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

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 GLD (Global Large Detector) Calorimeter … a candidate detector for ILC (International Linear Collider) particles readout absorber plate 1 cm x 5cm x 2 mm EM-scintillator-layer model

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

Requirements for the GLD Calorimeter Gain: ~ at least 10 5, preferably 10 6 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 mm 2 – 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 Thermostatic Chamber Green LED MPPC

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

C, V 0 vs. Temperature V 0 = aT +b C looks not sensitive to temperature, at least under < 20 o C V 0 is linear to temperature a = (5.67 ± 0.03) x10 -2 V/ o C b = 66.2 ± 0.1 V V 0 =aT+b

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

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

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 Measurement of uniformity in the sensor

Measurement with Microscopic Laser System １６００ pixel MPPC 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 ~25  m Laser spot

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

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

Gain Uniformity within a Pixel Higher gain in central part Gain variation in a sensitive region ~ 2.7% (RMS) Gain (x10 5 ) y-point (1  m pitch) x-point (1  m pitch) Edge of the sensor V bias = 70.0 V

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

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

Pixel-by-pixel Scan - Gain 3.2(x10 5 ) 3.8 (x10 5 ) edge of the sensor Edge pixels have higher gain Strange structure is seen, reason unknown Variation ~2.4%

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 V bias -V 0 (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