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The MPPC Study for the GLD Calorimeter Readout

Introduction Measurement of basic characteristics Gain, Noise Rate, Cross-talk Measurement of uniformity with microscopic laser Summary and plans. 2006/10/31 Takashi Maeda Institute of Physics, University of Tsukuba for KEK-DTP photon sensor group for the GLD Calorimeter group.

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The MPPC Study for the GLD Calorimeter Readout

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  1. Introduction Measurement of basic characteristics Gain, Noise Rate, Cross-talk Measurement of uniformitywith microscopic laser Summary and plans 2006/10/31 Takashi Maeda Institute of Physics, University of Tsukuba for KEK-DTP photon sensor group for the GLD Calorimeter group The MPPC Study for the GLD Calorimeter Readout

  2. EM-scintillator-layer model absorber plate 1 cm x 5cm x 2 mm 1 cm x 5cm x 2 mm particles readout GLD (Global Large Detector) Calorimeter… a candidate detector for ILC (International Linear Collider) • Sampling calorimeter with Pb/W - scintillator sandwich structure with WLSF readout • Particle Flow Algorithm (PFA) needs particle separation in the calorimeter • Fine granularity with strip/tile scintillators • Huge number of readout channels • ~10M (ECAL) + 4M (HCAL) ! • Used inside 3 Tesla solenoid Need a new photon sensor which is compact and low-cost, can operate in a strong magnetic field

  3. The Multi-Pixel Photon Counter (MPPC) …novel photon sensor being developed by Hamamatsu Photonics (HPK) 400 pixels ~ 1 mm 20~100 mm ~ 8 mm Depletion region ~ 2 mm Substrate Si Resistor 1600 pixels Bias voltage (~70V) Guard ring n- n+ p+ Al conductor p- substrate p+

  4. Requirements for the GLD Calorimeter • Gain: ~ at least 105, preferably 106 • Dynamic range: up to ~1000 p.e. (need > 2500 pixels) • to measure EM shower maximum • Single Photon Detection Efficiency: ~ 30 % • to identify MIP signals • Noise rate : < 1 MHz (threshold = 0.5 p.e.) • Good uniformity, small cross-talk • Timing Resolution ~ 1 nsec • Sensor area: 1.5 x 1.5 mm2 • to place a larger number of pixels • Should be stable against bias voltage / temperature / time

  5. Characteristics of the 1600-pixel MPPC • Evaluate performance as a function of bias voltage • Gain, Noise Rate, Cross-talk probability • Photon Detection Efficiency, Linearity(measurements still ongoing) • Temperature dependence is also measured • MPPC performance is known to be sensitive to temperature MPPC Green LED Thermostatic Chamber

  6. Gain measurement C : Pixel capacitance V0: Geiger-mode starting voltage 70V, 20℃ d ・30℃ ・25℃ ・20℃ ・15℃ ・10℃ ・0℃ ・-20℃ 2 pix. fired Pedestal 1 pix. fired S : ADC sensitivity = 0.25 pC/ADCcount A : Amp gain = 63 e : electron charge = 1.6 x10-19 C

  7. C, V0 vs. Temperature • C looks not sensitive to temperature, at least under < 20oC • V0 is linear to temperature V0=aT+b a = (5.67 ± 0.03) x10-2 V/oC b = 66.2 ± 0.1 V V0 = aT +b

  8. Vbias – V0(T) [V] Noise Rate 1MHz … rate of avalanche signals induced by thermal electrons ・30℃ ・25℃ ・20℃ ・15℃ ・10℃ ・0℃ ・-20℃ Lower temperature a Lower noise rate

  9. ・30℃ ・25℃ ・20℃ ・15℃ ・10℃ ・0℃ ・-20℃ Vbias – V0(T) [V] Cross-talk The cross-talk to adjacent pixels is caused by photons created in an avalanche. • Cross-talk probability looks stable with temperature in Vbias – V0 < 2.5V. Cross-talk probability ismeasured from dark noise rates :

  10. Measurement of uniformityin the sensor 1 pixel Using a microscopic laser system we perform • scan within a pixel • pixel-by-pixel scan to see the variation of • Gain • Hit probability • Cross-talk 1 pixel

  11. １６００ pixel MPPC ~25 m • Introduced by KEK-DTP • YAG Laser,  = 532 nm (green) • Pulse width~2nsec, rate ~ 8 kHz • Spot size ~ 1m • Light yield ~ 0.5 p.e. (not calibrated) • Can perform precise pinpoint scan with the well-focused laser • Measurement with Microscopic Laser System Laser spot

  12. Hit fraction Hit fraction vs. Bias Voltage Pedestal • Inject laser to center of a pixel. 1 pix. fired 2 pix. fired (cross-talk) The hit fraction depends on bias voltage, but is stabilized in Vbias > 70 V.

  13. Bias voltage ・ -71.0V ・ -70.0V ・ -69.5V ・ -69.0V Hit probability Uniformity within a Pixel 1 pixel • Fraction of sensitive region ～20% • Variation within a sensitive region ~9.2% (RMS) • The shape of sensitive region is not changed with bias voltage

  14. Gain (x105) y-point (1 mm pitch) x-point (1 mm pitch) Gain Uniformity within a Pixel Vbias = 70.0 V Edge of the sensor • Higher gain in central part • Gain variation in a sensitive region ~ 2.7% (RMS)

  15. Cross-talk Variation within a Pixel Sensitive region in a pixel Bias voltage ・ -71.0V ・ -70.0V ・ -69.5V ・ -69.0V Pedestal １ pix. fired 2 pix. fired (cross-talk) • Shape of the cross-talk probability • depends on bias voltage • Edge part shows larger cross-talk

  16. Pixel-by-pixel Scan - Hit fraction edge of the sensor Variation ~3.2% 20 x 20 pixels 0.55 Sensor 0.44

  17. Pixel-by-pixel Scan - Gain edge of the sensor • Edge pixels have • higher gain • Strange • structure is seen, • reason unknown • Variation ~2.4% 3.8 (x105) 3.2(x105)

  18. Summary • We are evaluating the MPPC performance from viewpoint of the GLD calorimeter readout use • Gain, Noise rate, Cross-talk are acceptable • The MPPC properties are sensitive to Vbias-V0(T)and temperature • Lower Noise rate and Cross-talk with lower temperature • The MPPC properties are observed to be uniform within a sensor. • Measure photon detection efficiency and Linearity • Perform same measurements for new MPPC samples and evaluate device-by-device variation(We just have been provided new samples by HPK) Plans

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