AnewapproachforimprovedtimeandpositionmeasurementsforTOFPETTimestampingofthephotoelectronsusinganalogueSiPMsKDoroudMCSWilliamsCorrespondingauthorEmailaddressKDoroudMARK2DetectordesignThetypicalmet ID: 891533
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1 ContentslistsavailableatNuclearInstrumen
ContentslistsavailableatNuclearInstrumentsandMethodsinPhysicsResearchAjournalhomepage:www.elsevier.com/locate/nima AnewapproachforimprovedtimeandpositionmeasurementsforTOF-PET:Time-stampingofthephoto-electronsusinganalogueSiPMsK.Doroud,M.C.S.Williams Correspondingauthor.E-mailaddress:(K.Doroud). MARK 2.DetectordesignThetypicalmethodofcouplingaphotodetectortoscintillatingcrystalsistohavetwomatchingmatrices,withaone-to-onecouplingbetweenthecrystalandthephotodetector.WeproposeanewgeometrybasedontheStripSiPMSiPMwiththescintillatorcutintoslabs.Fig.1showshoweachslabwillbecoupledtotheStripSiPM.TheSiPMarealsoknownasMultiPixelPhotonCounters(MPPC),sinceHamamatsuhasfabricatedtheStripSiPM,hereafterwerefertoitasStripMPPC.Ingureweshow16stripsreadingoutasingleslab;howeveritshouldbenotedthatthenumberofread-outstripsperslabstillneedstobeoptimised.Sixteenstripsallowsthetime-of-arrivalof16photonstobemeasuredindependently,sothatanalgorithmcanbederivedtoextractthebesttimeresolutionusingthese16values.Inaddition,theamountoflightfallingoneachstripcanbemeasuredsothatthedepthofinteractioncouldbeestimatedfromthisinformation;howeverthefront-endelectronicsusedforthesemeasurementsmeasurestheinputchargewithaTime-over-Threshold(ToT)techniqueandthiswasnotcienttoestimatethedepthofinteraction.3.ResultsfromprototypetestsetupAslabmoduleofsixLFScrystals(eachslabis3×11×15mm)hasbeengluedtoa16StripMPPCarray;thisisshowninFig.2Nasourcewasmountedbetweentheslabmoduleanda
2 referencedetectorconsistingofa3×3×15mm
referencedetectorconsistingofa3×3×15mmLFScrystalcoupledtoa3×3mmThissetupisshowninFig.3.TheStripMPPCswerereadoutwiththeNINOASICSIC,anultrafastamplier-discriminator.TheNINOthresholdissetattheamplitudeofhalfaphotoelectron.Thestripswerereadoutateachend,thus32channelsofNINOwereusedfortheslabmodulereadout.TheLVDSoutputsignalsfromtheNINOwereconvertedtoNIMlogicsignalsusingin-houseelectronics.TheseNIMlogicsignalswerefedtotheWaveCatchersystemmthatusesaswitchedcapacitorarraytorecordtheinputwaveform.A5pstimeresolutionwasobtained.BoththeleadingandtrailingedgeoftheNINOoutputwererecorded.Signalsfromeachstrip,discriminatedbytheNINOcreatestwo(onefromeachend);theaverageofthetime-stampsprovidesthe;whilethetimedierencegivesthepositionalongthestripwherethepulseoflightwasregistered.Eventsweretriggeredbyalargeenergydepositinthereferencecrystalandtheslabmodule.TheamplitudeofthelightpulsewasestimatedbytheNINOasicwithTime-over-ThresholdtechniqueandthiscontrolledthewidthoftheoutputLVDSsignal.TypicalToTspectrafor16stripsandthereferencedetectorareshowninFig.4.AToTcutof2×sigmaofthetofthetwoback-to-back511keVgammawasappliedtothedata.Foreachevent,thefollowingdataisproduced:(a)fortheslabmodule:16hittimes,16positions,16ToTvaluesand(b)forthereference,thehittimeandasingleToTvalue.These16hittimesweresortedintotimeorder;theCTRs(withrespecttothereferencecounter)arepresentedinFig.5.InFig.6CTRobtainedwhentheearliestseventime-stampsareaveraged.Thereferencecounterwas
3 alignedtobeback-to-backwithslab4inthecen
alignedtobeback-to-backwithslab4inthecentreofthemoduleasshowninFig.3.Thetimedibetweenthetwoendscanbeusedtoobtainthelocationalongthestriplengthwherethelightpulsehasbeenobserved:thiscorrespondstotheslabwherethe511keVgammaphotoninteracted.InFig.7weshowtheaveragedtimedierenceforthe16strips.ClearlythecorrectslabcanbeidentiedowingtotheexcellentpositionresolutionoeredbyStripMPPC.4.ConclusionsandoutlookTheStripMPPChasbeendevelopedwiththeideathatmultipleindividualphoto-sensorscanbeattachedtoascintillatingcrystal.Thisallowsthetime-stampingofrstarrivingphotons.The16measurementsoftheamplitudeofthelightcouldbeusedtoidentifythepositionofthe511keVinteraction(includingthedepthofinteraction);howeverweneedtoimplementabettermeasurementofthelightamplitudethanoeredbythesimpleToToftheNINOasic.Thestriparrayusedinthisstudyistherstprototypeofthisdevice.Toextractthefullpotentialwiththistechnique,moreperformantStripMPPCsareneeded.Inparticularthereisa200mdeadregionbetweeneachstripthatreducestheoverallphotondetectioneciency.Newprototypeshavebeenrecentlyfabricatedandareundertest.Thetechniquepresentedhere,dependsonthetimestampingof Fig.1.Schematicrepresentationofcouplingofstrip-SiPMstoanarrayofslabscintillatingcrystals. Fig.2.PhotographofsixLFSslabsgluedontoa16stripMPPCarray. Fig.3.Thecoincidencetimeresolutionmeasurementsetup. LutetiumFineSilicatemanufacturedbyZecotekPhotonicsInc.K.Doroud,M.C.S.Williams individualphoto-electrons.However,theNINOASIChasnotbeeno
4 ptimisedforthehighcapacitanceoftheMPPCan
ptimisedforthehighcapacitanceoftheMPPCandthusthebandwidthoftherststageisreduced.TheeectofthisisthattheNINOresonanaveragetimeofarrivaloftheinitialphotoelectrons(ratherthantherst)t).Anewfront-endASICisunderdesign,thatwillalsohaveanimprovedamplitudemeasurementcircuitry. Fig.4.TypicalspectraobtainedwithtwodetectorsoneachsideofaNasource:TheplotontheleftcorrespondstotheaverageToTspectrumof16strips.ThereferencedetectorconsistedofaHFF-MPPCattachedtoa3×3×3mmLFScrystal;theToTplotisshownontheright. Fig.5.CoincidencetimeresolutionofthestripMPPCscoupledtotheslab-module,arrangedintimeorder. Fig.6.Timespectracreatedbyaveragingtheearliesttimes( Fig.7.Positionoflightpulsealongthestripobtainedfromtheaverageof16timeK.Doroud,M.C.S.Williams ThisworkiscarriedoutintheframeoftheERCTICALproject(grantnumber338953;PI:PaulLecoq).TheauthorsappreciatethefundingthatthisprojectreceivesfromtheEU.[1]E.Venialgo,etal.,Timeestimationwithmultichanneldigitalsiliconphotomulti-pliers,Phys.Med.Biol.60(2015)2435.[2]K.Doroud.,M.C.S.Williams,K.Yamamoto,TheStripSiliconPhoto-Multiplier:aninnovationforenhancedtimeandpositionmeasurement,Nucl.Instrum.MethodsPhys.Res.A,2017.[3]Patent:ADetectorCongurationwithSemiconductorPhotomultiplierStripsanderentialReadout,WO2014180487(A1),2014-11-132014-11-13F.Anghinol,etal.,Nucl.Instrum.MethodsPhys.Res.A533(2004)183[5]D.Breton,J.Maalmi,E.Delagnes,UsingUltraFastAnalogMemoriesforFastPhoto-detectorReadOut,NDIP2011,Lyon.K.Doroud,M.C.S.William