Positron

Introduction

Whenaheatedpositroncollideswithanelectron,itmaytemporarilyformapositronandanegativeelectronbeforeannihilationoccurs.Theneutralboundsystem(e+e-)composedofelectronsiscalledpositronandisrepresentedbyPs.Itcanberegardedasanatom-likeorthesimplesthydrogen.Therearetwotypesofpositrons,oneisnormalo-Ps,e+ande-spinsareparallel,andtheotherissecondarystate,p-Ps,spinsareanti-parallel.TheaverageannihilationlifeofthenormalPsis10^(-7)s(long-livedtripletstate)andthesecondarystateis10^(-10)s(singlestate).Positronscanundergoelectronpick-upprocesses,spinexchangeprocesses,andcertainchemicalreactionprocesses.NormalPscanundergochemicalreactionssuchasoxidation,reduction,andcompoundformationwithmatrixmolecules.

Becauseelectronsandpositronswilleventuallyannihilatetoproducephotons,thehalf-lifeofpositroniumisveryshort.Accordingtothedifferentspinstatesofelectronsandpositrons,therearetwomaintypesofelectrons.Thesingletstate(S₀,thespinisopposite,thetotalspinis0)isthepara-positronium(para-positronium,abbreviatedasp-Ps),thetripletstate(S₁,thespinsareinthesamedirection,thetotalspinis1)orortho-positronium(abbreviatedaso-Ps).Inavacuum,thehalf-lifeofasingletelectronis125ps,andthentwophotons(511keV)areproducedafterannihilation;thehalf-lifeofatripletelectronis142ns,andtheannihilationproducesthreephotons,andsometimesmultiplephotons.Thetotalenergyofphotonsis1022keV,whichisthetotalmassofelectronsandpositrons.

Inthemedium,thehalf-lifeofthetripletelectronwillchangeaccordingly,whichprovidespeoplewithameanstostudythepropertiesofmatter.Theuseofpositronsorelectronevenelementstostudytheinternalpropertiesofmatterhasbecomeaverywidelyusedsubject.ItmainlyusesthefittingofPositronannihilationlifetimespectroscopy(PALS)inthemediumasameasurementmethod,anditisanon-destructivemethod.

Spinsingletandtriplet

Themassofthepositronis1.022MeV,whichistwicethemassoftheelectronminusthebindingenergyofafewelectronvolts.Thegroundstateofpositron,suchasthegroundstateofhydrogen,hastwopossibilitiesdependingontherelativeorientationofthespinsofelectronsandpositrons.

Singlestate,antiparallelspin(S=0,Ms=0)iscalledpair-positron(p-Ps).Ithasanaveragelifetimeof0.125nanoseconds(ns)andpreferentiallyattenuatesintotwogammarays,eachwithanenergyof511keV(atthecenterofmass).Bydetectingthesephotons,thelocationofthedecaycanbefound.Thisprocessisusedforpositronemissiontomography.Thecounter-positroncandecaytoanyeven-numberedphoton(2,4,6,...),buttheprobabilitydropssharplywiththenumber:thebranchratioattenuatedto4photonsis1.439(2)×10^(-6).

Thelifetimeofpositronsinvacuumisabout

Triplestateswithparallelspins(S=1,Ms=-1,0,1),Knownaso-positrons(o-Ps).Theaveragelifetimeis142.05±0.02ns,andthemainattenuationisthreegammarays.Otherdecaymodescanbeignored;forexample,thebranchratioofthefive-photonmodeis~1.0×10^(-6).

Thetruepositronlifetimecanberoughlycalculatedasfollows:

However,thecorrectionofO(α2)​​ismoreaccurateThecalculationresultsinavalueof7040nsfortheattenuationrate,whichcorrespondstoalifetimeof142nss.

Positronsinthe2Sstatearemetastableandhavealifetimeof1100nstopreventannihilation.Thepositrongeneratedinthisexcitedstatewillquicklyconnecttothegroundstate,whereannihilationwilloccurmorequickly.

Naturallyoccurring

Ifprotondecayisareality,high-energypositronswillbepredictedasthemainformofatomicmatterinthefutureuniverse.