次世代ガンマ線望遠鏡CTA用SiPMの最適化
田島宏康, 山根暢仁, 奥村 曉, 朝野 彰, 中村 裕樹, 日高 直哉, 他 CTA-Japan consortium
日本物理学会 2017年秋季大会
宇都宮大学, September 12–15, 2017

Cherenkov Telescope Array❖ Observations of gamma rays in 20 GeV – 300 TeV band
✤ Cherenkov light from electromagnetic shower produced by interaction ofgamma rays with atmosphere❖ Large collection area by placing many telescopes✤ ×10 better sensitivity than current instruments❖ Wide energy band coverage by three different sizes of telescopes✤ Large-sized telescope (LST): Φ = 23 m, 20 GeV – 1 TeV, 4 telescopes✤ Medium-sized telescope (MST): Φ = 10 – 12 m, 0.1 – 10 TeV, ~20 telescopes✤ Small-sized telescope (SST): Φ = 4 m, 1 – >300 TeV, 50 – 70 telescopes
all SSTs are placed at south site
LST23 mMST
10 – 12 m
SST−1M

ASTRI

GCT
G. Pérez, IAC, SMM

次世代ガンマ線望遠鏡CTA用SiPMの最適化
日本物理学会 2017年秋季大会, SEP 12–15, 2017, 宇都宮大学

2 /16

CTA SST Telescopes❖ SST-1M (single mirror)✤ Czech Republic, Ireland, Poland, Swiss❖ SST-2M (dual mirror)✤ Astrofisica con Specchi a Tecnologia Replicante Italiana (ASTRI)✦ Italy, Brazil, South Africa

SST-1M✤ Gamma-ray Cherenkov Telescope (GCT)✦ Australia, France, Germany, Japan, Netherlands, UKASTRIGCT

次世代ガンマ線望遠鏡CTA用SiPMの最適化
日本物理学会 2017年秋季大会, SEP 12–15, 2017, 宇都宮大学

3 /16

Dual Mirror SST Design Concept❖ Dual mirror design allowing use of compact camera✤ Schwarzschild-Couder (SC) optics✦ Short focal length to realize small plate scale (small camera, pixel)✦ Large field of view
• Greater telescope spacing (larger collection area)✦ Technically challenging✤ Small pixel (6–7 mm) photon sensor to reduce camera cost
✦ Multi-anode photomultiplier (MAPMT) or Silicon Photomultiplier (SiPM)✦ High density readout electronics (ASIC)GCTASTRIcamera

camera~4 m~4 m

次世代ガンマ線望遠鏡CTA用SiPMの最適化
日本物理学会 2017年秋季大会, SEP 12–15, 2017, 宇都宮大学

4 /16

Comparison with Single-Mirror Camera
ASTRI camera

SST-1M camera~50 cm10.9°
37 modules/camera
2,368 pixels/camera
0.19° (7 mm)/pixel

88 cm
9.1°
108 modules/camera
1,296 pixels
0.25° (24 mm)/pixel

credit: SST-1M
次世代ガンマ線望遠鏡CTA用SiPMの最適化
日本物理学会 2017年秋季大会, SEP 12–15, 2017, 宇都宮大学

GCT camera~35 cm9.1°

32 modules/camera
2,048 pixels/camera
0.15–0.18° (6–7 mm)/pixel
5 /16

Requirements for Photodetector
M1❖ Properties of Cherenkov photons from gamma-ray air shower✤ ~500 photons/m2 for 10 TeV gamma-ray shower✤ Several photons per pixel✤ Cherenkov photons peaks around ~350 nm

SecondarymirrorCamera
Focal Plane

Primarymirror

4.00 m2.00 m✦ Blue to near UV sensitivity is important

M2~0.35 m
3.05 m✤ Angular range for incident photon is 30–60°✤ Cherenkov photons arrives within few to few tens of ns✦ ns-timing is important❖ Night sky background (NSB) is the dominant background✤ Rate is >25 MHz/pixel
3.56 m✦ Dark count rate is not very important✦ [NSB] x [Optical crosstalk (OCT)]
can cause false triggers
due to accidental coincidences• Low OCT rate is important

Cherenkov
spectrum

NSBspectrum✤ NSB peaks above 550 nm✦ Low red sensitivity is preferred❖ Pixel size < 0.25 deg is required to obtain
good angular resolution of air showers✤ Pixel size ~ 6 mm with 4-m telescope
次世代ガンマ線望遠鏡CTA用SiPMの最適化
日本物理学会 2017年秋季大会, SEP 12–15, 2017, 宇都宮大学

300

400

500

600

700
6 /16

Photodetector❖ Silicon Photomultiplier is chosen as a photodetector for SST✤ Cost per channel✤ Photon detection efficiency✤ Tolerance against high rate environment (> 25 MHz per pixel)✤ Reliability
credit: HPK website❖ Major drawback of SiPM
✤ Optical crosstalk (OCT)✦ High rate night sky background (NSB) + OCT
can cause false triggers due to accidentalcoincidences✤ Gain dependence on the temperature✤ High sensitivities for red light (NSB wavelength)❖ Main objective of CTA SiPM development✤ Suppress OCT while retaining
photon detection efficiency (PDE)

credit: KETEK website✦ Add trenches✦ Optimize protection coating
次世代ガンマ線望遠鏡CTA用SiPMの最適化
日本物理学会 2017年秋季大会, SEP 12–15, 2017, 宇都宮大学

7 /16

Test Samples
Product ID

Pixel size Cell size

Technology

Short name

Fill factor

S12572-050C

3 mm

50 µm

Standard

REF-3050-S

62%

S13360-3050CS

3 mm

50 µm

LCT5

LCT5-3050-S

74%

S13360-3050VE

3 mm

50 µm

LCT5, 100 µm epoxy

LCT5-3050-E10074%

S13360-3050PE

3 mm

50 µm

LCT5, 300 µm epoxy

LCT5-3050-E30074%

S13360-6050CS

6 mm

50 µm

LCT5LCT5-6050-S

74%

S13360-3075CS

3 mm

75 µm

LCT5LCT5-3075-S

82%

S13360-6075CS

6 mm

75 µm

LCT5

LCT5-6075-S

82%

LVR-3050CS

3 mm

50 µm

LVR

LVR-3050-S

74%

LVR-6050CS

6 mm

50 µm

LVR

LVR-6050-S

74%

LVR-6075CS

6 mm

75 µm

LVR

LVR-6075-S

82%

LVR-7050CS

7 mm

50 µm

LVR

LVR-7050-S

74%

LVR2-6050CS

6 mm

50 µm

LVR2

LVR2-6050-S

74%

LVR2-6050CN

6 mm

50 µm

LVR2, no coating

LVR2-6050-N

74%

LVR2-7050CS

7 mm

50 µm

LVR2

LVR2-7050-S

74%

LVR2-7050CN

7 mm

50 µm

LVR2, no coating

LVR2-7050-N

74%

We have tested SensL and FBK SiPMs as well as Hamamatsu SiPMs.
Only Hamamatsu SiPMs are shown for easy comparisons
次世代ガンマ線望遠鏡CTA用SiPMの最適化
日本物理学会 2017年秋季大会, SEP 12–15, 2017, 宇都宮大学

LCT: Low Crosstalk
LVR: Low Breakdown Voltage
8 /16

SiPM Measurement Setup at Nagoya❖ Take waveform data by digital oscilloscope8✤ Offline data analysis✦ Digital filter to minimize
the effect of pile ups✦ Pulse analysis

V (mV)

6❖ Light output is monitored❖ Wavelength is fixed at 405 nmfor this measurement

4310–1050

100

150

200

250

300

350

400

450500t (ns)collimator

ND filter

LED52Oscilloscope(2.5 GSps)

Pulse Generator— Raw data
— Filtered data

7fiberdiffuser次世代ガンマ線望遠鏡CTA用SiPMの最適化
日本物理学会 2017年秋季大会, SEP 12–15, 2017, 宇都宮大学

SiPM amp

thermal chamber (25°C)
9 /16

PDE Measurements❖ We measure number of photons for short LED (or laser) pulses✤ Current measurement does not provide accurate PDE due tooptical crosstalk, delayed cross talk and after pulse❖ Number of photo electrons (p.e.) does not follow Poisson distribution
due to optical crosstalk✤ Probability of 0 p.e. is used to obtain the average to avoid effect of
optical crosstalk✤ Effect of dark count still need
to be taken into accountP (n) = e µ µn /n!
P (0) = e µµ = ln(P (0))
Ptrue (0) = PON (0)/POFF (0)❖ Common between Nagoya
and Catania
次世代ガンマ線望遠鏡CTA用SiPMの最適化
日本物理学会 2017年秋季大会, SEP 12–15, 2017, 宇都宮大学

0 p.e.

1 p.e.

2 p.e.

3 p.e.

10 /16

Optical Crosstalk Measurements❖ Assume 1 p.e. peak of dark signal is dominated by dark count✤ 2 p.e. peak consists of optical crosstalk from 1 p.e. and chancecoincidence of dark counts✦ Assume chance coincidence of dark counts follow Poisson statistics(small correction for most cases)N(

1.5 p.e.)
Ntotal= P (1)ROCT + P (2) + P (3) + · · ·Ntotal⇡ P (1)ROCT + P (2) + P (3),P (1) = µP (0),µ2P (2) =P (0),2µ3
P (3) =P (0),6N ( 1.5 p.e.)
ROCT ⇡µP (0)NtotalN(≧1.5 p.e.)µ2µ2
60 p.e.

次世代ガンマ線望遠鏡CTA用SiPMの最適化
日本物理学会 2017年秋季大会, SEP 12–15, 2017, 宇都宮大学

1 p.e.

2 p.e.

3 p.e.

11 /16

PDE vs. Over Voltage❖ If we take PDE normalized by fill factor as a function of relative
over voltage, the curve are very similar among different SiPM
❖ LVR is slightly better than others❖ Differences among individual SiPMs are small — 3050 ▲ REF— 3075 ◆ LCT5— 6050 ● LVR— 6075 ◼ LVR2— 7050 ◻︎︎ LVR2 (no coating)

λ = 405 nm
90(Photon Detection Efficiency)/(Fill Factor) (%)

70

Photon Detection Efficiency (%)

60
50403020

Before normalizationof PDE and OV

1000246
Over Voltage (%)

81012

次世代ガンマ線望遠鏡CTA用SiPMの最適化
日本物理学会 2017年秋季大会, SEP 12–15, 2017, 宇都宮大学

807060504030

With normalization ofPDE and OV

201000.00

0.05

0.10
0.15
0.20
(Over Voltage)/(Breakdown Voltage)

0.250.30

12 /16

Crosstalk Rate vs. Over Voltage❖ Factor out cell capacitance dependence of crosstalk rate by scaling
it with cell area and depth(assuming cell depth ∝ break down voltage)— 3050▲ REF— 3075— 6050— 6075— 7050✤ 3 mm pixel gives lower OCT than 6 mm pixel✦ OCT propagates partly via protection coating✤ LVR is worse than LCT5 and LVR2
✤ Differences among individual SiPMs are small
40

40

Before scalingby cell area and depth

302520151030
2520151055000246Over Voltage (V)8After scalingby cell area and depth

35

Scaled Optical Crosstalk Rate (%)

35

Optical Crosstalk Rate (%)◆ LCT5● LVR◼ LVR210

次世代ガンマ線望遠鏡CTA用SiPMの最適化
日本物理学会 2017年秋季大会, SEP 12–15, 2017, 宇都宮大学

0

2

4

6
Over Voltage (V)81013 /16

Crosstalk Dependence on CoatingThickness❖ Thicker coating or no coating give lower crosstalk✤ Further optimization of coating thickness is in progress
Crosstalk Rate vs Over Voltage
25
LCT5-3050 Epoxy 100 µm #1
LCT5-3050 Epoxy 100 µm #2
LCT5-3050 Epoxy 300 µm #665
LCT5-3050 Epoxy 300 µm #666
LCT5-3050 Silicone 450 µm #965
LCT5-3050 Silicone 450 µm #966
LVR2-6050 No coating #15
LVR2-6050 No coating #14
LVR2-7050 No coating #11

Optical Crosstalk Rate (%)

2015

10

500246
Over Voltage (V)次世代ガンマ線望遠鏡CTA用SiPMの最適化
日本物理学会 2017年秋季大会, SEP 12–15, 2017, 宇都宮大学

81014 /16

PDE vs. Crosstalk❖ LVR2-6050 and LVR2-7050 with no coating gives best performancefor OCT below 5%✤ Effect of OCT will be less than pile up of NSB in this regime❖ LVR-3050 with coating gives best performance for OCT above 5%✤ Further optimization of coating thickness is critical
70— 3050— 3075— 6050— 6075— 7050▲ REF◆ LCT5● LVR◼ LVR2◻︎︎ LVR2 (no coating)

Photon Detection Efficiency (%)

60
50
4030
20
10005

次世代ガンマ線望遠鏡CTA用SiPMの最適化
日本物理学会 2017年秋季大会, SEP 12–15, 2017, 宇都宮大学

101520
Optical Crosstalk Rate (%)

253015 /16

Summary❖ SiPM performance does not vary among individual devices
within the same batch❖ PDE dependence on the relative over voltage is very similar
among Hamamatsu SiPM types if PDE is normalized by fillfactor❖ OCT is affected by protection coating✤ Smaller pixel size and thicker coating reduce OCT rate
✤ No coating significantly reduces OCT rate

❖ LVR2-6050 and LVR2-7050 with no coating gives best
performance for OCT below 5%✤ Effect of OCT will be less than pile up of NSB in this regime❖ LVR-3050 with coating gives best performance for OCT
above 5%
❖ Prospects✤ Optimize the coating thickness to minimize OCT rate
✤ SST-2M will use >100k of 6-mm SiPMs over the next ~4 years
次世代ガンマ線望遠鏡CTA用SiPMの最適化
日本物理学会 2017年秋季大会, SEP 12–15, 2017, 宇都宮大学

16 /16