Chapter10-+Radiation+Detectors1.ppt

PROFESSIONAL SPECIAL ENGLISHCOURSENUCLEARENERGY Chapter10RADIATIONDETECTORS 輻射探測(cè)器 Measurementofradiationisrequiredinallfacets 方面 ofnuclearenergyinscientificstudies intheoperationofreactorsfortheproductionofelectricpower andforprotectionfromradiationhazard Detectorsareusedtoidentifytheradioactiveproductsofnuclearreactionsandtomeasureneutronflux Theydeterminetheamountofradioisotopesintheairwebreatheandthewaterwedrink ortheuptake 攝取 ofasampleofradioactivematerialadministered 給予 tothehumanbodyfordiagnosis 診斷 P96第二段 Thekindofdetectoremployed 使用 dependsontheparticlestobeobserved electrons gammarays neutrons ionssuchasfissionfragments orcombinations 化合物 Itdependsontheenergyoftheparticles Italsodependsontheradiationenvironmentinwhichthedetectoristobeused atoneendofthescale 測(cè)量數(shù)值范圍 isaminutetraceofaradioactivematerialandattheotherasourceoflargeradiationexposure Thetypeofmeasuringdevice asinallapplications ischosenfortheintended 預(yù)期的 purposeandtheaccuracydesired P96第三段 Thedemandsonadetectorarerelatedtowhatitiswewishtoknow a whetherthereisaradiationfieldpresent b thenumberofnuclearparticlesthatstrikeasurfacepersecondorsomeotherspecifiedperiodoftime c thetypeofparticlespresent andifthereareseveraltypes therelativenumberofeach d theenergyoftheindividualparticles and e theinstant 即刻 aparticlearrivesatthedetector 接上頁(yè) Fromthemeasurementofradiationwecandeduceproperties 性質(zhì) oftheradiationsuchasabilitytopenetrate 穿透 matterandtoproduceionization Wecanalsodeterminepropertiesofaradioactivesource includingdisintegrationrate 衰變率 halflife andamountofmaterial P96末段 Inthischapterwedescribetheimportantfeaturesofafewpopulartypesofdetectors Mostofthemarebasedontheionizationproducedbyincomingradiation Thedetectormayoperateinoneoftwomodes 模式 a current 電流 inwhichanaverageelectricalflowismeasured aswithanammeter 安培計(jì) and b pulse 脈沖 inwhichtheelectricalsignalsproducedbyindividualparticlesorraysareamplifiedandcounted Adetectoroperatinginthismodeisknownasacounter 計(jì)數(shù)器 10 1GASCOUNTERS 氣體計(jì)數(shù)器 Pictureagas filledchamberwithacentralelectrode anode 陽(yáng)極 electricallypositive andaconductingwall cathode 陰極 negative Theyaremaintainedatdifferentpotential asshowninFig 10 1 Ifachargedparticleorgammarayisallowedtoenterthechamber itmayproduceacertainamountofionizationinthegas Theresultantpositiveionsandelectronsareattractedtowardthenegativeandpositivesurfaces respectively P97第二段第六行 Ifthevoltageacrossthetubeislow thechargesmerelymigrate 移動(dòng) throughthegas theyarecollected andacurrentofshortduration apulse passesthroughtheresistor 電阻 andthemeter 儀表 計(jì) Moregenerally amplifyingcircuits 放大電路 arerequired Thenumberofcurrentpulsesisameasureofthenumberofincidentparticlesthatenterthedetector whichisdesignatedas 被稱作為 anionizationchamber 電離室 whenoperatedinthismode P97末段 Ifthevoltageisthenincreasedsufficiently electronsproducedbytheincidentradiation 入射輻射 throughionizationareabletogainenoughspeedtocausefurtherionizationinthegas Mostofthisactionoccursnearthecentralelectrode wheretheelectricfieldishighest Thecurrentpulsesaremuchlargerthanintheionizationchamberbecauseoftheamplificationeffect 接上頁(yè) Thecurrentisproportionaltotheoriginalnumberofelectronsproducedbytheincomingradiation andthedetectorisnowcalledaproportionalcounter 正比計(jì)數(shù)器 Onemaydistinguishbetween 區(qū)別 thepassageofbetaparticlesandalphaparticles whichhavewidelydifferentabilitytoionize Thetimeforcollectionisveryshort oftheorderofmicroseconds 微秒 P98 Ifthevoltageonthetubeisraisedstillhigher aparticleorrayofanyenergywillsetoff 引起 adischarge inwhichthesecondarychargesaresogreatinnumberthattheydominate 支配 theprocess Thedischargestopsofitsownaccord 自動(dòng)地 becauseofthegenerationneartheanodeofpositiveions whichreducetheelectricfieldtheretosuchanextentthatelectronsarenotabletocausefurtherionization Thecurrentpulsesarethenofthesamesize regardlessoftheeventthatinitiated 發(fā)動(dòng) them Inthismodeofoperation thedetectoriscalledaGeiger Muller GM counter 蓋革 米勒計(jì)數(shù)器 P99第三行 Aqualitative 定性的 distinctionbetweentheabovethreetypesofcountersisdisplayedgraphicallyinFig 10 2 whichisasemi logplotofthechargecollectedasafunctionofvoltage Wenotethatthecurrentvariesoverseveralordersofmagnitude 10 2NEUTRONDETECTORS 中子探測(cè)器 Inordertodetectneutrons whichdonotcreateionizationdirectly itisnecessarytoprovideameansforgeneratingthechargesthatcanionizeagas Advantageistakenofthenuclearreactioninvolvingneutronabsorptioninboron 硼 1n 10B 4He 7Li wheretheheliumandlithiumatomsarereleasedasions Oneformofboroncounter 硼計(jì)數(shù)器 isfilledwiththegasborontrifluoride BF3 三氟化物 andoperatedasanionizationchamberoraproportionalcounter Itisespeciallyusefulforthedetectionofthermalneutrons 熱中子 P100第三行 Forthedetectionoffastneutrons upintheMeVrange theprotonrecoilmethod 質(zhì)子反沖法 isused Thescatteringofaneutronbyhydrogenresultsinanenergyloss whichisanenergygainfortheproton Thusahydrogenous 氫的 含氫的 materialsuchasmethane CH4 甲烷 orH2itselfmayserveasthecountergas Theenergeticprotonsplaythesameroleasdidalphaparticlesandfissionfragmentsinthecountersdiscussedpreviously 10 3SCINTILLATIONCOUNTERS 閃爍計(jì)數(shù)器 Thenameofthisdetectorcomesfromthefactthattheinteractionofaparticlewithsomematerialsgivesrisetoascintillation 閃爍 orflash 閃光 oflight Thebasicphenomenonisfamiliar manysubstancescanbestimulated 激勵(lì) toglow 發(fā)光 visiblyonexposuretoultravioletlight 紫外線光 andtheimagesonacolortelevisionscreenaretheresultofelectronbombardment 接上頁(yè) Moleculesofmaterialsclassedasphosphors 磷 磷光體 areexcitedbyradiationsuchaschargedparticlesandsubsequentlyemitpulsesoflight Thesubstancesusedinthescintillationdetectorareinorganic 無(wú)機(jī)的 e g sodiumiodide 碘化鈉 orlithiumiodide ororganic 有機(jī)的 inoneofvariousforms crystalline 晶體 plastic liquid orgas P101第二段 Radiationworkersarerequiredtowearpersonaldetectorscalleddosimeters 劑量計(jì) inordertodeterminetheamountofexposuretoXorgammaraysorneutrons Amongthemostreliable 可靠的 andaccuratetypesisthethermo luminescentdosimeter TLD 熱釋光劑量計(jì) whichmeasurestheenergyofradiationabsorbed ItcontainscrystallinematerialssuchasCaF2orLiFwhichstoreenergyinexcitedstatesofthelattice 晶格 calledtraps 10 4SOLIDSTATEDETECTORS 固體探測(cè)器 Theuseofasolidratherthanagasinadetectorhastheadvantageofcompactness 致密性 duetotheshortrangeofchargedparticles Also whenthesolidisasemiconductor greataccuracyinmeasurementofenergyandarrivaltimeispossible Themechanismofionmotioninasoliddetectorisunique Visualizeacrystalsemiconductor suchassilicon 硅 orgermanium 鍺 asaregulararray 規(guī)則排列 offixedatomswiththeircomplementofelectrons P102第二段 Theelectricalconductivity 電導(dǎo)率 ofasemiconductorisverysensitivetothepresenceofcertainimpurities 雜質(zhì) Considersilicon chemicalvalence 化合價(jià) 4 with4electronsintheoutershell Introductionofasmallamountofvalence5materialsuchasphosphorus 磷 orarsenic 砷 givesanexcessofnegativecarriers andthenewmaterialiscalledn typesilicon Ifinsteadavalence3materialsuchasboronorgallium 鎵 isadded thereisanexcessofpositivecarriers andthesubstanceiscalledp typesilicon Whentwolayersofn typeandp typematerialsareputincontactandavoltageisapplied asinFig 10 5 接上頁(yè) P102倒數(shù)第八行末 Electronsaredrawnonewayandholestheother leavinganeutralor depleted 耗盡 region Mostofthevoltagedrop 電壓降 occursoverthatzone whichisverysensitivetoradiation Anincidentparticlecreateselectron holepairs 電子空穴對(duì) whicharesweptoutbytheinternalelectricfieldtoregisteras 表現(xiàn)為 acurrentpulse P103第三行 Onewaytoproduceasemiconductordetectorwithalargeactivevolume 活性區(qū) istointroducelithiumononesurfaceofaheatedcrystal 晶體 andapplyanelectricfield This drifts theLithroughthevolumewhichcompensates 補(bǔ)償 residualp typeimpurities Thisdetectormustbekeptpermanently 永存地 atliquidnitrogen 液氮 temperature l95 80C topreventredistributionofthelithium 接上頁(yè) P103第七行末 Apreferabledetectorformanyapplicationsismadeofa