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1、Gamma-andX-RayInteractionsinMatterIntroductionTherearefivetypesofinteractionswithmatterbyx-and?-rayphotonswhichmustbeconsideredinradiologicalphysics:ComptoneffectPhotoelectriceffectPairproductionRayleigh(coherent)scatteringPhotonuclearinteractionsIntroduction(cont.)Thefirstthreeofthesearethe
2、mostimportant,astheyresultinthetransferofenergytoelectrons,whichthenimpartthatenergytomatterinmany(usuallysmall)Coulomb-forceinteractionsalongtheirtracksRayleighscatteringiselastic;thephotonismerelyredirectedthroughasmallanglewithnoenergylossPhotonuclearinteractionsareonlysignificantforphoto
3、nenergiesaboveafewMeV,wheretheymaycreateradiation-protectionproblemsthroughthe(?,n)productionofneutronsandconsequentradioactivationComptonEffectAdescriptionoftheComptoneffectcanbeconvenientlysubdividedintotwoaspects:kinematicsandcrosssectionThefirstrelatestheenergiesandanglesoftheparticipati
4、ngparticleswhenaComptoneventoccurs;thesecondpredictstheprobabilitythataComptoninteractionwilloccurInbothrespectsitiscustomarytoassumethattheelectronstruckbytheincomingphotonisinitiallyunboundandstationaryComptonEffect(cont.)Theseassumptionsarecertainlynotrigorous,inasmuchastheelectronsallocc
5、upyvariousatomicenergylevels,thusareinmotionandareboundtothenucleusTheresultingerrorsremaininconsequentialinradiologicalphysicsapplications,becauseofthedominanceofthecompetingphotoelectriceffectundertheconditions(highZ,lowh?)whereelectronbindingeffectsarethemostimportantinComptoninteractions
6、ComptonEffect-KinematicsThefollowingfigureshowsaphotonofenergyh?collidingwithanelectronThephoton’sincidentforwardmomentumish?/c,wherecisthespeedoflightinvacuumThestationarytargetelectronhasnoinitialkineticenergyormomentumKinematicsoftheComptoneffectComptonEffect–Kinematics(cont.)Afterthecoll
7、isiontheelectrondepartsatangle?,withkineticenergyTandmomentumpThephotonscattersatangle?withanew,lowerquantumenergyh??andmomentumh??/cThesolutiontothecollisionkineticsisbaseduponconservationofbothenergyandmomentumEnergyconservationrequiresthatCompto
