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Gamma ray
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Artist's impression of an emission of a gamma ray (/γ/) from an atomic
nucleus
*Nuclear physics *
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Radioactive decay
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Alpha decay · Beta decay · *Gamma
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v • d
• e
This article is about the term's use in physics . For
other uses, see Gamma ray (disambiguation)
.
*Gamma radiation*, also known as *gamma rays* (denoted as γ ), is
electromagnetic radiation of high frequency (very short wavelength). They
are produced by sub-atomic particle interactions such as electron-positron
annihilation , neutral pion decay , radioactive decay , fusion , fission
or inverse Compton scattering in astrophysical processes. Gamma rays
typically have frequencies above 10^19 Hz, and therefore have energies
above 100 keV and wavelength less than 10 picometers , often smaller than
an atom . Gamma radioactive decay photons commonly have energies of a few
hundred keV, and are almost always less than 10 MeV in energy.
Because they are a form of ionizing radiation , gamma rays can cause
serious damage when absorbed by living tissue, and are therefore a health
hazard.
Paul Villard , a French chemist and physicist, discovered gamma radiation
in 1900, while studying radiation emitted from radium .^[1] Alpha and beta
"rays" had already been separated and named by the work of Ernest
Rutherford in 1899, and in 1903 Rutherford named Villard's distinct new
radiation "gamma rays."
In the past, the distinction between X-rays and gamma rays was based on
energy (or equivalently frequency or wavelength), with gamma rays being
considered a higher-energy version of X-rays. However, modern high-energy
(megavoltage ) X-rays produced by linear accelerators ("linacs") for
megavoltage treatment, in cancer radiotherapy usually have higher energy
than gamma rays produced by radioactive gamma decay . Conversely, one of
the most common gamma-ray emitting isotopes used in diagnostic nuclear
medicine , technetium-99m , produces gamma radiation of about the same
energy (140 KeV) as produced by a diagnostic X-ray machine, and
significantly lower energy than therapeutic photons from linacs. Because
of this broad overlap in energy ranges, the two types of electromagnetic
radiation are now usually 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), while gamma rays
are emitted by the nucleus or from other particle decays or annihilation
events. There is no lower limit to the energy of photons produced by
nuclear reactions, and thus ultraviolet and even lower energy photons
produced by these processes would also be defined as "gamma rays".^[2]
In certain fields such as astronomy, gamma rays and X-rays are still
sometimes defined by energy, or used interchangeably, since the processes
which produce them may be uncertain.
The Moon as seen by the Compton Gamma Ray Observatory , in gamma rays of
greater than 20 MeV. These are produced by cosmic ray bombardment of its
surface. The Sun, which has no similar surface of high atomic number to
act as target for cosmic rays, cannot be seen at all at these energies,
which are too high to emerge from primary nuclear reactions, such as solar
nuclear fusion.[1]
Contents
[hide ]
* 1 Units of measure and exposure
* 2 Properties
o 2.1 Shielding
o 2.2 Matter interaction
o 2.3 Light interaction
o 2.4 Gamma ray production
* 3 Health effects
* 4 Uses
o 4.1 Body response
o 4.2 Risk assessment
* 5 See also
* 6 References
* 7 External links
[edit ] Units of measure and exposure
The measure of gamma rays' ionizing ability is called the exposure:
* The coulomb per kilogram (C/kg) is the SI unit of ionizing radiation
exposure, and is the amount of radiation required to create 1 coulomb of
charge of each polarity in 1 kilogram of matter.
* The röntgen (R) is an obsolete traditional unit of exposure, which
represented the amount of radiation required to create 1 esu of charge of
each polarity in 1 cubic centimeter of dry air. 1 röntgen = 2.58×10^−4
C/kg
However, the effect of gamma and other ionizing radiation on living tissue
is more closely related to the amount of energy deposited rather than the
charge . This is called the absorbed dose :
* The gray (Gy), which has units of (J/kg), is the SI unit of absorbed
dose , and is the amount of radiation required to deposit 1 joule of
energy in 1 kilogram of any kind of matter. * The rad is the (obsolete)
corresponding traditional unit, equal to 0.01 J deposited per kg. 100 rad
= 1 Gy.
The equivalent dose is the measure of the biological effect of radiation
on human tissue. For gamma rays it is equal to the absorbed dose .
* The sievert (Sv) is the SI unit of equivalent dose, which for gamma rays
is numerically equal to the gray (Gy).
* The rem is the traditional unit of equivalent dose. For gamma rays it is
equal to the rad or 0.01 J of energy deposited per kg. 1 Sv = 100 rem.
[edit ] Properties
[edit ] Shielding
Shielding from gamma rays requires large amounts of mass. They are better
absorbed by materials with high atomic numbers and high density, although
neither effect is important compared to the total mass per area in the
path of the gamma ray. For this reason, a lead shield is only modestly
better (20-30%) as a gamma shield than an equal mass of another shielding
material such as aluminium, concrete, or soil; the lead's major advantage
is in its compactness.
The higher the energy of the gamma rays, the thicker the shielding
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") 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 (2½") of
concrete , or 9 cm (3½") of packed soil . However, the mass of this much
concrete or soil is only 20-30% larger than that of this amount of lead.
Depleted uranium is used for shielding in portable gamma ray sources, but
again the savings in weight over lead is modest, and the main effect is to
reduce shielding bulk.
[edit ] Matter interaction
The total absorption coefficient of aluminium (atomic number 13) for gamma
rays, plotted versus gamma energy, and the contributions by the three
effects. Over most of the energy region shown, the Compton effect
dominates.
The total absorption coefficient of lead (atomic number 82) for gamma
rays, plotted versus gamma energy, and the contributions by the three
effects. Here, the photoelectric effect dominates at low energy. Above 5
MeV, pair production starts to dominate
When a gamma ray passes through matter, the probability for absorption in
a thin layer is proportional to the thickness of that layer. This leads to
an exponential decrease of intensity with thickness. The exponential
absorption holds only for a narrow beam of gamma rays. If a wide beam of
gamma rays passes through a thick slab of concrete the scattering from the
sides reduces the absorption.
I(d) = I_0 \cdot e ^{-\mu d}. \,
Here μ = /n/σ is the absorption coefficient, measured in cm^−1 , /n/
the number of atoms per cm^3 in the material, σ the absorption cross
section in cm^2 and /d/ the thickness of material in cm.
In passing through matter, gamma radiation ionizes via three main
processes: the photoelectric effect , Compton scattering , and pair
production .
* *Photoelectric effect*: This describes the case in which a gamma photon
interacts with and transfers its energy to an atomic electron, ejecting
that electron from the atom. The kinetic energy of the resulting
photoelectron is equal to the energy of the incident gamma photon minus
the binding energy of the electron. The photoelectric effect is the
dominant energy transfer mechanism for x-ray and gamma ray photons with
energies below 50 keV (thousand electron volts ), 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 being emitted as a new, lower energy gamma photon with an
emission direction different from that of the incident gamma photon. The
probability of Compton scatter decreases with increasing photon energy.
Compton scattering is thought to be the principal absorption mechanism for
gamma rays in the intermediate energy range 100 keV to 10 MeV. Compton
scattering is relatively independent of the atomic number of the absorbing
material, which is why very dense metals 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 about 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 (1.02 MeV) appears
as the kinetic energy of the pair and the recoil nucleus. At the end of
the positron's range , it combines with a free electron. The entire mass
of these two particles 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.
[edit ] Light interaction
High-energy (from 80 to 500 GeV ) gamma rays arriving from far 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 their density may be estimated by
analyzing the incoming gamma-ray spectrums.^[3]
[edit ] Gamma ray production
See also: Gamma-ray generation
Gamma rays are often produced alongside other forms of radiation such as
alpha or beta . When a nucleus emits an α or β particle, the daughter
nucleus is sometimes left in an excited state. It can then jump down to a
lower energy state by emitting a gamma ray, in much the same way that an
atomic electron can jump to a lower energy state by emitting infrared ,
visible, or ultraviolet light.
Decay scheme of ^60 Co
Gamma rays, x-rays, visible light , and radio waves are all forms of
electromagnetic radiation . The only difference is the frequency and hence
the energy of the photons . Gamma rays are the most energetic. An example
of gamma ray production follows.
First ^60 Co decays to excited ^60 Ni by beta decay . Then the ^60 Ni
drops down to the ground state (see nuclear shell model ) by emitting two
gamma rays in succession (1.17 MeV then 1.33 MeV):
6027Co → 6028Ni^* + e^â»
+ ν_e +
*γ* + 1.17 MeV
6028Ni^* → 6028Ni
+ *γ* + 1.33 MeV
Another example is the alpha decay of ^241 Am to form ^237 Np ; this alpha
decay is accompanied by gamma emission. In some cases, the gamma emission
spectrum for a nucleus (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 can exist. The fact that an alpha spectrum can
have a series of different peaks with different energies reinforces the
idea that several nuclear energy levels are possible.
Image of entire sky in 100 MeV or greater gamma rays as seen by the EGRET
instrument aboard the CGRO spacecraft. Bright spots within the galactic
plane are pulsars while those above and below the plane are thought to be
quasars .
Because a beta decay is accompanied by the emission of a neutrino which
also carries energy away, the beta spectrum does not have sharp lines, but
instead is a broad peak. Hence from beta decay alone it is not possible to
probe the different energy levels found in the nucleus.
In optical spectroscopy, it is well known that an entity which emits light
can also absorb light at the same wavelength (photon energy). For
instance, a sodium flame can emit yellow light as well as absorb the
yellow light from a sodium vapor lamp . In the case of gamma rays, this
can be seen in Mössbauer spectroscopy. Here, a correction for the energy
lost by the recoil of the nucleus is made and the exact conditions for
gamma ray absorption through resonance can be attained.
This is similar to the Franck Condon effects seen in optical spectroscopy.
[edit ] Health effects
All ionizing radiation causes similar damage at a cellular level, but
because rays of alpha particles and beta particles are relatively
non-penetrating, external exposure to them causes only localized damage,
e.g. radiation burns to the skin. Gamma rays and neutrons are more
penetrating, causing diffuse damage throughout the body (e.g. radiation
sickness , increased incidence of cancer) rather than burns. External
radiation exposure should also be distinguished from internal exposure,
due to ingested or inhaled radioactive substances, which, depending on the
substance's chemical nature, can produce both diffuse and localized
internal damage. The most biological damaging forms of gamma radiation
occur in the gamma ray window , between 3 and 10 MeV, with higher energy
gamma rays being less harmful because the body is relatively transparent
to them. See cobalt-60 .
[edit ] Uses
Gamma-ray image of a truck taken with a VACIS (Vehicle and Container
Imaging System)
This property means that gamma radiation is often used to kill living
organisms, in a process called irradiation . Applications of this include
sterilizing medical equipment (as an alternative to autoclaves or chemical
means), removing decay-causing bacteria from many foods or preventing
fruit and vegetables from sprouting to maintain freshness and flavor.
Gamma-rays have the smallest wavelengths and the most energy of any wave
in the electromagnetic spectrum. These waves are generated by radioactive
atoms and in nuclear explosions. Gamma-rays can kill living cells, a fact
which medicine uses to its advantage, using gamma-rays to kill cancerous
cells.
Gamma-rays travel to us across vast distances of the universe, only to be
absorbed by the Earth's atmosphere. Different wavelengths of light
penetrate the Earth's atmosphere to different depths. Instruments aboard
high-altitude balloons and satellites like the Compton Observatory provide
our only view of the gamma-ray sky.
Due to their tissue penetrating property, gamma rays/X-rays have a wide
variety of medical uses such as in CT Scans and radiation therapy (/see
X-ray /). However, as a form of ionizing radiation they have the ability
to effect molecular changes, giving them the potential to cause cancer
when DNA is affected. The 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 .
Despite their cancer-causing properties, gamma rays are also used to treat
some types of cancer . In the procedure called gamma-knife surgery,
multiple concentrated beams of gamma rays are directed on 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 the surrounding tissues. (As an illustration of the radiation
origin-process contributing to its name, a similar technique which uses
photons from linacs rather than cobalt gamma decay, is called "Cyberknife
").
Gamma rays are also used for diagnostic purposes in nuclear medicine .
Several gamma-emitting radioisotopes are used, one of which is technetium
-99m. When 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. Such a technique can be employed to diagnose a wide
range of conditions (e.g. spread of cancer to the bones).
In the US, gamma ray detectors are beginning to be used as part of the
Container Security Initiative (CSI). These US$ 5 million machines are
advertised to scan 30 containers per hour. The objective of this technique
is to screen merchant ship containers before they enter US ports.
[edit ] Body response
After gamma-irradiation, and the breaking of DNA double-strands, a cell
can repair the damaged genetic material to the limit of its capability.
However, a study of Rothkamm and Lobrich has shown that the repairing
process works well after high-dose exposure but is much slower in the case
of a low-dose exposure.^[4]
[edit ] Risk assessment
The natural outdoor exposure in Great Britain ranges from 2 × 10^−7 to
4 × 10^−7 cSv/h (centisieverts per hour ).^[5] Natural exposure to
gamma rays is about 0.1 to 0.2 cSv per year, and the average total amount
of radiation received in one year per inhabitant in the USA is 0.36
cSv.^[6]
By comparison, the radiation dose from chest radiography is a fraction of
the annual naturally occurring background radiation dose,^[7] and the dose
from fluoroscopy of the stomach is, at most, 5 cSv on the skin of the
back.
For acute full-body equivalent dose, 100 cSv causes slight blood changes;
200–350 cSv causes nausea, hair loss, hemorrhaging and will cause death
in a sizable number of cases (10%–35%) without medical treatment; 500
cSv is considered approximately the LD_50 (lethal dose for 50% of exposed
population) for an acute exposure to radiation even with standard medical
treatment; more than 500 cSv brings an increasing chance of death;
eventually, above 750–1000 cSv, even extraordinary treatment, such as
bone-marrow transplants, will not prevent the death of the individual
exposed (see /Radiation poisoning /).^[/clarification needed /]
^[/citation needed /]
For low dose exposure, for example among nuclear workers, who receive an
average yearly radiation dose of 1.9 cSv,^[/clarification needed /] the
risk of dying from cancer (excluding leukemia ) increases by 2 percent.
For a dose of 10 cSv, that risk increase is at 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 .^[8]
[edit ] See also
* Radioactive decay * Nuclear fission /fusion * Annihilation * Gamma
spectroscopy * Gamma-ray astronomy * Gamma-ray burst * Mössbauer effect *
Gamma-ray generation
[edit ] References
1. *^ * L'Annunziata, Michael F. (2007). /Radioactivity: introduction and
history/. Amsterdam, Netherlands: Elsevier BV. pp. 55–58. ISBN
9780444527158 . 2. *^ * Shaw, R. W.; Young, J. P.; Cooper, S. P.; Webb,
O. F. (1999). "Spontaneous Ultraviolet Emission from ^233 Uranium/^229
Thorium Samples". /Physical Review Letters/ *82* (6): 1109–1111. doi
:10.1103/PhysRevLett.82.1109 . 3. *^ * Bock, R. K.; et al (2008-06-27).
"Very-High-Energy Gamma Rays from a Distant Quasar: How Transparent Is the
Universe?". /Science / *320* (5884): pp 1752–1754. doi
:10.1126/science.1157087 . ISSN 0036-8075 . 4. *^ * Rothkamm K. -
Evidence for a lack of DNA double-strand break repair in human cells
exposed to very low x-ray doses - Proceedings of the National Academy of
Science of the USA, 2003; 100 (9) : 5057-5062. 5. *^ * Department for
Environment, Food and Rural Affairs (Defra) UK – Keys facts about
radioactivity – 2003,
http://www.defra.gov.uk/environment/statistics/radioact/kf/rakf03.htm 6.
*^ * United Nations Scientific Committee on the Effects of Atomic
Radiation Annex E: Medical radiation exposures – Sources and Effects of
Ionizing – 1993, p. 249, New York, UN 7. *^ * US National Council on
Radiation Protection and Measurements – NCRP Report No. 93 – pp
53–55, 1987. Bethesda, Maryland, USA, NCRP 8. *^ * IARC – Cancer risk
following low doses of ionizing radiation – a 15-country study –
http://www.iarc.fr/ENG/Units/RCAa1.html
[edit ] External links
* Basic reference on several types of radiation
* Radiation Q & A
* GCSE information
* Radiation information
* Gamma ray bursts
* The Lund/LBNL Nuclear Data Search
- Contains
information on gamma-ray energies from isotopes.
* Mapping soils with airborne detectors
* Ndslivechart.png *The LIVEChart of
Nuclides - IAEA * with filter
on gamma-ray energy, in *Java *
or *HTML *
* Health Physics Society Public Education Website