. Formation of fluorescent gamma rays are a rapid subtype of radioactive gamma decay.

In certain cases, the excited nuclear state that follows the emission of a beta particle or other type of excitation, may be more stable than average, and is termed a metastable excited state, if its decay takes (at least) 100 to 1000 times longer than the average 10⁻¹² seconds. Such relatively long-lived excited nuclei are termed nuclear isomers, and their decays are termed isomeric transitions. Such nuclei have half-lifes that are more easily measurable, and rare nuclear isomers are able to stay in their excited state for minutes, hours, days, or occasionally far longer, before emitting a gamma ray. The process of isomeric transition is therefore similar to any gamma emission, but differs in that it involves the intermediate metastable excited state(s) of the nuclei. Metastable states are often characterized by high nuclear spin, requiring a change in spin of several units or more with gamma decay, instead of a single unit transition that occurs in only 10⁻¹² seconds. The rate of gamma decay is also slowed when the energy of excitation of the nucleus is small.^[5]

An emitted gamma ray from any type of excited state may transfer its energy directly to any electrons, but most probably to one of the K shell electrons of the atom, causing it to be ejected from that atom, in a process generally termed the photoelectric effect (external gamma rays and ultraviolet rays may also cause this effect). The photoelectric effect should not be confused with the internal conversion process, in which a gamma ray photon is not produced as an intermediate particle (rather, a «virtual gamma ray» may be thought to mediate the process).

Decay schemes

One example of gamma ray production due to radionuclide decay is the decay scheme for cobalt-60, as illustrated in the accompanying diagram. First, ⁶⁰
Co
decays to excited ⁶⁰
Ni
by beta decay emission of an electron of 0.31 MeV. Then the excited ⁶⁰
Ni
decays to the ground state (see nuclear shell model) by emitting gamma rays in succession of 1.17 MeV followed by 1.33 MeV. This path is followed 99.88% of the time:

60 27Co	→	60 28Ni*	+	e−	+	ν e	+	γ	+	1.17 MeV
60 28Ni*	→	60 28Ni					+	γ	+	1.33 MeV

Another example is the alpha decay of ²⁴¹
Am
to form ²³⁷
Np
; which is followed by gamma emission. In some cases, the gamma emission spectrum of the daughter nucleus is quite simple, (e.g. ⁶⁰
Co
/⁶⁰
Ni
) while in other cases, such as with (²⁴¹
Am
/²³⁷
Np
and ¹⁹²
Ir
/¹⁹²
Pt
), the gamma emission spectrum is complex, revealing that a series of nuclear energy levels exist.

Particle physics

Gamma rays are produced in many processes of particle physics. Typically, gamma rays are the products of neutral systems which decay through electromagnetic interactions (rather than a weak or strong interaction). For example, in an electron–positron annihilation, the usual products are two gamma ray photons. If the annihilating electron and positron are at rest, each of the resulting gamma rays has an energy of ~ 511 keV and frequency of ~ 1.24×10²⁰ Hz. Similarly, a neutral pion most often decays into two photons. Many other hadrons and massive bosons also decay electromagnetically. High energy physics experiments, such as the Large Hadron Collider, accordingly employ substantial radiation shielding.^{[citation needed]} Because subatomic particles mostly have far shorter wavelengths than atomic nuclei, particle physics gamma rays are generally several orders of magnitude more energetic than nuclear decay gamma rays. Since gamma rays are at the top of the electromagnetic spectrum in terms of energy, all extremely high-energy photons are gamma rays; for example, a photon having the Planck energy would be a gamma ray.

Other sources

A few gamma rays in astronomy are known to arise from gamma decay (see discussion of SN1987A), but most do not.

Photons from astrophysical sources that carry energy in the gamma radiation range are often explicitly called gamma-radiation. In addition to nuclear emissions, they are often produced by sub-atomic particle and particle-photon interactions. Those include electron-positron annihilation, neutral pion decay, bremsstrahlung, inverse Compton scattering, and synchrotron radiation.

Laboratory sources

In October 2017, scientists from various European universities proposed a means for sources of GeV photons using lasers as exciters through a controlled interplay between the cascade and anomalous radiative trapping.^[6]

Terrestrial thunderstorms

Thunderstorms can produce a brief pulse of gamma radiation called a terrestrial gamma-ray flash. These gamma rays are thought to be produced by high intensity static electric fields accelerating electrons, which then produce gamma rays by bremsstrahlung as they collide with and are slowed by atoms in the atmosphere. Gamma rays up to 100 MeV can be emitted by terrestrial thunderstorms, and were discovered by space-borne observatories. This raises the possibility of health risks to passengers and crew on aircraft flying in or near thunderclouds.^[7]

Solar flares

The most effusive solar flares emit across the entire EM spectrum, including γ-rays. The first confident observation occurred in 1972.^[8]

Cosmic rays

Extraterrestrial, high energy gamma rays include the gamma ray background produced when cosmic rays (either high speed electrons or protons) collide with ordinary matter, producing pair-production gamma rays at 511 keV. Alternatively, bremsstrahlung are produced at energies of tens of MeV or more when cosmic ray electrons interact with nuclei of sufficiently high atomic number (see gamma ray image of the Moon near the end of this article, for illustration).

Pulsars and magnetars

The gamma ray sky (see illustration at right) is dominated by the more common and longer-term production of gamma rays that emanate from pulsars within the Milky Way. Sources from the rest of the sky are mostly quasars. Pulsars are thought to be neutron stars with magnetic fields that produce focused beams of radiation, and are far less energetic, more common, and much nearer sources (typically seen only in our own galaxy) than are quasars or the rarer gamma-ray burst sources of gamma rays. Pulsars have relatively long-lived magnetic fields that produce focused beams of relativistic speed charged particles, which emit gamma rays (bremsstrahlung) when those strike gas or dust in their nearby medium, and are decelerated. This is a similar mechanism to the production of high-energy photons in megavoltage radiation therapy machines (see bremsstrahlung). Inverse Compton scattering, in which charged particles (usually electrons) impart energy to low-energy photons boosting them to higher energy photons. Such impacts of photons on relativistic charged particle beams is another possible mechanism of gamma ray production. Neutron stars with a very high magnetic field (magnetars), thought to produce astronomical soft gamma repeaters, are another relatively long-lived star-powered source of gamma radiation.

Quasars and active galaxies

More powerful gamma rays from very distant quasars and closer active galaxies are thought to have a gamma ray production source similar to a particle accelerator. High energy electrons produced by the quasar, and subjected to inverse Compton scattering, synchrotron radiation, or bremsstrahlung, are the likely source of the gamma rays from those objects. It is thought that a supermassive black hole at the center of such galaxies provides the power source that intermittently destroys stars and focuses the resulting charged particles into beams that emerge from their rotational poles. When those beams interact with gas, dust, and lower energy photons they produce X-rays and gamma rays. These sources are known to fluctuate with durations of a few weeks, suggesting their relatively small size (less than a few light-weeks across). Such sources of gamma and X-rays are the most commonly visible high intensity sources outside the Milky Way galaxy. They shine not in bursts (see illustration), but relatively continuously when viewed with gamma ray telescopes. The power of a typical quasar is about 10⁴⁰ watts, a small fraction of which is gamma radiation. Much of the rest is emitted as electromagnetic waves of all frequencies, including radio waves.

Gamma-ray bursts

The most intense sources of gamma rays, are also the most intense sources of any type of electromagnetic radiation presently known. They are the «long duration burst» sources of gamma rays in astronomy («long» in this context, meaning a few tens of seconds), and they are rare compared with the sources discussed above. By contrast, «short» gamma-ray bursts of two seconds or less, which are not associated with supernovae, are thought to produce gamma rays during the collision of pairs of neutron stars, or a neutron star and a black hole.^[9]

The so-called long-duration gamma-ray bursts produce a total energy output of about 10⁴⁴ joules (as much energy as the Sun will produce in its entire life-time) but in a period of only 20 to 40 seconds. Gamma rays are approximately 50% of the total energy output. The leading hypotheses for the mechanism of production of these highest-known intensity beams of radiation, are inverse Compton scattering and synchrotron radiation from high-energy charged particles. These processes occur as relativistic charged particles leave the region of the event horizon of a newly formed black hole created during supernova explosion. The beam of particles moving at relativistic speeds are focused for a few tens of seconds by the magnetic field of the exploding hypernova. The fusion explosion of the hypernova drives the energetics of the process. If the narrowly directed beam happens to be pointed toward the Earth, it shines at gamma ray frequencies with such intensity, that it can be detected even at distances of up to 10 billion light years, which is close to the edge of the visible universe.

Properties

Penetration of matter

Due to their penetrating nature, gamma rays require large amounts of shielding mass to reduce them to levels which are not harmful to living cells, in contrast to alpha particles, which can be stopped by paper or skin, and beta particles, which can be shielded by thin aluminium. Gamma rays are best absorbed by materials with high atomic numbers (Z) and high density, which contribute to the total stopping power. Because of this, a lead (high Z) shield is 20–30% better as a gamma shield than an equal mass of another low-Z shielding material, such as aluminium, concrete, water, or soil; lead’s major advantage is not in lower weight, but rather its compactness due to its higher density. Protective clothing, goggles and respirators can protect from internal contact with or ingestion of alpha or beta emitting particles, but provide no protection from gamma radiation from external sources.

The higher the energy of the gamma rays, the thicker the shielding made from the same shielding material is required. Materials for shielding gamma rays are typically measured by the thickness required to reduce the intensity of the gamma rays by one half (the half value layer or HVL). For example, gamma rays that require 1 cm (0.4 inch) of lead to reduce their intensity by 50% will also have their intensity reduced in half by 4.1 cm of granite rock, 6 cm (2.5 inches) of concrete, or 9 cm (3.5 inches) of packed soil. However, the mass of this much concrete or soil is only 20–30% greater than that of lead with the same absorption capability. Depleted uranium is used for shielding in portable gamma ray sources, but here the savings in weight over lead are larger, as a portable source is very small relative to the required shielding, so the shielding resembles a sphere to some extent. The volume of a sphere is dependent on the cube of the radius; so a source with its radius cut in half will have its volume (and weight) reduced by a factor of eight, which will more than compensate for uranium’s greater density (as well as reducing bulk).^{[clarification needed]} In a nuclear power plant, shielding can be provided by steel and concrete in the pressure and particle containment vessel, while water provides a radiation shielding of fuel rods during storage or transport into the reactor core. The loss of water or removal of a «hot» fuel assembly into the air would result in much higher radiation levels than when kept under water.

Matter interaction

When a gamma ray passes through matter, the probability for absorption is proportional to the thickness of the layer, the density of the material, and the absorption cross section of the material. The total absorption shows an exponential decrease of intensity with distance from the incident surface:

where x is the thickness of the material from the incident surface, μ= nσ is the absorption coefficient, measured in cm⁻¹, n the number of atoms per cm³ of the material (atomic density) and σ the absorption cross section in cm².

As it passes through matter, gamma radiation ionizes via three processes:

• The photoelectric effect: This describes the case in which a gamma photon interacts with and transfers its energy to an atomic electron, causing the ejection of that electron from the atom. The kinetic energy of the resulting photoelectron is equal to the energy of the incident gamma photon minus the energy that originally bound the electron to the atom (binding energy). The photoelectric effect is the dominant energy transfer mechanism for X-ray and gamma ray photons with energies below 50 keV (thousand electronvolts), but it is much less important at higher energies.
• Compton scattering: This is an interaction in which an incident gamma photon loses enough energy to an atomic electron to cause its ejection, with the remainder of the original photon’s energy emitted as a new, lower energy gamma photon whose emission direction is different from that of the incident gamma photon, hence the term «scattering». The probability of Compton scattering decreases with increasing photon energy. It is thought to be the principal absorption mechanism for gamma rays in the intermediate energy range 100 keV to 10 MeV. It is relatively independent of the atomic number of the absorbing material, which is why very dense materials like lead are only modestly better shields, on a per weight basis, than are less dense materials.
• Pair production: This becomes possible with gamma energies exceeding 1.02 MeV, and becomes important as an absorption mechanism at energies over 5 MeV (see illustration at right, for lead). By interaction with the electric field of a nucleus, the energy of the incident photon is converted into the mass of an electron-positron pair. Any gamma energy in excess of the equivalent rest mass of the two particles (totaling at least 1.02 MeV) appears as the kinetic energy of the pair and in the recoil of the emitting nucleus. At the end of the positron’s range, it combines with a free electron, and the two annihilate, and the entire mass of these two is then converted into two gamma photons of at least 0.51 MeV energy each (or higher according to the kinetic energy of the annihilated particles).

The secondary electrons (and/or positrons) produced in any of these three processes frequently have enough energy to produce much ionization themselves.

Additionally, gamma rays, particularly high energy ones, can interact with atomic nuclei resulting in ejection of particles in photodisintegration, or in some cases, even nuclear fission (photofission).

Light interaction

High-energy (from 80 GeV to ~10 TeV) gamma rays arriving from far-distant quasars are used to estimate the extragalactic background light in the universe: The highest-energy rays interact more readily with the background light photons and thus the density of the background light may be estimated by analyzing the incoming gamma ray spectra.^[10][11]

Gamma spectroscopy

Gamma spectroscopy is the study of the energetic transitions in atomic nuclei, which are generally associated with the absorption or emission of gamma rays. As in optical spectroscopy (see Franck–Condon effect) the absorption of gamma rays by a nucleus is especially likely (i.e., peaks in a «resonance») when the energy of the gamma ray is the same as that of an energy transition in the nucleus. In the case of gamma rays, such a resonance is seen in the technique of Mössbauer spectroscopy. In the Mössbauer effect the narrow resonance absorption for nuclear gamma absorption can be successfully attained by physically immobilizing atomic nuclei in a crystal. The immobilization of nuclei at both ends of a gamma resonance interaction is required so that no gamma energy is lost to the kinetic energy of recoiling nuclei at either the emitting or absorbing end of a gamma transition. Such loss of energy causes gamma ray resonance absorption to fail. However, when emitted gamma rays carry essentially all of the energy of the atomic nuclear de-excitation that produces them, this energy is also sufficient to excite the same energy state in a second immobilized nucleus of the same type.

Applications

Gamma rays provide information about some of the most energetic phenomena in the universe; however, they are largely absorbed by the Earth’s atmosphere. Instruments aboard high-altitude balloons and satellites missions, such as the Fermi Gamma-ray Space Telescope, provide our only view of the universe in gamma rays.

Gamma-induced molecular changes can also be used to alter the properties of semi-precious stones, and is often used to change white topaz into blue topaz.

Non-contact industrial sensors commonly use sources of gamma radiation in refining, mining, chemicals, food, soaps and detergents, and pulp and paper industries, for the measurement of levels, density, and thicknesses.^[12] Gamma-ray sensors are also used for measuring the fluid levels in water and oil industries.^[13] Typically, these use Co-60 or Cs-137 isotopes as the radiation source.

In the US, gamma ray detectors are beginning to be used as part of the Container Security Initiative (CSI). These machines are advertised to be able to scan 30 containers per hour.

Gamma radiation is often used to kill living organisms, in a process called irradiation. Applications of this include the sterilization of medical equipment (as an alternative to autoclaves or chemical means), the removal of decay-causing bacteria from many foods and the prevention of the sprouting of fruit and vegetables to maintain freshness and flavor.

Despite their cancer-causing properties, gamma rays are also used to treat some types of cancer, since the rays also kill cancer cells. In the procedure called gamma-knife surgery, multiple concentrated beams of gamma rays are directed to the growth in order to kill the cancerous cells. The beams are aimed from different angles to concentrate the radiation on the growth while minimizing damage to surrounding tissues.

Gamma rays are also used for diagnostic purposes in nuclear medicine in imaging techniques. A number of different gamma-emitting radioisotopes are used. For example, in a PET scan a radiolabeled sugar called fluorodeoxyglucose emits positrons that are annihilated by electrons, producing pairs of gamma rays that highlight cancer as the cancer often has a higher metabolic rate than the surrounding tissues. The most common gamma emitter used in medical applications is the nuclear isomer technetium-99m which emits gamma rays in the same energy range as diagnostic X-rays. When this radionuclide tracer is administered to a patient, a gamma camera can be used to form an image of the radioisotope’s distribution by detecting the gamma radiation emitted (see also SPECT). Depending on which molecule has been labeled with the tracer, such techniques can be employed to diagnose a wide range of conditions (for example, the spread of cancer to the bones via bone scan).

Health effects

Gamma rays cause damage at a cellular level and are penetrating, causing diffuse damage throughout the body. However, they are less ionising than alpha or beta particles, which are less penetrating.

Low levels of gamma rays cause a stochastic health risk, which for radiation dose assessment is defined as the probability of cancer induction and genetic damage.^[14] High doses produce deterministic effects, which is the severity of acute tissue damage that is certain to happen. These effects are compared to the physical quantity absorbed dose measured by the unit gray (Gy).^[15]

Body response

When gamma radiation breaks DNA molecules, a cell may be able to repair the damaged genetic material, within limits. However, a study of Rothkamm and Lobrich has shown that this repair process works well after high-dose exposure but is much slower in the case of a low-dose exposure.^[16]

Risk assessment

The natural outdoor exposure in the United Kingdom ranges from 0.1 to 0.5 µSv/h with significant increase around known nuclear and contaminated sites.^[17] Natural exposure to gamma rays is about 1 to 2 mSv per year, and the average total amount of radiation received in one year per inhabitant in the USA is 3.6 mSv.^[18] There is a small increase in the dose, due to naturally occurring gamma radiation, around small particles of high atomic number materials in the human body caused by the photoelectric effect.^[19]

By comparison, the radiation dose from chest radiography (about 0.06 mSv) is a fraction of the annual naturally occurring background radiation dose.^[20] A chest CT delivers 5 to 8 mSv. A whole-body PET/CT scan can deliver 14 to 32 mSv depending on the protocol.^[21] The dose from fluoroscopy of the stomach is much higher, approximately 50 mSv (14 times the annual background).

An acute full-body equivalent single exposure dose of 1 Sv (1000 mSv), or 1 Gy, will cause mild symptoms of acute radiation sickness, such as nausea and vomiting; and a dose of 2.0–3.5 Sv (2.0–3.5 Gy) causes more severe symptoms (i.e. nausea, diarrhea, hair loss, hemorrhaging, and inability to fight infections), and will cause death in a sizable number of cases —- about 10% to 35% without medical treatment. A dose of 5 Sv^[22] (5 Gy) is considered approximately the LD₅₀ (lethal dose for 50% of exposed population) for an acute exposure to radiation even with standard medical treatment. A dose higher than 5 Sv (5 Gy) brings an increasing chance of death above 50%. Above 7.5–10 Sv (7.5–10 Gy) to the entire body, even extraordinary treatment, such as bone-marrow transplants, will not prevent the death of the individual exposed (see radiation poisoning).^[23] (Doses much larger than this may, however, be delivered to selected parts of the body in the course of radiation therapy.)

For low-dose exposure, for example among nuclear workers, who receive an average yearly radiation dose of 19 mSv,^{[clarification needed]} the risk of dying from cancer (excluding leukemia) increases by 2 percent. For a dose of 100 mSv, the risk increase is 10 percent. By comparison, risk of dying from cancer was increased by 32 percent for the survivors of the atomic bombing of Hiroshima and Nagasaki.^[24]

Units of measurement and exposure

The following table shows radiation quantities in SI and non-SI units:

Ionizing radiation related quantities view ‧ talk ‧ edit

Quantity	Unit	Symbol	Derivation	Year	SI equivalent
Activity (A)	becquerel	Bq	s−1	1974	SI unit
curie	Ci	3.7 × 1010 s−1	1953	3.7×1010 Bq
rutherford	Rd	106 s−1	1946	1,000,000 Bq
Exposure (X)	coulomb per kilogram	C/kg	C⋅kg−1 of air	1974	SI unit
röntgen	R	esu / 0.001293 g of air	1928	2.58 × 10−4 C/kg
Absorbed dose (D)	gray	Gy	J⋅kg−1	1974	SI unit
erg per gram	erg/g	erg⋅g−1	1950	1.0 × 10−4 Gy
rad	rad	100 erg⋅g−1	1953	0.010 Gy
Equivalent dose (H)	sievert	Sv	J⋅kg−1 × WR	1977	SI unit
röntgen equivalent man	rem	100 erg⋅g−1 x WR	1971	0.010 Sv
Effective dose (E)	sievert	Sv	J⋅kg−1 × WR × WT	1977	SI unit
röntgen equivalent man	rem	100 erg⋅g−1 × WR × WT	1971	0.010 Sv

The measure of the ionizing effect of gamma and X-rays in dry air is called the exposure, for which a legacy unit, the röntgen was used from 1928. This has been replaced by kerma, now mainly used for instrument calibration purposes but not for received dose effect. The effect of gamma and other ionizing radiation on living tissue is more closely related to the amount of energy deposited in tissue rather than the ionisation of air, and replacement radiometric units and quantities for radiation protection have been defined and developed from 1953 onwards. These are:

• The gray (Gy), is the SI unit of absorbed dose, which is the amount of radiation energy deposited in the irradiated material. For gamma radiation this is numerically equivalent to equivalent dose measured by the sievert, which indicates the stochastic biological effect of low levels of radiation on human tissue. The radiation weighting conversion factor from absorbed dose to equivalent dose is 1 for gamma, whereas alpha particles have a factor of 20, reflecting their greater ionising effect on tissue.
• The rad is the deprecated CGS unit for absorbed dose and the rem is the deprecated CGS unit of equivalent dose, used mainly in the USA.

Distinction from X-rays

The conventional distinction between X-rays and gamma rays has changed over time. Originally, the electromagnetic radiation emitted by X-ray tubes almost invariably had a longer wavelength than the radiation (gamma rays) emitted by radioactive nuclei.^[25] Older literature distinguished between X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10⁻¹¹ m, defined as gamma rays.^[26] Since the energy of photons is proportional to their frequency and inversely proportional to wavelength, this past distinction between X-rays and gamma rays can also be thought of in terms of its energy, with gamma rays considered to be higher energy electromagnetic radiation than are X-rays.

However, since current artificial sources are now able to duplicate any electromagnetic radiation that originates in the nucleus, as well as far higher energies, the wavelengths characteristic of radioactive gamma ray sources vs. other types now completely overlap. Thus, gamma rays are now usually distinguished by their origin: X-rays are emitted by definition by electrons outside the nucleus, while gamma rays are emitted by the nucleus.^{[25][27][28][29]} Exceptions to this convention occur in astronomy, where gamma decay is seen in the afterglow of certain supernovas, but radiation from high energy processes known to involve other radiation sources than radioactive decay is still classed as gamma radiation.

For example, modern high-energy X-rays produced by linear accelerators for megavoltage treatment in cancer often have higher energy (4 to 25 MeV) than do most classical gamma rays produced by nuclear gamma decay. One of the most common gamma ray emitting isotopes used in diagnostic nuclear medicine, technetium-99m, produces gamma radiation of the same energy (140 keV) as that produced by diagnostic X-ray machines, but of significantly lower energy than therapeutic photons from linear particle accelerators. In the medical community today, the convention that radiation produced by nuclear decay is the only type referred to as «gamma» radiation is still respected.

Due to this broad overlap in energy ranges, in physics the two types of electromagnetic radiation are now often defined by their origin: X-rays are emitted by electrons (either in orbitals outside of the nucleus, or while being accelerated to produce bremsstrahlung-type radiation),^[31] while gamma rays are emitted by the nucleus or by means of other particle decays or annihilation events. There is no lower limit to the energy of photons produced by nuclear reactions, and thus ultraviolet or lower energy photons produced by these processes would also be defined as «gamma rays».^[32] The only naming-convention that is still universally respected is the rule that electromagnetic radiation that is known to be of atomic nuclear origin is always referred to as «gamma rays», and never as X-rays. However, in physics and astronomy, the converse convention (that all gamma rays are considered to be of nuclear origin) is frequently violated.

In astronomy, higher energy gamma and X-rays are defined by energy, since the processes that produce them may be uncertain and photon energy, not origin, determines the required astronomical detectors needed.^[33] High-energy photons occur in nature that are known to be produced by processes other than nuclear decay but are still referred to as gamma radiation. An example is «gamma rays» from lightning discharges at 10 to 20 MeV, and known to be produced by the bremsstrahlung mechanism.

Another example is gamma-ray bursts, now known to be produced from processes too powerful to involve simple collections of atoms undergoing radioactive decay. This is part and parcel of the general realization that many gamma rays produced in astronomical processes result not from radioactive decay or particle annihilation, but rather in non-radioactive processes similar to X-rays.^{[clarification needed]} Although the gamma rays of astronomy often come from non-radioactive events, a few gamma rays in astronomy are specifically known to originate from gamma decay of nuclei (as demonstrated by their spectra and emission half life). A classic example is that of supernova SN 1987A, which emits an «afterglow» of gamma-ray photons from the decay of newly made radioactive nickel-56 and cobalt-56. Most gamma rays in astronomy, however, arise by other mechanisms.

See also

• Annihilation
• Galactic Center GeV excess
• Gaseous ionization detectors
• Very-high-energy gamma ray
• Ultra-high-energy gamma ray

Notes

References

External links

• Basic reference on several types of radiation Archived 2018-04-25 at the Wayback Machine
• Radiation Q & A
• GCSE information
• Radiation information Archived 2010-06-11 at the Wayback Machine
• Gamma-ray bursts
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Смотреть что такое ГАММАИЗЛУЧЕНИЕ в других словарях:

Гамма — третья буква греческого алфавита. В ионийской системе счисления обозначала число 3. Произошла от финикийской буквы гамл 𐤂. От самой же гаммы произошли многие другие латинские и кириллические буквы (G, C, Г…).

Строчная греческая буква гамма имеет большое распространение в научной нотации (в математике, физике, астрономии).

В геометрии при указании углов обычно используются по порядку альфа, бета и гамма.

Гамма-излучение (γ-лучи) — электромагнитное излучение в предельно коротковолновом диапазоне.

«Гамма» часто является частью названия звёзд (γ Centauri — Гамма Центавра), обычно указывает на их относительную светимость (альфа — наиболее яркая звезда в созвездии, бета и гамма тусклее).

Символ «Греческая строчная буква гамма» был утвержден как часть Юникода версии 1.1 в 1993 г.

Этот текст также доступен на следующих языках:
English;

Га́мма-излуче́ние (гамма-лучи, γ-лучи) — вид электромагнитного излучения, характеризующийся чрезвычайно малой длиной волны — менее 2⋅10⁻¹⁰ м — и, вследствие этого, ярко выраженными корпускулярными и слабо выраженными волновыми свойствами. Относится к ионизирующим излучениям, то есть к излучениям, взаимодействие которых с веществом способно приводить к образованию ионов разных знаков.

Гамма-излучение представляет собой поток фотонов, имеющих высокую энергию (гамма-квантов). Условно считается, что энергии квантов гамма-излучения превышают 10⁵ эВ, хотя резкая граница между гамма- и рентгеновским излучением не определена. На шкале электромагнитных волн гамма-излучение граничит с рентгеновским излучением, занимая диапазон более высоких частот и энергий. В области 1—100 кэВ гамма-излучение и рентгеновское излучение различаются только по источнику: если квант излучается в ядерном переходе, то его принято относить к гамма-излучению; если при взаимодействиях электронов или при переходах в атомной электронной оболочке — к рентгеновскому излучению. С точки зрения физики, кванты электромагнитного излучения с одинаковой энергией не отличаются, поэтому такое разделение условно.

Гамма-излучение испускается при переходах между возбуждёнными состояниями атомных ядер (см. Изомерный переход; энергии таких гамма-квантов лежат в диапазоне от ~1 кэВ до десятков МэВ), при ядерных реакциях, при взаимодействиях и распадах элементарных частиц (например, при аннигиляции электрона и позитрона, распаде нейтрального пиона и т. д.), а также при отклонении энергичных заряженных частиц в магнитных и электрических полях (см. Синхротронное излучение, Тормозное излучение). Энергия гамма-квантов, возникающих при переходах между возбуждёнными состояниями ядер, не превышает нескольких десятков МэВ. Энергии гамма-квантов, наблюдающихся в космических лучах, могут превосходить сотни ГэВ.

Гамма-излучение было открыто французским физиком Полем Вилларом в 1900 году при исследовании излучения радия. Три компоненты ионизирующего излучения радия-226 (в смеси с его дочерними радионуклидами) были разделены по направлению отклонения частиц в магнитном поле: излучение с положительным электрическим зарядом было названо α-лучами, с отрицательным — β-лучами, а электрически нейтральное, не отклоняющееся в магнитном поле излучение получило название γ-лучей. Впервые такая терминология была использована Э. Резерфордом в начале 1903 года. В 1912 году Резерфорд и Эдвард Андраде^[en] доказали электромагнитную природу гамма-излучения.

Физические свойства

[1]
Гамма-лучи, в отличие от α-лучей и β-лучей, не содержат заряженных частиц и поэтому не отклоняются электрическими и магнитными полями и характеризуются большей проникающей способностью при равных энергиях и прочих равных условиях. Гамма-кванты вызывают ионизацию атомов вещества. Основные процессы, возникающие при прохождении гамма-излучения через вещество:

• Фотоэффект — энергия гамма-кванта поглощается электроном оболочки атома, и электрон, совершая работу выхода, покидает атом (который становится положительно ионизированным).
• Комптон-эффект — гамма-квант рассеивается при взаимодействии с электроном, при этом образуется новый гамма-квант, меньшей энергии, что также сопровождается высвобождением электрона и ионизацией атома.
• Эффект образования пар — гамма-квант в электрическом поле ядра превращается в электрон и позитрон.
• Ядерный фотоэффект — при энергиях выше нескольких десятков МэВ гамма-квант способен выбивать нуклоны из ядра.

Детектирование

[2]
Зарегистрировать гамма-кванты можно с помощью ряда ядерно-физических детекторов ионизирующего излучения (сцинтилляционных, газовых, полупроводниковых и т. д.).

Использование

[3]
Области применения гамма-излучения:

• Гамма-дефектоскопия — контроль изделий просвечиванием γ-лучами.
• Пищевая промышленность: консервирование пищевых продуктов (гамма-стерилизация для увеличения срока хранения).
• Медицина: стерилизация медицинских материалов и оборудования; лучевая терапия; радиохирургия.
• Гамма-каротаж в геофизике.
• Приборы для измерения расстояний: уровнемеры, гамма-высотомеры на космических аппаратах.
• Гамма-астрономия.

Биологические эффекты

[4]
Облучение гамма-квантами в зависимости от дозы и продолжительности может вызвать хроническую и острую лучевую болезнь. Стохастические эффекты облучения включают различные виды онкологических заболеваний. В то же время гамма-облучение подавляет рост раковых и других быстро делящихся клеток при локальном воздействии на них. Гамма-излучение является мутагенным и тератогенным фактором.

Защита

Защитой от гамма-излучения может служить слой вещества. Эффективность защиты (то есть вероятность поглощения гамма-кванта при прохождении через неё) увеличивается при увеличении толщины слоя, плотности вещества и содержания в нём тяжёлых ядер (свинца, вольфрама, обеднённого урана и пр.).

В таблице ниже указаны параметры слоя половинного ослабления^[en] гамма-излучения с энергией 1 МэВ для различных материалов:

Подробнее: Материал защиты, Плотность, г/см³ …

Материал защиты	Плотность, г/см³	Слой половинного ослабления, см	Масса 1 см² слоя половинного ослабления, г
Свинец	11,35	0,8	9,08
Бетон	1,5-3,5	3,8-6,9	10,35-13,3
Сталь	7,5-8,05	1,27	9,53-10,22
Железо	7,86	1,5	11,79
Алюминий	2,82	4,3	12,17
Вольфрам	19,3	0,33	6,37
Вода	1,00	~10	10
Обеднённый уран	19,5	0,28	5,46
Воздух	0,0013	~8500	11,05

Хотя эффективность поглощения и зависит от материала, первоочередное значение имеет просто удельный вес.

Морфемный разбор слова:

Однокоренные слова к слову:

Гамма-излучение (гамма-лучи, γ-лучи), виды, образование, биологическая опасность и защита

Гамма-излучение (гамма-лучи, γ-лучи), виды, образование, биологическая опасность и защита.

Гамма-излучение (гамма-лучи, γ-лучи) — это вид электромагнитного излучения, характеризующийся чрезвычайно малой длиной волны – менее 2⋅10 −10 м – и, вследствие этого, ярко выраженными корпускулярными и слабо выраженными волновыми свойствами.

Гамма-излучение (гамма-лучи, γ-лучи) и его виды:

Гамма-излучение (гамма-лучи, γ-лучи) — вид электромагнитного излучения, характеризующийся чрезвычайно малой длиной волны – менее 2⋅10 −10 м – и, вследствие этого, ярко выраженными корпускулярными и слабо выраженными волновыми свойствами.

Гамма-излучение относится к ионизирующим излучениям, то есть к излучениям, взаимодействие которых с веществом способно приводить к образованию ионов разных знаков.

Гамма-излучение (в узком смысле) – это проникающее электромагнитное излучение, возникающее при спонтанных превращениях («распаде») атомных ядер многих естественных или искусственно созданных радиоактивных элементов (радионуклидов).

В более широком смысле гамма-излучением называется любое электромагнитное излучение с квантовыми энергиями от нескольких сотен килоэлектронвольт и выше, независимо от характера их возникновения.

Название гамма-лучей происходит от деления ионизирующего излучения на альфа-излучение, бета-излучение и гамма- излучение в соответствии с их возрастающей способностью проникать в материю. Альфа- и бета-лучи состоят из заряженных частиц и поэтому взаимодействуют с материей значительно сильнее, чем незаряженные фотоны или кванты гамма-излучения. Соответственно, последние имеют значительно более высокую проникающую способность.

Альфа-излучение (α-лучи) – это поток ядер атомов гелия-4, имеющих положительный заряд. Ядро атома гелия-4 (α-частица) – 4 ₂He 2+ образовано двумя протонами и двумя нейтронами.

Бета-излучение (β-лучи) являют собой поток электронов – е – (частиц с отрицательным зарядом) или позитронов – p (соответственно, частиц с положительным зарядом).

– мягкое гамма-излучение (с энергиями фотонов от нескольких сотен килоэлектронвольт до нескольких мегаэлектронвольт),

– гамма-излучение средних энергий (с энергиями фотонов от нескольких мегаэлектронвольт до десятков мегаэлектронвольт),

– гамма-излучение высоких энергий (с энергиями фотонов от нескольких десятков мегаэлектронвольт до 10 11 электронвольт),

– гамма-излучение сверхвысоких энергий (с энергиями фотонов свыше 10 11 электронвольт).

На шкале электромагнитных волн гамма-излучение граничит с жестким рентгеновским излучением. При этом четкая граница между гамма-излучением и жестким рентгеновским излучением не определена.

Гамма-излучение было открыто французским физиком Полем Вилларом в 1900 году при исследовании излучения радия. Он поместил радий-226 (в смеси с его дочерними радионуклидами) в магнитное поле. В результате компоненты излучения были разделены на три составляющие по направлению отклонения частиц в магнитном поле: излучение с положительным электрическим зарядом было названо α-лучами, с отрицательным — β-лучами, а электрически нейтральное, не отклоняющееся в магнитном поле излучение получило название γ-лучей. Впервые такую терминологию использования предложил Э. Резерфорд в начале 1903 года.

Возникновение и образование гамма-излучения:

Гамма-излучение возникает:

– при переходах между возбуждёнными состояниями атомных ядер в стабильное (при т.н. изомерном переходе);

– при взаимодействиях и распадах элементарных частиц (например, при аннигиляции электрона и позитрона, распаде нейтрального пиона и т. д.),

– при отклонении энергичных заряженных частиц в магнитных и электрических полях.

Гамма излучение также возникает в космическом пространстве.

Биологический эффект и опасность гамма-излучения:

Гамма-излучение является мутагенным и тератогенным фактором. Оно поражает ДНК клеток человека.

Свойства гамма-излучения:

– гамма-лучи, в отличие от α-лучей и β-лучей, не содержат заряженных частиц и поэтому не отклоняются электрическими и магнитными полями;

– гамма-лучи характеризуются большей проникающей способностью (по сравнению с α- и β- лучами) при равных энергиях и прочих равных условиях;

– гамма-излучение при прохождении через вещество вызывает ионизацию атомов вещества;

Фотоэффект или фотоэлектрический эффект – явление взаимодействия света или любого другого электромагнитного излучения (например, гамма-излучения) с веществом, при котором энергия фотонов передаётся электронам вещества (энергия фотона поглощается электроном оболочки атома). В конденсированных (твёрдых и жидких) веществах выделяют внешний (поглощение фотонов сопровождается вылетом электронов за пределы тела) и внутренний (электроны, оставаясь в теле, изменяют в нём своё энергетическое состояние, переходят из связанного состояния в свободное без вылета наружу) фотоэффект. При внутреннем фотоэффекте как следствие поглощения фотона образуется пара носителей заряда: электрон в зоне проводимости и дырка в валентной зоне. Концентрация носителей заряда приводит к возникновению фотопроводимости (повышению электропроводности полупроводника или диэлектрика) или возникновению электродвижущей силы. Фотоэффект в газах состоит в ионизации атомов или молекул под действием излучения.

При взаимодействии гамма-кванта с веществом происходит поглощение энергии гамма-кванта электроном оболочки атома, и электрон, совершая работу выхода, покидает атом (который становится положительно ионизированным).

Вероятность фотоэффекта прямо пропорциональна 5-й степени атомного номера химического элемента и обратно пропорциональна 3-й степени энергии гамма-излучения. Фотоэффект, как правило, преобладает при энергиях гамма-кванта от нескольких сотен килоэлектронвольт и менее.

Комптон-эффект – явление некогерентного рассеяния электромагнитного излучения (например, фотонов, гамма-квантов) на свободных электронах, сопровождающееся уменьшением частоты электромагнитного излучения (увеличением длины волны). Часть энергии фотонов и гамма-квантов после рассеяния передается электронам.

При взаимодействии гамма-кванта с электроном образуется новый гамма-квант, меньшей энергии, что также сопровождается высвобождением электрона и ионизацией атома.

Эффект образования (рождения) пар – явление, при котором возникают пары частица-античастица. Эффект образования (рождения) пар является обратным процессу аннигиляции,

Гамма-квант, взаимодействуя с электромагнитным полем атомного ядра, превращается в электрон и позитрон.

Рождение электрон-позитронных пар при взаимодействии гамма-кванта энергии выше 3 МэВ с электромагнитным полем ядра является преобладающим процессом взаимодействия гамма-квантов с веществом. При более низких энергиях гамма-квантов действуют в основном комптоновское рассеяние и фотоэффект. А при энергиях гамма-кванта ниже 1,022 МэВ эффект рождения пар вообще отсутствует.

Ядерный фотоэффект – явление испускания ядрами атомов нуклонов (протонов и нейтронов) при ядерных реакциях, происходящих при поглощении гамма-квантов ядрами атомов.

Ядерный фотоэффект действует при энергиях гамма-кванта выше нескольких десятков МэВ.

Применение гамма-излучения:

– в гамме-дефектоскопии: контроль качества изделий просвечиванием γ-лучами;

– в пищевой промышленности при консервировании пищевых продуктов: гамма-стерилизация для увеличения срока хранения;

– в приборах для измерения расстояний: уровнемеры, гамма-высотомеры на космических аппаратах;

Источник

Значение слова «гамма-излучение»

Источник (печатная версия): Словарь русского языка: В 4-х т. / РАН, Ин-т лингвистич. исследований; Под ред. А. П. Евгеньевой. — 4-е изд., стер. — М.: Рус. яз.; Полиграфресурсы, 1999; (электронная версия): Фундаментальная электронная библиотека

Гамма-квантами являются фотоны с высокой энергией. Считается, что энергии квантов гамма-излучения превышают 105 эВ, хотя резкая граница между гамма- и рентгеновским излучением не определена. На шкале электромагнитных волн гамма-излучение граничит с рентгеновским излучением, занимая диапазон более высоких частот и энергий. В области 1—100 кэВ гамма-излучение и рентгеновское излучение различаются только по источнику: если квант излучается в ядерном переходе, то его принято относить к гамма-излучению; если при взаимодействиях электронов или при переходах в атомной электронной оболочке — к рентгеновскому излучению. С точки зрения физики, кванты электромагнитного излучения с одинаковой энергией не отличаются, поэтому такое разделение условно.

Гамма-излучение испускается при переходах между возбуждёнными состояниями атомных ядер (см. Изомерный переход, энергии таких гамма-квантов лежат в диапазоне от

1 кэВ до десятков МэВ), при ядерных реакциях (например, при аннигиляции электрона и позитрона, распаде нейтрального пиона и т. д.), а также при отклонении энергичных заряженных частиц в магнитных и электрических полях (см. Синхротронное излучение). Энергия гамма-квантов, возникающих при переходах между возбуждёнными состояниями ядер, не превышает нескольких десятков МэВ. Энергии гамма-квантов, наблюдающихся в космических лучах, могут превосходить сотни ГэВ.

Гамма-излучение было открыто французским физиком Полем Вилларом в 1900 году при исследовании излучения радия.

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Как защититься от гаммы излучения человеку — применение

Гамма излучение представляет собой довольно серьезную опасность для человеческого организма, да и для всего живого в общем.

Это электромагнитные волны с очень маленькой длиной и высокой скоростью распространения.

Чем же они так опасны, и каким образом можно защититься от их воздействия?

О гамме излучение

Все знают, что атомы всех веществ содержат в себе ядро и электроны, которые вращаются вокруг него. Как правило, ядро – это довольно стойкое образование, которому трудно нанести повреждения.

При этом существуют вещества, ядра которых неустойчивы, и при некотором воздействии на них происходит излучение их составляющих. Такой процесс называется радиоактивным, он имеет определенные составляющие, названные по первым буквам греческого алфавита:

Стоит отметить, что радиационный процесс подразделяется на два вида в зависимости от того, что именно в результате выделяется.

Гамма излучение – это поток энергии в виде фотонов. Процесс разделения атомов под воздействием радиации сопровождается образованием новых веществ. При этом атомы вновь образовавшегося продукта имеют довольно нестабильное состояние. Постепенно при взаимодействии элементарных частиц возникает восстановление равновесия. В результате происходит выброс лишней энергии в виде гаммы.

Проникающая способность такого потока лучей очень высока. Оно способно проникать через кожные покровы, ткани, одежду. Более тяжелым будет проникновение через металл. Чтобы задержать такие лучи необходима довольно толстая стена из стали или бетона. Однако длина волныγ-излучения очень мала и составляет меньше 2·10 −10 м, а ее частота находится в диапазоне 3*1019 – 3*1021 Гц.

Гамма частицами являются фотоны с довольно высокой энергией. Исследователи утверждают, что энергия гаммы излучения может превышать показатель 10 5 эВ. При этом граница между рентгеновскими и γ-лучами далеко не резкая.

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Открыл гамма излучение французский физик Поль Виллар в 1900 году, проводя исследование излучения радия.

Чем опасно гамма-излучение

Гамма излучение является наиболее опасным, нежели альфа и бета.

Механизм действия:

Гамма излучения опасно тем, что такое взаимодействие человека с лучами не ощущается им ни в коей мере. Дело в том, что каждый орган и система человеческого организма реагирует по-разному на γ-лучи. Прежде всего, страдают клетки, способные быстро делиться.

Системы:

Оказывается негативное влияние и на генетическом уровне. Кроме того, такое излучение имеет свойство накапливаться в человеческом организме. При этом в первое время оно практически не проявляется.

Где применяется гамма-излучение

Несмотря на негативное влияние, ученые нашли и положительные стороны. В настоящее время такие лучи применяются в различных сферах жизни.

Гамма излучение — применение:

Излучение гамма частиц нашло свое применение в медицине. Используется оно в терапии онкологических больных. Такой метод имеет название «лучевая терапия» и основывается на воздействии лучей на быстро делящиеся клетки. В результате при правильном использовании появляется возможность уменьшить развитие патологических клеток опухоли. Однако такой метод, как правило, применяется в том случае, когда другие уже бессильны.

Отдельно стоит сказать о влияние его на мозг человека

Современные исследования позволили установить, что мозг постоянно испускает электрические импульсы. Ученые считают, что гамма излучения возникает в те моменты, когда человеку приходится работать с разной информацией одновременно. При этом небольшое количество таких волн ведет к уменьшению запоминающей способности.

Как защититься от гамма-излучения

Какая же защита существует, и что сделать, чтобы уберечься от этих вредных лучей?

В современном мире человек окружен различными излучениями со всех сторон. Однако гамма частицы из космоса оказывают минимальное воздействие. А вот то, что находится вокруг представляет гораздо большую опасность. Особенно это относится к людям, работающим на различных атомных станциях. В таком случае защита от гамма излучения состоит в применении некоторых мер.

Стоит отметить, что коэффициент ослабления гамма излучения зависит от того, из какого материала сделан защитный барьер. Так, например, лучшим металлом считается свинец в виду его свойства поглощать излучение в большом количестве. Однако он плавится при довольно низких температурах, поэтому в некоторых условиях используется более дорогой металл, например, вольфрам или тантал.

Еще один способ обезопасить себя – это измерить мощность гамма излучения в Вт. Кроме того, мощность измеряется также в зивертах и рентгенах.

Норма гамма излучения не должна превышать 0,5 микрозиверта в час. Однако лучше если этот показатель не будет выше 0,2 микрозиверта в час.

Чтобы измерить гамма излучение, применяется специальное устройство – дозиметр. Таких приборов существует довольно много. Часто используется такой аппарат, как «дозиметр гамма излучения дкг 07д дрозд». Он предназначен для оперативного и качественного измерения гамма и рентгеновского излучения.

У такого устройства есть два независимых канала, которые могут измерять МЭД и Эквивалент дозировки. МЭД гамма излучения это мощность эквивалентной дозировки, то есть количество энергии, которую поглощает вещество в единицу времени с учетом того, какое воздействие лучи оказывают на человеческий организм. Для этого показателя также существуют определенные нормы, которые обязательно должны быть учтены.

Излучение способно негативно влиять на организм человека, однако даже для него нашлось применение в некоторых сферах жизни.

Видео: Гамма-излучение

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Гамма излучение как пишется

§ 120. Следующие разряды существительных и сочетания существительных пишутся через дефис

1. Сочетания двух существительных, в которых первая часть обладает самостоятельным склонением:

а) сочетания-повторы разного типа, парные конструкции, сочетания соотносительных или близких по значению слов, дефисное написание которых определяется § 118, пп. 2, 3, 4 общих правил, напр.: умница-разумница, волк-волчище, горе-злосчастие, полусон-полуявь, друг-приятель, имя-отчество, куп- ляпродажа, марксизм-ленинизм;

б) сочетания с однословными приложениями, следующими за определяемым словом, напр.: баба-яга, ванька-встанька, город-герой, ковёр-самолёт, лён-долгунец, мать-героиня, пти- ца-носорог, рак-отшельник, рыба-попугай, скатерть-самобранка (устойчивые сочетания); дом-новостройка, журналист-между- народник, писатель-эмигрант, студент-медик, собака-ищейка, солдат-новобранец, садовод-любитель, студент-первокурсник, мать-старуха, девочка-красавица,
Маша-резвушка (свободные сочетания); с неизменяемой второй частью: парад-алле, лотерея-аллегри, программа-максимум, программа-минимум. См. также § 123, п. 2, примечание.

Примечание 1. Неизменяемое приложение может быть передано цифрами, напр.: Олимпиада-80, «Восток-2» (космический корабль), «Спрут-4» (телесериал).

Примечание 2. О раздельном написании сочетаний с однословными приложениями, следующими за определяемым словом, типа птица иволга, город Москва, господин министр см. § 122, п. 1.

Примечание 3. О замене дефиса перед приложением знаком тире см. корректирующие правила, § 154, пп. 1, 2, 5.

в) сочетания с однословными приложениями, предшествующими определяемому слову, напр.: старик-отец, красавица-дочка, умница-сын, герой-лётчик, мудрец-писатель, проказница-мартышка, самодурка-мачеха, трудяга-следователь, профан-редактор, пройдоха-управляющий. Такие приложения носят оценочный характер.

Сочетания этого типа с собственными именами обычно пишутся раздельно: старик Державин (П.), крошка Цахес (персонаж одноименной повести Гофмана), простак Ваня и т. п.; но: матушка-Русь (Некр.).

2. Сочетания с приложениями, в которых первая часть представляет собой несклоняемое существительное, напр.: кафе-автомат, каноэ-одиночка, меццо-сопрано, пальто-пелерина, ревю-оперетта, реле-станция, франко-вагон.

К ним относятся также: а) сочетания названий нот со словами диез, бемоль, бекар: до-диез, соль-диез, ми-бемоль, ля-бемоль, ля-бекар и т. п.; б) сочетания с первыми частями брутто, нетто, соло: брутто-вес, нетто-баланс, соло-вексель и т. п.; в) названия производственных марок и изделий типа Ту-104, Ил-18.

Примечание. О раздельном написании названий нот со словами мажор и минор см. § 122, п. 6.

3. Сложные слова с несклоняемой первой частью, выраженной существительным в им. п. ед. ч., имеющим окончание, напр.: ага-хан, горе-охотник, луна-парк, чудо-богатырь, эхо-импульс.

Сюда относятся также термины с названиями греческих букв в качестве начальных элементов, напр.: агьфа-частица, бета-распад, гамма-излучение, дельта-древесина, каппа-фактор, лямбда-характеристика, сигма-функция, тета-ритм.

Из этого правила имеется много исключений. Слитно пишутся по традиции все названия химических соединений такого строения, напр.: бромацетон, бутилкаучук, винилацетилен, метилбензол, метилкаучук, хлорацетон, хлорбензол, этилбензол, этилцеллюлоза. Примеры других слитных написаний: вымпелфал, костьутиль, лотлинь, планкарта, фальцаппарат, четвертьфинал, штормтрап, ялбот.

Слова с первыми частями диско- (муз.), макси-, миди-, мини- (как отступление от правила § 117, п. 3), напр.: диско-клуб, диско-музыка, макси-мода, миди-юбка, мини-платье, мини-трактор, мини-футбол, мини-ЭВМ.

Следующие группы существительных, образуемых с соединительными гласными (как отступление от правила § 119, п. 3):

а) названия сложных единиц измерения, напр.: койко-место, машино-место, пассажиро-километр, тонно-километр, самолёто-вылет, станко-час, человеко-день;

б) русские названия промежуточных стран света: северо-восток, северо-запад, юго-восток, юго-запад, а также северо-северо-восток, северо-северо-запад, юго-юго-восток, юго-юго-запад;

7. Группа слов, обозначающих преимущественно должности и звания, с первыми частями вице-, камер-, контр-, лейб-, обер-, статс-,
унтер-, флигель-, штаб-, штабе-, а также экс- (в значении ‘бывший’), напр.: вице-губернатор, вице-канцлер, вице-консул, вице-президент, вице-премьер, вице-чемпион; камер-юнкер, камер-паж; контр-адмирал; лейб-гвардия, лейб-гусар, лейб-драгун, лейб-медик; обер-бургомистр, обер-мастер, обер-офицер, обер-прокурор; статс-дама, статс-секретарь; унтер-офицер; флигель-адъютант; штаб-квартира, штаб-лекарь, штаб-офицер, штаб-ротмистр; штабс-капитан; экс-президент, экс-министр, экс-директор, экс-чемпион, экс-вице-премьер.

Примечание. Слова экстерриториальный и экспатриация, где приставка экс- имеет другое значение, пишутся слитно. Так же пишутся музыкальные термины обертон и унтертон.

8. Названия, имеющие форму словосочетаний со служебным словом (поскольку они состоят из трех частей, то пишутся с двумя дефисами): иван-да-марья, мать-и-мачеха, не-тронь-меня (растения), любишь-не-любишь (игра).

Примечание. О написании географических названий типа Ростов-на-Дону см. § 126, п. 6.

Примечание 1. Слитное написание сочетаний с пол- определяется правилом § 119, п. 6; раздельное — корректирующим правилом § 153.

Примечание 2. В наречиях с первой частью впол- (см. § 136, п. 5) слитное написание не зависит от последующей буквы: вполоборота, вполуха и т. п.

Примечание 3. Слова с первой частью полу- пишутся слитно (см. § 117, п. 1).

10. Существительные, образованные от пишущихся через дефис нарицательных существительных, напр.: вице-президентство, генерал-губернаторство, камер-юнкерство, приват-доцентура, тред-юнионизм, унтер-офицерство, унтер-офицерша (от вице-президент, генерал-губернатор, камер-юнкер, приват-доцент, тред-юнион, унтер-офицер ). Исключения: зюйдвестка, пингпонгист, сальтоморталист, шахермахерство, яхтклубовец.

Примечание. О слитном написании слов, образованных от пишущихся через дефис собственных имен с прописной буквой во второй части, см. § 119, п. 5.

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ГАММА-ИЗЛУЧЕНИЕ

Что такое ГАММА-ИЗЛУЧЕНИЕ, ГАММА-ИЗЛУЧЕНИЕ это, значение слова ГАММА-ИЗЛУЧЕНИЕ, происхождение (этимология) ГАММА-ИЗЛУЧЕНИЕ, синонимы к ГАММА-ИЗЛУЧЕНИЕ, парадигма (формы слова) ГАММА-ИЗЛУЧЕНИЕ в других словарях

гамма-излучение

га́мма-излуче́ние

Коротковолновое электромагнитное излучение, испускаемое радиоактивными веществами.

ГАММА-ИЗЛУЧЕНИЕ

гамма-излучение

ГА́ММА-ИЗЛУЧЕ́НИЕ, гамма-излучения, ср. ( спец. ). Коротковолновое электромагнитное излучение, испускаемое радиоактивными веществами.

гамма-излучение

Коротковолновое электромагнитное излучение, возникающее при распадах радиоактивных ядер, элементарных частиц, при их превращениях в другие частицы в результате столкновения частицы и отвечающей ей античастицы и т. д.

гамма-излучение

фотонное излучение, возникающее при изменении энергетического состояния атомных ядер, ядерных превращениях или при аннигиляции частиц; обладает выраженным биологическим действием.

гамма-излучение

«One faces the future with one’s past.»
Pearl S. Buck

«Cherish your human connections: your relationships with friends and family.»
Joseph Brodsky

«I will honor Christmas in my heart, and try to keep it all the year.»
Charles Dickens

«Maybe Christmas, the Grinch thought, doesn’t come from a store.»
Dr. Seuss

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