1. 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

2. 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

3. 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+

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
Green LED
MPPC
Thermostatic Chamber

6. 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

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. 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

9. 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.

10. 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

11. 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

12. 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.

13. 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

14. 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)

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