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.Large area shield scatters toward detec-Absorption of photontor some of the radiation originally travelingScatter of photonaway from detector and thus gives less attenu-ation than is measured in good geometry ex-periment (Fig.2.11).Primary or uncollided photonSource DetectorLarge area shieldReport 49; Slaback et al., 1997).There are times, however, when the fall-off may not be describable in terms of a single value for the half-valuelayer.When relevant experimental data are not available, or when theproblem is too complex to be treated by half-value layer concepts, the ac-tual procedure is to make a calculation using the good geometry values,and increase the results by a factor that takes account of the productionof secondary radiation due to scattering.This factor is generally known asthe buildup factor and its use is described in texts on radiation shielding(Chilton et al., 1984).The scattered radiation and, accordingly, the buildup factor increasesrelative to the unscattered or primary radiation as the shield thickness in-creases.It is the dominant radiation in the thicker shields.Some values ofthe buildup factor are given in Table 2.5.Note that buildup factors are considerably higher in shields composedof the lighter elements (water, concrete) than the heavier elements (lead).The reason is that the energies of the scattered photons are lower than theenergy of the primary radiation, and attenuation and absorption increaseas the energy decreases at energies characteristically emitted by radio-nuclides.The increase in absorption with lower energies is more marked inelements with higher atomic number because of the photoelectric effect;hence the buildup factor in lead decreases much more slowly than thebuildup factor in water as the shield thickness increases. |50 TWO Principles of Protection against Ionizing ParticlesTable 2.5 Values of buildup factors.Number of half-value layers in shieldaE (MeV) 1.44 2.88 5.77 10.08Buildup factors in water0.255 3.09 7.14 23.0 72.90.5 2.52 5.14 14.3 38.81.0 2.13 3.71 7.68 16.2Buildup factors in iron0.5 1.98 3.09 5.98 11.71.0 1.87 2.89 5.39 10.2Buildup factors in lead0.5 1.24 1.42 1.69 2.001.0 1.37 1.69 2.26 3.02Source: Jaeger et al., 1968.a.The number of half-value layers = 1.44 × attenuation coefficient × shield thickness.8.6 Protection from Gamma Sources Time, Distance, ShieldingThe three key words time, distance, and shielding introduced in con-nection with protection from beta particles apply equally well to protec-tion against gamma photons.Short working times and maximum workingdistances are as effective in reducing exposure from gamma photons asfrom beta particles.As with beta particles, the degree by which the inten-sity from a small source is reduced at increasing distances is obtained bythe inverse-square law.Because gamma photons are so much more pene-trating than beta particles, they require more shielding, the amount de-pending, of course, on the size of the source.If large gamma sources are tobe handled, thick shields are required; often special manipulators and leadglass windows are used.We must keep in mind one basic difference between beta and gammashielding.Beta particles are charged ionizing particles and have a maxi-mum range.Thus a shield built to stop beta particles from a particularradionuclide will stop the particles from any source consisting of that nu-clide, regardless of the source strength.11 On the other hand, a gammashield always allows a fraction of the gamma photons to get through, sincethey are uncharged ionizing particles.The fraction decreases, of course, asthe thickness of the shield increases.11.There may still be a secondary effect from bremsstrahlung. |9 Heavy Charged Ionizing Particles 51Suppose the fraction of gamma photons that penetrate a shield is 1 per-cent.If 1,000 are incident, 10 will penetrate, and if 100 are incident, 1 willpenetrate, on the average.These numbers may be insignificant.On theother hand, if 1015 photons are incident on the shield, 1013 will getthrough.This could have serious consequences.With gamma photons, as with all uncharged ionizing particles, a shieldthat is just thick enough to provide protection for one level of activity willnot be thick enough for levels that are significantly higher.The protectionoffered by a gamma shield must always be evaluated in terms of the sourcestrength, and no shield should be trusted until its adequacy has beenverified for the source to be shielded.9 Heavy Charged Ionizing ParticlesThe electron is the most common but not the only charged ionizing parti-cle encountered in work with radiation sources.12 For our purposes, theother particles may be characterized by their charge and their mass whenthey are at rest.Because they are so small, it is not convenient to expresstheir mass in grams.Instead, we shall express their mass relative to the elec-tron mass (an electron has a mass of 9.1 × 10-28 g) or in terms of the en-ergy equivalent of their mass.We often express the mass in terms of the en-ergy equivalent because it is possible for the rest mass of a particle to beconverted into an equivalent amount of energy and vice versa.The equiva-lence is given mathematically by Einstein s equation, energy = mass × (ve-locity of light)2.The rest energy of an electron is 0.51 MeV.9.1 The Alpha Particle A Heavy Particle with High LinearEnergy Transfer and High Capacity for Producing DamageThe alpha particle is an energetic helium nucleus, consisting of twoneutrons and two protons.It is therefore heavier than the electron by a fac-tor of over 7,300 and has double the charge.It is commonly emitted in theradioactive decay of the heaviest nuclides in the periodic table.Examplesof naturally occurring alpha emitters are uranium, thorium, radium, andpolonium.An artificially produced alpha emitter, plutonium, is likely tobe the main component of fuel in the nuclear power plants of the future.The alpha particles emitted by these nuclides possess kinetic energiesranging between 4 MeV and 9 MeV.The corresponding speeds are be-12.Material in this section on particles not encountered by the reader may be omitted ina first reading. |52 TWO Principles of Protection against Ionizing Particlestween 1.4 and 2.1 × 109 cm/sec.They are much less than the speeds ofbeta particles in the same energy range, which are quite close to the speedof light [ Pobierz caÅ‚ość w formacie PDF ]
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.Large area shield scatters toward detec-Absorption of photontor some of the radiation originally travelingScatter of photonaway from detector and thus gives less attenu-ation than is measured in good geometry ex-periment (Fig.2.11).Primary or uncollided photonSource DetectorLarge area shieldReport 49; Slaback et al., 1997).There are times, however, when the fall-off may not be describable in terms of a single value for the half-valuelayer.When relevant experimental data are not available, or when theproblem is too complex to be treated by half-value layer concepts, the ac-tual procedure is to make a calculation using the good geometry values,and increase the results by a factor that takes account of the productionof secondary radiation due to scattering.This factor is generally known asthe buildup factor and its use is described in texts on radiation shielding(Chilton et al., 1984).The scattered radiation and, accordingly, the buildup factor increasesrelative to the unscattered or primary radiation as the shield thickness in-creases.It is the dominant radiation in the thicker shields.Some values ofthe buildup factor are given in Table 2.5.Note that buildup factors are considerably higher in shields composedof the lighter elements (water, concrete) than the heavier elements (lead).The reason is that the energies of the scattered photons are lower than theenergy of the primary radiation, and attenuation and absorption increaseas the energy decreases at energies characteristically emitted by radio-nuclides.The increase in absorption with lower energies is more marked inelements with higher atomic number because of the photoelectric effect;hence the buildup factor in lead decreases much more slowly than thebuildup factor in water as the shield thickness increases. |50 TWO Principles of Protection against Ionizing ParticlesTable 2.5 Values of buildup factors.Number of half-value layers in shieldaE (MeV) 1.44 2.88 5.77 10.08Buildup factors in water0.255 3.09 7.14 23.0 72.90.5 2.52 5.14 14.3 38.81.0 2.13 3.71 7.68 16.2Buildup factors in iron0.5 1.98 3.09 5.98 11.71.0 1.87 2.89 5.39 10.2Buildup factors in lead0.5 1.24 1.42 1.69 2.001.0 1.37 1.69 2.26 3.02Source: Jaeger et al., 1968.a.The number of half-value layers = 1.44 × attenuation coefficient × shield thickness.8.6 Protection from Gamma Sources Time, Distance, ShieldingThe three key words time, distance, and shielding introduced in con-nection with protection from beta particles apply equally well to protec-tion against gamma photons.Short working times and maximum workingdistances are as effective in reducing exposure from gamma photons asfrom beta particles.As with beta particles, the degree by which the inten-sity from a small source is reduced at increasing distances is obtained bythe inverse-square law.Because gamma photons are so much more pene-trating than beta particles, they require more shielding, the amount de-pending, of course, on the size of the source.If large gamma sources are tobe handled, thick shields are required; often special manipulators and leadglass windows are used.We must keep in mind one basic difference between beta and gammashielding.Beta particles are charged ionizing particles and have a maxi-mum range.Thus a shield built to stop beta particles from a particularradionuclide will stop the particles from any source consisting of that nu-clide, regardless of the source strength.11 On the other hand, a gammashield always allows a fraction of the gamma photons to get through, sincethey are uncharged ionizing particles.The fraction decreases, of course, asthe thickness of the shield increases.11.There may still be a secondary effect from bremsstrahlung. |9 Heavy Charged Ionizing Particles 51Suppose the fraction of gamma photons that penetrate a shield is 1 per-cent.If 1,000 are incident, 10 will penetrate, and if 100 are incident, 1 willpenetrate, on the average.These numbers may be insignificant.On theother hand, if 1015 photons are incident on the shield, 1013 will getthrough.This could have serious consequences.With gamma photons, as with all uncharged ionizing particles, a shieldthat is just thick enough to provide protection for one level of activity willnot be thick enough for levels that are significantly higher.The protectionoffered by a gamma shield must always be evaluated in terms of the sourcestrength, and no shield should be trusted until its adequacy has beenverified for the source to be shielded.9 Heavy Charged Ionizing ParticlesThe electron is the most common but not the only charged ionizing parti-cle encountered in work with radiation sources.12 For our purposes, theother particles may be characterized by their charge and their mass whenthey are at rest.Because they are so small, it is not convenient to expresstheir mass in grams.Instead, we shall express their mass relative to the elec-tron mass (an electron has a mass of 9.1 × 10-28 g) or in terms of the en-ergy equivalent of their mass.We often express the mass in terms of the en-ergy equivalent because it is possible for the rest mass of a particle to beconverted into an equivalent amount of energy and vice versa.The equiva-lence is given mathematically by Einstein s equation, energy = mass × (ve-locity of light)2.The rest energy of an electron is 0.51 MeV.9.1 The Alpha Particle A Heavy Particle with High LinearEnergy Transfer and High Capacity for Producing DamageThe alpha particle is an energetic helium nucleus, consisting of twoneutrons and two protons.It is therefore heavier than the electron by a fac-tor of over 7,300 and has double the charge.It is commonly emitted in theradioactive decay of the heaviest nuclides in the periodic table.Examplesof naturally occurring alpha emitters are uranium, thorium, radium, andpolonium.An artificially produced alpha emitter, plutonium, is likely tobe the main component of fuel in the nuclear power plants of the future.The alpha particles emitted by these nuclides possess kinetic energiesranging between 4 MeV and 9 MeV.The corresponding speeds are be-12.Material in this section on particles not encountered by the reader may be omitted ina first reading. |52 TWO Principles of Protection against Ionizing Particlestween 1.4 and 2.1 × 109 cm/sec.They are much less than the speeds ofbeta particles in the same energy range, which are quite close to the speedof light [ Pobierz caÅ‚ość w formacie PDF ]