Gamma ray

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

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

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.

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. For instance, a gamma ray will pass through 1 cm of aluminium, while an alpha ray 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.

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 (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.

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:

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 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 (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.

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 (1.02 MeV) appears as the kinetic energy of the pair and the recoil nucleus. The positron has a very short lifetime (about 10^-8 seconds). 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 (or positrons) 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:

{}^{60}\hbox{Co}\;\to\;^{60}\hbox{Ni*}\;+\;e^-\;+\;\overline{\nu}_e. Then the ⁶⁰Ni drops down to the ground state (see nuclear shell model) by emitting two gamma rays in succession:

{}^{60}\hbox{Ni*}\;\to\;^{60}\hbox{Ni}\;+\;\gamma.

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, (eg ⁶⁰Co/⁶⁰Ni) while in other cases, such as with (²⁴¹Am/²³⁷Np and ¹⁹²Ir/¹⁹²Pt), 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 (photon energy). 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.

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 (e.g. spread of cancer to the bones).

Gamma ray detectors are also starting to be used in Pakistan as part of the Container Security Initiative (CSI). These States dollar|US$" target="_blank" >^*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.
In both Gundam Seed and Seed Destiny gamma ray technology is incorperated 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.

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

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