CTA report 158:

Evaluation of a multi-pixel silicon
photomultiplier for a Medium-Sized
Telescope upgrade
Anatolii Zenin, Hiroyasu Tajima, Akira Okumura,
Yuki Nakamura

ISEE, Nagoya University

Weakly Interacting Massive
Particle (WIMP)
Baer, Howard et al., "Dark matter production in the early Universe:
beyond the thermal WIMP paradigm", Phys. Rept. 555 (2015)• "WIMP miracle"

Mass and cross section calculated
from the dark matter abundancecorresponds to independent WIMP
predictions by several promisingparticle physics models

Overlap• WIMPs are expected to annihilate
and produce gamma rays

1
• E ⇡ 10 EDM can be detectedmost eﬃciently<latexit sha1_base64="b1dWJxeDPQ0ObXj7JdrgMGKwBhE=">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</latexit>2

Cherenkov Telescope Array andatmospheric Cherenkov imaging

Very-high-energy
Gamma Ray
(20 GeV – 300 TeV)Electromagnetic Cascade

Large-Sized Telescope:
20 GeV – 150 GeV

Cherenkov Photons

Medium-Sized Telecope:
150 GeV – 5 TeV
Suitable for search for WIPMs
in the TeV mass range.
Cameras currently use PMTs
Small-Sized Telecope:
5 TeV – 300 TeV

K. Bernlöhr

3

MST camera
Credit: CTA website

High photon detection eﬃciency and high signal tonoise ratio are required to detect faint Cherenkov
flashes produced by gamma-ray showers
Photon density can be about ~300 photons per
square meter in the case of 1 TeV showers (~50
photon signals per pixel)
Night sky background (NSB) rate is about
200 MHz (~2 photon signals / pixel for a 10 ns
shower) in the case of MST
Credit: CTA website

Credit: A. Okumura

50 mm~1800 pixels

4

MST camera
High photon detection eﬃciency and high signal tonoise ratio are required to detect faint Cherenkov
flashes produced by gamma-ray showers
Photon density can be about ~300 photons per
square meter in the case of 1 TeV showers (~50
photon signals per pixel)
Night sky background (NSB) rate is about
200 MHz (~2 photon signals / pixel for a 10 ns
shower) in the case of MST
Credit: CTA website

Credit: CTA website

Light
Concentrator

PMT
Credit: A. Okumura
Credit: A. Okumura

50 mm~1800 pixels

5Reason to employ SiPMs
MST3

σ v (cm s-1)

By using SiPM we can extend
observation time allocated fordark matter search by up to afactor of 7, which will provide:

Predicted sensitivity to signal from dark matter annihilation(combination of expected annihilation channels)• Increased sensitivity

10

− 23

rs

10− 24

i

erm

F
10

y
6
T
A
L
-

− 25

50
2
S

HES

h

Expected cross section• Sensitivity in higher DM
mass range

10

− 26

CTA 500h(pessimistic)

10− 27 −1
101

Possible improvement(optimistic)10

DM mass (TeV)

Credit: A. Okumura

Features of silicon photomultipliers (SIPMs)○NSB tolerant. Operable under full moon (100xNSB)○High PDE (up to 58%)
○Compact, low voltage (60V)
×Too sensitive to NSB in > 550 nm range
×Optical crosstalk, high dark count rate

6

Comparison with current
photodetectors• Can not evaluate justby looking at PDE andQE because of light
concentrators• Measurement isnecessary due tocomplex angulardependence of QE and
PDE• Limited number of LED

Available LED
325 375
colors (nm): 310 340
402
arb. unit

Viability criterion: same or
better total Cherenkov
light yield and SNR

5

4.5

498
465

558

CTA SiPM PDE

CTA PMT QE

635

NSB-heavy
region

4
3.5
3
2.5

100 GeV γ
Cherenkov
spectrum2
1.5
1

La Palma
NSB spectrum

0.5
0
200

300

400

500

600

700
wavelength, nm

colors. Interpolation bysimulation is necessary
7Measurement setup
Evaluatedphotodetector

~4 ns
flash
Optical axis
Diﬀuser

2.55 mReadout
system8

Measurement setup
Optical axis

Rotation axis
(θ angle)

Camera pixel
Rotation stage

Monitor PMT

Credit: A. Okumura

9Simulated devices

ROBAST tool was used forthe ray-tracing simulation
Credit: A. Okumura

50mm
Credit: A. Okumura

PMT

SiPM

QE data by Hamamatsu Photonics

Geometry (data sheet)

Angular sensitivity dependence(measurement)
Positional sensitivity dependence(measurement)
Anode collection eﬃciency(95% assumed)

Refractive indices (measurements)
Absorption (measurement)
PDE data by Hamamatsu Photonics
Interference eﬀects are ignored

Light concentrator
Simulated reflectance of a special
high-reflectance coating
10Collection efficiencies ofcamera pixels• Simulation is• Measurement results
are "folded" withrespect to 0 deg.
• Marked points

represent data taken
for negative angles

λCollection eﬃciency (a.u.)

normalized to match
on-axis (0 deg.)
SiPM data

465 nm2.5

PMT simulation
SiPM simulation
PMT measurement

2

SiPM measurement

SiPM
1.5

PMT
1

0.5

0

0

5

10

15

20

25

30

35
40
Ѳ angle, deg

11discrepancy is~17% at 402 nm

SiPM simulation

2.5

PMT measurement

SiPM

SiPM measurement

2

1.5

PMT

PMT simulation
SiPM simulation

3

PMT measurement
SiPM measurement

2.5

2

1.5

1
1
0.5

0

Collection eﬃciency (a.u.)
λ• Most of thediscrepancy likely
comes fromuncertain PDE of
SiPM

PMT simulation

λCollection eﬃciency (a.u.)• The largest

340 nm

310 nm

0.5

0

5

10

15

20

25

30

0

35
40
Angle (deg)

0

5

10

4

PMT simulation
SiPM simulation

3.5

PMT measurement
SiPM measurement

3

15

20

25

30

35
40
angle, deg

375 nm

325 nm2.5
2Collection eﬃciency (a.u.)λ

•Simulation shows
general consistency
with experimentλCollection eﬃciency (a.u.)Collection efficiencies ofcamera pixels

PMT simulation

2.2

SiPM simulation

2

PMT measurement

1.8

SiPM measurement

1.6
1.4
1.2
1

1.5

0.8
0.6

1

0.4
0.5

0.2
0

0

5

10

15

20

25

30

35
40
angle, deg

0

0

5

10

15

20

25

30

35
40
angle, deg

12

PMT simulation
SiPM simulation

1.6

PMT measurement

SiPM

1.4

1

PMT

Collection eﬃciency (a.u.)
λ

9

6
5
4

2

0.2

1
10

15

SiPM measurement

7

0.4

5

SiPM simulation
PMT measurement

30

PMT simulation

8

0.6

0

• Most of the

SiPM measurement

1.2

0.8

discrepancy is~17% at 402 nm

discrepancy likely
comes fromuncertain PDE of
SiPM

1.8

Collection eﬃciency (a.u.)λ• The largest

558 nm402 nm20

25

30

PMT simulation
SiPM simulation
PMT measurement

2

0

5

10

15

20

25

30

35
40
angle, deg

635 nm

465 nm
2.5

0

35
40
angle, deg

SiPM measurement

1.5

1Collection eﬃciency (a.u.)λ•Simulation shows
general consistency
with experimentλCollection eﬃciency (a.u.)Collection efficiencies ofcamera pixels

40

PMT simulation
SiPM simulation

35

PMT measurement
SiPM measurement

30
25
20
15
10

0.5
5
0

0

5

10

15

20

25

30

35
40
angle, deg

0

0

5

10

15

20

25

30

35
40
angle, deg

13

Light yields of camera pixels(300-750 nm)
collection efficiency (a.u.)

NSB collection

PMT simulation

0.5

SiPM simulation

0.4

0.3

0.6

PMT simulation
SiPM simulation

0.5

Total light yields:

0.4

0.3
0.2

<latexit sha1_base64="I6h9HboSgppACwhQ/XatOFBl+Js=">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</latexit>

YCher (SiPM)= 1.60
YCher (PMT)

0.2

0.1

0

0.1

0

5

10

15

20

25

30

0

35
40
angle, deg

0

5

10

15

20

25

30

35
40
Angle (deg)<latexit sha1_base64="4nF2NEmBmE7P2endbZ4vxta9WhI=">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</latexit>

YNSB (SiPM)= 3.65
3.55
YNSB (PMT)

Photon distributions
a.u.

collection efficiency (a.u.)

Cherenkov collection

Simulated Cherenkov ph. dist.

2

monitor PMT

Simulated NSB ph. dist.

1.8
1.6
1.4
1.2

1

Higher Cherenkov
photon yield at the costof a large increase insensitivity to NSB

0.8
0.6
0.4
0.2
0
0

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5

10

15

20

25

30

35
40
angle (deg)Spdecreases by 1.2
NSB14Light yields of camera pixels(300-750 nm), with a filter
PMT simulation

0.3

SiPM simulation

NSB collection
collectiomn efficiency (a.u.)collection efficiency (a.u.)

Cherenkov collection

PMT simulation

0.14

SiPM simulation

Total light yields with afilter on a SiPM:

0.12

0.25

0.2

0.1

0.08

0.15
0.06
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0.1

0.04

0.05

0.02

0

5

10

15

20

25

30

0

35
40
Angle (deg)

0

5

10

15

20

25

30

35
40
Angle (deg)<latexit sha1_base64="cc57WM96Nl4SKAl/isB4LTXkfes=">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</latexit>

YNSB (SiPM)= 0.88
0.98
YNSB (PMT)

Assumed optical filter
PDE

0

YCher (SiPM)= 1.001.07
YCher (PMT)

0.6

0.5

Which means SiPM
might surpass PMT interms of performanceeven under normal
NSB conditions

PDE w/o a filter

0.4

0.3

PDE with a filter

0.2

0.1

0

NSB spectrum
300

400

500

600

700
wavelength, nm

15Summary
Our results:• Automated measurement setup for comparison of relative SiPM / PMT performance• Ray-tracing simulation with adjustable parameters that shows general consistency
with the measurement

Our estimation indicates that SiPM can detect the same amount of Cherenkov
photons and less NSB light, which demonstrates viability of the SiPM option.
Current predictions (with a filter):• An increase of 7% in SNR• Almost no change in Cherenkov light yield
To do:• Have to understand the discrepancy between the simulation and measurement andimprove the simulation

16