Chapter8- Particle Accelerators.ppt

NUCLEARENERGY PROFESSIONAL SPECIAL ENGLISHCOURSE Chapter8ParticleAccelerators 粒子加速器 Chapter8ParticleAccelerators 粒子加速器 Adevicethatprovidesforcesonchargedparticlesbysomecombinationofelectricandmagneticfieldsandbringstheionstohighspeedandkineticenergyiscalledanaccelerator Manytypeshavebeendevelopedforthestudyofnuclearreactionsandbasicnuclearstructure withanever increasing 持续增长 demandforhigherparticleenergy Inthischapterweshallreviewthenature 性质 oftheforcesonchargesanddescribethearrangement 结构 andprincipleofoperationofseveralimportantkindsofparticleaccelerators 8 1ELECTRICANDMAGNETICFORCES 电磁力 Letusrecallhowchargedparticlesareinfluenced 感应 byelectricandmagneticfields First visualize 想象 apairofparallelmetalplatesseparatedbyadistancedasinthesamplecapacitor 电容器 showninFig 8 l P73第二段第三行中 Apotentialdifference 势差 VandelectricfieldE V dareprovidedtotheregionoflowgaspressurebyadirect current 直流电 voltagesupplysuchasabattery 电池 Ifanelectronofmassmandchargeeisreleasedatthenegativeplate 阴极板 itwillexperienceaforceEe anditsaccelerationwillbeEe m Itwillgainspeed andonreachingthepositiveplate 阳极板 itwillhavereachedakineticenergymv2 2 Ve Next letusintroduceachargedparticleofmassm chargee andspeedvintoaregionwithuniformmagneticfield 均匀磁场 B asinFig 8 2 P73末行尾 Ifthechargeentersinthedirectionofthefieldlines 场力线 itwillnotbeaffected butifitentersperpendicularly p p n dikjul 垂直地 tothefield itwillmoveatconstantspeedonacircle Itsradius calledtheradiusofgyration d ai rei n 回旋半径 isr mv eB suchthatthestrongerthefieldorthelowerthespeed thesmallerwillbetheradiusofmotion Lettheangularspeed 角速度 be omega equaltov r Usingtheformulaforr wefind eB m Ifthechargeentersatsomeotherangle itwillmoveinapathcalledahelix 螺旋状 likeawiredoorspring 金属丝制的门弹簧 P75首段 Finally letusreleaseachargeinaregionwherethemagneticfieldBischangingwithtime Iftheelectronwereinsidethemetalofacircularloop 圆环 ofwireofareaAasinFig 8 3 itwouldexperienceanelectricforceinducedbythechangeinmagneticflux 磁通量 BA Thesameeffectwouldtakeplacewithoutthepresenceofthewire ofcourse 8 2HIGH VOLTAGEMACHINES 高压设备 Onewaytoaccelerateionstohighspeedistoprovidealargepotentialdifferencebetweenasourceofchargesandatarget Ineffect thephenomenonoflightning 闪电 inwhichadischarge 放电 fromchargedcloudstotheearthtakesplace isproducedinthelaboratory Twodevicesofthistypearecommonlyused Thefirstisthevoltagemultiplier 电压倍增器 orCockroft Waltonmachine 考 瓦氏加速器 Thesecondistheelectrostaticgenerator 静电发生器 orVandeGraaffaccelerator 范德格拉夫静电加速器 Fig 8 5 Aninsulated 绝缘的 metalshellisraisedtohighpotentialbybringingitchargeonamovingbelt 传动带 permittingtheaccelerationofpositivechargessuchasprotonsordeuterons Particleenergiesoftheorderof5MeVarepossible withaverysmallspreadinenergy 8 3LINEARACCELERATOR 直线加速器 Ratherthan 胜于 givingachargeonelargeaccelerationwithahighvoltage itcanbebroughttohighspeedbyasuccessionof 一连串 accelerationsthroughrelativelysmallpotentialdifferences asinthelinearaccelerator sketchedinFig 8 6 P73第三行末 Itconsistsofaseriesof 一连串的 acceleratingelectrodes 电极 intheformoftubeswithalternatingelectricpotentialsappliedasshown Anelectronoriongainsenergyinthegapsbetweentubesand drifts 漂流 withoutchangeofenergywhileinsidethetube wherethefieldisnearlyzero Bythetimethechargereachesthenextgap thevoltageisagaincorrectfor 做 的校正 acceleration 8 4CYCLOTRONANDBETATRON Successiveelectricalaccelerationbyelectrodes 电极 andcircularmotion 圆周运动 withinamagneticfieldarecombinedinthecyclotron Fig 8 7 一 CYCLOTRON saikl tr n 回旋加速器 P77第四行末 Twohollow 空的 metalboxescalled dees D字盒 intheshapeoftheletterD aresuppliedwithalternatingvoltages 交流电压 incorrectfrequencyandoppositepolarity 极性 Inthegapbetweendees aniongainsenergyasinthelinearaccelerator 直线加速器 thenmovesonacirclewhileinsidethefield freeregion 无电场区 guidedbythemagneticfield EachcrossingofthegapwithpotentialdifferenceVgivesimpetus 推动力 totheionwithanenergygainVe andtheradiusofmotionincreasesaccordingtor v where eB mistheangularspeed 角速度 接上页 Theuniquefeature 特色 特点 ofthecyclotronisthatthetimerequiredforonecompleterevolution 周转 周期 T 2 isindependentoftheradiusofmotionoftheion Thusitispossibletouseasynchronized si kr naiz 同步的 alternatingpotentialofconstantfrequency angularfrequency 2 toprovideaccelerationattherightinstant 二 BETATRON 电子感应加速器 P78第三行 Electronsarebroughttohighspeedsintheinductionaccelerator 感应加速器 orbetatron Achangingmagneticflux 磁通量 providesanelectricfield 电场 andaforceonthecharges whiletheyareguidedinapathofconstantradius Figure8 8showsthevacuumchamber 真空室 intheformofadoughnut 圆环圈 placedbetweenspeciallyshapedmagneticpoles 磁极 bi t tr n bei Theforceonelectronsofchargeeisinthedirectiontangential tan d n l 切线的 totheorbitofradiusr Therateatwhichtheaveragemagneticfield 磁场 withintheloopchangesis B t providedbyvaryingthecurrentinthecoilsoftheelectromagnet 电磁铁线圈 Themagnitude 大小 数量 oftheforceis 8 5SYNCHROTRON 同步加速器 Amajorsteptowardhigherparticleenergiesresultedfromtheinventionofthesynchrotron si kr tr n Itconsistsoftheperiodicaccelerationoftheparticlesbyradio frequencyelectricfields butwithatime varyingmagneticfieldthatkeepsthechargesonacircularpath Ions thatareoutofstep arebroughtbackintostep i e theyaresynchronized P80首段 Bytheuseofacceleratorsofgreatersophistication s fisti kei n 先进复杂 andhigherparticleenergy manynewsubnuclearparticles 亚核粒子 suchasmesons mes n mi 介子 andxi sigma andlambda particleshavebeendiscoveredandtheinternalstructureofnucleihasbecomebetterunderstood 8 6PRODUCTIONOFNUCLEARFUELS 核燃料生产 Itispossiblethatacceleratorscanbeuseddirectlytohelpsolvetheenergyproblem ExperimentsatCaliforniaradiationlaboratoriesshowedthatlargeneutronyieldswereachievedintargetsbombardedbychargedparticlessuchasdeuteronsorprotonsofseveralhundredsMeVenergy P80第三段接上页 Newdramatic 戏剧性的 生动的 nuclearreactionsareinvolved Oneisthestrippingreaction 剥裂反应 Fig 8 9 a inwhichadeuteronisbrokenintoaprotonandaneutronbytheimpact 碰撞 冲击 onatargetnucleus Anotheristheprocessofspallation sp le n 裂变 蜕变 inwhichanucleusisbrokenintopiecesbyanenergeticprojectile Figure8 9 b showshowacascade k sked 喷流 ofnucleonsisproducedbyspallation Athirdis evaporation I v p rei n 蒸发 inwhichneutronsflyout 冲出 ofanucleuswithsome100MeVofinternalexcitationenergy 激发能 seeFig 8 9 c Theaverageenergyofevaporationofneutronsisabout3MeV Theexcitednucleusmayundergo 经历 fission whichreleasesneutrons andfurtherevaporationfromthefissionfragmentscanoccur P80第三段接上页 Ithasbeenpredicted 预测 thatasmanyas50neutronscanbeproducedbyasinglehigh energy 500MeV deuteron Theneutronscouldbecapturedinisotopessuchasuranium 238orthorium 232toproducenewnuclearfuelsasdiscussedinSection6 3 Ithasalsobeensuggestedthatpartially 部分地 burnednuclearfuelcanbeexposedtoneutronsfromanacceleratortargettobringthefissileisotope 裂变同位素 content 含量 uptooperablelevel P80第四段 8 7SUMMARY Chargedparticlessuchaselectronsandionsoflightelementsarebroughttohighspeedandenergybyparticleaccelerators whichemployelectricandmagneticfieldsinvariousways Inthehigh voltagemachine