Chapter10-+Radiation+Detectors1.ppt

PROFESSIONAL SPECIAL ENGLISHCOURSENUCLEARENERGY Chapter10RADIATIONDETECTORS 辐射探测器 Measurementofradiationisrequiredinallfacets 方面 ofnuclearenergyinscientificstudies intheoperationofreactorsfortheproductionofelectricpower andforprotectionfromradiationhazard Detectorsareusedtoidentifytheradioactiveproductsofnuclearreactionsandtomeasureneutronflux Theydeterminetheamountofradioisotopesintheairwebreatheandthewaterwedrink ortheuptake 摄取 ofasampleofradioactivematerialadministered 给予 tothehumanbodyfordiagnosis 诊断 P96第二段 Thekindofdetectoremployed 使用 dependsontheparticlestobeobserved electrons gammarays neutrons ionssuchasfissionfragments orcombinations 化合物 Itdependsontheenergyoftheparticles Italsodependsontheradiationenvironmentinwhichthedetectoristobeused atoneendofthescale 测量数值范围 isaminutetraceofaradioactivematerialandattheotherasourceoflargeradiationexposure Thetypeofmeasuringdevice asinallapplications ischosenfortheintended 预期的 purposeandtheaccuracydesired P96第三段 Thedemandsonadetectorarerelatedtowhatitiswewishtoknow a whetherthereisaradiationfieldpresent b thenumberofnuclearparticlesthatstrikeasurfacepersecondorsomeotherspecifiedperiodoftime c thetypeofparticlespresent andifthereareseveraltypes therelativenumberofeach d theenergyoftheindividualparticles and e theinstant 即刻 aparticlearrivesatthedetector 接上页 Fromthemeasurementofradiationwecandeduceproperties 性质 oftheradiationsuchasabilitytopenetrate 穿透 matterandtoproduceionization Wecanalsodeterminepropertiesofaradioactivesource includingdisintegrationrate 衰变率 halflife andamountofmaterial P96末段 Inthischapterwedescribetheimportantfeaturesofafewpopulartypesofdetectors Mostofthemarebasedontheionizationproducedbyincomingradiation Thedetectormayoperateinoneoftwomodes 模式 a current 电流 inwhichanaverageelectricalflowismeasured aswithanammeter 安培计 and b pulse 脉冲 inwhichtheelectricalsignalsproducedbyindividualparticlesorraysareamplifiedandcounted Adetectoroperatinginthismodeisknownasacounter 计数器 10 1GASCOUNTERS 气体计数器 Pictureagas filledchamberwithacentralelectrode anode 阳极 electricallypositive andaconductingwall cathode 阴极 negative Theyaremaintainedatdifferentpotential asshowninFig 10 1 Ifachargedparticleorgammarayisallowedtoenterthechamber itmayproduceacertainamountofionizationinthegas Theresultantpositiveionsandelectronsareattractedtowardthenegativeandpositivesurfaces respectively P97第二段第六行 Ifthevoltageacrossthetubeislow thechargesmerelymigrate 移动 throughthegas theyarecollected andacurrentofshortduration apulse passesthroughtheresistor 电阻 andthemeter 仪表 计 Moregenerally amplifyingcircuits 放大电路 arerequired Thenumberofcurrentpulsesisameasureofthenumberofincidentparticlesthatenterthedetector whichisdesignatedas 被称作为 anionizationchamber 电离室 whenoperatedinthismode P97末段 Ifthevoltageisthenincreasedsufficiently electronsproducedbytheincidentradiation 入射辐射 throughionizationareabletogainenoughspeedtocausefurtherionizationinthegas Mostofthisactionoccursnearthecentralelectrode wheretheelectricfieldishighest Thecurrentpulsesaremuchlargerthanintheionizationchamberbecauseoftheamplificationeffect 接上页 Thecurrentisproportionaltotheoriginalnumberofelectronsproducedbytheincomingradiation andthedetectorisnowcalledaproportionalcounter 正比计数器 Onemaydistinguishbetween 区别 thepassageofbetaparticlesandalphaparticles whichhavewidelydifferentabilitytoionize Thetimeforcollectionisveryshort oftheorderofmicroseconds 微秒 P98 Ifthevoltageonthetubeisraisedstillhigher aparticleorrayofanyenergywillsetoff 引起 adischarge inwhichthesecondarychargesaresogreatinnumberthattheydominate 支配 theprocess Thedischargestopsofitsownaccord 自动地 becauseofthegenerationneartheanodeofpositiveions whichreducetheelectricfieldtheretosuchanextentthatelectronsarenotabletocausefurtherionization Thecurrentpulsesarethenofthesamesize regardlessoftheeventthatinitiated 发动 them Inthismodeofoperation thedetectoriscalledaGeiger Muller GM counter 盖革 米勒计数器 P99第三行 Aqualitative 定性的 distinctionbetweentheabovethreetypesofcountersisdisplayedgraphicallyinFig 10 2 whichisasemi logplotofthechargecollectedasafunctionofvoltage Wenotethatthecurrentvariesoverseveralordersofmagnitude 10 2NEUTRONDETECTORS 中子探测器 Inordertodetectneutrons whichdonotcreateionizationdirectly itisnecessarytoprovideameansforgeneratingthechargesthatcanionizeagas Advantageistakenofthenuclearreactioninvolvingneutronabsorptioninboron 硼 1n 10B 4He 7Li wheretheheliumandlithiumatomsarereleasedasions Oneformofboroncounter 硼计数器 isfilledwiththegasborontrifluoride BF3 三氟化物 andoperatedasanionizationchamberoraproportionalcounter Itisespeciallyusefulforthedetectionofthermalneutrons 热中子 P100第三行 Forthedetectionoffastneutrons upintheMeVrange theprotonrecoilmethod 质子反冲法 isused Thescatteringofaneutronbyhydrogenresultsinanenergyloss whichisanenergygainfortheproton Thusahydrogenous 氢的 含氢的 materialsuchasmethane CH4 甲烷 orH2itselfmayserveasthecountergas Theenergeticprotonsplaythesameroleasdidalphaparticlesandfissionfragmentsinthecountersdiscussedpreviously 10 3SCINTILLATIONCOUNTERS 闪烁计数器 Thenameofthisdetectorcomesfromthefactthattheinteractionofaparticlewithsomematerialsgivesrisetoascintillation 闪烁 orflash 闪光 oflight Thebasicphenomenonisfamiliar manysubstancescanbestimulated 激励 toglow 发光 visiblyonexposuretoultravioletlight 紫外线光 andtheimagesonacolortelevisionscreenaretheresultofelectronbombardment 接上页 Moleculesofmaterialsclassedasphosphors 磷 磷光体 areexcitedbyradiationsuchaschargedparticlesandsubsequentlyemitpulsesoflight Thesubstancesusedinthescintillationdetectorareinorganic 无机的 e g sodiumiodide 碘化钠 orlithiumiodide ororganic 有机的 inoneofvariousforms crystalline 晶体 plastic liquid orgas P101第二段 Radiationworkersarerequiredtowearpersonaldetectorscalleddosimeters 剂量计 inordertodeterminetheamountofexposuretoXorgammaraysorneutrons Amongthemostreliable 可靠的 andaccuratetypesisthethermo luminescentdosimeter TLD 热释光剂量计 whichmeasurestheenergyofradiationabsorbed ItcontainscrystallinematerialssuchasCaF2orLiFwhichstoreenergyinexcitedstatesofthelattice 晶格 calledtraps 10 4SOLIDSTATEDETECTORS 固体探测器 Theuseofasolidratherthanagasinadetectorhastheadvantageofcompactness 致密性 duetotheshortrangeofchargedparticles Also whenthesolidisasemiconductor greataccuracyinmeasurementofenergyandarrivaltimeispossible Themechanismofionmotioninasoliddetectorisunique Visualizeacrystalsemiconductor suchassilicon 硅 orgermanium 锗 asaregulararray 规则排列 offixedatomswiththeircomplementofelectrons P102第二段 Theelectricalconductivity 电导率 ofasemiconductorisverysensitivetothepresenceofcertainimpurities 杂质 Considersilicon chemicalvalence 化合价 4 with4electronsintheoutershell Introductionofasmallamountofvalence5materialsuchasphosphorus 磷 orarsenic 砷 givesanexcessofnegativecarriers andthenewmaterialiscalledn typesilicon Ifinsteadavalence3materialsuchasboronorgallium 镓 isadded thereisanexcessofpositivecarriers andthesubstanceiscalledp typesilicon Whentwolayersofn typeandp typematerialsareputincontactandavoltageisapplied asinFig 10 5 接上页 P102倒数第八行末 Electronsaredrawnonewayandholestheother leavinganeutralor depleted 耗尽 region Mostofthevoltagedrop 电压降 occursoverthatzone whichisverysensitivetoradiation Anincidentparticlecreateselectron holepairs 电子空穴对 whicharesweptoutbytheinternalelectricfieldtoregisteras 表现为 acurrentpulse P103第三行 Onewaytoproduceasemiconductordetectorwithalargeactivevolume 活性区 istointroducelithiumononesurfaceofaheatedcrystal 晶体 andapplyanelectricfield This drifts theLithroughthevolumewhichcompensates 补偿 residualp typeimpurities Thisdetectormustbekeptpermanently 永存地 atliquidnitrogen 液氮 temperature l95 80C topreventredistributionofthelithium 接上页 P103第七行末 Apreferabledetectorformanyapplicationsismadeofa