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1、22.101AppliedNuclearPhysics(Fall2004)Lecture19(11/22/04)GammaInteractions:ComptonScattering_______________________________________________________________________References:R.D.Evan,AtomicNucleus(McGraw-HillNewYork,1955),Chaps23–25..W.E.Meyerhof,Elemen
2、tsofNuclearPhysics(McGraw-Hill,NewYork,1967),pp.91108.W.Heitler,QuantumTheoryofRadiation(Oxford,1955),Sec.26.________________________________________________________________________Weareinterestedintheinteractionsofgammarays,electromagneticradiationspr
3、oducedbynucleartransitions.Thesearetypicallyphotonswithenergiesintherangeof~0.1–10Mev.Theattenuationoftheintensityofabeamofgammaraysinanabsorber,sketchedinFig.17.1,followsatrueexponentialvariationwiththedistanceofpenetration,whichisunlikethatofchargedp
4、articles,Fig.17.1.Attenuationofabeamofgammaradiationthroughanabsorberofthicknessx.−µxI(x)=Ie(17.1)oTheinteractionisexpressedthroughthelinearattenuationcoefficientµwhichdoesnotdependonxbutdoesdependontheenergyoftheincidentgamma.Byattenuationonemeanseith
5、erscatteringorabsorption.Sinceeitherprocesswillremovethegammafromthebeam,theprobabilityofpenetratingadistancexisthesameastheprobabilityoftravelingadistancexwithoutanyinteraction,exp(-µx).Theattenuationcoefficientis1thereforetheprobabilityperunitpathofi
6、nteraction;itiswhatwewouldcallthemacroscopiccrosssectionΣinthecaseofneutroninteraction.Thereareseveraldifferentprocessesofgammainteraction.Eachprocesscanbetreatedasoccurringindependentlyofeachother;forthisreasonµisthesumoftheindividualcontributions.The
7、secontributions,ofcourse,arenotequallyimportantatanygivenenergy.Eachprocesshasitsownenergyvariationaswellasdependenceontheatomicnumberoftheabsorber.Wewillfocusourdiscussionsonthethreemostimportantprocessesofgammainteraction,Comptonscattering,photoelect
8、riceffect,andpairproduction.Thesecanbeclassifiedbyobjectwithwhichthephotoninteractsandthetypeofprocess(absorptionorscattering).Asshowninthematrixbelow,photoelectricistheabsorptionofaphotonfollowedbytheejectionofanatomicelectron.Comptons
