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1、正電子湮沒技術(shù) 及其應(yīng)用PredictionofpositronExistenceofpositronspredictedbyPAMDiracin1928fromrelativistictheoryofelectrons.英國物理學(xué)家P.Dirac(1902---1984)Dirac’sequationimplies:positronmass=electronmasspositroncharge=+ePredictionofantimatterDiscoveryofantimatterAnderson(1932)discoveredthepositronpredicte
2、dbyDiracDiscoveryofantimatterPositivelychargedelectronsdetectedincosmicrayspassingthroughacloudchamberimmersedinamagneticfieldAcloudchamber(Wilsonchamber)wasnormallyusedatthattimetodetecttracksofchargedparticles.Itcontainedasupersaturatedvapour.Whenachargedparticleentersthechamber,itcoll
3、ideswithairoralcoholvapouratoms,producingfreeions(ionisationprocess).Vapourinthechambercondensesaroundthesefreeions,formingdroplets.Thedropletsarewhatformthetrail.ExpansiontypecloudchamberOriginalWilsonchambercloudchamberTracksofparticlesinthecloudchamberParticletrackcanbephotographed.Cl
4、oudchambercanbeplacedinamagneticfield,thusallowingthemeasurementoftheparticlemomentumwhichisinverselyproportionaltothecurvature(曲率)ofthetrackinthemagneticfield.Particlemomentumcomponentperpendiculartothefieldisp(MeV/c)=0.3?10-3B(gauss)r(cm)whereristheradiusofcurvature.Differenttypesofpar
5、ticleswillleavedifferenttrails.Alphaparticles,whicharerelativelyheavy,willproducestraightdensetrails(left).Slowelectronsleavewispy,irregulartrails(centre).Tracksofcosmic-rayparticlesareshownontheright.正電子的基本物理特性正電子是電子的反粒子。又稱陽電子，，一般用符號e+表示。兩者除電荷符號相反外,其他性質(zhì)（靜止質(zhì)量、電荷的電量、自旋）都相同。正電子的來源（Sourceof
6、positron）放射性核素的正β+衰變energypositronbdecayEg=1277keV22Na宇宙射線中的正電子電子對效應(yīng)(Paire+-e-Production)ApassingphotoncanbeperturbedbytheEM-fieldofanatom,causingthephotontodecayintoandelectron(e-)andapositron(e+).Electromagneticenergyisconvertedintomatter(massenergy).Thisiscalledpaircreation.Chargecons
7、erved:0?e-+e+Energyconserved:Minimumphotonenergy(hνmin)iswhenMomentumconserved:Nucleusrequiredtoconservelinearmomentum.(agammaray)能量≥1.02MeV的γ射線與原子核作用可能產(chǎn)生一對正-負(fù)電子。能量轉(zhuǎn)化成質(zhì)量M=E/C2M＋γ→M+e++e-1.02MeVmeme基本條件：Eγ?1.02MeV電子對效應(yīng)(續(xù)上)正-負(fù)電子云霧室徑跡圖正電子湮沒(PositronAnnihilation)正電子進(jìn)入物質(zhì)在短時間內(nèi)迅
