Encyclopedia
Gamma rays are an energetic form of
electromagnetic radiation produced by
radioactive decay or other nuclear or subatomic processes such as
electron-positron annihilation.
Explanation
Gamma rays form the highest-energy end of the
electromagnetic spectrum. They are often defined to begin at an
energy of 10 keV, corresponding to a minimum frequency of 2.42 EHz, or a maximum wavelength of 124
pm, although electromagnetic radiation from around 10 keV to several hundred keV is also referred to as hard
X-rays. It is important to note that there is no physical difference between gamma rays and X-rays of the same energy — they are two names for the same electromagnetic radiation, just as
sunlight and moonlight are two names for visible
light. Rather, gamma rays are distinguished from X-rays by their origin.
Gamma ray is a term for high-energy electromagnetic radiation produced by nuclear transitions, while
X-ray is a term for high-energy electromagnetic radiation produced by energy transitions due to accelerating electrons. Because it is possible for some electron transitions to be of higher energy than some nuclear transitions, there is an overlap between what we call low energy gamma rays and high energy X-rays.
Gamma rays are a form of
ionizing radiation; they are more penetrating than either
alpha or
beta radiation , but less ionizing. For instance, a gamma ray will pass through 1 cm of
aluminium, while an alpha particle will be stopped by even a single sheet of paper.
Gamma sources are used for a range of applications in both
medicine and
industry. For further details see commonly used gamma emitting isotopes.
High energy synchrotron radiation nowadays covers the lower part of the
Gamma Ray spectrum from an artificial, tuneable source.
Shielding
Shielding for γ rays requires large amounts of mass. The material used for shielding takes into account that gamma rays are better absorbed by materials with high atomic number and high density. Also, the higher the energy of the gamma rays, the thicker the shielding required. Materials for shielding gamma rays are typically illustrated by the thickness required to reduce the intensity of the gamma rays by one half . For example, gamma rays that require 1 cm of
lead to reduce their intensity by 50% will also have their intensity reduced in half by 6 cm of
concrete or 9 cm of packed dirt.
Interaction with matter
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:
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 , 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 , an energy spectrum which includes most gamma radiation present in a nuclear explosion. Compton scattering is relatively independent of the atomic number of the absorbing material.
- Pair Production: By interaction via the Coulomb force, in the vicinity of the nucleus, the energy of the incident photon is spontaneously converted into the mass of an electron-positron pair. A positron is the anti-matter equivalent of an electron; it has the same mass as an electron, but it has a positive charge equal in strength to the negative charge of an electron. Energy in excess of the equivalent rest mass of the two particles appears as the kinetic energy of the pair and the recoil nucleus. The positron has a very short lifetime . At the end of its range, it combines with a free electron. The entire mass of these two particles is then converted into two gamma photons of 0.51 MeV energy each.
The secondary electrons produced in any of these three processes frequently have enough energy to produce many ionizations up to the end of range.
The exponential absorption described above holds, strictly speaking, only for a narrow beam of gamma rays. If a wide beam of gamma rays passes through a thick slab of concrete, the scattering in from the sides reduces the absorption.
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 level by emitting a gamma ray in much the same way that an atomic electron can jump to a lower level by emitting visible light or
ultraviolet radiation.
Gamma rays, x-rays, visible
light, and UV rays 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
60Co decays to excited
60Ni by
beta decay:
Then the
60Ni drops down to the ground state by emitting two gamma rays in succession:
Gamma rays of 1.17 MeV and 1.33 MeV are produced.
Another example is the alpha decay of
241Am to form
237Np; this alpha decay is accompanied by gamma emission. In some cases, the gamma emission spectrum for a nucleus is quite simple, while in other cases, such as with , 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.
Because a beta decay is accompanied by the emission of a
neutrino which also carries away energy, the beta spectrum does not have sharp lines, but instead it 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 . For instance, a sodium flame can emit yellow light as well as absorb the yellow light from a
sodium vapour 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 Frank Condon effects seen in optical spectroscopy.
Uses
The powerful nature of gamma rays have made them useful in the sterilization of medical equipment by killing
bacteria. They are also used to kill bacteria and insects in foodstuffs, particularly meat, marshmallows, pie, eggs, and vegetables, to maintain freshness.
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 . However, as a form of
ionizing radiation they have the ability to effect molecular changes, particularly to
DNA, giving them the potential to cause
cancer.
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 focus the radiation on the growth while minimising damage to the surrounding tissues.
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 .
Gamma ray detectors are also starting to be used in Pakistan as part of the Container Security Initiative . These
US$5 million machines are advertised to scan 30 containers per hour. The objective of this technique is to pre-screen merchant ship containers before they enter U.S. ports.
In fiction
- An accident involving gamma rays transformed the scientist Bruce Banner into the Incredible Hulk in the Marvel comic of the same name. Many of the Hulk's villains and allies were also affected by gamma rays, and the standard in Marvel comics seems to be that the effects of gamma mutation on an individual depend on the subsconscious desires and psychological quirks of that individual .
- In both Gundam Seed and Seed Destiny gamma ray technology is incorporated in the space cannon G.E.N.E.S.I.S.
- In David Weber's Honorverse, grasers are powerful gamma-radiation-powered energy weapons.
- Metroids, creatures in the popular series of the same name, go through a large metamorphosis when exposed to gamma-radiation.
- EVE Online, a space-based mmorpg for the pc, has a group of weapons technology that uses various EM radiations as lasers, some of which are gamma lasers.
History
Gamma rays were discovered by the French chemist and physicist, Paul Ulrich Villard in 1900 while he was studying
uranium. Working in the chemistry department of the
École Normale in rue d'Ulm,
Paris with self-constructed equipment, he found that the rays were not bent by a
magnetic field.
For a time, it was assumed that gamma rays were particles. The fact that they were rays was demonstrated by the British Physicist,
William Henry Bragg in 1910 when he showed that the rays ionized gas in a similar way to X-rays.
In 1914,
Ernest Rutherford and Edward Andrade showed that gamma rays were a form of electromagnetic radiation by measuring their wavelengths using
crystal diffraction. The wavelengths are similar to those of X-rays and are very short, in the range 10
-11m to 10
-14m. It was
Rutherford who coined the name 'gamma rays', after naming 'alpha' and 'beta' rays; the natures of the different rays were unknown at that time.
Gamma-ray astronomy did not develop until it was possible to get our detectors above all or most of the atmosphere, using balloons or spacecraft. The first gamma-ray telescope, carried into orbit on the Explorer XI satellite in 1961, picked up fewer than 100 cosmic gamma-ray photons.
Perhaps the most spectacular discovery in gamma-ray astronomy came in the late 1960s and early 1970s. Detectors on board the Vela satellite series, originally military satellites, began to record bursts of these rays, not from Earth, but from deep space.
References
- http://imagers.gsfc.nasa.gov/ems/gamma.html
- http://www.sciencedaily.com/releases/2005/01/050128222047.htm
- http://en.wikipedia.org/wiki/Radiation
- http://www.rerf.or.jp/eigo/radefx/basickno/whatis.htm
- http://www.meds.com/pdq/radio.html
- http://www.cancer.gov/cancertopics/factsheet/Therapy/radiation
- http://www.gcsechemistry.com/pwav46.htm
- http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PHPAEN000008000011004954000001&idtype=cvips&gifs=yes
- http://www.saic.com/products/security/relocatable-vacis/relocatable-vacis-faq.html
- http://www.physics.isu.edu/radinf
- http://www.astro.caltech.edu/~ejb/faq.html
See also
