Suppression of crosstalk in SiPMs forthe next generation gamma-rayobservatory, CTA16aS41-3

Hiro Tajima, Akira Okumura, Yuki Nakamura, Anatolii Zenin,
Nagoya University (for the CTA-Japan Consortium)
Japan Physical Society Meeting
Shinshu University, September 16, 2018

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

Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan2 /14

Requirements for Photon Sensors in CTA❖ 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✦ Blue to near UV sensitivity is important✤ Cherenkov photons arrives within a few to few tens of ns✦ ns-timing is important❖ Night sky background (NSB) is the dominant background✤ Rate is >25 MHz/pixel
Cherenkovspectrum✦ Dark count rate is not important✦ [NSB] x [Optical crosstalk (OCT)]
can cause false triggers
due to accidental coincidences• Low OCT rate is important

NSBspectrum✤ NSB peaks above 550 nm✦ Low red sensitivity is preferred
Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

300

400

500

600

700
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Photon Sensor❖ Silicon Photomultiplier is chosen as a photon sensor 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 accidental
coincidences✤ 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)

Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

credit: KETEK website

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Summary of Past Studies❖ Thicker coating or no coating give lower crosstalk
Crosstalk Rate vs Over Voltage
25
LCT5-3050 Epoxy 100 µm #1
LCT5-3050 Epoxy 100 µm #2LCT5-3050 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 (%)

2015100 µm300 µm

450 µm10No coating

500

246
Over Voltage (V)Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

8105 /14

Propagation of Crosstalk Photons❖ No coating (or very thin coating)✤ Reflected photons come back to the original cell

❖ Intermediate thickness✤ Photons reflected by the air interface may produce avalanches
in other cells❖ Very thick coating✤ Photons reflected by the air interface may get out of the device✤ Smaller device may have lower crosstalk rate

❖ How about the crosstalk from the backside?

Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

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Propagation of Crosstalk Photons❖ No coating (or very thin coating)✤ Reflected photons come back to the original cell

❖ Intermediate thickness✤ Photons reflected by the air interface may produce avalanches
in other cells❖ Very thick coating✤ Photons reflected by the air interface may get out of the device✤ Smaller device may have lower crosstalk rate

❖ How about the crosstalk from the backside?

Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

6 /14

Propagation of Crosstalk Photons❖ No coating (or very thin coating)✤ Reflected photons come back to the original cell

❖ Intermediate thickness✤ Photons reflected by the air interface may produce avalanches
in other cells❖ Very thick coating✤ Photons reflected by the air interface may get out of the device✤ Smaller device may have lower crosstalk rate

❖ How about the crosstalk from the backside?

Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

6 /14

Propagation of Crosstalk Photons❖ No coating (or very thin coating)✤ Reflected photons come back to the original cell

❖ Intermediate thickness✤ Photons reflected by the air interface may produce avalanches
in other cells❖ Very thick coating✤ Photons reflected by the air interface may get out of the device✤ Smaller device may have lower crosstalk rate

❖ How about the crosstalk from the backside?

Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

6 /14

OCT Dependence on Device/Cell Sizes❖ We have systematically investigated the OCT rate withvarying device size, cell size, and with and without coating✤ OCT rate is expected to be proportional to the number ofelectron-holes pairs (=charge) produced in an avalanche✦ proportional to a product of [cell capacitance] and [over voltage]✤ Find out propagation properties of crosstalk photons
Product ID

Device size

Cell size

Coating

Fill factor

S14520-3050VS

3 mm

50 µm

300 µm74%

S14520-3050VN

3 mm

50 µm

None

74%

S14520-3075VS

3 mm

75 µm

300 µm82%

S14520-3075VN

3 mm

75 µm

None

82%

S14520-6050VS

6 mm

50 µm

300 µm74%

S14520-6050VN

6 mm

50 µm

None

74%

S14520-6075VS

6 mm

75 µm

300 µm82%

S14520-6075VN

6 mm

75 µm

None

82%

Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

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SiPM Measurement Setup at Nagoya❖ Take waveform data by digital oscilloscope
8✤ 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 nm
for this measurement
Pulse Generator
Tektronix AFG 3251

LEDND filter

fiber

5432
10−1Oscilloscope
Tektronix MSO4054B

diffuser— Raw data
— Filtered data

7050

100

150

200

250

300

350

400

450

500t (ns)SiPM(Monitor)
ZFL-500LN-S+collimator

SiPM (DUI or Reference)climate chamber (25°C)
Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

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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 effects 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)0 p.e.

Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

1 p.e.

2 p.e.

3 p.e.

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PDE vs Relative Over Voltage

Average p.e.

Averege p.e. for Monitor SiPM
0.36

0.35
0.34

0.33

0.32

0.31

(Photon Detection Efficiency)/(Fill Factor) (%)❖ PDEs were measured for 2 devices for each type
❖ PDEs were measured twice for one device❖ Measured PDEs were very consistent, which indicatesvarying light intensity is properly compensated by themonitor SiPM
60504030— 3 mm— 6 mm
● 50 µm◼ 75 µm○◻︎︎ no coating

20100.30204060

80100120140160

180 200 220Measurement number

00Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

0.05

0.1

0.15
0.20.25(Over Voltage)/(Breakdown Voltage)

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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 within ∆tPS (~3 ns in our setup)
Probability to have no optical crosstalk

N(N(1.5 p.e.)= 1 (1 ROCT )e fDC tPS
0.5 p.e.)✓◆N ( 1.5 p.e.)
ROCT = 11/e fDC tPS
N ( 0.5 p.e.)

Poisson probability to have nooverlapping dark count within ∆tPS

<latexit sha1_base64="CtKuJlq36iMZ8HPcThLvX+ECQb4=">AAAC8nicpVJLbxMxEPYurxAeTeHIxWpElRyyrBGISgWpUnrgRAM0baU4RF5nNrHqfWDPIkWr/RlcOIAQV34NN/4N3jSt6OOAxEiWPn3fzPjzjKNcK4th+Nvzr12/cfNW43bzzt1799da6w8ObFYYCUOZ6cwcRcKCVikMUaGGo9yASCINh9Fxv9YPP4GxKkv3cZHDOBGzVMVKCnTUZN1rbPLYCFm+6fAZfKQseM63eSJwbpIyDyCoutWpFl6h0VeU9WiH9d5NTpW9/n7VpfCh7MVn3G6/4rugUVA84wbvq4rz5ub5ymVDriFG1/S/rHGjZnPsPvk3J81Jqx0G4TLoZcBWoE1WMZi0fvFpJosEUpRaWDtiYY7jUhhUUkPV5IWFXMhjMYORg6lIwI7L5coq+tgxUxpnxp0U6ZL9u6IUibWLJHKZtUl7UavJq7RRgfHWuFRpXiCk8uSiuNAUM1rvn06VAYl64YCQRjmvVM6FGzO6X1IPgV188mVw8DRgYcDePmvvvFyNo0EekQ3SIYy8IDvkNRmQIZFe5n32vnrffPS/+N/9HyepvreqeUjOhf/zD0TE6LM=</latexit>N(≧0.5 p.e.)N(≧1.5 p.e.)ƒDC: Dark count rate∆tPS: Double-pulse separation time

0 p.e.

Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

1 p.e.

2 p.e.

3 p.e.

11 /14

Crosstalk Rate vs. Over Voltage❖ Dark count pileup correction works well
❖ Factor out cell capacitance dependence of crosstalk rate by scaling itwith cell area and depth— 3 mm(assuming cell depth ∝ break down voltage)

45

Optical Crosstalk Rate (%)

Optical Crosstalk Rate (%)✤ 3 mm device gives slightly lower OCT than 6 mm device✤ OCT rate scales very well with cell capacitance with coating✦ Not so without coating✤ Differences among individual SiPMs are small
Before DCR pileup correction

403530254030
25201515101055246810Over Voltage (V)Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

After DCR pileup correction

352000— 6 mm
● 50 µm◼ 75 µm○◻︎︎ no coating00246810Over Voltage (V)

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Crosstalk Rate vs. Over Voltage❖ Dark count pileup correction works well
❖ Factor out cell capacitance dependence of crosstalk rate by scaling itwith cell area and depth— 3 mm(assuming cell depth ∝ break down voltage)

40

Scaled Optical Crosstalk Rate (%)

Optical Crosstalk Rate (%)✤ 3 mm device gives slightly lower OCT than 6 mm device✤ OCT rate scales very well with cell capacitance with coating✦ Not so without coating✤ Differences among individual SiPMs are small
Before scalingby cell capacitance

353025201530

After scalingby cell capacitance

2520151010550035— 6 mm● 50 µm◼ 75 µm○◻︎︎ no coating

246810Over Voltage (V)Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan00246810
Over Voltage (V)

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Fit to Otte Function❖ Optical crosstalk rate should proportional to the charge
produced in the avalanche and avalanche trigger probability
0exp[ COtte
VOV /VBR ])ROCT = COCT · (Fill factor) · (Cell size)2 /VBR · VOV · (1<latexit sha1_base64="Ol/4XzGnvXOahhwxeCAo1yA224k=">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</latexit>

cell capacitor

Avalanche probability✤ COCT is smaller for 75 µm cells (smaller crosstalk efficiency)✤ COtte is smaller without coating
Optical Crosstalk Rate (%)✦ Avalanche seed is produced in the region where it is harder totrigger avalanche
Product ID

COCT

COtte

S14520-3050VS

0.1

5S14520-6050VS

0.09

10

S14520-3075VS

0.06

18

S14520-6075VS

0.07

1720

S14520-3050VN

0.09215

S14520-6050VN

0.07

310

S14520-3075VN

0.03

5

S14520-6075VN0.03

5Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

403530PDE saturating voltage— 3 mm— 6 mm
● 50 µm◼ 75 µm○◻︎︎ no coating

25

500

pile up rate due to25 MHz night sky background246810Over Voltage (V)

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Fit to Otte Function❖ Optical crosstalk rate should proportional to the charge
produced in the avalanche and avalanche trigger probability
0exp[ COtte
VOV /VBR ])ROCT = COCT · (Fill factor) · (Cell size)2 /VBR · VOV · (1<latexit sha1_base64="Ol/4XzGnvXOahhwxeCAo1yA224k=">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</latexit>

cell capacitor

Avalanche probability✤ COCT is smaller for 75 µm cells (smaller crosstalk efficiency)✤ COtte is smaller without coatingOptical Crosstalk Rate (%)✦ Avalanche seed is produced in the region where it is harder totrigger avalanche
Product ID

COCTCOtte

S14520-3050VS

0.1

5S14520-6050VS

0.09

10

S14520-3075VS

0.06

18

S14520-6075VS

0.07

1720

S14520-3050VN

0.09

215

S14520-6050VN

0.07

310

S14520-3075VN

0.03

5

S14520-6075VN

0.03

5Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

403530PDE saturating voltage— 3 mm— 6 mm
● 50 µm◼ 75 µm○◻︎︎ no coating

25

500pile up rate due to
25 MHz night sky background246810Over Voltage (V)

13 /14

Fit to Otte Function❖ Optical crosstalk rate should proportional to the charge
produced in the avalanche and avalanche trigger probability
0exp[ COtte
VOV /VBR ])ROCT = COCT · (Fill factor) · (Cell size)2 /VBR · VOV · (1<latexit sha1_base64="Ol/4XzGnvXOahhwxeCAo1yA224k=">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</latexit>

cell capacitor

Avalanche probability✤ COCT is smaller for 75 µm cells (smaller crosstalk efficiency)✤ COtte is smaller without coating
Optical Crosstalk Rate (%)✦ Avalanche seed is produced in the region where it is harder totrigger avalanche
Product ID

COCT

COtte

S14520-3050VS

0.1

5S14520-6050VS

0.09

10

S14520-3075VS0.0618

S14520-6075VS

0.07

1720

S14520-3050VN

0.09

215

S14520-6050VN

0.07

310

S14520-3075VN

0.03

5

S14520-6075VN

0.03

5Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan

4035
30

PDE saturating voltage— 3 mm— 6 mm
● 50 µm◼ 75 µm○◻︎︎ no coating

25

500pile up rate due to25 MHz night sky background246810Over Voltage (V)

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Summary❖ Optical crosstalk rate is significantly affected by protection coating✤ Smaller device size and thicker coating reduce OCT rate
✤ Larger cell size increase OCT rate, but not proportional to the cell area
✤ No coating significantly reduces OCT rate
✦ OCT rate does not scale with the cell size
✦ OCT seeds are produced in the region where avalanche trigger probability islow❖ Prospects✤ 6 mm device with 75 µm cell without coating may be the best choice for CTA
Photon Detection Efficiency (%)✦ Without protective coating, damages on the SiPM surface are of major concerns• We put a piece of protective film to avoid 50damages during the assembly handling• Damages during observations can be40
avoided by the UV transparent shieldon the camera front
30
This shield will also act as a filter for red— 3 mmlights.)— 6 mm20✤ Further reduction of OCTby suppressing crosstalk due to photons
reflected at the backside of SiPM

1000

Suppression of crosstalk in SiPMs for CTA
JPS meeting, SEP 16, 2018, Shinshu University Japan● 50 µm◼ 75 µm○◻︎︎ no coating
51015202530
3540
Optical Crosstalk Rate (%)

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