原子光谱学(X射线)教学教程电子教案

Chapter16X-raysX-rayswerediscoveredlateinthenineteenthcenturybytheGermanphysicistWilhelmRoentgen.Roentgenwasstudyinglow-pressuredischargesinagasandnoticedthattheanodeofhisdischargetubewasthesourceofenergeticandpenetratingradiation.Thepenetratingpropertiesofx-raysareofgreatpracticalimportantandhavebeenutilizedbymedicalprofessionalmostsincetheyearoftheirdiscovery.X-rayareproducedwhenhigh-velocityelectronsstrikeasolidtarget.(usuallymetal)Chapter16X-raysξ16-1Thegenerateofx-rays1.DeviceforgeneratingX-ray1.theconditionforgeneratingX-rayare:(1).High-velocityelectronsbeam(2).themetaltargetThemetaltargetresistthemotionoftheelectronsandtransfertheirkineticenergytotheX-raysenergy.X-raysarehigh-frequencyelectromagneticwavewhosepossessthewavelengthbetweensome0.05—100Å.Anelectrondischargethroughalow-pressuregasmakesrelativelysmallenergiesavailableforexcitation.Theoutmostatomicelectronisexcitedandtheresultingspectrummainlyinthevisibleandinfraredregion.Amodeofexcitationthatcanejectelectronsfrominnerlevelsresultinx-raysemission.Letusbrieflydescribetheusualmethodofexcitinginnershellelectrons.AndtheninterpretThesituationintermofquantumenergylevels.Chapter16X-raysThenumbersoftheemittedxrayphotonsandthetotalenergyisapproximatelyproportionaltothesquareoftheatomicnumberofthetargetmaterial.HighvoltageappliedtothetubeV:100-102kvcurrentacrossthetubei:102-103mA2.XraygeneratorAtypicalX-raytubeisillustratedinfigurebelow.Abeamofelectronsemittedfromahotcathodeinavacuumisacceleratedbyhigh-voltageandstrikeametaltargetanode.

Fig16-1Am102kvcathodeanodeChapter16X-raysTheimpactenergyofthebeamofelectronsejectsomeelectronsfromtheinnershellsoftheatomsinthetarget,enablingelectronsfromtheoutershellstomaketransitionstotheavailableinnershellsvacancies.TheenergylostbyelectronsthatmakethetransitionsisgivenupasXrays,whoseemissionaccompaniesthetransition.3.ActualfocusareaandeffectivefocusareausuallyeffectivefocusareaissmallerthanActualfocusarea,andisapproximatelyshapetoarectangle.(seeFig16-2)smallerfocusareaforphotographyandlargersmallerfocusareaformedicaltreatment.2.Theintensityandhardnessofxray(1)xrayintensitywhereNiisthenumberofthexrayphotonwithenergyhνi,twowaytoincreaseintensityI:a.tubecurrent

i↑→

Ni

↑→I↑(convenienttobemeasured)b.TubevoltageV↑→νi↑→I↑(hardtobemeasured)Soweusetubecurrent

itomarkingxrayintensitywiththeunitmA.Chapter16X-rays(2)xrayhardnessxrayhardnessmeansxray’spenetratingability,andonlydependsonthewavelengthofthexray.Thatis:V↑→νi↑→penetratingability↑.SoweusetubevoltageVtomarkingxray’shardnesswiththeunitkv.Hardnesstubevoltage（kv）

λmin（Å）

usage verysoft5∼202.50∼0.62

photographyandskintreatmentsoft

20∼100 0.62∼0.12

photographyandfluoroscopyhard

100∼250 0.12∼0.05 deepertissuetreatmentveryhard≥250 ≤0.05 deeptissuetreatmentChapter16X-raysξ16-2xrayspectrumThetotalx-rayemissioniscomposedoftwoseparateparts.Thewavelengthofthecharacteristicx-raythatdependonthematerialoftarget.ThecontinuouspartoftheX-rayradiation,knownasbremsstrahlung(Germanfor“brakingradiation”).Doesnotdependontargetmaterial,butonlyonthemagnitudeofthehighvoltageappliedtothetube.1.Continuous

spectrum(1)principle:itwasknownasbraking

radiation.Asthenameimplies,bremsstrahlunghasitsorigininthebrakingeffectofthetargetontheelectronsintheincidentbeam.Thetargetslowtheseelectrons,andsomeoftheenergylostisgivenoffdirectlyasXray.ThemostenergeticX-rayphotonthatispossiblehasaquantumenergyequaltothekineticenergyofanelectroninthebeam.Thekineticenergyinturnarisefromthetransportofanelectronchargethroughthehighvoltagepotentialdifference.Wethereforehave:maximumphotonenergyChapter16X-rayskineticenergyofelectronCombiningtheseresults,wecanwriteaformulaforthewavelengthofthemostenergeticphoton.h=Planck’sconstantc=velocityoflighte=electronchargeU=voltageacrossthetube2.characteristicx-rayThenotation(symbol)fornamedXraysisshowninFig16-2.Shelldesignation

O

Nαβ

MMseries

αβγL

Lseries

αβγδK

Kseries

Figure16-2Thenotation(symbol)fornamedXraysisshowninFig16-2,

Shelldesignation

ON

αβM

Mseries

αβγLLseries

αβγδ

KKseries

Figure16-2EachseriesofXraylinesinnamedfortheshellinwhichtheexcitationprocesscausesthevacancy.TheKseriesresultsfromtheanelectroninanuppershellmakingadownwardtransitiontoaKshellvacancy,TheLseriesfromadownwardtransitiontoaLshellvacancy,andsoon.AGreeklettersubscript(α,β,γandsoon)indicatethepositionofthelineintheseries.Chapter16X-raysTheKαlineisthefirstlineintheKseries,whichresultfromLshellelectronmakingatransitiontoav