SHIELDING MATERIALSDOE-HDBK-1017/2-93Plant MaterialsGamma rays are attenuated by processes which are functions of atomic number and mass (thatis they all involve interactions near the nucleus or interactions with the electrons around thenucleus). Gamma shielding is therefore more effectively performed by materials with highatomic mass number and high density. One such material is lead. Lead is dense and has about82 electrons for each nucleus. Thus, a gamma would interact more times in passing througheight inches of lead then passing through the same thickness of a lighter material, such as water.As the gamma interacts with the shielding material, it loses energy and eventually disappears.Lead and lead alloys have been used to some extent in nuclear reactor shields and have an addedadvantage of ease of fabrication. Because of its low melting point, lead can be used only wherethe temperatures do not exceed its melting point.Iron, although a medium weight element, also functions well as a gamma attenuator. For gammarays with energies of 2 MeV, roughly the same mass of iron as of lead is required to remove aspecific fraction of the radiation. At higher and lower energies, however, the mass-attenuationefficiency of lead is appreciably greater than that of iron. In many cases, the selection of ironis based on structural, temperature, and economic considerations.Water is a poor material for shielding gamma rays; however, large amounts will serve toattenuate gamma radiation.Concrete, as discussed previously, is also a good attenuator of gamma rays and is superior towater. This is mainly a result of the presence of moderately high mass number elements, suchas calcium and silicon. As a general shield material, there is much to recommend aboutconcrete; it is strong, inexpensive, and adaptable to both block and monolithic types ofconstruction.Alphaand BetaRadiationAlpha particles, being the largest particles of radiation and having a +2 charge, interact withmatter more readily than other types of radiation. Each interaction results in a loss of energy.This is why the alpha has the shortest range of all the types of radiation. Alpha particlesgenerally are stopped by a thin sheet of paper. As a comparison, a 4 MeV alpha particle willtravel about 1 inch in air, whereas a 4 MeV beta particle will travel about 630 inches in air.Because it deposits all of its energy in a very small area, the alpha particle travels only a shortdistance.The beta particle is more penetrating than the alpha. However, because of the -1 charge, the betaparticle interacts more readily than a non-charged particle. For this reason, it is less penetratingthan uncharged types of radiation such as the gamma or neutron. The beta particle can generallybe stopped by a sheet of aluminum. Because the beta travels farther than the alpha, it depositsits energy over a greater area and is, therefore, less harmful than the alpha if taken internally.All materials described under neutron and gamma radiation are also effective at attenuating betaradiation.MS-05Page 20Rev. 0
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