正电子概况ppt[兼容模式]

正电子在材料科学中的应用叶邦角中国科学技术大学理学院PDF文件使用pdfFactory试用版本创建3ÿf3www.fineprint.cn内容安排q讲课（~20学时）q见习（6课时）q实验（4课时）q调研（6课时）q个人调研报告（4课时，每人20分钟）PDF文件使用pdfFactory试用版本创建ÿwww.fineprint.cn讲课内容q正电子概况(I)q正电子概况(II)q正电子技术与发展q正电子湮灭技术q正电子储存技术q正电子在半导体材料中的应用q正电子在纳米多孔材料中的应用q正电子用于研究表面和界面q正电子在高聚物中的应用PDF文件使用pdfFactory试用版本创建ÿwww.fineprint.cn正电子概况(I)第一讲正电子发现正电子的特性正电子产生和正电子源正电子偶素PDF文件使用pdfFactory试用版本创建ÿwww.fineprint.cn一、正电子发现1897年J.J.Thomson发现电子1913年E.Rutherforda粒子散射1919年E.Rutherford质子,F.Hess气球实验(宇宙线)1932年Chadwick发现了中子1931-33年Pauly，Fermi对原子核的b衰变谱的解释：中微子(56年实验证实）1932年Anderson发现反电子(e+)1935年H.Yukawa预言存在p介子，Mp~1/7Mp1936年Anderson在宇宙线中发现“介子”（m-轻子）PDF文件使用pdfFactory试用版本创建ÿwww.fineprint.cnElectronscirclethenucleusduetotheElectricforceBohrAtom&RadiationAllowedOrbits12345n=Electroninlowest“allowed”energylevel(n=1)Electroninexcitedstate(n=5)Before12345ElectronfallstothelowestenergylevelAfterRadiatedphotonPDF文件使用pdfFactory试用版本创建fÿfwww.fineprint.cnAtomicRadiationThedifferenceinenergy,ΔE,isgivenby:DE=E5–E1=hn=Ephotonh=Planck’sconstant=6.6x10-34[Js]ν=frequencyoflight[hz]TheenergyofthelightisDIRECTLYPROPORTIONALtothefrequency,ν.Recallthatthefrequency,ν,isrelatedtothewavelengthby:c=nl(n=c/l)So,higherfrequencyèhigherenergyèlowerwavelengthThisiswhyUVradiationbrownsyourskinbutvisiblelightdoesnot!Itisnow“known”thatwhenanelectronisinan“excitedstate”,itspontaneouslydecaystoalower-energystablestate.Beforen=1n=2n=3n=4n=5EnergyElectroninexcitedstate(higherPE)E5E4E2E3E1E5E4E3E2E1Aftern=1n=2n=3n=4n=5EnergyElectroninloweststate(lowerPE)E5E4E2E3E1Oneexamplecouldbe:PDF文件使用pdfFactory试用版本创建ÿÿwww.fineprint.cnHydrogenatomenergy“levels”QuantumphysicsprovidesthetoolstocomputethevaluesofE1,E2,E3,etc…Theresultsare:En=-13.6/n2EnergyLevelEnergyEn(eV)1-13.62-3.43-1.514-0.855-0.54So,thedifferenceinenergybetweenthe3rdand1stquantumstateis:Ediff=E3–E1=-1.51–(-13.6)=12.09(eV)Whenthis3à1atomictransitionoccurs,thisenergyisreleasedintheformofelectromagneticenergy.TheseresultsDODEPENDONTHETYPEOFATOMORMOLECULE12345PDF文件使用pdfFactory试用版本创建www.fineprint.cnSomeOtherQuantumTransitionsInitialStateFinalStateEnergydiff.[eV]Energydiff.[J]Wavelength[nm]Region2110.21.6x10-18121X-ray3112.11.9x10-18102X-ray4112.82.0x10-1897X-ray321.893.0x10-19654Red422.554.1x10-19485Aqua522.864.6x10-19432VioletPDF文件使用pdfFactory试用版本创建www.fineprint.cnCosmicRaysqCosmicRaysareenergeticparticlesthatimpingeonouratmosphere.qTheycomefromalldirections.qWhenthesehighenergyparticlesstrikeatoms/moleculesinouratmosphere,theyproduceasprayofparticles.qMany“exotic”particlescanbecreated.Aslongastheyarenotsomassiveastoviolateenergyconservation.qSomeoftheseparticlesareunstableand“decay”quicklyintootherstableparticles.qAnyoftheseexoticparticlescanreachthesurfaceoftheearth.PDF文件使用pdfFactory试用版本创建ÿwww.fineprint.cnDiscoveriesinCosmicRaysØ1932:Discoveryoftheantiparticleoftheelectron,thepositron.Confirmedtheexistenceandpredictionthatanti-matterdoesexist!!!Ø1937:Discoveryofthemuon.It’sverymuchlikea“heavyelectron”.Ø1947:Discoveryofthepion.We’lltouchonthesetoday…andsomeotherthings…PDF文件使用pdfFactory试用版本创建ÿwww.fineprint.cnDirac•In1929PaulDiracmanagedtocombinequantummechanicsandspecialrelativityTheenergyofaparticlecanbegivenby:•Butthisequationleavesbothapositiveandanegativesolution22422cpcmEo+=2242cpcmEo+±=PDF文件使用pdfFactory试用版本创建www.fineprint.cnPDF文件使用pdfFactory试用版本创建www.fineprint.cnNegativeEnergyqRatherthanignoringthenegativeenergystate,aswouldbeexpectedinclassicalphysics,Diracdecidedtodealwiththeprobleminaquantummechanicalsense.qTheamplitudeofastreamofparticlescanberepresentedbytheplanewavefunction:qButthisequationcanalsorepresentparticleswithnegativeenergyandmomentumtravellingbackintime.hpxEtiAe/)(--=yPDF文件使用pdfFactory试用版本创建33www.fineprint.cnPositronDiscoveryLeadplatePositronA“CloudChamber”iscapableofdetectingchargedparticlesastheypassthroughit.Thechamberissurroundedbyamagnet.Themagnetbendspositivelychargedparticlesinonedirection,andnegativelychargedparticlesintheotherdirection.Byexaminingthecurvatureaboveandbelowtheleadplate,wecandeduce:(a)theparticleistravelingupwardinthisphotograph.(b)it’schargeispositiveUsingotherinformationabouthowfarittraveled,itcanbededucedit’snotaproton.CloudChamberPhotographLargercurvatureofparticleaboveplatemeansit’smovingslower(lostenergyasitpassedthrough)It’saparticlewho’smassissameaselectronbuthaspositivechargeèPOSITRON!PDF文件使用pdfFactory试用版本创建f駿fwww.fineprint.cnExpansiontypecloudchamberOriginalWilsonchamberqAndersonusedcloudchamberina15kGmagneticfield.qThechamberwasdividedintotwopartsbya6-mmleadplate.PDF文件使用pdfFactory试用版本创建ÿwww.fineprint.cnTracksofparticlesinthecloudchamberPDF文件使用pdfFactory试用版本创建ÿwww.fineprint.cnC.D.Anderson.Thepositiveelectron,Phys.Rev.,43,491(1933).PDF文件使用pdfFactory试用版本创建ÿwww.fineprint.cnSignificanceofPositronDiscoveryThepositrondiscoverywasthefirstevidenceforANTIMATTER.Thatis,thepositronhasessentiallyallthesamepropertiesasanelectron,except,it’schargeispositive!Ifanelectronandapositroncollide,theyANNIHILATEandformpureenergy(EMRadiation).Thisconversionofmatterintoenergyisacommoneventinthelifeofphysici