Using MPPCs for T2K Fine Grain Detector Fabrice Retière (TRIUMF) for the FGD group University of British Columbia, Kyoto University, University of Regina,TRIUMF and University of Victoria 1
T2K Fine Grain Detector MPPC � Element of T2K near detector � Active target for neutrino 1 mm Y11 fiber interaction � Elements • Plastic scintillator bar 0.96 mm (POPOP) � 2 meter long µ ν • Light collection with Wavelength Shifting fiber • Readout by Hamamatsu p MPPC • ~10,000 channels 2
FGD physics requirements � 100% efficiency for MIP crossing a bar � Particle identification • By dE/dx for particle crossing the FGD • By range, especially for stopping protons � Large energy released (10 MIPs) • By detecting Michel positrons for stopping π + � Position resolution • Bar width & no information along the bar � Timing resolution • ~ 3ns per neutrino interaction for matching with photons in calorimeter 3
MPPC basic Gain Delta V : 400pixel No.050/100 3 10 × 1400 parameters 1200 1000 15C � Gain > 2 10 5 20C 800 • i.e. 1PE = 2 10 5 e- 25C 600 � Way above typical electronics 400 69.5V noise 200 0.5 1 1.5 2 2.5 Delta V [V] � Photo-detection efficiency PDE Delta V : 400pixel No.050/100 2.2 • Comparable or better than PDE relative to PMT 2 1.8 PMT 1.6 � But need to measure PDE for 1.4 Cross-talk and 1.2 After-pulsing free proper wavelength 1 0.8 69.5V 0.6 0.5 1 1.5 2 2.5 Delta V [V] S. Gomi et al. (Kyoto University) 4
Photo-electron per MIP MPPC fulfill requirements � Beam test at TRIUMF • 120 MeV/c particles � Electrons are minimum ionizing � Worst case scenario • No fiber mirroring • End of the bar � More than 10 direct PE even at 69.5V • No need to run at higher voltage � Issue of Fiber-MPPC coupling still being addressed • New coupler • 1.2x1.2 mm 2 MPPC M. Bryant (UBC), P.Kitching (TRIUMF), S. Yen (TRIUMF) 5
MPPC fulfilling requirements � Quantum efficiency • For 100% efficiency need more than 10 PE per MIP � Go for at least 15 PEs per MIP � Energy resolution. Not directly a MPPC issue • Driven by photon statistics (~25% for 15 PE) � Increase quantum efficiency would help � Timing resolution. • Not a MPPC issue in principle (fast) � Dynamic range • 400 pixels provide more than 50 MIPs dynamic range due to saturation � Nuisances: Dark noise, cross-talk, after-pulsing 6
Reading out MPPCs � Compromise between timing resolution and integration time • Desirable to measure all pulses continuously during beam spill (5 µ s) and about 2 muon decay constant (2.2 µ s) after spill • Chose a waveform digitization solution � Use the Switch Capacitor Array designed for Time Projection Chamber (AFTER ASIC) � Fairly slow shaper (100 ns rise time) � 50 MHz sampling frequency � 512 time bin ~ 10 µ s total integration time 7
Waveforms from MPPC coupled to AFTER ASIC Laser beams ~5 PE ~20PE Dark noise hit Dark noise hit ~60 PE ~60 PE 8
Fulfilling the dynamic range and energy resolution requirements � For calibration need � ASIC noise ~ 2,000 e- • MPPC gain ~ 5 10 5 to identify 1 PE peak • Noise set to 0.2 PE • 0.2 PE noise ⇒ attenuate by ~50 � Maximum dynamic � ASIC dynamic range range = 400 PE • After-pulsing may = 600 fC • Dynamic range 0.2 PE increase beyond 400 pixel to ~200 PE • Need another channel with higher attenuation 9
Coupling AFTER ASIC to MPPC Values of R and C Charge sum are only indicative � Issues • Attenuation Charge pump � High/low input to ASIC 70 V • Low input capacitance R low = 0 Ω � Low electronic noise R bias ~ 100 k Ω • Noise from resistors C low = 1 pF • MPPC recovery MPPC AFTER 10-15 cm ASIC � Require small R bias with purely C dec = 1 nF R high = 0 Ω capacitive termination • Minimize reflections (50 Ω line) • Pulse shape C high = 10 pF DAC � Solution -5V to +5V • Not clear yet. Some answer R gnd = 10 k Ω from Spice simulations • Building a specific 8 channel C gnd = 40 pF prototype 10 D. Bishop, L. Kurchaninov, K. Mizouchi (TRIUMF)
Pulse shape 70 V and recovery R gnd = 10 k Ω R bias 100 k Ω MPPC C gnd = 100 pF 10-15 cm C dec = 1 nF AFTER ASIC C high = 3.3 pF 100 pF to ground 11 N. Jain (Darmouth), and T. Lindner (UBC)
Timing resolution � Obtained by fitting Configuration Resolution for MIP (20 PE) waveforms • Fit rising edge only Simulations + 3 ns � Source of fluctuations waveform fit • Photon arrival time � Fiber and scintillator Data + full 5 ns decay constants waveform fit � Waveform distortion • Dark noise 4 ± 1 ns Data + rising • After-pulsing edge fit � Need to measure after- pulsing to evaluate effect 12 T. Lindner, S. Oser (UBC)
Beyond the gross features Estimating the MPPC Nuisances � Dark noise • Add pulses. Increase data size � But useful for gain calibration • At <500 kHz, does not affect timing and energy resolution � Cross-talk • Marginal worsening of energy resolution (if <20%) • Increase number of PE � May skew timing resolution � After-pulsing • Worsen timing resolution when fitting full waveform 13
Measuring Dark noise, cross-talk and after-pulsing � Fast recovery biasing scheme: no resistance in series � Trigger on Dark noise hits (~0.3 PE threshold) � Use fast amplifier (CAEN N978) � Use 1 GHz digitizer (CAEN V1789) � Search for pulses • Extract MPPC*Amplifier response function • Search for pulses based on rise time + fall time + amplitude criteria • Fit by a superposition of response functions � Add more pulses if poor fit (partial pulse overlap) • Pulse finding is the main source of systematic errors 14
Typical waveforms with after- pulsing test setup 1.06 PE @ 649 0.96 PE @ 653 Pulse adde 0.90 PE @ 669 3.21 PE @ 719 appropriate ∏ Data (70V, 25C) ▼ pulse finder ∏ First pass refit ∏ Refit after splitting 2 missed pulses 2.0 PE (cross-talk) 15
Amplitude vs time for all pulses 70 V, 25C Trigger pulse Cross-talk = 1-N(1PE)/Ntot 16
Hit amplitude vs time 70 V, 25C Trigger pulses Cross-talk region Expected 8.75 ns recovery time constant ⇒ could be lengthened 17
Reducing after-pulsing by playing with recovery time � It is possible to reduce after-pulsing by increasing the recovery time • Resistance in series with bias • Introduce dead time after the pulse � Is there an acceptable compromise? • For the FGD, readout issue may force us to run with a long recovery time � After-pulsing is then automatically reduced � FGD approach • Run a low bias voltage: after-pulsing ~ 10% 18
Separating Dark Noise and after-pulsing 70V 69.5V � Count all hits All hits All hits • No cross-talk • Sensitive to multiple after- pulse � Histogram the time of the 1 st 70 V 69.5 V hit after trigger 1 st hit after trigger 1 st hit after trigger • No cross-talk • No multiples 2 exponential fit 1 exponential fit � But more complicated fit 19
After-pulsing fit results � Fit is impaired by low statistics • 69.5V and 70V have more statistics • Long time constant hard to pin down � Increase of constant in all hits expected • Short time constant 20-30 ns � Dominate the after- pulsing 20
Competing contributions Total after-pulsing Is dark noise really saturating and the visible increase due to after-pulsing? 21
Conclusions � MPPC + AFTER combination fulfill FGD requirements � MPPC nuisances are under control for the FGD application • After-pulsing is dominant � Run MPPC at low bias to avoid significant after-pulsing � Not a problem. Quantum efficiency is large enough � Investigating interplay between recovery, pulse shape, and after-pulsing • Is there an optimum design? 22
Back-up 23
Measuring after-pulsing with gate technique B. Kirby (UBC) 24
Measuring after-pulsing with average Average of all pulses 1PE response function technique Dark noise contribution (fit) Average after dark noise subtraction R. Tacik (U. Regina) After-pulsing contribution (fit) Cross-talk contribution (fit) 25
1 st hit timing distribution fit function ⎡ − ⎤ − t t Ap − ⋅ − ⋅ = − − ⋅ + τ τ DN t DN t ⎢ ⎥ dN / dt e * ( 1 Ap Ap e ) DN e e τ ⎣ ⎦ DN = dark noise rate Ap = After-pulsing probability τ = After-pulsing time constant 26
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