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Carl David Anderson Biographical Memoir

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Carl David Anderson Biographical Memoir NATIONAL ACADEMY OF SCIENCES C A R L D A V I D A NDERSON 1905—1991 A Biographical Memoir by WILLIAM H. PICKERING Any opinions expressed in this memoir are those of the author(s) and do not necessarily reflect the views of the National Academy of Sciences. Biographical Memoir COPYRIGHT 1998 NATIONAL ACADEMIES PRESS WASHINGTON D.C. CourtesyofInstituteArchives,CaliforniaInstituteofTechnology,Pasadena CARLDAVIDANDERSON September3,1905–January11,1991 BYWILLIAMH.PICKERING ESTKNOWNFORHISdiscoveryofthepositiveelectron, Borpositron,CarlDavidAndersonwasawardedthe NobelPrizeinphysicsin1936atagethirty-one.Thedis- coveryofthepositronwasthefirstofthenewparticlesof modernphysics.Electronsandprotonshadbeenknown andexperimentedwithforaboutfortyyears,anditwas assumedthatthesewerethebuildingblocksofallmatter. Withthediscoveryofthepositron,anexampleofanti- matter,allmanneroftheoreticalandexperimentalpossi- bilitiesarose.TheRoyalSocietyofLondoncalledCarl’s discovery“oneofthemostmomentousofthecentury.” BornonSeptember3,1905,inNewYorkCity,Carlwas theonlysonofSwedishimmigrantparents.Hisfather,the seniorCarlDavidAnderson,hadbeenintheUnitedStates since1896.WhenCarlwassevenyearsold,thefamilyleft NewYorkforLosAngeles,whereCarlattendedpublic schoolsandin1923enteredtheCaliforniaInstituteof Technology.Caltechhadopeneditsdoorsin1921with RobertA.Millikan,himselfaNobellaureate,aschiefex- ecutive.TogetherwithchemistArthurA.Noyesandas- tronomerGeorgeElleryHale,Millikanestablishedthehigh standardofexcellenceandthesmallstudentbodythat todaycontinuestocharacterizeCaltech. 3 4 BIOGRAPHICALMEMOIRS Carlwasanexcellentstudent.Originallyinterestedin electricalengineering,hechangedtophysicsafterattend- ingaclasswithProfessorIkeBowen.Inhisjunioryearhe wasoneoftwostudentsawardedthehighlycovetedtravel prizeforscholasticachievement.Thisprizeconsistedofa grantsufficientfortwostudentstotravelinEuropeforsix months,fromMarchoftheirjunioryeartothefollowing September,withanitinerarymadeuplargelyoftheirown choosing.Ofthecandidatesconsideredfortheprize,six wereselectedtoattendaspecialclassonthearts,history, andcultureofEurope,givenbyJohnMacArthur,deanof freshmen.Thiswasintendedtohelpthewinnersplanan interestingtrip.Thechosentwospentmostoftheirtime visitingtheprescribedroundofmuseumsandcathedrals, buttheydidgettomeetscientistsH.A.LorentzandH. Kamerlingh-Onnes. Carlgraduatedin1927intheoptionof“physicsengi- neering.”HereceivedhisPh.D.magnacumlaudein1930 inthesameoption.Histhesisonthespatialdistribution ofelectronsejectedfromgasesbyXrays,resultedina PhysicalReviewpublication.(Caltechdroppedtheoption “physicsengineering”in1931andreplaceditwith“ap- pliedphysics”in1934.) MillikanbecameCarl’sgraduateadvisor.Inthosedays graduatestudentshadagreatdealoffreedomintheir research.Consequently,Carlreceivedverylittledirection fromMillikan,butheadvisedCarluponreceivinghisPh.D. togetaNationalResearchCouncilfellowshipandgoto someotherschooltobroadenhisexperience.Carlapplied toComptoninChicago,butafterbeingaccepted,Millikan changedhismindandurgedhimtoremainatCaltechfor atleastanotheryeartostudytheComptonscatteringof cosmicrays.Millikanthoughtthatcosmicrayswereprima- rilyphotonsandthisexperimentwouldgivehiminforma- tionontheenergyoftheradiation. CARLDAVIDANDERSON 5 CarlhadwantedtoextendhisX-raythesisworktogamma rays.Inthis,fortunately,heagreedwithMillikan’sdesire toworkwithcosmicrays,because,exceptforgammarays fromthoriumC”,thesenaturalraysdonothaveenough energytoproducepositrons. Millikanhadbecomeinterestedincosmicraysinthe early1930s.CosmicradiationwasfirststudiedbyVictor HessinAustria,butwasnotwellunderstood.Tostudythe radiationMillikanorganizedthreegroupsatCaltech,which weretouseelectroscopes,Geigercounters,andcloudcham- bersastools.CarlAndersonhandledthecloudchamber investigations.H.VictorNeherdevelopedelectroscopes.I wasresponsiblefortheGeigercounters.TheWilsoncloud chamberisashortcylinderwithglassendplatescontain- ingagassaturatedwithwatervapor.Thepressureisdropped suddenlysothatthegasexpandsandcoolstoasupersatu- ratedstate.Ifanionizingparticlehasjustpassedthrough thechamber,therewillbeatrailofwaterdropletsonthe ionsalongitspath.Thesedropletsarephotographed.The densityofthedropletsisameasureoftheionizationpro- ducedand,therefore,ofthenatureoftheionizingpar- ticle.ForinventingthechamberC.T.R.Wilsonwasawarded aNobelPrizein1927.Carlimprovedhisownchamberby usingapistonexpandingintoavacuumtodropthepres- sureveryrapidlyandbyusingamixtureofwaterand alcoholinthechamber.Consequently,heobtainedmuch betterphotographsthanmostothercloudchamberex- perimenters. Andersonbuilthischamberonthetopfloorofthe aeronauticsbuildingatCaltech,wheretherewasadequate electricpowertorunthelargeelectromagnetsurrounding hischamber.Themagnetprovidedafieldof25,000gauss overanarea6inchesindiameter.Ionizingparticlestravel- ingacrossthisfieldbendintocircularpaths.Bymeasur- 6 BIOGRAPHICALMEMOIRS ingthecurvatureofthetracksinthechamber,hecalcu- latedthemomentumoftheparticlescausingthetrackand determinedthepolarityofthechargeontheparticle.To confirmthedirectionoftraveloftheparticles,heplaced aleadplateacrossthechamber.Particlespassingthrough theplateemergedwithlowerthantheinitialenergy,and thereforethedirectionoftravelwasconfirmed. Atfirst,thecloudchamberexpansionswererandomly timedandmanyphotographsshowednotracks.Later,Geiger counterswereusedtotriggeranexpansionafteracounter recordedthepassageofaparticle.Theresultwasamarked increaseinusefuldata. Earlypicturesshowingtracksofcosmicrayparticleswere asurprise.Thecosmicraysproducedshowersofparticles bothpositivelyandnegativelycharged.Thetwopolarities oftrackshowedthesamedropletdensity.Iftheparticles wereelectronsandprotons,theprotontrackwouldhave beenmuchdenser,exceptatveryhighenergies.Thisex- perimentalresultworriedAndersonaswellasMillikan. Theyevenspeculatedthatthepositivelychargedparticles wereactuallyelectronstravelingintheoppositedirection. TosettlethisquestionCarlputtheleadplateinhischam- ber.Aparticlepassingthroughtheplatelosesenergyand thusthedirectionoftravelisuniquelydetermined. In1932Carlrecordedthehistoricphotographofaposi- tivelychargedelectronpassingthroughtheleadplatein thecenterofthecloudchamber.Thechangeincurvature ofthepathonthetwosidesoftheleadplateshowedthe directionoftravel.Itwasdefinitelyapositivelycharged particle.Surprisingly,theparticlewastravelingupward. Severalwellknownscientists,includingNielsBohr,were veryskepticalofthisresult,butitwassoonconfirmed whenP.M.S.BlackettandG.P.S.Occhialinipublished theirdataintheMarch1933ProceedingsoftheRoyalSociety. CARLDAVIDANDERSON 7 Theyproposedthatthepositronappearedasoneofapair ofpositiveandnegativeelectronsproducedwhenagamma raywasconvertedintomatter.Thepositronhadavery shortlifebeforebeingabsorbedbyacollisionwithan electron,whichproducedtwogammarayphotonsof511 kevenergyeach. Andersonsaidthathediscussedtheproblemofthefor- mationanddisappearanceofthepositronwithJ.Robert Oppenheimerand,inretrospect,wassurprisedthatOppie didnotcomeupwiththispair-productionmechanism.He alsocommentedthatitwasverydifficulttounderstand Oppenheimer’sanswerstohisquestions.Later,whenhe talkedwithRichardP.Feynman,justtheoppositewastrue. Feynmanwasclearandprecise. Anderson’spositronandJ.Chadwick’sneutron,discov- eredandreportedintheProceedingsoftheRoyalSocietyin 1932,werethefirstnewfundamentalparticles.Chadwick alsoreceivedaNobelPrizein1935,ayearbeforeAnder- son.Inafewyearsthesediscoveriesledtothe“zoo”of strangeparticlesofmodernphysics. MillikanhadcarriedelectroscopesandGeigercounters tovariousplacessothathecouldusetheearthasagiant magnettoanalyzetheenergiesofprimarycosmicrays. Carlandhisgraduatestudent,SethNeddermeyer,deter- minedtofollowMillikan’sleadandtaketheircloudcham- bertohighaltitudesandvariouslatitudes.Thecloudcham- berwasmountedonanoldflatbedtruckand,withgreat difficulty,driventothesummitofPike’sPeak.Infact, theyweretowedupmostoftheway.Thetwoexperiment- ersfoundevidenceforanewshort-livedparticleinterme- diateinmassbetweentheelectronandtheproton.This wasoriginallycalledthemesotron,butisnowknownas themumeson.PhotographstakeninPasadenaandin Panamaconfirmedtheexistenceofthisnewparticle. 8 BIOGRAPHICALMEMOIRS IntheDepressiondaysofthe1930sfinancingscientific researchwasdifficult.ThePike’sPeaktripwasdoneona shoestring.Firsttheyfounda1932ton-and-a-halfChevrolet truck,whichtheypurchasedfor$400.Theythenfounda flatbedtrailerandimprovisedahitch.Someoldpacking casesweremadeintoahousingfortheequipment.Arriv- inginColoradoSprings,theyhadthemotoroverhauled andtheclutchreplaced.Evenso,theycouldn’tquitemake ittothetollgateupthemountain.ThePike’sPeakCom- panytowedthemup,probablytocleartheroad. Theirtroubleswerejustbeginning.Theyhadanold Cadillacenginegeneratorsetthatwouldn’tproducead- equatepowerat14,000feet.Whentheytookthegenera- tordowntoColoradoSpringstobefixed,theChevrolet truckbrokedownbecausetherepairworkhadnotbeen doneproperly.However,asluckwouldhaveit,anew Chevrolettesttruckappeared,whichwasbeingmonitored onatripupthemountain.Itcarriedasapassengera GeneralMotorsvice-president,whostoppedtoinquireabout thescientist’sproblem.Hethenverykindlyhadthetruck towedupthehillandsubsequentlyreplacedtheengine freeofcharge.Buttheirtroublesweren’tover.Theycouldn’t affordtopaythe$2.50pernightforthesix-weekstayin theshedontopofthemountain,sotheyboughtaChevy Roadsterfor$50andstayedataroadcrewbunkhouse halfwaydownthemountain.Theywereobviouslytwo dedicatedscientistsandtheywerefirst-classexperiment- ers.DuringtheirmonthandahalfonPikes’Peakthey tookabout10,000photographs. ShortlybeforeAmericaenteredWorldWarII,Compton contactedAndersonandinvitedhimtobethedirectorof whatsoonbecametheatomicbombproject.Comptonsug-
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