Using MPPCs for T2K Fine Grain Detector Fabrice Retire (TRIUMF) - - PowerPoint PPT Presentation

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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

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T2K Fine Grain Detector

Element of T2K near detector Active target for neutrino interaction Elements

• Plastic scintillator bar

(POPOP)

2 meter long

• Light collection with

Wavelength Shifting fiber

• Readout by Hamamatsu

MPPC

• ~10,000 channels

µ p 1 mm Y11 fiber 0.96 mm MPPC ν

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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

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MPPC basic parameters

Gain > 2 105

• i.e. 1PE = 2 105 e-

Way above typical electronics noise

Photo-detection efficiency

• Comparable or better than

PMT

But need to measure PDE for proper wavelength

• S. Gomi et al. (Kyoto University)

Delta V [V] 0.5 1 1.5 2 2.5 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2

PDE Delta V : 400pixel No.050/100

Delta V [V] 0.5 1 1.5 2 2.5 200 400 600 800 1000 1200 1400

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10 ×

Gain Delta V : 400pixel No.050/100

15C 20C 25C PDE relative to PMT 69.5V 69.5V Cross-talk and After-pulsing free

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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 mm2 MPPC
• M. Bryant (UBC), P.Kitching (TRIUMF), S. Yen (TRIUMF)

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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

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Reading out MPPCs

Compromise between timing resolution and integration time

• Desirable to measure all pulses continuously

during beam spill (5 µs) and about 2 muon decay constant (2.2 µs) 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 µs total integration time

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Waveforms from MPPC coupled to AFTER ASIC

Laser beams Dark noise hit Dark noise hit ~60 PE ~20PE ~5 PE ~60 PE

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Fulfilling the dynamic range and energy resolution requirements

For calibration need to identify 1 PE peak

• Noise set to 0.2 PE

Maximum dynamic range = 400 PE

• After-pulsing may

increase beyond 400 pixel

ASIC noise ~ 2,000 e-

• MPPC gain ~ 5 105
• 0.2 PE noise ⇒

attenuate by ~50

ASIC dynamic range = 600 fC

• Dynamic range 0.2 PE

to ~200 PE

• Need another channel

with higher attenuation

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Coupling AFTER ASIC to MPPC

Issues

• Attenuation

High/low input to ASIC

• Low input capacitance

Low electronic noise

• Noise from resistors
• MPPC recovery

Require small Rbias with purely capacitive termination

• Minimize reflections (50 Ω line)
• Pulse shape

Solution

• Not clear yet. Some answer

from Spice simulations

• Building a specific 8 channel

prototype

MPPC Charge pump 70 V

10-15 cm

DAC

• 5V to +5V

Charge sum Rbias ~ 100 kΩ Cdec = 1 nF AFTER ASIC Clow = 1 pF Chigh = 10 pF Cgnd = 40 pF Rgnd = 10 kΩ Rhigh = 0 Ω Rlow = 0 Ω

Values of R and C are only indicative

• D. Bishop, L. Kurchaninov, K. Mizouchi (TRIUMF)

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Pulse shape and recovery

MPPC 70 V

10-15 cm Rbias 100 kΩ Cdec = 1 nF Chigh = 3.3 pF Cgnd = 100 pF Rgnd = 10 kΩ

AFTER ASIC 100 pF to ground

• N. Jain (Darmouth), and T. Lindner (UBC)

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Timing resolution

Obtained by fitting waveforms

• Fit rising edge only

Source of fluctuations

• Photon arrival time

Fiber and scintillator decay constants

Waveform distortion

• Dark noise
• After-pulsing

Need to measure after- pulsing to evaluate effect 4±1 ns Data + rising edge fit 5 ns Data + full waveform fit 3 ns Simulations + waveform fit Resolution for MIP (20 PE) Configuration

• T. Lindner, S. Oser (UBC)

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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

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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

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Typical waveforms with after- pulsing test setup

2 missed pulses Pulse adde appropriate 2.0 PE (cross-talk) ∏ Data (70V, 25C) ▼ pulse finder ∏ First pass refit ∏ Refit after splitting

1.06 PE @ 649 0.96 PE @ 653 0.90 PE @ 669 3.21 PE @ 719

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Amplitude vs time for all pulses

Trigger pulse Cross-talk = 1-N(1PE)/Ntot 70 V, 25C

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Hit amplitude vs time

70 V, 25C Expected 8.75 ns recovery time constant ⇒ could be lengthened Trigger pulses Cross-talk region

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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%

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Separating Dark Noise and after-pulsing

Count all hits

• No cross-talk
• Sensitive to

multiple after- pulse

Histogram the time of the 1st hit after trigger

• No cross-talk
• No multiples

But more complicated fit

70V All hits 69.5V All hits 70 V 1st hit after trigger 69.5 V 1st hit after trigger 2 exponential fit 1 exponential fit

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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

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Competing contributions

Total after-pulsing Is dark noise really saturating and the visible increase due to after-pulsing?

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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?

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Back-up

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Measuring after-pulsing with gate technique

• B. Kirby (UBC)

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Measuring after-pulsing with average technique

Average of all pulses 1PE response function Dark noise contribution (fit) Average after dark noise subtraction After-pulsing contribution (fit) Cross-talk contribution (fit)

• R. Tacik (U. Regina)

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1st hit timing distribution fit function

t DN t t t DN

e e Ap DN e Ap Ap e dt dN

⋅ − − − ⋅ −

+ ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⋅ − − =

τ τ

τ ) 1 ( * /

DN = dark noise rate Ap = After-pulsing probability τ = After-pulsing time constant