DOE-HDBK-1019/1-93INTERACTION OF RADIATION WITH MATTERAtomic and Nuclear PhysicsGammaRadiationGamma radiation is electromagnetic radiation. It is commonly referred to as a gamma ray andis very similar to an x-ray. The difference is that gamma rays are emitted from the nucleus ofan atom, and x-rays are produced by orbiting electrons. The x-ray is produced when orbitingelectrons move to a lower energy orbit or when fast-moving electrons approaching an atom aredeflected and decelerated as they react with the atom's electrical field (called Bremsstrahlung).The gamma ray is produced by the decay of excited nuclei and by nuclear reactions. Becausethe gamma ray has no mass and no charge, it is difficult to stop and has a very high penetratingpower. A small fraction of the original gamma stream will pass through several feet of concreteor several meters of water.There are three methods of attenuating gamma rays. The first method is referred to as thephoto-electric effect. When a low energy gamma strikes an atom, the total energy of the gammais expended in ejecting an electron from orbit. The result is ionization of the atom and expulsionof a high energy electron. This reaction is most predominant with low energy gammasinteracting in materials with high atomic weight and rarely occurs with gammas having an energyabove 1 MeV. Annihilation of the gamma results. Any gamma energy in excess of the bindingenergy of the electron is carried off by the electron in the form of kinetic energy.The second method of attenuation of gammas is called Compton scattering. The gamma interactswith an orbital or free electron; however, in this case, the photon loses only a fraction of itsenergy. The actual energy loss depending on the scattering angle of the gamma. The gammacontinues on at lower energy, and the energy difference is absorbed by the electron. Thisreaction becomes important for gamma energies of about 0.1 MeV and higher.At higher energy levels, a third method of attenuation is predominant. This method ispair-production. When a high energy gamma passes close enough to a heavy nucleus, the gammacompletely disappears, and an electron and a positron are formed. For this reaction to take place,the original gamma must have at least 1.02 MeV energy. Any energy greater than 1.02 MeVbecomes kinetic energy shared between the electron and positron. The probability of pair-production increases significantly for higher energy gammas.NP-01Page 66Rev. 0
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