The MPPC Study for the GLD Calorimeter Readout

• 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

• 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 sensorwhich 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 plate1 cm x 5cm x 2 mm

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-

Guard ring n-

Al conductorp+ 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 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

Thermostatic Chamber

Green LED MPPC

Gain measurement

)( oBias VVe

CGain

・30℃・25℃・20℃・15℃・10℃・ 0℃・ -20℃

d

eA

dSGain

S : ADC sensitivity = 0.25 pC/ADCcount

A : Amp gain = 63e : electron charge = 1.6 x10-19 C

C : Pixel capacitanceV0: Geiger-mode starting voltage

Pedestal

1 pix. fired

2 pix. fired

70V, 20℃

C, V0 vs. Temperature

V0 = aT +b

• C looks not sensitive to temperature, at least under < 20oC

• V0 is linear to temperature

a = (5.67 ± 0.03) x10-2 V/oCb = 66.2 ± 0.1 V

V0=aT+b

Noise Rate … rate of avalanche signals induced by thermal electrons

Vbias – V0(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 Vbias – V0 < 2.5V.

.).5.0(

.).5.1(

epRate

epRatePcrosstalk

The cross-talk to adjacent pixelsis caused by photons created inan avalanche.

Cross-talk probability ismeasured from dark noise rates :

・30℃・25℃・20℃・15℃・10℃・ 0℃・ -20℃

Vbias – V0(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

1 pixel

Measurement of uniformityin 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

~25m

Laser spot

)(

.).5.0(

allN

epNP

ev

evHit

Hit fraction vs. Bias Voltage• Inject laser to center of a pixel.

The hit fraction depends on bias voltage,but is stabilized in Vbias > 70 V.

Pedestal

1 pix. fired

2 pix. fired (cross-talk)

Hit

frac

tion

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

prob

abili

ty

Gain Uniformity within a Pixel

•Higher gain in central part•Gain variation in a sensitive region ~ 2.7% (RMS)

Gain (x105)

y-po

int

(1

m p

itch)

x-point (1 m pitch)

Edge of the sensor

Vbias = 70.0 V

Cross-talk Variation within a Pixel

.)2(.)1(

.)2(

pixNpixN

pixNP

evev

evXtalk

• 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

0.44

0.55

edge of the sensor

Variation ~3.2%

20 x 20 pixels

Sensor

Pixel-by-pixel Scan - Gain

3.2(x105)

3.8 (x105)

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 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