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

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 particles readout absorber plate 1 cm x 5cm x 2 mm EM-scintillator-layer model Need a new photon sensor which is compact and low-cost, can operate in a strong magnetic field

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

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

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 Green LED MPPC Thermostatic Chamber

Gain measurement C : Pixel capacitance V0: Geiger-mode starting voltage 70V, 20℃ d 30oC 25oC 20oC 15oC 10oC 0oC -20oC 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

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

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

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

Measurement of uniformity in 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 fraction Cross-talk 1 pixel

Measurement with Microscopic Laser System Introduced by KEK-DTP YAG Laser,  = 532 nm (green) Pulse width ~ 2 nsec, 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 1600 pixel MPPC ~25 m Laser spot

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

Uniformity within a Pixel Bias voltage -71.0V -70.0V -69.5V -69.0V Hit probability 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

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

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

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

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)

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