Gamma ray

:This article is about electromagnetic radiation. For the power metal band, see Gamma Ray (band)

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Gamma rays (often denoted by the Greek letter gamma, γ) are an energetic form of electromagnetic radiation produced by radioactivity or other nuclear or subatomic processes such as electron-positron annihilation.

Related Topics:
Gamma - Electromagnetic radiation - Radioactivity - Electron-positron annihilation

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Gamma rays form the highest-energy end of the electromagnetic spectrum. They are often defined to begin at an energy of 10 keV, a frequency of/ 2.42 EHz, or a 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.

Related Topics:
Electromagnetic spectrum - Energy - KeV - EHz - Pm - X ray - Sunlight - Moonlight - Light

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Gamma rays are a form of ionizing radiation; they are more penetrating than either alpha or beta radiation (neither of which is electromagnetic radiation), but less ionizing.

Related Topics:
Ionizing radiation - Alpha - Beta

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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 (the half value layer or HVL). For example, gamma rays that require 1 cm (0.4 inches) of lead to reduce their intensity by 50% will also have their intensity reduced in half by 6 cm (2½ inches) of concrete or 9 cm (3½ inches) of packed dirt.

Related Topics:
Atomic number - Lead - Concrete

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Gamma rays from nuclear fallout would probably cause the largest number of casualties in the event of the use of nuclear weapons in a nuclear war. An effective fallout shelter reduces human exposure at least 1000 times.

Related Topics:
Nuclear fallout - Nuclear weapon - Nuclear war - Fallout shelter

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Gamma rays are less ionizing than either alpha or beta rays. However, reducing human danger requires thicker shielding. They produce damage similar to that caused by X-rays, such as burns, cancer, and genetic mutations.

Related Topics:
Ionizing - X-rays - Cancer - Mutations

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In terms of ionization, gamma radiation interacts with matter via three main processes: the photoelectric effect, Compton scattering, and pair production.

Related Topics:
Photoelectric effect - Compton scattering - Pair production

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Photoelectric Effect: This describes the case in which a gamma photon interacts with and transfers all of its energy to an orbital 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 thought to be 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.

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Compton Scattering: This is an interaction in which an incident gamma photon loses enough energy to an orbital 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 (megaelectronvolts), 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.

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Pair Production: By interaction in the vicinity of the coulomb force of the nucleus, the energy of the incident photon is spontaneously converted into the mass of an electron-positron pair. A positron is the antimatter equivalent of an electron; it has the same weight 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 (1.02 MeV) appears as the kinetic energy of the pair and the recoil nucleus. The electron of the pair, frequently referred to as the secondary electron, is densely ionizing. The positron has a very short lifetime. It combines within 10-8 seconds with a free electron. The entire mass of these two particles is then converted into two gamma photons of 0.51 MeV energy each.

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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 ultraviolet radiation.

Related Topics:
Daughter nucleus - Ultraviolet

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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.

Related Topics:
Light - Electromagnetic radiation - Frequency - Energy - Photons

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An example of gamma ray production follows.

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First cobalt-60 decays to excited nickel-60 by beta decay:

Related Topics:
Cobalt - Nickel - Beta decay

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Then the nickel-60 drops down to the ground state (see nuclear shell model) by emitting a gamma ray:

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Gamma rays of 1.17 MeV and 1.33 MeV are produced.

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