1. 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

2. 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

3. ~ 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)

4. 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

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 Thermostatic Chamber Green LED MPPC

6. 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 x10 -19 C C : Pixel capacitance V 0 : Geiger-mode starting voltage Pedestal 1 pix. fired 2 pix. fired 70V, 20 ℃

7. 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

8. 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

9. 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]

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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)

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

17. 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%

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 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