Gamma ray

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Artist's impression of an emission of a gamma ray (γ) from an atomic nucleus

Gamma rays (denoted as γ) are 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 10¹⁹ 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. 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), the latter being considered a higher-energy version of the former. However, high-energy X-rays produced by linear accelerators ("linacs") and astrophysical processes now often have higher energy than gamma rays produced by radioactive gamma decay. In fact, one of the most common gamma-ray emitting isotopes used in 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 the therapeutic treatment X-rays produced by linac machines in cancer radiotherapy. Because of this overlap in energy ranges, the two types of electromagnetic radiation are now usually defined by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus (that is, produced by gamma decay), 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".[1] In certain fields such as astronomy, gamma rays and X-rays are still sometimes defined by energy, as the processes which produce them may be uncertain.

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.

Properties

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.

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.

Here μ = nσ is the absorption coefficient, measured in cm⁻¹, n the number of atoms per cm³ in the material, σ the absorption cross section in cm² 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.

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.[2]