APD signals

1. The Timing Counterof the Meg Experiment: design and commissioning M. De Gerone* on behalfof the MEG Timing Counter Group *INFN & Universityof Genoa The Timing Counter of the MEG experiment is designed to deliver trigger information and to accurately measure the time of flight of the e+ to search for the rare decay e+. It consists of two sectors located up and downstream the decay target. Each sector is made of two layers: scintillating fibers read by APDsfor trigger and track reconstruction in the inner side, scintillating bars with PMTsreadout for trigger and time measurement in the outer one. Here the design criteria , the calibrations and the performances of the detector reached during two years of run are presented. MEG Experiment: sensitive search for e+ branching ratio. Clearsignatureof the event: photon and positronwithsameenergy (equaltohalf the muon mass) simultaneouslydetectedback-to-back. The TC mustprovide a fast and efficient trigger signal, togheterwith a precise measurementofpositron timing and impact point. The detector issplit in 2identicalmodules, placessimmetricallyrespectto the target tomaximizegeometricalacceptance. Eachmoduleisdivided in 2sub-detectorsarranged in 2layers: the longitudinal (outer) one and the transverse (inner) one, satisfyingdifferentrequirements. Bothsub-detectors are based on fast opticaldevices. Top viewof the MEG layout • The Transverse detector ismadeofanarrayof 256 scintillatingfiberscoupledtoAvalanchePhotoDiodes (APD). • The smallsizeof the fibers (5X5mm) results in goodspatialresolution and perfectly match APD sensitive area. • Fiber light transmissionisimprovedusing a reflectivecoat. • APDs are biassedverycloseto the breakdownvoltagetoobtainorder 103gains. • Frontendelectronicsismounted on detector: signalsfrombunchesof 16 adjacentfibers are sent to trigger system, single fiberssignals are discriminated and provide offline hitmapreconstruction. • The longitudinal detector ismadeby 15 scintillatingbars (Bicron BC404) withdimensions 4x4x80cm3, read-outby a coupleof fine meshPMTs (2” Hamamatsu R5924), stacked in a barrellikeshape. • The averagedtimeofPMTsprovides a high resolutionmeasurementof the impact time, whilepositron impact pointisreconstructedusingpmtchargeratio in online trigger algorithm and timedifference (offline). • A DoubleThresholdDiscriminator system (DTD) isusedtooptimize timing resolution and eventselectionavoidingefficiency loss. Low thresholdis put just outsidenoiseleveltoachieve best timeresolution. Eventsfromsignalpositrons are selectedbyputting a high thresholdaccordingto Landau spectrum (1coupleofthresholdvaluesforallPMTs, required PMT equalization). Picture of Timing Counterwithfibersexposed Trigger (online performances) Eventreconstruction (offline performances) APD signals Having the analogpulsesacquired at 1.6Gs/swe can obtain the meanshapeofeach PMT waveform (template): the TimeWalkisevaluatedfitting the t(V) shape in the regionof interest (i.e. forsignalgreaterthan high levelthreshold) with the function wherexis a normalizedamplitude. Foreach PMT anevent-by-eventcorrectionisappliedusing the analogsignal information and the calculatedfitcoefficients. APD signalsfor a typical MEG event (in this case, wehave2bunchesof 16 APDs hit)… …andcorresponding offline Zreconstruction (Zbar-ZAPD) A PMTscharge/amplitudeequalizationisnecessaryalsotohave a reliablezestimation at trigger levelbyusingchargeratio. The equalizationisachievedusingcosmicrayshitting the center of the bars and tuninggainstoobtainalmostequal (~10%) Landau’speaks. Furtherrefiningmade online at trigger level. Inter barsoffsets due to electronic chain are tobetakeninto account byaligning the cosmicraysdistribution on TC. Effectivevelocityforeach bar isextrapolatedfrom impact pointdistribution and reconstructed bar lenght. Time [ns] The trigger resolutionisevaluatedcomparing the online reconstructed impact pointwith the offline one. Weobtain 7,3 cm r.m.s.asshowed in the picture. Cosmicrayshitmapasseen on TC, before and after the allignment NormalizedAmplitude • Time vs normalizedamplitude plot withfitsuperimposed Finalperformances Zresolutions are calculatedusing triple barsevents, evaluating the distributionsof We obtain σz ~ 1.2cm. Timeresolution upper limitisextrapolatedfrom multiple bar hit: timedifferencesdistributionsforadjacentbars in double and triple barscoincidencesfrom MEG data havesigmas in the range 60-90 ps. The uniformityof the detector isquitegood, everybars show resolutions σt < 80ps for triple bars event. High thresholdstudies: scanfor HLT valuetoobtainefficienteventselection.