核專業(yè)英語的學(xué)習(xí)教案

核專業(yè)英語的學(xué)習(xí)教案第1頁/共28頁Chapter8ParticleAccelerators

(粒子加速器)Adevicethatprovidesforcesonchargedparticlesbysomecombinationofelectricandmagneticfieldsandbringstheionstohighspeedandkineticenergyiscalledanaccelerator.Manytypeshavebeendevelopedforthestudyofnuclearreactionsandbasicnuclearstructure,withanever-increasing(持續(xù)增長)

demandforhigherparticleenergy.Inthischapterweshallreviewthenature(性質(zhì))

oftheforcesonchargesanddescribethearrangement(結(jié)構(gòu))

andprincipleofoperationofseveralimportantkindsofparticleaccelerators.第2頁/共28頁8.1ELECTRICANDMAGNETIC

FORCES(電磁力)Letusrecallhowchargedparticlesareinfluenced(感應(yīng))

byelectricandmagneticfields.First,visualize(想象)

apairofparallelmetalplatesseparatedbyadistancedasinthesamplecapacitor(電容器)

showninFig.8.l.

第3頁/共28頁P73第二段第三行中Apotentialdifference(勢差)

VandelectricfieldE=V/dareprovidedtotheregionoflowgaspressurebyadirect-current(直流電)

voltagesupplysuchasabattery(電池).Ifanelectronofmassmandchargeeisreleasedatthenegativeplate(陰極板),itwillexperienceaforceEe,anditsaccelerationwillbeEe/m.Itwillgainspeed,andonreachingthepositiveplate(陽極板)

itwillhavereachedakineticenergymv2/2

=Ve.第4頁/共28頁Next,letusintroduceachargedparticleofmassm,chargee,andspeedvintoaregionwithuniformmagneticfield(均勻磁場)

B,asinFig.8.2.第5頁/共28頁P73末行尾Ifthechargeentersinthedirectionofthefieldlines(場力線),itwillnotbeaffected,butifitentersperpendicularly(垂直地)

tothefield,itwillmoveatconstantspeedonacircle.Itsradius,calledtheradiusofgyration(旋轉(zhuǎn)),isr=mv/eB,suchthatthestrongerthefieldorthelowerthespeed,thesmallerwillbetheradiusofmotion.Lettheangularspeed(角速度)

be(omega),equaltov/r.Usingtheformulaforr,wefind=eB/m.Ifthechargeentersatsomeotherangle,itwillmoveinapathcalledahelix(螺旋狀)

,likeawiredoorspring(金屬絲制的門彈簧).第6頁/共28頁P75首段Finally,letusreleaseachargeinaregionwherethemagneticfieldBischangingwithtime.Iftheelectronwereinsidethemetalofacircularloop(圓環(huán))

ofwireofareaAasinFig.8.3,itwouldexperienceanelectricforceinducedbythechangeinmagneticflux(磁通量)

BA.Thesameeffectwouldtakeplacewithoutthepresenceofthewire,ofcourse.第7頁/共28頁8.2HIGH-VOLTAGEMACHINES

(高壓設(shè)備)Onewaytoaccelerateionstohighspeedistoprovidealargepotentialdifferencebetweenasourceofchargesandatarget.Ineffect,thephenomenonoflightning(閃電),inwhichadischarge(放電)

fromchargedcloudstotheearthtakesplace,isproducedinthelaboratory.Twodevicesofthistypearecommonlyused.

Thefirstisthevoltagemultiplier(電壓倍增器)

orCockroft-Waltonmachine.第8頁/共28頁Thesecondistheelectrostaticgenerator(靜電發(fā)生器)

orVandeGraaffaccelerator

(Fig.8.5).Aninsulated(絕緣的)

metalshellisraisedtohighpotentialbybringingitchargeonamovingbelt(傳動帶),permittingtheaccelerationofpositivechargessuchasprotonsordeuterons.Particleenergiesoftheorderof5MeVarepossible,withaverysmallspreadinenergy.第9頁/共28頁8.3

LINEARACCELERATOR

(直線加速器)Ratherthan(勝于)

givingachargeonelargeaccelerationwithahighvoltage,itcanbebroughttohighspeedbyasuccessionof(一連串)

accelerationsthroughrelativelysmallpotentialdifferences,asinthelinearaccelerator,sketchedinFig-8.6.

第10頁/共28頁P73第三行末Itconsistsofaseriesof(一連串的)

acceleratingelectrodes(電極)

intheformoftubeswithalternatingelectricpotentialsappliedasshown.Anelectronoriongainsenergyinthegapsbetweentubesand“drifts(漂流)”withoutchangeofenergywhileinsidethetube,wherethefieldisnearlyzero.Bythetimethechargereachesthenextgap,thevoltageisagaincorrectfor(做…的校正)

acceleration.第11頁/共28頁8.4CYCLOTRONAND

BETATRON

(一)CYCLOTRON(回旋加速器)Successiveelectricalaccelerationbyelectrodes(電極)

andcircularmotion(圓周運動)

withinamagneticfieldarecombinedinthecyclotron.(Fig.8.7)第12頁/共28頁P77第四行末Twohollow(空的)

metalboxescalled“dees(D字盒)”(intheshapeoftheletterD)aresuppliedwithalternatingvoltages(交流電壓)

incorrectfrequencyandoppositepolarity(極性).Inthegapbetweendees,aniongainsenergyasinthelinearaccelerator(直線加速器),thenmovesonacirclewhileinsidethefield-freeregion(無電場區(qū)),guidedbythemagneticfield.EachcrossingofthegapwithpotentialdifferenceVgivesimpetus(推動力)

totheionwithanenergygainVe,andtheradiusofmotionincreasesaccordingtor=v/ω,whereω=eB/mistheangularspeed(角速度).第13頁/共28頁接上頁Theuniquefeature(特色,特點)

ofthecyclotronisthatthetimerequiredforonecompleterevolution(周轉(zhuǎn),周期),T=2π/ω,isindependentoftheradiusofmotionoftheion.Thusitispossibletouseasynchronized(同步的)

alternatingpotentialofconstantfrequencyν,angularfrequencyω=2πν,toprovideaccelerationattherightinstant.第14頁/共28頁(二)BETATRON(電子感應(yīng)加速器)(P78第三行)Electronsarebroughttohighspeedsintheinductionaccelerator(感應(yīng)加速器)

orbetatron.Achangingmagneticflux(磁通量)

providesanelectricfield(電場)

andaforceonthecharges,whiletheyareguidedinapathofconstantradius.Figure8.8showsthevacuumchamber(真空室)

intheformofadoughnut(圓環(huán)圈)

placedbetweenspeciallyshapedmagneticpoles(磁極).

第15頁/共28頁第16頁/共28頁Theforceonelectronsofchargeeisinthedirectiontangential(切線的)

totheorbitofradiusr.Therateatwhichtheaveragemagneticfield(磁場)

withintheloopchangesis△B/△t,providedbyvaryingthecurrentinthecoilsoftheelectromagnet(電磁石卷).Themagnitude(大小,數(shù)量)

oftheforceis第17頁/共28頁8.5SYNCHROTRON(同步加速器)Amajorsteptowardhigherparticleenergiesresultedfromtheinventionofthesynchrotron.Itconsistsoftheperiodicaccelerationoftheparticlesbyradio-frequencyelectricfields,butwithatime-varyingmagneticfieldthatkeepsthechargesonacircularpath.Ions(thatareoutofstep)arebroughtbackintostep,i.e.,theyaresynchronized.第18頁/共28頁P80首段Bytheuseofacceleratorsofgreatersophisticationandhigherparticleenergy,manynewsubnuclearparticles(亞核粒子)

suchasmesons(介子)

andxi,sigma,andlambdaparticleshavebeendiscoveredandtheinternalstructureofnucleihasbecomebetterunderstood.第19頁/共28頁8.6PRODUCTIONOFNUCLEARFUELS

(核燃料生產(chǎn))Itispossiblethatacceleratorscanbeuseddirectlytohelpsolvetheenergyproblem.ExperimentsatCaliforniaradiationlaboratoriesshowedthatlargeneutronyieldswereachievedintargetsbombardedbychargedparticlessuchasdeuteronsorprotonsofseveralhundredsMeVenergy.第20頁/共28頁P80第三段接上頁Newdramatic(戲劇性的,生動的)

nuclearreactionsareinvolved.Oneisthestrippingreaction(剝裂反應(yīng)),Fig.8.9(a),inwhichadeuteronisbrokenintoaprotonandaneutronbytheimpact(碰撞,沖擊)

onatargetnucleus.Anotheristheprocessofspallation(裂變,蛻變)inwhichanucleusisbrokenintopiecesbyanenergeticprojectile.Figure8.9(b)showshowacascade(噴流)

ofnucleonsisproducedbyspallation.第21頁/共28頁Athirdis“evaporation(蒸發(fā))”inwhichneutronsflyout(沖出)

ofanucleuswithsome100MeVofinternalexcitationenergy,seeFig.8.9(c).Theaverageenergyofevaporationofneutronsisabout3MeV.Theexcitednucleusmayundergo(經(jīng)歷)

fission,whichreleasesneutrons,andfurtherevaporationfromthefissionfragmentscanoccur.P80第三段接上頁第22頁/共28頁第23頁/共28頁Ithasbeenpredicted(預(yù)言)

thatasmanyas50neutronscanbeproducedbyasinglehigh-energy(500MeV)deuteron.Theneutronscouldbecapturedinisotopessuchasuranium-238orthorium-232toproducenewnuclearfuelsasdiscussedinSection6.3.Ithasalsobeensuggestedthatpartially(部分地)

burnednuclearfuelcanbeexposedtoneutronsfromanacceleratortargettobringthefissileisotope(裂變同位素)

content(滿足)

uptooperablelevel.P80第四段第24頁/共28頁8.7SUMMARYChargedparticlessuchaselectronsandionsoflightelementsarebroughttohighspeedandenergybyparticleaccelerators,whichemployelectricandmagneticfieldsinvariousways.Inthehigh-voltagemachinesabeamofionsisaccelera