Development and evaluation of incoherent scattering spectroscopy for spatial and spectral characterization of high-energy x-ray beams

128 Pages·2000·4.1 MB·English

by  Li, Texin, 1967-

DEVELOPMENTANDEVALUATIONOFINCOHERENT
SCATTERINGSPECTROSCOPYFORSPATIALANDSPECTRAL
CHARACTERIZATIONOFHIGH-ENERGYX-RAYBEAMS
By
TEXINLI
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOLOFTHE
UNIVERSITYOFFLORIDAINPARTIALFULFILLMENTOFTHE
REQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
2000
ACKNOWLEDGMENTS
Theauthorisverygratefultoallthepeoplewhosecontributionshavemade
thisresearchprojectsuccessful.
SpecialthanksgotoDr.SamimAnghaieforhisconstantfinancialsupport
andvaluablesuggestionsthathelpedtoguidethedirectionofthisproject.Dr.Travis
Knight,Mr.WardDougherty,Dr.GaryChen,Mrs.SarahWhite,andespeciallyDr.
MorganWhitehelpedinsimulation.TheauthorisgreatlyindebtedtoDr.Angelo
Ferrariforhissuperbexperimentalskills.Dr.WolfgangTome,Dr.SiyongKim,and
Mr.JoonHoParkalsohelpedintheexperimentwork.Dr.ChihrayLiuprovidedthe
relevantreferencesandpertinentcomments.Dr.WesleyBolchwaskindenoughto
permittheauthortoruncodesathisworkstationsandMr.DidierRajonhelpedin
settinguptheaccount.
Theauthorwouldalsoliketotakethischancetoexpresshissinceregratitude
tootherclassmatesandfacultymembersattheNuclearandRadiologicalEngineering
DepartmentandtheRadiationOncologyDepartmentfortheirconstructive
suggestionsandtheirhelpfulcomments.
TheauthorthanksDr.IainC.Trevena,Mr.ColettePigeon-Jolicoeur,andMr.
DaveWillinsonatMDSNordionforprovidingthenecessarydataneededforthe
MCNPsimulation.
11
TABLEOFCONTENTS
page
ACKNOWLEDGMENTS
ii
LISTOFTABLES v
LISTOFFIGURES vi
ABSTRACT ix
CHAPTERS
1 INTRODUCTION 1
2 THEORYOFINCOHERENTSCATTERINGMETHOD 18
Scattering 18
CoherentScattering 19
IncoherentScattering 21
ChoiceofScatteringAngle 24
EffectofCollimator 26
ChoiceofShieldingMaterials 32
ChoiceofScatteringMaterials 33
DetectorResponse 37
3 MATERIALSANDMETHODS 41
Cobalt-60TherapyMachine 41
HighPurityGermaniumDetector 44
Electronics 45
ExperimentalSetup 47
MeasurementofSpectra 48+
EffectofCollimator 49+
4 RESULTSANDDISCUSSION 50
ScatteredSpectra 50
EffectofCollimator 55
iii
5 MONTECARLOSIMULATION 60
ScatteredSpectrumofCobalt-60BeamSimulation 62
ModelingofCobalt-60Machine 64
PrimarySpectrumSimulationofCobalt-60Machine 66
ScatteredSpectrumSimulationofCobalt-60Machine 67
EffectoftheCollimator 69
6 APPLICATIONTOMEDICALLINEARACCELERATOR 76
DescriptionofMedicalLinearAccelerator 76
MeasurementofSpectrum 80
MonteCarloSimulation 84
7 CONCLUSIONS 91
APPENDICES
A MCNPINPUTFILEFORCOBALT-60MACHINESIMULATION 95
B MCNPINPUTFILEFORSL25LINACSIMULATION 100
REFERENCESCITED Ill
BIOGRAPHICALSKETCH 115
IV
LISTOFTABLES
Table page
2-1.Minimumscatteringangles(indegree) 26
2-2.Thegeometriclinebroadening(inkeV)of100keVPhotons 28
2-3.Thegeometriclinebroadening(inkeV)of1000keVPhotons 29
2-4.Thegeometriclinebroadening(inkeV)of10MeVPhotons 30
6-1.PeaksobservedinneutronirradiatedHPGedetector 82
v
LISTOFFIGURES
Figure page
2-1.Atomicformfactorversusmomentumtransferparameterforelements
carbonandlead(normalizedtoZ2) 20
2-2.Incoherentscatteringfunctionsversusmomentumtransferparameter
forelementscarbonandlead(normalizedtoZ) 23
2-3.Optimalscatteringanglefordifferentdetectingrangesandincident
photonenergy 27
2-4.Geometriclinebroadeningfor100keVphotons 29
2-5.Geometriclinebroadeningfor1000keVphotons 30
2-6.Geometriclinebroadeningfor10MeVphotons 31
2-7.ThePhotonCrossSectionsforLead 32
2-8.Differentialatomiccoherentandincoherentcrosssectionsfor100keV
photonsincarbon 34
2-9.Differentialatomiccoherentandincoherentcrosssectionsfor1250keV
photonsincarbon 35
2-10.Differentialatomiccoherentandincoherentcrosssectionsfor5000keV
photonsincarbon 35
2-11.Differentialatomiccoherentandincoherentcrosssectionsfor100keV
photonsinlead 36
2-12.Differentialatomiccoherentandincoherentcrosssectionsfor1250keV
photonsinlead 36
2-13.Differentialatomiccoherentandincoherentcrosssectionsfor5000keV
photonsinlead 37
vi
33--1.Aschematicelectronicsystemforgammaspectroscopy 45
4-
2.TheExperimentalsetupofincoherentscatteringmethod(nottoscale) 47
1.Measuredscatteredspectrumfromcobalt-60machine 51
4-2.Thereconstructedspectrumfromthemeasuredscatteredspectrum(only
twopeaks)areshown 52
4-3.Primaryphotonpeaksfromcobalt-60 53
4-
45--4.Cobalt-60spectrumwithleadshielding 55
4-5.Cobalt-60spectrumwithoutleadshielding 56
4-6.Cesium-137spectrumwithleadshielding 57
7.Cesium-137spectrumwithoutleadshielding 58
1. MCNPsimulationgeometryforpointbeamandsizebeam 62
5-2.Simulatedspectrumfromcobalt-60pointsourcewith90°scattering
(pointbeamcase) 63
5-3.Simulatedspectrumfromcobalt-60machinewith90°scattering
(sizebeamcase) 64
5-4.SourcedesignofTheratron1000cobalt-60therapymachine 65
5-5.Simulatedprimaryspectrumformcobalt-60machine 66
5-6.TheactualMCNPgeometryofthecobalt-60machineheadand
themeasurementsetup 68
5-7.Theactualsimulationofthecobalt-60machinewith90°scattering 68
5-8.MNCPsimulatedspectrafromcobalt-60with5.0cmthickcopper
collimator,ristheradiusofthepinholeofthecollimator 70
5-9.Simulatedspectrafromcobalt-60with5.0cmthickleadcollimator,
ristheradiusofthepinholeofthecollimator 71
5-10. Photoncrosssectionsforcopperintheenergyrangeof10keV-MeV 72
5-11. Photoncrosssectionforleadintheenergyrangeof10keV-2MeV 73
vii
5-
6-12.Simulatespectrumfromcobalt-60with20.0cmleadcollimator,r=0.6cm 74
1.Photoneutronproductioncrosssectionsfortungsten 80
6-2.Photonneutronproductioncrosssectionsinlead 81
6-3.NeutroninducedspectralcharacteristicsofHPGedetectorinthemazeofa
radiationtherapyroomwithalinearacceleratoroperatingat20MV 83
6-4.GeometryofMCNPsimulationofSL25Linac 85
6-5.MCNPsimulatedprimaryfluxspectrathroughthescattersurfaceandin
thescatter 86
6-6.Comparisonoftheoreticallypredictedscatteredfluxspectrumfromprimaryflux
inthescattererandMCNPsimulatedscatteredfluxspectrum 87
6-7.ComparisonofreconstructedprimaryfluxspectrumandMCNPsimulated
primaryfluxspectrum 88
6-8.Measurementresolutionversusreconstructedresolutionwith90°scattering
bydifferentincidentenergy 89
6-9.Measurementresolutionversusreconstructedresolutionforincident
energyof20MeVwithdifferentscatteringangles 90
vm
AbstractofDissertationPresentedtotheGraduateSchoolofthe
UniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
DEVELOPMENTANDEVALUATIONOFINCOHERENT
SCATTERINGSPECTROSCOPYFORSPATIALANDSPECTRAL
CHARACTERIZATIONOFHIGH-ENERGYX-RAYBEAMS
By
TexinLi
December,2000
Chairman:SamimAnghaie
MajorDepartment:NuclearandRadiologicalEngineering
Whiletheknowledgeaboutthespectralinformationfromhighenergyx-ray
machinesisfundamentalinsomeapplications,likemegavoltagephotontherapy,how
togettheinformationpresentsadauntingtask.Thedifficultyliesinthefactthatthe
detectorisalwaysinundatedwithbillionsofparticles,andwillbesaturatedinstantly.
Inaddition,thedetectingefficiencyfallsoffdrasticallyathighenergy.Toremedy
thissituation,thescatteringmethodisproposedandhasbeentriedonseveral
occasions.Thescatteringmethodcaneffectivelyreducethehighintensityandthe
energyspectrumisshiftedtoasuitablerange.However,inapplyingthismethodtoa
practicalsituation,attentionshouldbepaidtosomecomplicatingfactors.
Inthisresearchproject,acompleteformulationfortheincoherentscattering
methodisgiven;thescatteringmethodisthenappliedtoacobalt-60radiationtherapy
IX
machine,andthescatteredspectrumismeasuredandthentransformedbacktothe
primaryspectrum.WhentheMonteCarlotechnique(MCNP)isusedtosimulatethe
scatteredspectrum,goodagreementisobserved.Thebroadeningeffectinherentto
thescatteringmethodisintroducedanddemonstratedquantitativelyforthefirsttime.
Thebroadeningeffectisthecauseofenergyresolutionlosswhenthemeasured
scatteredspectrumistransformedbacktotheincidentspectrum.Also,theeffectof
thecollimatorisobservedexperimentallyandMonteCarlo(MCNP)simulation
confirmstheobservation.Methodsareproposedtominimizethisunwanted
complicationbasedontheradiologicalpropertiesofmaterialsandthephysicaldesign
ofthecollimator.
Theincoherentscatteringmethodhasbeenappliedtomedicalelectron
acceleratorsusingtheMonteCarlo(MCNP)technique.Theprimaryandthescattered
spectraareobtainedunderidealexperimentalconditions.TheMCNPsimulated
scatteredspectrumistransformedbacktoprimaryspectrumandgoodagreementis
observed.
x