gamma radiation


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gamma radiation,

high-energy photons emitted as one of the three types of radiation resulting from natural radioactivityradioactivity,
spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation. The energy produced by radioactivity has important military and industrial applications.
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. It is the most energetic form of electromagnetic radiationelectromagnetic radiation,
energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field.
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, with a very short wavelength (high frequency). Wavelengths of the longest gamma radiation are less than 10−10 m, with frequencies greater than 1018 hertz (cycles per sec). Gamma rays are essentially very energetic X raysX ray,
invisible, highly penetrating electromagnetic radiation of much shorter wavelength (higher frequency) than visible light. The wavelength range for X rays is from about 10−8 m to about 10−11
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; the distinction between the two is not based on their intrinsic nature but rather on their origins. X rays are emitted during atomic processes involving energetic electrons. Gamma radiation is emitted by excited nuclei (see nucleusnucleus,
in physics, the extremely dense central core of an atom. The Nature of the Nucleus
Composition

Atomic nuclei are composed of two types of particles, protons and neutrons, which are collectively known as nucleons.
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) or other processes involving subatomic particles; it often accompanies alpha or beta radiation, as a nucleus emitting those particles may be left in an excited (higher-energy) state. The applications of gamma radiation are much the same as those of X rays, both in medicine and in industry. In medicine, gamma ray sources are used for cancer treatment and for diagnostic purposes. Some gamma-emitting radioisotopes are also used as tracers (see radioactive isotoperadioactive isotope
or radioisotope,
natural or artificially created isotope of a chemical element having an unstable nucleus that decays, emitting alpha, beta, or gamma rays until stability is reached.
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). In industry, principal applications include inspection of castings and welds. Data from artificial satellites and high-altitude balloons have indicated that a flux of gamma radiation is reaching the earth from outer space, thus opening up the field of research known as gamma-ray astronomygamma-ray astronomy,
study of astronomical objects by analysis of the most energetic electromagnetic radiation they emit. Gamma rays are shorter in wavelength and hence more energetic than X rays (see gamma radiation) but much harder to detect and to pinpoint.
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.

Gamma Radiation

 

shortwave electromagnetic radiation. On the scale of electromagnetic waves it is contiguous to hard X rays, occupying the next higher frequency range. It has extremely small wavelengths (λ ≤ 10-8 cm) and, as a result, pronounced corpuscular properties—that is, it behaves like a stream of particles—gamma quanta, or photons—with energy hv (v is the frequency of the radiation, and h is Planck’s constant).

Gamma radiation occurs during the decay of radioactive nuclei and elementary particles, upon the annihilation of particle-antiparticle pairs, and also during the passage of fast charged particles through a substance.

The gamma radiation accompanying the decay of radioactive nuclei is emitted during the transition of a nucleus from a more excited energy state to a less excited state or to the ground state. The energy of a gamma quantum is equal to the energy difference Δ& of the states between which the transition takes place (see Figure 1). The emission of a gamma

Figure 1

quantum by a nucleus, unlike other types of radioactive transformations (alpha decay or beta decay), does not entail a change in its atomic charge or mass number. The width of gamma-ray lines is usually very small (~ 10-2 electron volt, or eV). Since the separation between levels (from several keV to several MeV) is many times greater than the widths of the lines, gamma radiation has a line spectrum—that is, it consists of a series of discrete lines. The energies of the excited states of nuclei can be established by studying the gamma-radiation spectra.

Gamma quanta with larger energies are emitted during the decay of certain elementary particles. For example, when a quiescent π0-meson decays, gamma radiation with an energy of ~70 MeV is produced. The gamma radiation from the decay of elementary particles also has line spectra. However, decaying elementary particles often move at velocities comparable to the speed of light c. As a result, Doppler broadening of the line occurs, and the gamma-radiation spectrum is spread over a wide range of energies.

The gamma radiation produced when fast charged particles pass through a substance is caused by the deceleration of the particles in the Coulomb field of the atomic nuclei. The braking gamma radiation, like braking X radiation, is characterized by a continuous spectrum whose upper boundary coincides with the energy of the charged particle, such as an electron. Braking gamma radiation with a maximum energy of up to several dozen giga electron volts (GeV) is produced in charged-particle accelerators.

In interstellar space, gamma radiation may occur as a result of collisions between quanta of soft, long-wave electromagnetic radiation, such as light, and electrons that have been accelerated by the magnetic fields of cosmic objects. In this case the fast electron transfers its energy to the electromagnetic radiation, and the visible light is transformed into harder gamma radiation.

An analogous phenomenon can take place under terrestrial conditions when high-energy electrons produced in accelerators encounter photons of visible light in the intense light beams created by lasers. The electron transfers its energy to the light photon, which becomes a gamma quantum. Thus, it is possible in practice to transform individual photons of light into high-energy gamma quanta.

Gamma radiation has great penetrating power—that is, it can go through matter of great thickness without appreciable attenuation. The main processes that take place upon the interaction of gamma radiation with matter are photoelectric absorption (the photoelectric effect), Compton scattering (the Compton effect), and the formation of electron-positron pairs. In the photoelectric effect a gamma quantum is absorbed by one of the electrons of an atom, and the energy of the gamma quantum is transformed (minus the binding energy of the electron in the atom) into the kinetic energy of the electron that escaped from the atom. The probability of the photoelectric effect is directly proportional to the fifth power of the atomic number of the element and inversely proportional to the third power of the gamma-radiation energy. Thus, the photoelectric effect is predominant for gamma quanta of low energy ( 100 keV) in heavy atoms (lead or uranium).

In the Compton effect a gamma quantum is scattered by one of the weakly bound electrons in an atom. The gamma quantum does not disappear, as in the photoelectric effect, but only changes with regard to energy (wavelength) and direction of propagation. As a result of the Compton effect, the narrow beam of gamma rays becomes broader and the radiation becomes softer (of longer wavelength). The intensity of Compon scattering is proportional to the number of electrons per cubic centimeter of the substance; therefore the probability of this process is proportional to the atomic number. The Compton effect becomes noticeable in substances with low atomic numbers and at gamma-radiation energies exceeding the binding energy of the electrons in the atoms. Thus, in the case of lead the probability of Compton scattering is comparable with the probability of photoelectric absorption at an energy of 0.5 MeV. For aluminum the Compton effect predominates at much lower energies.

If the energy of a gamma quantum exceeds 1.02 MeV, the production of electron-positron pairs in the electrical field of the nuclei becomes possible. The probability of pair production is proportional to the square of the atomic number and increases with an increase in hv. Consequently, when hv ~ 10 MeV, the main process in any substance is found to be pair production (see Figure 2). The reverse process, electron-positron annihilation, is a source of gamma radiation.

Figure 2. Absorption coefficient of gamma radiation in lead as a function of the energy ε of the gamma quanta

The attenuation of gamma radiation in a substance is usually characterized by the absorption coefficient, which indicates the thickness x of an absorber that reduces the intensity I0 of an incident beam of gamma radiation by a factor of e:

I = I0e-μ0x

Here μ0 is the linear absorption coefficient for gamma rays in cm-1. A mass absorption coefficient equal to the ratio of μ0 to the density of the absorber is introduced. In these cases the thickness is measured in g/cm2.

An exponential law of the attenuation of gamma radiation is valid for a narrow, directional beam of gamma rays when either the absorption or the scattering process removes the gamma radiation from the primary beam. However, at high energies (hv > 10 MeV) the passage of gamma rays through a substance is more complicated. The secondary electrons and positrons have high energy and can therefore, in turn, create gamma radiation through the annihilation and braking processes. Thus, a series of alternating generations of secondary gamma radiation, electrons, and positrons forms in the substance—that is, a cascade shower develops. The number of secondary particles in such a shower at first increases with thickness and then attains a maximum. However, absorption processes then begin to prevail over the particle multiplication processes, and the shower is extinguished. The shower-producing capability of gamma radiation depends on the ratio between its energy and the so-called critical energy, beyond which for a given substance there is virtually no possibility of developing a shower. The lighter the substance, the higher this energy, &cr (for air it is 50 MeV; for lead, 5 MeV).

The energy of gamma radiation is measured in experimental physics with various types of gamma spectrometers, the majority of which are based on measurement of the energy of secondary electrons. The main types of gamma spectrometer are magnetic, scintillation, semiconductor, and crystal-diffraction.

The study of the spectra of nuclear gamma radiation yields important information regarding nuclear structure. Observation of the effects associated with the influence of external mediums on the characteristics of nuclear gamma radiation is used in studies of the properties of solids. Gamma radiation is used in technology—for example, to detect flaws in metal parts (gamma-ray flaw detection). It is used in radiation chemistry to initiate chemical transformations such as polymerization processes. It is also utilized in the food industry for sterilization. The principal sources of gamma rays are natural and artificial isotopes such as 226Ra, 60Co, and 137Cs, as well as electron accelerators.

E. M. LEIKIN

Effect on the organism. The effect of gamma radiation on the organism is similar to that of other forms of ionizing radiation. It can cause radiation injury to an organism, including its destruction. The nature of the effect of gamma radiation depends on the energy of the gamma quanta and the spatial features of the irradiation (for example, external or internal). The relative biological effectiveness (RBE) of gamma radiation (the effectiveness of hard X rays is taken as 1) is 0.7-0.9. Under industrial conditions (chronic influence in small doses) the RBE of gamma radiation is taken as 1.

Gamma radiation is utilized in medicine to treat tumors and to sterilize rooms, apparatus, and medical preparations. It is also used to obtain mutations from which economically useful types are selected. Highly productive types of microorganisms (for example, for producing antibiotics) and plants are bred in this manner.

REFERENCES

Al’fa-, beta- i gamma-spektroskopiia, issue 1. Edited by K. Siegbahn. Moscow, 1969. (Translated from English.)
Eksperimental’naia iadernaia fizika, vol. 1. Edited by E. Segrè. Moscow, 1955. (Translated from English.)
Gamma-luchi. Moscow-Leningrad, 1961.
Glasstone, S. Atom. Atomnoe iadro. Atomnaia energiia. Moscow, 1961. (Translated from English.)

gamma radiation

[′gam·ə ‚rād·ē′ā·shən]
(nucleonics)
Radiation of gamma rays.

gamma radiation

1. electromagnetic radiation emitted by atomic nuclei; the wavelength is generally in the range 1 × 10--10 to 2 × 10--13 metres
2. electromagnetic radiation of very short wavelength emitted by any source, esp the portion of the electromagnetic spectrum with a wavelength less than about 1 × 10--11 metres
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