Time-of-Propagation Cherenkov counter for particle

arXiv:physics/9904009v1[physics.ins-det]6Apr1999DPNU-99-08Mar.9,1999Time-of-PropagationCherenkovcounterforparticleidentificationM.Akatsu,M.Aoki,K.Fujimoto,Y.Higashino,M.Hirose,K.Inami,A.Ishikawa,T.Matsumoto,K.Misono,I.Nagai,T.Ohshima,A.Sugi,A.Sugiyama,S.Suzuki,M.TomotoandH.OkunobPhysicsDepartment,NagoyaUniversityChikusa,Furo,Nagoya464-8602,JapanbIPNS,HighEnergyAcceleratorResearchOrganization(KEK),Ibaraki305-0801,JapanAbstractWedescribehereanewconceptofaCherenkovdetectorforparticleidentificationbymeansofmeasuringtheTime-of-Propagation(TOP)ofCherenkovphotons.PreprintsubmittedtoElsevierPreprint2February20081IntroductionParticle-identification(PID)capabilityplaysanessentialroleinexperimentsatB-factories.Especially,π/Kidentificationinthemomentumrangeupto4GeV/coverawideangularregioniscruciallyimportantfortheprimaryphysicsgoalstomeasureCPviolation.IntheBelledetectoratKEK-B,acom-binationofAerogelCherenkovcounters,TOFcountersanddE/dXmeasure-mentinacentraldriftchamber(CDC)providesPIDinformationforchargedparticles[1].Althoughthepresentdetectorsystemcoversmostofthemo-mentumregionofthechargedparticles,thePIDpowerforaparticlewithmomentumabove3GeV/cisnotsufficientlysatisfactory.Concerningthefu-tureupgradeofPIDdevices,wediscussheretheeffectivenessofmeasuringboththeTime-of-Propagation(TOP)andtheemission-angleoftheCherenkovphotons.ThetimeresolutionoftheTime-of-Flight(TOF)counterusingaplasticscin-tillatorisinherentlylimitedbythefollowingeffectsbesidesthetransit-timespread(TTS)ofaphototube:(1)afinitedecay-timeofphotonemission,and(2)adifferentphotonpropagation-lengthorpropagation-timeinthescin-tillatortoaphototube,dependingonthephotonemissionangle.Thus,theconventionalTOFcountermeasuresthearrivaltimeoftheearliestphotonsoutofmany,wherebytheremaininglargeamountofphotonsarenoteffec-tivelyusedforthetimemeasurement.Whileeffect(1)cannotbereducedasfarasthescintillationisconcerned,effect(2)canberesolvedbythemea-suringthearrivaltimesoftheearliestphotonsasafunctionofthephotonemissionangle,since,inthetotal-reflection-typescintillatorbar,mostpho-tonspropagateinthescintillatorwhilekeepingtheiroriginalemissionangles.Thistwo-dimensionalinformationcouldimprovethetimeresolution,depend-ingonthenumberofangularsegmentationsofthephotondetectorandonthenumberofphotonscollectedateachsegment.WhenCherenkovradiationisutilized,althoughitsnumberofproducedpho-tonsismuchsmallerthaninthescintillationcase,effect(1)can,inprinciple,bedisregarded.Inaddition,sincetheCherenkovphotonemission-angleisuniquelydeterminedbytheparticlevelocity(β)andsincethepropagationtimeofphotonsinalightguideofaninternal-total-reflectiontypecanbecalculatedasafunctionofthephotonemissionangle,ameasurementofacorrelationbetweenTOPandthephotonemissionanlgecouldprovidePIDinformationbyitself,asweproposebelow.2IntheDIRCconcept[2,3],Cherenkovphotonsproducedbyaquartzradiatoraretransportedtoanendoftheradiatorbymeansofinternaltotalreflection,anditsringimageismagnifiedandprojectedontoaphoton-detectorplaneplacedatthebarend.Abasicstudyofthisconceptclearlyprovedthataphotoniseffectivelytransportedinthelongquartzbar,preservingitsorigi-nalphotondirection.TheBabardetectoratSLACadoptedthisconceptforPID,andintroducedalargestand-offphotondetectionsysteminordertoineffectuatethefinitesizeofthequartzcross-sectionontotheringimage[3].Ontheotherhand,Kamaeetal.[4]proposedacompactfocussingtypeofDIRCusingasmallmirrorinsteadofalargestand-offsystem.Thisapproachsuitsparticlesincidentnormaltothequartzbar,butforinclinedparticlestheringimageisdistortedduetothefinitequartzcross-section.Tomakethisimage-fusingeffectinsignificant,thesizeandgeometryofthefocussingmirrorbecomeunfeasible[5].WeproposehereanewconceptfortheCherenkov-ringimagedetector,theTOPdetector.Thetwo-dimensionalinformationoftheringimageisrepre-sentedbytheTOPoftheCherenkovphotonanditshorizontalangle(Φ;see,Fig.1).TheuseofTOPinformationwithanappropriatefocussingmirrormakestheimage-fusingeffectbedisregardedandcompactificationcanbere-alizedwhilemaintainingthehighenoughPIDability.Wedescribebelowthebasicconcept,characteristicfeatures,practicalconfigurationsandsimulationresultsontheTOPdetector,anddiscusssomeissusesformakingitarealisticdevice.WeshouldmentionapaperbyHonsheidetal.[6],whichwenoticedafterourdetailedanalysishadbeencompleted.ItdiscussestheCherenkovCorrelatedTiming(CCT)detector:CCT[6,7]isakindofTOFcounterwithasinglephototubereadoutfordetectingearlyarrivingphotonsofCherenkovradiation.Inref.[6]asimilarconceptasourscanbefound,althoughnodetailedstudyispresentedandthefocussingmirrorapproachisnotintroduced.2ConceptualDesignandExpectedPropertiesTheprincipalstructureoftheTOPcounterinthe(x,y,z)coordinatesystemisillustratedinFig.1.Whenachargedparticlepassesthroughtheradiatorbar,Cherenkovphotonsareemittedinaconicaldirectiondefinedbytheemissionangle(θC),wherecosθC=1/nβ,n=refractiveindex.Then,photonspropagatetobothoreitherendsbymeansoftotalreflectionontheinternalbarsurface.3Photonspropagatingbackwardarereflectedbyaflatmirrorattheend.Attheforwardend,photonsarehorizontallyfocusedbyabutterfly-shapedmirrorontoapla