Chapter48-Radiation:IonizingINTRODUCTIONRobertN.Cherry,Jr.Ionizingradiationiseverywhere.Itarrivesfromouterspaceascosmicrays.Itisintheairasemissionsfromradioactiveradonanditsprogeny.Naturallyoccurringradioactiveisotopesenterandremaininalllivingthings.Itisinescapable.Indeed,allspeciesonthisplanetevolvedinthepresenceofionizingradiation.Whilehumansexposedtosmalldosesofradiationmaynotimmediatelyshowanyapparentbiologicaleffects,thereisnodoubtthationizingradiation,whengiveninsufficientamounts,cancauseharm.Theseeffectsarewellknownbothinkindandindegree.Whileionizingradiationcancauseharm,italsohasmanybeneficialuses.Radioactiveuraniumgenerateselectricityinnuclearpowerplantsinmanycountries.Inmedicine,xraysproduceradiographsfordiagnosisofinternalinjuriesanddiseases.Nuclearmedicinephysiciansuseradioactivematerialastracerstoformdetailedimagesofinternalstructuresandtostudymetabolism.Therapeuticradiopharmaceuticalsareavailabletotreatdisorderssuchashyperthyroidismandcancer.Radiotherapyphysiciansusegammarays,pionbeams,electronbeams,neutronsandothertypesofradiationtotreatcancer.Engineersuseradioactivematerialinoilwellloggingoperationsandinsoilmoisturedensitygauges.Industrialradiographersusexraysinqualitycontroltolookatinternalstructuresofmanufactureddevices.Exitsignsinbuildingsandaircraftcontainradioactivetritiumtomakethemglowinthedarkintheeventofapowerfailure.Manysmokedetectorsinhomesandcommercialbuildingscontainradioactiveamericium.Thesemanyusesofionizingradiationandradioactivematerialsenhancethequalityoflifeandhelpsocietyinmanyways.Thebenefitsofeachusemustalwaysbecomparedwiththerisks.Therisksmaybetoworkersdirectlyinvolvedinapplyingtheradiationorradioactivematerial,tothepublic,tofuturegenerationsandtotheenvironmentortoanycombinationofthese.Beyondpoliticalandeconomicconsiderations,benefitsmustalwaysoutweighriskswhenionizingradiationisinvolved.IonizingRadiationIonizingradiationconsistsofparticles,includingphotons,whichcausetheseparationofelectronsfromatomsandmolecules.However,sometypesofradiationofrelativelylowenergy,suchasultravioletlight,canalsocauseionizationundercertaincircumstances.Todistinguishthesetypesofradiationfromradiationthatalwayscausesionization,anarbitrarylowerenergylimitforionizingradiationusuallyissetaround10kiloelectronvolts(keV).Directlyionizingradiationconsistsofchargedparticles.Suchparticlesincludeenergeticelectrons(sometimescallednegatrons),positrons,protons,alphaparticles,chargedmesons,muonsandheavyions(ionizedatoms).ThistypeofionizingradiationinteractswithmatterprimarilythroughtheCoulombforce,repellingorattractingelectronsfromatomsandmoleculesbyvirtueoftheircharges.Indirectlyionizingradiationconsistsofunchargedparticles.Themostcommonkindsofindirectlyionizingradiationarephotonsabove10keV(xraysandgammarays)andallneutrons.X-rayandgamma-rayphotonsinteractwithmatterandcauseionizationinatleastthreedifferentways:1.Lower-energyphotonsinteractmostlyviathephotoelectriceffect,inwhichthephotongivesallofitsenergytoanelectron,whichthenleavestheatomormolecule.Thephotondisappears.2.Intermediate-energyphotonsmostlyinteractthroughtheComptoneffect,inwhichthephotonandanelectronessentiallycollideasparticles.Thephotoncontinuesinanewdirectionwithreducedenergywhilethereleasedelectrongoesoffwiththeremainderoftheincomingenergy(lesstheelectron’sbindingenergytotheatomormolecule).3.Pairproduct...
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