G4光学模型.docx
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G4光学模型.docx
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G4光学模型
5.2.5OpticalPhotonProcesses
Aphotonisconsideredtobe optical whenitswavelengthismuchgreaterthanthetypicalatomicspacing.InGEANT4opticalphotonsaretreatedasaclassofparticledistinctfromtheirhigherenergy gamma cousins.Thisimplementationallowsthewave-likepropertiesofelectromagneticradiationtobeincorporatedintotheopticalphotonprocess.Becausethistheoreticaldescriptionbreaksdownathigherenergies,thereisnosmoothtransitionasafunctionofenergybetweentheopticalphotonandgammaparticleclasses.
TheGEANT4catalogueofprocessesatopticalwavelengthsincludesrefractionandreflectionatmediumboundaries,bulkabsorptionandRayleighscattering.ProcesseswhichproduceopticalphotonsincludetheCerenkoveffect,transitionradiationandscintillation.OpticalphotonsaregeneratedinGEANT4withoutenergyconservationandtheirenergymustthereforenotbetalliedaspartoftheenergybalanceofanevent.
Theopticalpropertiesofthemediumwhicharekeytotheimplementationofthesetypesofprocessesarestoredasentriesina G4MaterialPropertiesTable whichislinkedtothe G4Material inquestion.Thesepropertiesmaybeconstantsortheymaybeexpressedasafunctionofthephoton'swavelength.Thistableisaprivatedatamemberofthe G4Material class.The G4MaterialPropertiesTable isimplementedasahashdirectory,inwhicheachentryconsistsofa value andakey.Thekeyisusedtoquicklyandefficientlyretrievethecorrespondingvalue.Allvaluesinthedictionaryareeitherinstantiationsof G4double ortheclass G4MaterialPropertyVector,andallkeysareoftype G4String.
A G4MaterialPropertyVector iscomposedofinstantiationsoftheclass G4MPVEntry.The G4MPVEntry isapairofnumbers,whichinthecaseofanopticalproperty,arethephotonmomentumandcorrespondingpropertyvalue.The G4MaterialPropertyVector isimplementedasa G4std:
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vector,withthesortingoperationdefinedasMPVEntry1 constG4intNUMENTRIES=32; G4doubleppckov[NUMENTRIES]={2.034*eV,......,4.136*eV}; G4doublerindex[NUMENTRIES]={1.3435,......,1.3608}; G4doubleabsorption[NUMENTRIES]={344.8*cm,......,1450.0*cm]; G4MaterialPropertiesTable*MPT=newG4MaterialPropertiesTable(); MPT->AddConstProperty("SCINTILLATIONYIELD",100./MeV); MPT->AddProperty("RINDEX",ppckov,rindex,NUMENTRIES}; MPT->AddProperty("ABSLENGTH",ppckov,absorption,NUMENTRIES}; scintillator->SetMaterialPropertiesTable(MPT); Sourcelisting5.2.4 Opticalpropertiesaddedtoa G4MaterialPropertiesTable andlinkedtoa G4Material. 5.2.5.1GenerationofPhotonsin processes/electromagnetic/xrays -CerenkovEffect TheradiationofCerenkovlightoccurswhenachargedparticlemovesthroughadispersivemediumfasterthanthegroupvelocityoflightinthatmedium.Photonsareemittedonthesurfaceofacone,whoseopeninganglewithrespecttotheparticle'sinstantaneousdirectiondecreasesastheparticleslowsdown.Atthesametime,thefrequencyofthephotonsemittedincreases,andthenumberproduceddecreases.Whentheparticlevelocitydropsbelowthelocalspeedoflight,theradiationceasesandtheemissionconeanglecollapsestozero.Thephotonsproducedbythisprocesshaveaninherentpolarizationperpendiculartothecone'ssurfaceatproduction. Theflux,spectrum,polarizationandemissionofCerenkovradiationinthe AlongStepDoIt methodoftheclass G4Cerenkov followwell-knownformulae,withtwoinherentcomputationallimitations.Thefirstarisesfromstep-wisesimulation,andthesecondcomesfromtherequirementthatnumericalintegrationcalculatetheaveragenumberofCerenkovphotonsperstep.Theprocessmakesuseofa G4PhysicsTable whichcontainsincrementalintegralstoexpeditethiscalculation. ThetimeandpositionofCerenkovphotonemissionarecalculatedfromquantitiesknownatthebeginningofachargedparticle'sstep.Thestepisassumedtoberectilineareveninthepresenceofamagneticfield.Theusermaylimitthestepsizebyspecifyingamaximum(average)numberofCerenkovphotonscreatedduringthestep,usingthe SetMaxNumPhotonsPerStep(constG4intNumPhotons) method.TheactualnumbergeneratedwillnecessarilybedifferentduetothePoissoniannatureoftheproduction.Inthepresentimplementation,theproductiondensityofphotonsisdistributedevenlyalongtheparticle'stracksegment,eveniftheparticlehasslowedsignificantlyduringthestep. Thefrequentlyverylargenumberofsecondariesproducedinasinglestep(about300/cminwater),compelledtheideainGEANT3.21ofsuspendingtheprimaryparticleuntilallitsprogenyhavebeentracked.DespitethefactthatGEANT4employsdynamicmemoryallocationandthusdoesnotsufferfromthelimitationsofGEANT3.21withitsfixedlargeinitialZEBRAstore,GEANT4neverthelessprovidesforananalogousfunctionalitywiththepublicmethodSetTrackSecondariesFirst.AnexampleoftheregistrationoftheCerenkovprocessisgiveninsourcelisting #include"G4Cerenkov.hh" voidExptPhysicsList: : ConstructOp(){ G4Cerenkov*theCerenkovProcess=newG4Cerenkov("Cerenkov"); G4intMaxNumPhotons=300; theCerenkovProcess->SetTrackSecondariesFirst(true); theCerenkovProcess->SetMaxNumPhotonsPerStep(MaxNumPhotons); theParticleIterator->reset(); while((*theParticleIterator)()){ G4ParticleDefinition*particle=theParticleIterator->value(); G4ProcessManager*pmanager=particle->GetProcessManager(); G4StringparticleName=particle->GetParticleName(); if(theCerenkovProcess->IsApplicable(*particle)){ pmanager->AddContinuousProcess(theCerenkovProcess); } } } Sourcelisting5.2.5 RegistrationoftheCerenkovprocessin PhysicsList. 5.2.5.2GenerationofPhotonsin processes/electromagnetic/xrays -Scintillation Everyscintillatingmaterialhasacharacteristiclightyield, SCINTILLATIONYIELD,andanintrinsicresolution, RESOLUTIONSCALE,whichgenerallybroadensthestatisticaldistributionofgeneratedphotons.AwiderintrinsicresolutionisduetoimpuritieswhicharetypicalfordopedcrystalslikeNaI(Tl)andCsI(Tl).Ontheotherhand,theintrinsicresolutioncanalsobenarrowerwhentheFanofactorplaysarole.Theactualnumberofemittedphotonsduringastepfluctuatesaroundthemeannumberofphotonswithawidthgivenby ResolutionScale*sqrt(MeanNumberOfPhotons).Theaveragelightyield, MeanNumberOfPhotons,hasalineardependenceonthelocalenergydeposition,butitmaybedifferentforminimumionizingandnon-minimumionizingparticles. Ascintillatorisalsocharacterizedbyitsphotonemissionspectrumandbytheexponentialdecayofitstimespectrum.InGEANT4thescintillatorcanhaveafastandaslowcomponent.Therelativestrengthofthefastcomponentasafractionoftotalscintillationyieldisgivenbythe YIELDRATIO.Scintillationmaybesimulatedbyspecifyingtheseempiricalparametersforeachmaterial.Itissufficienttospecifyintheuser's DetectorConstruction classarelativespectraldistributionasafunctionofphotonenergyforthescintillatingmaterial.Anexampleofthisisshowninsourcelisting5.2.6. constG4intNUMENTRIES=9; G4doubleScnt_PP[NUMENTRIES]={6.6*eV,6.7*eV,6.8*eV,6.9*eV, 7.0*eV,7.1*eV,7.2*eV,7.3*eV,7.4*eV}; G4doubleScnt_FAST[NUMENTRIES]={0.000134,0.004432,0.053991,0.241971, 0.398942,0.000134,0.004432,0.053991, 0.241971}; G4doubleScnt_SLOW[NUMENTRIES]={0.000010,0.000020,0.000030,0.004000, 0.008000,0.005000,0.020000,0.001000, 0.000010}; G4Material*Scnt; G4MaterialPropertiesTable*Scnt_MPT=newG4MaterialPropertiesTable(); Scnt_MPT->AddProperty("FASTCOMPONENT",Scnt_PP,Scnt_FAST,NUMENTRIES); Scnt_MPT->AddProperty("SLOWCOMPONENT",Scnt_PP,Scnt_SLOW,NUMENTRIES); Scnt_MPT->AddConstProperty("SCINTILLATIONYIELD",5000./MeV); Scnt_MPT->AddConstProperty("RESOLUTIONSCALE",2.0); Scnt_MPT->AddConstProperty("FASTTIMECONSTANT",1.*ns); Scnt_MPT->AddConstProperty("SLOWTIMECONSTANT",10.*ns); Scnt_MPT->AddConstProperty("YIELDRATIO",0.8); Scnt->SetMaterialPropertiesTable(Scnt_MPT); Sourcelisting5.2.6 Specificationofscintillationpropertiesin DetectorConstruction. Incaseswherethescintillationyieldofascintillatordependsontheparticletype,differentscintillationprocessesmaybedefinedforthem.Howthisyieldscalestotheonespecifiedforthematerialisexpressedwiththe ScintillationYieldFactor intheuser's PhysicsList asshowninsourcelisting5.2.7.Inthosecaseswherethefasttoslowexcitationratiochangeswithparticletype,themethod SetScintillationExcitationRatio canbecalledforeachscintillationprocess(seetheadvancedunderground_physicsexample).Thisoverwritesthe YieldRatio obtainedfromthe G4MaterialPropertiesTable. G4Scintillation*theMuonScintProcess=newG4Scintillation("Scintillation"); theMuonScintProcess->SetTrackSecondariesFirst(true); theMuonScintProcess->SetScintillationYieldFactor(0.8); theParticleIterator->reset(); while((*theParticleIterator)()){ G4ParticleDefinition*particle=theParticleIterator->value(); G4ProcessManager*pmanager=particle->GetProcessManager(); G4StringparticleName=particle->GetParticleName(); if(theMuonScintProcess->IsApplicable(*particle)){ if(particleName=="mu+"){ pmanager->AddProcess(theMuonScintProcess); pmanager->SetProcessOrderingToLast(theMuonScintProcess,idxAtRest); pmanager->SetProcess
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