Study of the MPPC Performance - contents - Introduction Fundamental properties microscopic laser scan –check variation within a sensor Summary and plans Sep France-Japan Joint Meeting Satoru Uozum T. Maeda, H. Yamazaki, Y. Sudo for the GLD Calorimeter group 1

~ 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 type photon sensor being developed by Hamamatsu 2

Required performance for the GLD Calorimeter Gain: ~ Best to have 10 6, at least 10 5 Dynamic range: can measure ~1000 p.e. – satisfactory to measure EM shower maximum – need > 2500 pixels Photon Detection Efficiency ~ 30 % – to distinguish MIP signal Noise rate : < 1 MHz (threshold = 0.5 p.e., threshold =1.5 p.e is also acceptable) good uniformity, small cross-talk Timing Resolution ~ 1 nsec – Necessary for bunch ID, slow neutron separation Sensor area: 1.5 x 1.5 mm – to put more number of pixels Should be stable against bias voltage / temperature / time 3

To achieve our goal, we are studying basic properties of the MPPC collaborating with Hamamatsu. Now we are measuring performance of the latest 1600 pixel MPPC sample. Based on its results, we provide feedback to Hamamatsu to have improved samples. Our R&D Status Evaluate performance of the MPPC prototypes Provide feedback to HPK Improved samples from HPK 4

Basic Properties of the 1600 pixel MPPC Check fundamental performance against bias voltage –Gain, Noise Rate, Cross-talk probability –Photon Detection Efficiency, Response curve (measurements still ongoing) Temperature dependence is also measured –MPPC performance is known to be sensitive to temperature Thermostatic Chamber Green LED MPPC 5

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 capacity V 0 : Breakdown voltage 6

C, V 0 against Temperature V 0 = aT +b a = (5.67 ± 0.03) x10 -2 b = 66.2 ± 0.1 C looks not sensitive to temperature in under < 20 o C V 0 is linear to temperature 7

Noise Rate … rate of fake avalanche signal induced by thermal electrons Over voltage [V] = V bias – V 0 (T) ・ 30 ℃ ・ 25 ℃ ・ 20 ℃ ・ 15 ℃ ・ 10 ℃ ・ 0 ℃ ・ - 20 ℃ Lower temperature  Lower noise rate 8

Cross-talk Cross-talk probability looks stable with temperature in over voltage < 2.5V. ・ 30 ℃ ・ 25 ℃ ・ 20 ℃ ・ 15 ℃ ・ 10 ℃ ・ 0 ℃ ・ - 20 ℃ Over voltage [V] = V bias – V 0 (T) 9 Cross-talk to adjacent pixels is caused by photons created in an avalanche. Cross-talk probability is measured from dark noise rates :

Measurement with Microscopic Laser １６００ pixel MPPC YAG Laser, = 532 nm (green) Pulse width ~ 2 nsec, rate ~ 8 kHz Spot size ~ 1  m Light yield = 0~1 p.e. (reduced by filters) Can perform precise pinpoint scan with the well-focused laser ~25  m Laser spot （ before inserting filters ） 10

Using the laser, we perform scan within a pixel pixel-by-pixel scan to see the variation of Gain Photon Detection Efficiency Cross-talk 11

Efficiency v.s. Bias Voltage Inject laser to center of a pixel. The efficiency depends on bias voltage, but is stabilized in V bias > 70 V. Pedestal １ pix. fired 2 pix. fired (cross-talk) 12

Efficiency Variation within a Pixel Fraction of sensitive region ～ 20% Variation within a sensitive region ~9.2% 1 pixel The shape of sensitive region is not changed with bias voltage Bias voltage ・ -71.0V ・ -70.0V ・ -69.5V ・ -69.0V 13

Gain Variation within a Pixel Gain (x10 5 ) Center part have higher gain Gain variation in a sensitive region ~ 2.7% y-point (1  m pitch) x-point (1  m pitch) Edge of a sensor V bias = 70.0 V 14

Cross-talk within a Pixel Shape of the cross-talk probability depends on bias voltage Edge part show larger cross-talk Bias voltage ・ -71.0V ・ -70.0V ・ -69.5V ・ -6 ９.0V Sensitive region in a pixel Pedestal １ pix. fired 2 pix. fired (cross-talk) 15

Pixel-by-pixel Scan Scan pixels in quadrant of a sensor (The quadrant is divided into 4 sub-regions for some technical reasons) Inject laser at the center of each pixel 20 x 20 pixels Sensor 16

Pixel-by-pixel Scan - Efficiency edge of a sensor 17 Total variation ~4.0 ％

Pixel-by-pixel Scan - Gain 3.2(x10 5 ) 3.8 (x10 5 ) edge of a sensor Edge pixels have higher gain Strange oblique structure is seen, reason unknown Total variation ~3.2 ％ 18

Summary We are evaluating the MPPC performance from the viewpoint of the GLD calorimeter readout use The MPPC properties are sensitive to over voltage (=V bias -V 0 (T)), thus affected by temperature change  Need of accurate voltage / temperature control As a result of the microscopic laser scan, the MPPC properties are observed to be uniform within a sensor. Measure P.D.E. and Response curve (Input light yield v.s. output pulse height), I-V curve Figure out device-by-device variation –20 samples will be delivered in next week Plans 19