CTA報告110：焦点面光検出器の光検出効率改善に向けた集光装置の開発
奥村 曉、小野 祥弥A、Dang Viet TanA、朝野 彰、片桐 秀明A、田島 宏康、手嶋 政廣B, C、中村 裕樹、林田 将明B、
山本 常夏D、吉田 龍生A、他 CTA Consortium
名古屋大学 宇宙地球環境研究所（ISEE）
茨城大理A、東大宇宙線研B、マックスプランク物理C、甲南大理工D
2016 年 9 月 22 日

cherenkov
telescopearray

日本物理学会

oxon@mac.com

Cherenkov Telescope Array (CTA)
cherenkov
telescopearray

The Next-generation Very-high-energy(VHE) Gamma-ray Observatory© G. Pérez, IAC, SMM
2Cherenkov Telescope Array (CTA)
Large-Sized
Telescope (LST)

Medium-Sized
Telescope (MST)

4@North + 4@SouthD = 23 m
FOV = 4.5°
E = 20–200 GeV

15@N + 25@S
D = 12 m
FOV = 8°E = 100 GeV – 10 TeV

North

Schwarzschild–Couder
Telescope (SCT)

Small-Sized Telescope(SST)

25@S
D = 9.6 m
FOV = 8°E = 200 GeV – 10 TeV

GCT (SC)~35@S
ASTRI (SC)~35@S
Davis–Cotton ~20@S
D ~4 m
FOV ~9°E = 5 TeV – 300 TeV

South

1 km

LST
MST
SCT
SST

3 km
Array Layout Examples© G. Pérez, IAC, SMM
3Large-Sized Telescope (LST)
23 m diameter × 4Will achieve 20 GeVenergy threshold
High statistics gammaray astronomy
28 m23 m‣

Gamma-ray burst‣

AGN‣

Connection to Fermi/LAT (20–300 GeV)

‣Origin of extragalactic
cosmic rays

The higher photon
collection eﬃciency isthe better
4

The LST Camera~2.2 m1885 PMTs

LST “Mini” Camera at ICRR
Comprises 1855 high-QE PMTs (Peak QE ~40%) with hemispherical cathode
Covered by hexagonal light concentrators to reduce ineﬀective area
“Winston cones” have been used in Cherenkov telescopes5

(TV-yp2)z2<•(12)

2 R 2,

where R2 is the radius of the exit aperture, so that22

Ideas (1) – Bézier Curve Profilen 2 R 2 > n1 R, sinOmax,(13)(a)

which is identical with Eq. (10) for rotationally sym2D Compoundmetricsvstems. Parabolic Cone (CPC)

ZdS+z Z axis

-max. -2-(Hexagonal Case R = 1.0, n = 1.0

θ cross

100

θ max

80

60

Winston Cone

2. Construction of an ideal light collector for the case of
constant index of refraction. In this example, 0Amas=160.

FIG.

mm

20mm

Quadratic Bézier

ρ 1 = 2040ρ 1 = 30

Collection Efficiency (%)

or Winston Conexl

A. Okumura / Astroparticle Phy

Cubic Bézier

Equations (7) and (10) provide the desired restriction
Winstonon the exit aperture, imposed by theinitial 1970
angulardivergence, for optical systems that are not necessarily

005

10
15
2025
Incident Angle θ (deg)3035

Okumura 2012

imageSo-calledforinig. a Winston cone accepts only photons(c) with angle of incidence within a cutoﬀ(R = 0.9, n = 1.5

angle (in 2D space)

IDEAL LIGHT COLLECTORS)

For a 3-dimensional hexagonal cone, use of a100Bézier curve profile outperformsθ cross θa max
We call an optical system that has the smallest
parabolicprofilef numberallowedby the sine inequality an ideal lightcollector. As is well known from the theory of lenses, a6

Ideas (2) – ABS Cone + Thin Specular Films
Okumura+ (2015)

(a)

(b)

(c)
t ~0.1 mm

t 0.5 mm
50 mm

ABS Resin conewith Thin Specular Films

Light weight
Very high reflectance (~95–99%) in a wide wavelength (300–800 nm) and angle (20–70deg) ranges that cannot be achieved by aluminum coating
Minimum dead area (~0.1 mm) thanks to the film thickness
700)

Ideas (3) – ESR Films + Multilayer Coating(b)Reflectance (%)

102
100
98

30°

20°

Okumura+ (2015)

40°
50°

96
94

60°

92

70°

P

90

RYA
NI
RELIM

50 mm
PMT

88UV
86
Enhanced
84 Coating
82
80
300

350

400

450

500
550
600
Wavelength (nm)Base film: 3M Vikuiti™ Enhanced Specular Reflector (ESR) (insulating, R >95%, >400 nm)
Multilayer coating: 54 layers (Ta2O5 and SiO2) to enhance UV reflectance≳95% reflectance in wide ranges of angle and wavelength8

PMT Anode Sensitivity

Okumura+ (2015)

Anode Sensitivity

Cathode Sensitivity

10.8

q = 22 mm

0.6

Angular Dependence
Relative Anode Sensitivity

Averaged Relative Sensitivity

Positional Dependence
1.3
1.2
1.1
1

Y

0.4

Y

0.9

Dy1

0.8
0.2

Dy2
Side View

0<20<10Front View

01020
Light Source Position Y (mm)0.7
−90−60−3003060
90Angle of Incidence (deg)CTA PMT (R11920-100-20) has positional and angular dependence of anode sensitivity
The cone profile was optimized considering these dependence and film reflectance
Note: Collection eﬃciency of a light concentrator cannot be defined9Performance

Measurements by Dang Viet Tan (Ibaraki)

100

100%

Rotational Dependence (365nm)
465 nm
365 nm90%

310 nm80

ryaniPrelim

6040

Relative Anode Sensitivity (%)

Relative Anode Sensitivity (%)

Color Dependence
100806040

310 nm
365 nm200−4020465 nm−30−20−100

φ = 0°
φ = 10°φ = 20°φ = 30°10203040Angle of Incidence (deg)0−40−30−20−10010203040Angle of Incidence (deg)The anode sensitivity normalized to a 25-mm mask shows 90 to >100%at the most important angles

25 mm

5–10% better than Al + multilayer coating on plastic

mask

Trade-oﬀ between detection eﬃciency and cost/labor/uniformity

PMT

φ50 mm10

Small-Sized Telescope (SST)
4 m diameter × 70

Primary
MirrorSecondaryMirror‣

3 designs proposed‣

SiPM based cameras‣

Spread over a few km2High-energy frontierup to 300 TeV
Origins of Galactic
cosmic rays

CameraPrototype of Gamma-rayCherenkov Telescope (GCT)‣PeVatron(s) search‣

Supernova remnants

The cheaper unit priceis the better for alarge eﬀective area
11

SiPM Cameras (SCT and SSTs)~40 cm11,328 pixels

2,048 pixels~80 cmSCT Camera

SST Camera

First use of Schwarzschild-Couder optics (aspherical primary and secondary)
Small plate-scale, small angular resolution (~4–6 arcmin), and large FOV (8–9°)
require a fine pixel size of ~6 mm and multi-channel photodetectors
12

SiPM Dead Space and Fresnel Reflection
Hamamatsu S12642-050
Primary0.20 mm Gap

Secondary
MPPC (Multi-Pixel Photon Counter) arrays

S13361-6050 series

Photon detection efficiency vs. wavelength (typical example)
50

Photon detection efficiency (%)

3 mm

Taken from HPK catalog(Ta=25 °C)

40

30

Camera20

100200

300

400

500

600

700

800

900

1000

Wavelength (nm)
KAPDB0318EA

Photon detection efﬁciency does not include crosstalk or afterpulses.

~15% photons are lost due to the pixel gaps (typically 0.2 mm)
6 × 106(Ta=25 °C)

Gain
Crosstalk probability
Photon detection
efficiency (λ=450 nm)

60ty, photon detection efficiency (%)

Overvoltage specifications of gain, crosstalk probability, photon detection efficiency (typical example)

Angle of incidence ranges from 30 to 60 degrees in SC optics, resulting inhigher Fresnel reflection than parabola or Davies–Cotton
Gain

5 × 106504 × 10640

3 × 106

302 × 1062013

Lens Array for SiPM

WHWLens Array

3.2 mmHSiPM
3.0 mmLens Array Simulation

Silicone is highly UV transparent and can be molded
Simulated a silicone lens array with various lens shape parameters by ROBAST(Okumura+ 2016, see http://robast.github.io/)14

Simulation Results
Positional Distribution of Detected Photons
161.4

29.5
2928.5+14.5%

1.2

1412110280.80.6

6

26.5 0.442620.2
25.52533003.0 mm

2.5

250

2200

1.5

150

1100

0.5

508

27.5
27

Efficiency (%)
Y (mm)

30valleyH (mm)Efficiency(%)

Relative Increase (%)

000

0.2

0.4

0.6

0.8

11.2
1.4
valleyW (mm)

000.5

11.5

2

2.5

3X (mm)0PDE can be improved by 14.5% (e.g., PDE 40% → 46%)
Photons are concentrated on the inner region of individual SiPM pixels
Currently prototyping a lens array made of UV-transparent glass
15

Summary

Developed a hexagonal light concentrator for CTA LargeSized Telescopes using highly reflective specular films
Achieved 80–100% collection eﬃciency for various colors
The film can be used for other detectors such as
scintillators
An idea of a light concentrator (lens array) for an SiPM
array has been also studied and proposed
Prototyping a lens array made of UV-transparent glass

16