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.11.1 The Pattern of the Imparted Energy in a MediumThe ionization patterns produced by various particles incident on a me-dium such as water are shown in Figure 2.17.Beta particles are depicted ascontinuously producing ionization (from the moment they enter the me-dium) along a tortuous path marked by occasional large-angle scatteringsas well as smaller deflections.The large-angle scatterings occur when thebeta particles pass close by an atomic nucleus and encounter strong electri-cal forces.These large-angle scatterings do not produce ionization, butthey cause the beta particle to emit x-ray photons.There are also sidetracks of ionization caused by high-speed electrons (known as delta rays)that are ejected from the orbits of atoms.The depths of penetration of in-dividual beta particles vary depending on their energy and the degree ofscattering.The maximum penetration is equal to the range of the beta par-ticles in the medium.The particles impart their energy as excitation as wellas ionization of the atoms in the medium.Alpha particles are shown traveling in a straight line and stopping in a |11 The Absorbed Dose 612.17 Ionization patterns produced14C-²-by various particles in tissue (not Maxrange0.29mm0.154 MeV maxdrawn to scale).Max range 8 mm32P-²-1.71 MeV max210Po-±Range 0.037 mm5.3 MeV³125I-³0.35 MeV³Average dist.to collision 3.3 cm60Co-³1.33 MeVAverage dist.to collision 16.4 cmn1 MeV neutronAverage dist.to collision 2.6 cm1 MeV proton Range 0.028 mm3100 A° between primary ionizationsbeta particleslow ionizationdensity largeSpacings ofmoleculeindividualionizationsalpha particleshigh ionizationdensity 15 A° between ionizationsmuch shorter distance than beta particles penetrate.The ionizations, al-ways closely spaced, become even more closely spaced near the end of therange.The spacings of the individual ionizations of alpha and beta parti-cles are compared with the dimensions of a large molecule at the bottom ofthe figure.Gamma photons are shown as proceeding without interaction untilthere is a collision in which a high-speed electron is ejected from an atom. |62 TWO Principles of Protection against Ionizing ParticlesThe ejected electrons may have energies up to the maximum energy of thegamma photon minus the energy required to eject the electrons from theirshells.The maximum-energy electrons result from complete absorption ofthe photon, and the lower-energy electrons result from partial loss of theenergy of the photon in a scattering.The scattered gamma photon pro-ceeds until it undergoes further interactions.Photons of two energies are shown for comparison.The gamma photonfrom cobalt-60 on the average travels a longer distance (16.4 cm) than thelower-energy gamma photon from iodine-125 (3.3 cm) between interac-60tions.The electrons ejected by the Co photons are also more energetic,on the average, and travel greater distances before being stopped.Fast neutrons are shown to proceed without interaction until there is anelastic collision with a nucleus, followed by recoil of the nucleus and scat-tering of the neutron at reduced energy.The recoil nucleus, because of itslarge mass, has high linear energy transfer and short range.A track of themost important recoil nucleus in tissue, the proton, is shown diagrammati-cally under the neutron track.11.2 Definition of Absorbed DoseThe basic quantity that characterizes the amount of energy imparted tomatter is the absorbed dose.The mean absorbed dose in a region is deter-mined by dividing the energy imparted to the matter in that region by themass of the matter in that region.From the discussion of patterns of ionization presented in the last sec-tion, it is obvious that patterns are not uniform throughout a region, andtherefore the absorbed dose is different in different parts of the region.Aproper application of this concept requires a knowledge not only of themanner in which the energy is imparted but also of the significance of thismanner with regard to the production of injury to tissue.In evaluating ab-sorbed dose, the radiation protection specialist must select the critical re-gion in which he wishes to evaluate the energy imparted.He may averagethe energy imparted over a fairly large region.If the energy pattern is verynonuniform, he may select that part of the region where the energy im-parted is a maximum to obtain a maximum dose.If the region where thedose is a maximum is very small, he may decide that the dose in that regionis not as significant as a dose determined by considering a larger region.Weshall examine the various possibilities in later examples.11.3 The Gray The SI Unit for Absorbed DoseAbsorbed doses are expressed in units of grays or in prefixed formsof the gray, such as the milligray (1/1,000 gray) and the microgray (1/1,000,000 gray). |12 The Equivalent Dose 63The gray is equal to 1 joule of energy absorbed per kilogram of matter(1 J/kg).Since we express here the energies of the radiations in terms ofMeV, we can also say that the gray (Gy) is equal to 6.24 × 109 MeV/g.The milligray is equal to 6.24 × 106 MeV/g.19The gray was adopted by the International Commission on RadiationUnits and Measurements (ICRU) in 1975 as the special name for the stan-dard international (SI) unit of absorbed dose (NCRP, 1985b).It usedthe SI base units of joules for energy and kilograms for mass in definingthe gray.The gray replaced the rad, defined as 100 ergs per gram and nowreferred to as the traditional unit.The rad is equal to 0.01 J/kg or 1centigray (cGy).SI units are used exclusively in all reports of the ICRU, the Interna-tional Commission on Radiological Protection (ICRP), and the U.S.Na-tional Council on Radiation Protection and Measurements (NCRP).Ab-sorbed doses are often expressed in centigrays in the literature to makethem numerically equal to the dose values given in rads in the past.Harmful levels of radiation dose are generally expressed in terms ofgrays.For example, over a gray must be imparted in a short period over asubstantial portion of the body before most individuals will show sig-nificant clinical symptoms (Saenger, 1963).Occupational absorbed dosesfrom x and gamma radiation are limited to a maximum of 50 milligraysper year
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